1988 Plymouth Horizon low gas mileage

You're asking for something I don't think ever existed. I have a couple of DRB2s, and two versions of the OTC Monitor 4000 which is almost identical, and was made by the same people. You might check eBay for an owner's manual for one of those.

The reason there is no manual is the operation is pretty self-explanatory. You press buttons based on menu choices. I have a DRB3 and there was an operating manual for that, but I got it from attending a Chrysler class on that scanner. Again, the menus are easy to follow and understand. If you plugged your scanner in already, you've seen how easy it is to follow.

What you're asking is equivalent to a wanne-be carpenter asking how to run a hammer. That's the easy part. What he needs to learn is what to do with the hammer once he has it.

What you should be asking is how to interpret the readings on the scanner, and that's what you're after with the diagnostic books. The service manual just tells you how to perform specific procedures. Chrysler produced a lot of diagnostic manuals for specific systems. I have hundreds of them at home, but they aren't needed once you become familiar with the circuits. Since I understand electrical theory, I don't need those manuals to know WHAT to test, but what I liked is they had drawings showing connectors and which wires were the circuits I was after. The tests they tell you to do, with "yes" and "no" results, are based on doing those tests with the DRB2, but you have to understand that those tests are for diagnosing the causes of the diagnostic fault codes, not all the various complaints that don't set codes. For example, there would not be a test for "poor fuel mileage". There would be a test for "oxygen sensor not switching properly from rich to lean". When there's no fault codes, you have to use the readings shown on the scanner to figure out the cause of the problem.

Chrysler actually does a pretty good job of providing system operation descriptions in the regular service manuals at the beginning of the various sections. Reading through those will help you figure out where to look, what to test, and what the test results mean.

To get the fuel mileage you listed, you should be seeing a lot of black smoke from the tail pipe. The MAP sensor has the biggest say in the fuel metering calculations. The Engine Computer just modifies those calculations slightly based on readings from other sensors. The first thing is to look at the vacuum reading and / or voltage for the MAP sensor while the engine is running. Next, if those incorrectly indicate the engine is under load or accelerating, double-check or pinch off the main vacuum hoses to see if vacuum goes up. A vacuum leak will result in low vacuum readings which equates to high load and the need for more fuel.

Most of the sensor are run on 5.0 volts, and there are mechanical stops, (throttle position sensor), or electronic circuitry that limits the signal voltage to roughly 0.5 to 4.5 volts. Anything outside that range is what triggers a diagnostic fault code. The MAP sensor can be expected to read around 4.2 to 4.4 volts with the ignition switch on and the engine not running. There is no vacuum, and that reading represents barometric pressure. Once the engine is running, intake manifold vacuum will bring that signal voltage down to, ... Uhm, ... Somewhere around 1.5 volts. The only way to get more vacuum is during coasting from highway speed.

Be aware some of these cars had turbocharged engines. In those the intake manifold goes into a pressure situation at times and the MAP sensor was different to accommodate that. If you get incorrect readings from the MAP sensor and you don't find any vacuum leaks, try a different sensor and be sure it's for a naturally-aspirated engine. 99 percent of used sensors are the right one for your engine.

The original MAP sensors used a piezoelectric crystal connected to a diaphragm. Tugging on the crystal produces a voltage just like in an older phonograph cartridge. GM developed that part and had tons of trouble with it, ... So they sold them to Chrysler. It's doubtful you still have the original one on your car. As I type this, I'm sitting in my daily driver '88 Grand Caravan, (which is 27 years old), and has an amazing number of original parts still working fine, but the MAP sensor had to be replaced when the van was still under warranty. The redesigned sensor uses a "strain gauge" which is a long thin wire wrapped around a core. As the diaphragm tugs on the wire, that wire changes length very slightly, and that causes it to change resistance very slightly. The circuitry amplifies that change and the result is what you'll see on the scanner. While this isn't actually done, that sensor is so sensitive to changes in vacuum, it could be used to measure engine speed by detecting the tiny increases in vacuum each time a piston takes a gulp of air.

There are a number of ways the MAP sensor can set a fault code, but those codes aren't very specific on '95 and older vehicles. One problem you might look for is if the hose going to the sensor is long and has dips in it where fuel vapors could condense. If the sensor is mounted by the right strut tower, just pull one end off and hold it up to see if any gas runs out. That gas will dampen the response of the sensor and can lead to setting a fault code related to slow response. There's a fix for that.

Boy, I can see we're going to become friends in a hurry. The points you made are the same points about newer vehicles that have me keeping my old rusty trusty '88 Grand Caravan on the road. I have a friend with a body shop who specializes in rebuilding smashed one and two-year-old Dodge trucks. He doesn't actually buy them. He has people in the area who call him with what they want, he finds them, fixes them, gets them inspected, then charges just for going to other states to get them and for repairing them. He has been threatening to build me a newer Caravan for years because the carpet is the only thing holding the front to the rear of my van, but I don't want anything to do with all the unnecessary, unreliable, expensive computers that I have to go to the dealer for. If you think the information for your Horizon is top secret, you should try working on GM products! They have some of the poorest customer business practices. According to a national-level trainer, Hyundai, Toyota, and Chrysler are the top three in the world for putting a customer's best interest ahead of short-term profits.

Now, for your car, understand that nothing on it is top secret. It's just that it isn't common knowledge. The fact that you already understand basic electrical theory puts you way ahead of most of the people I work with here. It is also the hardest thing for students to learn because the type of person who likes working on mechanical things learns best by observing how parts interact, and manipulating them in various ways. You can't do that with electricity, therefore, it's a mystery and scary. In my classroom, I compared everything electrical to water flowing in pipes and rivers, and I had real good success. Only one student had trouble, so we spent from 5:00 p.M. To 11:00 p.M. One night in the classroom until he "got it". Now he works for one of the premier auto electrical shops in my city.

I rolled my eyes when I read your comment about testing parts. We don't do that or have the time to do that. In fact, I saw a coworker get fired for doing that AFTER the problem was diagnosed and repaired. The other instructor I used to work with taught how to test parts because he never actually was a mechanic or learned how to efficiently diagnose problems. He taught strictly by the textbook, which no other instructor does.

In your case I'll let you test a part AFTER you've diagnosed it as defective and you simply want to double-check yourself or understand how the part failed and why it caused the symptoms it did. As far as testing every part, one-at-a-time, that would take you hours on your car, and it would take weeks on newer models. For now, you let the Engine Computer do its self-tests. It will tell you not which part to replace, as too many uninformed people think. It will tell you which circuit needs to be looked at. When a part is referenced in a fault code, that part is actually the cause of that code only about half of the time. This is where people say, "gee, the computer said to replace that part. I replaced it three times and the code keeps on setting". That's when they get me involved.

There are a lot of articles on this site that explain how sensors and some circuits work. Later I'll send you a private message with the name of my web site for basic electrical. Right now I'm still working on basic sensors and how the circuits work. You might also visit a nearby community college with an Automotive program. They will have older copies of their text books in their libraries. The information is totally relevant. We just get new versions every few years with the old information rearranged because for program certification, one of the requirements is our text books can't be more than five years old.

The other problem with producing a book like you're looking for is they can't possibly cover all the variations of things that can go wrong. If you look at your Honda book, there might be a dozen things listed for a specific complaint. What do you do when you go through the entire list and you haven't solved the problem? This would be like asking for written directions to follow a route to a city in a different state, then you find out the main route and the two alternatives are all closed for road construction, or none of those roads take you past the attraction the kids want to see. You need a road map so you can pick the best route AFTER you know where you want to end up.

A text book will tell you how the various circuits work, but again, if they tell you what to expect for a sensor voltage, you're going to find something different on an engine that's running perfectly fine. You have to take in the theory, which you can get from the Chrysler service manual, then use that knowledge to do your own diagnosis and evaluation of the test readings.

By the way, I never allowed my students to use those troubleshooting charts where you answer "yes" or "no" to a question, then follow the flow chart. Those are for people who don't have any idea what they're doing, and that ain't us. I had a dozen "bugged" cars for them to diagnose, and I proved to them in every case that once they diagnosed two causes of a dead charging system on a car, for example, they didn't even need the diagram to troubleshoot the third and fourth problems. This is where the "on-the-job-experience" pays off.

The systems on your car are real basic and easy to understand, but it still gets complicated when there's more than a failed part to consider, like on newer cars. You have to think about wires chewed up by mice, corroded connector terminals, wires rubbed through the insulation, things that were modified by previous owners, etc. Dealership mechanics don't run into those types of things very often.

I'm going to try to find some student workbooks at home that might help you. I had a house fire last year and moved all that stuff into my shop, so it's buried somewhere. Remember though, they deal with fault codes and the tests to solve them. If you don't have any codes, we'll have to go by symptoms and some tests I would do if I was working on it.

Finally, you might also ask an instructor if they have any older reference material that you can have. My office was always piled high with stacks of stuff I couldn't bring myself to throw out, but I would gladly give them to members of the community. After all, it was their property taxes that were paying my wages. We also had real high-level classes put on once per month by Carquest and their trainer who came from another state. Those were REAL expensive and were mainly meant for mechanics from independent shops who didn't get the manufacturer's training. Our instructor owns a shop in Illinois that specializes in the one car out of a hundred that everyone else has given up on. He networks with a lot of manufacturer's trainers, and together, they solve those elusive problems, then he designs a class to cover that car brand or system. A lot of that was so far over my head, but thanks to those books he put together, I can use them for reference now that those cars have been around for a few years and we've seen a few of them. He used to give me boxes of his leftover books to give away to students after he was no longer teaching that class. The problem is a lot of his information involved reading gas analyzers and seeing how the exhaust responds to things he forces to change. That usually caused my eyes to glaze over. He was covering stuff so far beyond what is covered in any community college program.

By the way, as a final parting comment, I thought I had reached the pinnacle of success when I was hired as the suspension and alignment specialist at a very nice family-owned Chrysler dealership, but it became obvious real quickly that the real experts were at the independent shops. I had the luxury of factory-sponsored training and a help hotline to call, but I was limited to repair procedures that were approved by Chrysler. That's fine if you have to replace a thousand-dollar wiring harness that's under warranty, but if the customer is paying the bill, you'll want to simply repair the broken wire. The independent guys had all the more cost-effective solutions and access to aftermarket parts. I also saw the same models over and over so I became real familiar with them, but the other guys couldn't say the same thing when they had to work on trade-in cars of a different brand. That's where my previous seven years experience at an independent shop payed off.

B the way, my father was "Sir". My students and I were on a first-name basis because they were young adults, (and we were all a little goofy). We do not stand on formality. If you just mentally bow down to your computer before your next reply to me, that will be sufficient!

That experience of which you speak is a real sore point with me. To be qualified to teach in the Wisconsin Technical College System, you either need seven years experience, or a college degree and two years experience. I replaced my instructor from the late '70s when he retired. The other instructor retired a year later. I was on the hiring committee for his replacement. We had the most excellent candidate, but we also had a nut-job of a vice president out of California who, for the first time in the history of our college, over-ruled our decision and picked a guy with no experience. He didn't even own a set of basic tools. The students hated him and his unrealistic requirements, and to this day I still hear horror stories from former students. The school had to hire two instructors to do parts of his job because the shop owners who hire the graduates were complaining that the kids couldn't do anything of value. But, ... They did have typical sensor resistance values memorized! No professional needs to do that. The Engine Computer does that and sets a fault code to tell us where to look.

I'm not going to argue with your success, but to get the ball rolling, I'll explain how the throttle position sensor works. It is fed with 5.0 volts and ground from the computer. In between those two terminals is a carbon strip. It's a resistor of around 5,000 ohms. The actual value is totally unimportant. In fact, no two sensors are ever exactly alike. The signal wire is connected to the movable contact that slides across that carbon strip. If this were a normal "potentiometer" like in audio equipment, running the sensor mechanically from idle to wide-open-throttle would cause 0.0 volts to show up on the signal wire at idle, and 5.0 volts at wide-open-throttle, but there's a catch.

There are mechanical stops inside the sensor so the movable contact can never go below 0.5 volts or above 4.5 volts. That is what you'll see on your scanner, but those values are approximate, and again, not critical. You might find 0.73 volts and 4.2 volts, for example, so don't get all "wrapped around the axle" if they're off a little.

The fun starts when there's a break in one of the circuits. That is usually a broken wire or corroded connector terminal, or, if you want to be devious, like I was, you take the sensor apart, separate a connection, then run wires from them to a switch. Now the students have a "bug" to diagnose. I suspect you understand electrical theory well enough to know what happens with open circuits. Suppose the ground wire for the TPS is cut. There would be no current flow through the carbon strip, no voltage would be dropped, and you'd find 5.0 volts on all three terminals at the sensor's connector. 5.0 volts is outside of that acceptable 0.5 to 4.5 volt range for signal voltage, and that's what the computer looks at to know when to set a fault code.

If there's a cut in the 5.0 feed wire to the sensor, you would find 0.0 volts on all three terminals at the sensor. 0.0 volts is also an unacceptable condition and will cause a fault code to set. The problem on a car as old as this one is the fault codes are very basic and will just say something to the affect of "incorrect TPS voltage" or "TPS voltage high or low". This is where you know now the circuit to look at, but you have to take at least one voltage reading to get more detail. These codes get a lot more detailed on '96 and newer cars, and there can be a dozen different codes for one sensor.

This brings me to what you should have found on your TPS with the intermittent contact. If the signal wire is cut, or there's dirt under the movable contact, you have no idea what voltage will be seen by the computer. Due to the interconnected circuitry inside the computer, the signal voltage is going to "float" to some random value. If that voltage is between 0.5 and 4.5 volts, the computer will accept it and try to run on it. To prevent that, all signal wires use a "pull-up" or a "pull-down" resistor inside the computer. When everything is working properly, the value of that resistor is so high that it has no affect on the circuit at all. However, when there's a break in that signal wire, the pull-up resistor places 5.0 volts on that terminal and the computer recognizes that as an unacceptable condition. That is what tells it to set a fault code. Most cars use pull-up resistors but there are some that use pull-down resistors to place 0.0 volts on the signal terminal.

This is what should have happened on your car if there were dead spots in the TPS. The clinker is there is always a list of conditions that must be met to set a fault code. If there's already a code set for something the computer needs as a reference to test the TPS, that test may be suspended, and the second defect won't be detected until the first problem is fixed.

You can see this on your scanner by simply unplugging the TPS. The signal circuit will be open, the pull-up resistor will put 5.0 volts on that computer terminal, and you'll see that 5.0 volts on the scanner.

There is one more clinker worth mentioning that affects some older GM cars. Any time the computer determines it can't trust a sensor's readings, it will disregard them and inject an approximate value to run on based on the readings of the other sensors. This is why sometimes you'll hear an engine runs better when you disconnect a sensor. The problem on some older GM cars is the scanner displays that injected value, not the defective value. If you unplug the TPS and have an open circuit on the signal wire, you'll measure 5.0 volts there with your voltmeter, but the scanner may show.70 volts. This is where you have to be smarter than the scanner. You'll still have the correct fault code, and you'll know the circuit to test. You just have to understand that your voltmeter is more accurate than the scanner.

There's one more point of value related to Chrysler throttle position sensors. When you replace one, or if you disconnect the battery or let it run dead, the Engine Computer loses its memory. It will have to relearn "minimum throttle" before it will know when it must be in control of idle speed. Until that occurs, the engine may be hard to start unless you hold the accelerator pedal down 1/4", you won't get the nice idle flare-up to 1500 rpm at start-up, and it will tend to stall at stop signs. To meet the conditions for the relearn to take place, drive at highway sped with the engine warmed up, then coast for at least seven seconds without touching the pedals. The extended high intake manifold vacuum indicates your foot is off the accelerator pedal, and the current signal voltage is what it is going to see at idle. If you install a new sensor with a lower voltage at idle, the computer will learn that right away. If you install one with a higher voltage at idle, the computer will always assume you're holding the accelerator pedal down a little, so it will let you be in control. Once you coast off an exit ramp or down a hill, the computer will learn the new sensor's reading at idle.

I agree with your comments about computers, but it's not all the manufacturer's fault. Most of the blame goes to our idiot politicians who write laws about things they don't understand. There's no denying computers have given us good things like Engine Computers, Anti-lock Brake Computers, and Air Bag Computers, but the technology is outpacing our need for it. There is no reason we need to involve a computer with wipers, head lights, and power windows. If a driver is too stupid to know how to adjust their power seat and they need a computer to do that for them, fine, give them that choice, but don't make all of us buy that unnecessary technology. The fact it can be done doesn't mean it's appropriate use of technology.

The diagnostic fault codes of which you speak are not top secret. The confusing part may be due to the fact Chrysler moved them around in the service manuals for a few years. You might find them listed in the "Fuel System" section, the "Engine Performance" section, or the "Emissions" section.

We have the codes listed here too. The '96 and newer OBD2 codes standardized them among all manufacturers. Those are here:

https://www.2carpros.com/trouble_codes/obd2/P0100

The '95 and older Chrysler-specific codes are here:

https://www.2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks

Actually, if you Google the Midwest Renewable Energy Association, you'll find all kinds of things like this. Their annual show is 40 miles from my house. (Bought my first solar panels there 18 years ago which I still use when I repair and display car radios at the nation's second largest old car show, 55 miles from me). My friend is always talking about Sterling engines but my eyes just glaze over. There are a lot of cars there each year that have been modified by their owners but they all have major drawbacks. One of the most popular is running diesel engines on cooking oil, but you still have to start the engine on diesel. One woman is so proud of her 2000 Ford Ranger that she paid $22,000.00 to have converted to an all-electric truck! That is not a typo. And for that ridiculous cost, it takes overnight to charge the batteries, they fill the box to the point she can't even fit a sack of groceries in there, and the range is a whopping 25 miles on a charge. What I am proud of is my '88 Grand Caravan that in ten minutes I can put enough cheap energy in the tank to let me drive anywhere for two weeks. She will have to live to 150 to get her money out of that investment. She might make it, but we know the truck won't.

I've had a lot of discussions and listened in with people a lot smarter than me, and in every case there are negatives that the average car owner would find unacceptable. You can't change habits overnight. Compressed natural gas has been around for over 20 years running the fleet of trucks for a local plumbing company, but it's just now that consumer fueling stations are starting to pop up. Not everyone is going to run out and buy a new car for tens of thousands of dollars so they can save a little on fuel costs.

By the way, for the last truck my friend built for himself, a 2012 Dodge diesel Megacab, he turned it into an 8' box with a dually rear axle. He took 60 percent of the truck's frame, and 60 percent of a new frame right from Chrysler that was never installed, (came through a salvage yard in Detroit), and put them together. The 8' box came from there too. It still had the evidence of where they were experimenting with running a diesel on compressed natural gas. So there's lots of stuff going on out there. It's just that there's too many drawbacks yet.

Remember too that automotive marketing is extremely competitive. If a manufacturer could advertise one extra inch of headroom, one extra horsepower, one extra mile per gallon, or for heaven's sake, one more cup holder, you can be sure they would do it.

I heard from a former coworker about someone who mistakenly ended up with a new 1980 Ford Bronco that got 30 mpg. Ford insisted he sell it back but he refused, and had to hide it. I mentioned that to an instructor at a Ford school I attended, and he knew all about it. The reason it was supposed to be top secret is they didn't want the dealers to be inundated with requests they couldn't fulfill. Turns out that since gasoline cools the intake valves, and the lead was a lubricant, they were having valve problems with that design. Have you ever watched an Indy-car race where they can adjust the fuel / air ratio from the steering wheel, and how many of them win or lose a race based on fuel mileage? Why not just run with that leanest possible mixture? It doesn't slow the car down very much. Also ever wonder why in NASCAR when a driver runs out of fuel and coasts into the pits, very often it's not too many laps before the engine blows up? The damage was done by running too lean and the valves overheated. The fuel does more than just make power.

It takes a specific amount of energy to move a specific amount of weight at a certain speed. We could all get 54 miles per gallon in a Prius if you don't mind getting out of the way of people on bicycles trying to pass you, or you could get a Horizon Miser with a carburetor and have fun zipping around all over. For the rest of us, we have to reduce weight, friction, or wind resistance, or reduce the power to just what is needed to maintain the top speed, but it will take forever to get there. A lot of college clubs build cars that get over 300 miles to the gallon, but no one would buy a copy. There's no room for a passenger or groceries, no air conditioning, and certainly no heated butt-roasting seats!

You have to remember too that it wasn't that long ago that a car was considered worn out at 150,000 miles. Today it's just getting broken in.

Every year I told my students they were free to prefer any brand of car they liked. They just had better not tell me theirs is better than mine unless they can tell me why.

In my case, I started out preferring Chryslers when I learned as a youngster that so many parts easily interchanged between years, engines, and models. For some cars you almost need to know the paint color to get the right PCV valve!

As for my van, the speedometer is driven by an ultra-reliable cable. If the speed sensor fails, I simply won't have cruise control or lock-up torque converter. I have an '89, a '94, and a '95 Grand Caravan. All of those will have no speedometer and no transmission shifting if the speed sensor fails. I can drive cross-country with no cruise control if I have to. Can't do that with the three newer vans.

My transmission is the three-speed hydraulically-controlled transmission based on the really tough 727. I use this van to drag around a tandem axle enclosed trailer that's bigger and heavier than the van. The newer ones with the computer-controlled four speeds would explode before I got to the end of my driveway.

As the suspension and alignment specialist at a very nice family-owned Chrysler dealership for ten years, I own all the special tools for replacing pressed-in front wheel bearings that Chrysler used to use, but for '87 and '88 minivans they experimented with bolt-on bearings, but only on the Grands with 15" wheels. If I have to replace a bearing while sitting in a snow bank at home, I want it to be a relatively painless bolt-on affair.

With 15" you got bigger brakes. I don't even need the trailer brakes hooked up to stop the procession.

Power locks; no computer
Power mirrors; no computer
Power seat; no computer
Delayed wipers, front and rear; no computer
Automatic transmission; no computer
Power steering; no computer
Interior lights; no computer
Power lift gate release; no computer
Head lights with high and low beams; no computer
Struts and shock absorbers; no computer
Brake lights and turn signals; no computer
Fuel level gauge, temperature gauge, oil pressure gauge, voltmeter; no computer
Day / night rear-view mirror that I am smart enough to know how to dim when I want it to, not when some computer tells me I want it to.
Back-up lights that turn on automatically with a simple switch; no computer
Electric starter motor with a neutral safety switch in the circuit; no computer
Front and rear heaters and AC units, four speed fan; no computer
No anti-theft system which are real effective at keeping owners from driving their cars.
CD / cassette radio that I can get the service manual for.
Oh, I do have an electronic module for the overhead compass / thermometer, but I can still drive the van if that ever fails. Try doing that if the electronic instrument cluster fails, which is real common.

Everything on that list involves at least one computer on every new car, and just to be sure there is sufficient use of unnecessary technology, we have computers to control the computers. It makes me sick to know that I can't buy a new van like this one, and I might have to make it last another ten to twenty years. I don't mind paying for someone to provide services that I can't do or don't care to do, but I'm a mechanic, and as such I refuse to pay another mechanic to do my job. The problem is there is way too much that can only be fixed by replacing expensive parts, including computers, then they must be programmed by the dealer. I will never own a vehicle that I am forced to go to someone else, period.

I've been amazed for 25 years how everything comes in pairs. Two cars need struts in the same day. Two leaking radiators in the same day. Now it's with these questions. I just replied to another fellow about a friend I tried to help a couple of nights ago. He's trying to repair an $8000.00 computer board for a half million dollar tree harvester. A cable got pinched and it blew a surface-mount transistor right off the board, then we found a roasted 8-pin IC. We spent two hours taking the machine apart so we could look at the good board in that one and get part numbers. I have a hot-air soldering station and the equipment to replace those tiny parts, but for the shock to destroy those parts like that, you know more parts are going to be damaged and have no external clues. They're trying to round up some other failed boards that we can swipe parts from, but I seriously doubt we're going to be able to fix this one. Still, they seem to appreciate my enthusiasm.

You can bet the owner of that machine is crying. How in the word did we ever manage to cut trees down before we had computers to tell us how to do it?

I have to assume the engineers are insane.

The reality is the production cost of these computers, once the development is done, is extremely low, so on their end, it costs them very little to give us touch screens, doors that lock themselves at 15 mph, interior lights that fade out slowly, and other similar toys and gimmicks. What they don't seem to care about is the difficulty in diagnosing this stuff when it fails, and you know with the environment it lives in, it's going to fail, and the cost of repairing these systems. Remember the kid's game "Mouse Trap"? The goal is to take something simple and turn it into something unnecessarily complicated. Well, with all the manufacturing technology we have today, they should be able to build a car that is ultra-reliable and trouble-free, but how can they make any money on that?