I found out what the body control module is

We work with all kinds of car owners from professional mechanics with many years of experience all the way down to those who can barely put in gas on their own. Often they just need reassurance and their hand held, or they need to know when and how to find a specialist.

I agree with your assessment of how some people communicate. I was way at the top, . . . of the bottom third of my high school graduating class, but I did pay attention in class, and I can put together a coherent sentence. You gotta remember for a lot of people we help, English is their second language, so allowances must be made. English was the second language for my stepfather who came here from Poland after WWII, and he became the chief editor of a newspaper. He could still be hard to understand at times, but you have to give him credit for putting in the work to live here in the U.S.

As a side note, when you take an Automotive course at a community college, it includes at least one Communications class. We get paid, and customers get charged, for what we write on the back of the repair orders. It's in the mechanic's best interest to be accurate and clear, otherwise some of what he does, he does for free.

One of my pet peeves is when people are too lazy to list all the related symptoms or clues they've observed, or they can't be bothered to list the engine size, then they get angry if we don't come up with the right solution right away. Most people are understanding and will try to work with us. Even then, it's very common for us to spend a couple of hours formatting drawings and diagrams for uploading, and typing long, detailed replies, then we never hear back. We don't know if we did any good or what the final solution was.

I didn't answer your previous question because there were too many ways to do that, and none would always apply. My best comment is the engineers can't design a car to do what they did for almost a hundred years without adding on a complicated, unnecessary, unreliable computer. We've had power locks, power windows, cruise control, air conditioning, horns, and interior lights since almost forever, but starting around the early 1990s, none of those systems work unless there's a computer involved. Chrysler had a delayed ignition key light that had a time delay back in the 1960s. It used an indestructible, inexpensive thermal timer switch that never failed. Today we must have interior lights that fade out gradually. That requires a computer. We had power door locks in the '60s that worked off a switch on the door. Today owners demand doors that lock automatically at a certain road speed or when you shift out of "park". That requires a computer. Driver's can't be bothered to push the "lock" button on their own. They also can't be expected to turn on their head lights in gloomy or rainy weather. We must have daytime running lights that run the high beams at 80 percent of normal brightness so they blind you in your rear-view mirror. That 80 percent involves "pulse-width dimming" which requires a computer. The same thing could have been done by turning on the low beams with a ten-dollar relay tied to the ignition switch. All of these computers live in a dusty, humid environment where there's huge temperature swings and lots of vibration. Those things are the deadly enemies of electronics, so by all means, lets see how much more of it we can hang on our cars.

All of these "features" require a Body Computer which adds greatly to the cost and complexity, and makes diagnosis take longer and repairs more expensive.

Computers on cars today are a fact of life that we have to live with, but now the manufacturers have figured out all kinds of ways to make them cost owners a real lot more money for repairs. I would not want to go back to the days before Engine Computers since they clean up the exhaust, but they do cause almost as much trouble as they prevent. I wouldn't have any complaint at all if we could just go to a salvage yard, buy a used computer, and throw it into a different vehicle. Some of this new programming they came up with was supposed to address the problem of car theft, but it's mostly costing legitimate owners, and cars are still being stolen.

Sorry for getting off track. Your Body Computer controls a lot of functions, and they vary depending on the optional equipment your car has. Ford can call them their "Generic Electronic Module, (GEM). On some Dodge trucks it was called their "Central Timer Module". "Body Controller" is another name you will see.

The sensors you asked about gets even more involved, and confusing. The Engine Computer needs to know road speed so it can initiate some emissions tests at higher speeds where the driver won't notice subtle changes in engine performance. The Transmission Computer needs to know road speed to calculate shifts points. The Body Computer wants to know road speed to know when to lock the doors. The Air Bag Computer needs to know road speed so it doesn't deploy the air bags at low speeds where they aren't necessary or effective. Can't forget the instrument cluster that can't run the speedometer without knowing road speed. That's just five of the computers that would each need a vehicle speed sensor, (VSS).

The Engine Computer obviously needs a coolant temperature sensor to know when to advance ignition timing, when to turn on the radiator fan, and when to add in the oxygen sensor readings to the fuel metering calculations. The Transmission Computer modifies shift schedules based on engine temperature, so it wants to know coolant temperature. If you have a temperature gauge on the instrument cluster, it needs to know coolant temperature. Even your Climate Control Computer makes vent adjustments based on coolant temperature. That's four computers that need to know the same thing, but there aren't four coolant temperature sensors on the engine.

Instead, there's just one sensor for most common circuits, then its information is shared between modules. That is done with a pair of wires called the "data buss". Today there's typically one coolant temperature sensor on the engine. It is connected to the Engine Computer. The transmission could have a fluid temperature sensor on it. It also usually has the vehicle speed sensor somewhere on its tail housing. Intake air temperature sensor is usually somewhere on the fresh air tube leading to the throttle body. It could instead be on the throttle body itself, built into the mass air flow sensor on all brands except older Chryslers, or it could be inside the Engine Computer where fresh air flows through, and may be called the "battery temperature sensor". That was an older Chrysler thing. A number of computers make adjustments based on incoming air temperature, or "ambient air temperature", but there's usually just one of those on the car.

The point I'm getting to is you can't ask how many sensors there are for any one computer, even when you count how many are bolted to the engine, the transmission, or are somewhere else. The only time you need to know which computer a sensor is wired to is when you're diagnosing that circuit. The easiest way to follow this is to look at just the Engine Computer. Suppose a dozen sensors are connected to it. Besides their signal voltages, that computer also knows engine speed, engine load, injector pulse width, (length of time they're pulsed open), throttle position, throttle direction of change, throttle rate of change, when it's at idle, and when it's at wide-open-throttle. It even knows when there's unburned oxygen in the exhaust. All of that data is compiled into a packet of digital information that is transmitted onto the data buss hundreds of times per second. Those two data buss wires are spliced and connected to every other computer module on the car. There can easily be over four dozen of them. They all receive that packet of information, then each computer can pick off what it needs to look at, and it will disregard the rest.

Once the Engine Computer is done sending its data, a different computer gets a turn at doing the same thing. The Transmission Computer, for example, will broadcast which gear it has put the transmission in, what its fluid temperature is, even how much wear has taken place in each of its clutch packs. Now every computer gets to see that information and use just the parts of it that's needed. The Overhead Compass Module, for example, doesn't care about clutch wear, so it disregards that data. Once the Transmission Computer is done sending its packet of information, the next computer does the same thing. In this way, every computer has the opportunity to know what every sensor is reporting, and how every system is operating, regardless if it needs to know that or not. This sharing of information is repeated hundreds of times per second. It is why every computer doesn't have to have its own dedicated sensor. Every sensor's readings get sent to every computer.

The best example of this is the instrument cluster. Many years ago they would have their own coolant temperature sensor, fuel level sensor, oil pressure sensor, and sometimes one or two others wired directed to it. Today there are no sensors wired to the clusters. They get all their data over the data buss.

To add to the misery, for the last dozen or so years, there can be two or three separate data busses on a car. There started being too much information and too many computers for one buss to handle. You may see a reference to a "Buss A", "Buss B", and "Buss C" in one service manual. Liken that to three entirely independent conversations taking place in a tavern. Some of the people might shift around and interact with people from a different conversation to share information, but then they go back to their original group.

Sometimes you'll see a reference to a "low-speed data buss" or a "high-speed data buss". The high-speed buss is just like with home computers where you pay more to get higher speeds, but you'd only do that when necessary. Air Bag Computers are the best example of one that uses the high-speed buss. It has to receive crash sensor data, and send out bag firing voltages instantly. To be most effective, decisions and actions must take place within a few microseconds. Engine Computers typically have two data busses they use. Circuits that involve emissions require immediate updating. Those actions related to turning on a fan relay or emissions solenoid can use the slower buss where a delay of a fraction of a second is meaningless. Same with the overhead compass or instrument cluster. Those can update through the slowest data buss there is, and it will still be hundreds of times faster than our brains can perceive.

All of this has nothing to do with oil changes. There is no sensor involved in when to change the oil. Some vehicles today do have some kind of dash indication of when the oil should be changed, but that is done through software in one of the computers. My truck has a digital gauge to show how much life is left in the oil, but that is calculated by looking at the amount of short-trip driving, long-trip driving, average engine temperature, average intake air temperature, and things like that. It even looks at the on-dash trailer brake setting when it sees a trailer is connected.

There's also no correlation between a failed sensor and the engine locking up. Seized engines are rare. Failed sensors occur all the time. This would be like saying the likelihood of having a flat tire is based on the position of your power seat. People come up with all kinds of ways to figure out when to have the oil changed. Every three months is too often if the vehicle is only driven one day a week, but it's not often enough if it's driven just a few miles at time, multiple times per day. We've always been taught almost all engine wear takes place in the few minutes when the engine warming up. That's when parts haven't yet expanded to fit properly, mainly pistons. Very little wear takes place once the engine has reached its operating temperature. That's why semi trucks can reach well over a million miles with no major repair work. They're always warmed up.

The thing to look at is what the oil has to do. Its job is to "isolate moving parts from each other". That refers to bearings and journals, and other parts that would come in contact with each other. Beyond that, it's the additives that are important and that wear out. One we've all heard about is detergents. That scrubs off carbon and other deposits. Another is "dispersants". That additive helps to keep the deposits in suspension so the oil can carry them away to the filter. Without dispersants, the bad stuff would settle at the bottom of the oil pan where eventually it could form sludge and block the oil pick-up and starve the engine of oil.

Corrosion inhibitors neutralize the acids that form from combustion gas blowby past the piston rings. Most of that blowby gets pulled out by the PCV system to be burned, but there's always a little that gets in the oil. Seal conditioners are mostly to keep the front and rear crankshaft seals soft and pliable.

Everyone you talk with will have their own story about a favorite oil brand and when the oil should be changed. I have two comments in that regard. First, it is common to hear someone's engine developed a severe oil leak right after changing brands or when switching to synthetic oil. It's not the oil, but the additives that might be responsible for that. When you drain five quarts of oil out during a normal oil change, there's still about two quarts stuck in the passages and oil pump that don't drain out. That may still have some additives in it. A different brand of oil might have just as good additives, but they are not compatible with those in the old oil. A perfect example of that is a detergent in the new oil that attacks or degrades the seal conditioner in the old oil, then the seals start to leak. Sometimes that leak will clear up over time if you stick with that new brand of oil. Sometimes the leak will slow down, but never totally clear up even when switching back to the old brand. The same is true when switching between regular and synthetic oil. Best advice here is to stick with one brand you like.

For my last comment of value related to oil, every container will have two designations to indicate whether it's compatible for your engine. For the most part this is a factor of the age of your vehicle. You'll see something to the effect of "SG / CH" on the label. Car manufacturers are constantly improving metal alloys and technology in their engines. Part of that improvement requires the oil to meet a very specific set of conditions to provide the needed protection. When a major advancement is made, they will specify what is required of the oil, then the oil producers have to change their formulations to meet those requirements. A good way to look at this is to say with each oil update, it gets "better". If the old formulation was okay for your engine, the new formulation will work just fine. It's the other way you can't go. If you buy a new car today, containers of oil you had stockpiled in your garage for many years will not meet the new car's requirements. It doesn't mean the engine will fail. It means you won't be getting all the protection you could have. That might shorten the engine's life a little. Of bigger concern now is variable valve timing. That's a new technology that greatly increases fuel mileage and horsepower, but it puts more responsibility on the oil. These systems have valving that will be adversely affected by using the wrong oil.

To get back to those classifications, in my "SG / CH" example, the "S" stands for "spark-fired engines", meaning gas engines. The "C" stands for "compression-fired engines", meaning diesels. Every time the oil's formulation goes through a significant improvement or change, the second letter increases by one, so an oil listed as "SG" is the next evolution after the old formulation that was listed as "SF". The next time there's a major advancement, which could be a few years down the road, it will be listed as "SH". I don't even know what they're up to now. We don't have to concern ourselves with that. Any oil you can buy anywhere will be of the newest classification. Once they develop the newest formula, they stop producing the older one. And remember, the newer formulas are always more than good enough for older engines. That's an age thing, not a mileage thing. There are special oils available for very high-mileage engines, regardless of how old they are. I've had a few Dodge Caravans with well over 400,000 miles, and I only used the least expensive store brand oil I could find. It may be true you get what you pay for, but in my case, that store brand oil was produced by a well-known company and was more than good enough for my needs.

Basically what I'm saying is three-month oil change intervals probably shouldn't be what you plan. Instead, look at the type of driving you do, how many miles you've driven since the last oil change, and how many of those miles were with the engine warmed up. Also consider if you drive on a lot of dusty roads and how often the engine has to pull a heavy load, whether that's a trailer, a carload of people, or it goes up and down a lot of mountains. We used to recommend an oil change every 3,000 miles, but today with the much better additives, it's not uncommon for a manufacturer to specify as much as every 10,000 miles for their vehicles. In the 1960s and '70s, 3,000 miles could mean a year for a lot of people. Today many of us put on that many miles in a month. If you do this type of calculations, it still might turn out to be an oil change roughly every three months, which is fine.

Be aware, some manufacturers, Ford in particular, used to specify a really high oil change interval to make their cost of regular maintenance appear to be much lower than that of their competitors, but when you checked the owner's manual, it showed that was for "normal", or "light" service. We used to joke about it. When you read further, there were so many conditions and limitations that it was impossible to meet the "normal" driving. No matter how you drove or how far, it always came down to using the "severe" schedule, which was identical to those of every other manufacturer, but that's not what they advertised.

Check out these articles too when you have time:

https://www.2carpros.com/articles

There's also well over 700 videos on YouTube that show how to do a lot of common repairs and services.

The "green silicone plate" you're thinking of is the circuit board all the components are soldered onto. Some also have the connector terminals soldered to them. In tvs and vcrs those boards usually have copper "traces" or wires, on both sides. I got tricked once when building a "bugged" car for my students to diagnose. I drilled through the board to run a wire, then found out they can have two additional layers sandwiched in between. Never saw anything like that in 35 years of repairing tvs. With tvs, we typically had just one circuit board to deal with, and we diagnosed down to the failed transistor or other component, then replaced just them. We never do that with computers on cars, partly because we can't get diagrams for them in service manuals, partly because we can't get the specialty parts, partly because we don't have the training to diagnose the failed part, and a lot because if we mess up the repair, it could leave a customer sitting on the side of the road, ... In the middle of winter, ... On a dark Saturday night, ... In the middle of nowhere, ... With no cell service.

Your assessment of ignition and fuel systems is essentially correct, but you're diving too deeply into the design element. The fuel pump is just a motor-driven pump that builds fuel pressure. That pressure is regulated with a simple spring-loaded valve that used to be on the fuel rail on the engine, which also fed the injectors. They had a fuel return line going right back into the tank to dump the excess fuel there. Today most designs put that regulator right in the tank where we don't even see it.

Injectors and ignition systems are triggered by the Engine Computer. Those computers look at all the sensor data and operating conditions to calculate ignition and injector timing for best performance with lowest emissions.

When you look at any computer or suspect one of not working properly, a good way to look at it is what I learned about testing integrated circuits in tv repair. It's a black box with unknown stuff inside, but there are a number of things they all need. First, they must have a ground and a power source, meaning a voltage supply. ICs usually just have one of each, but there can be more. Automotive computers almost always have multiples of each. It's only the Chrysler products that I can recite from memory, but other brands are similar. Their Engine Computers will have four ground wires. One is called the "signal" ground and one is called the "power" ground. Those names can be misleading because they don't refer to their function. They refer to the types of current that flows through them. The power ground carries current returning to the battery for high-power circuits to include injectors, ignition coils, and solenoid and relay coils. There is always a little resistance in a wire and in a connection. A tiny amount of voltage is "dropped" across a resistance when current flows through it. That little voltage drop can't be avoided and is totally insignificant to those circuits.

The signal ground carries the return current for the sensors. That current is extremely small, so for all practical purposes, there is no voltage drop to speak of. The reason for the two separate ground circuits is if everything shared just the one wire, the voltage drops created by the injectors and ignition coils would appear on the sensor circuits. They would interact, and even a few hundredths of a volt error can mean a real lot to the computer. For example, for a long time, Chrysler was the only manufacturer that didn't need a mass air flow sensor to make their engines run right. They used the MAP sensor to measure engine load. That's just a fancy vacuum sensor, but they are very precise. Once the computer learns the "personality" of that sensor, it adjusts fuel metering calculations according to hundredths of a volt signal change, where the "normal" or acceptable range is 0.5 to 4.5 volts. If the two grounds were shared on one wire, each time an injector or ignition coil fired, there would appear possibly as much as a tenth of a volt, give or take, on that wire. That would raise every sensor's ground from a perfect 0.00 volts to 0.10 volt, and that would increase its signal voltage. That would cause all those sensors to report the wrong signal voltages.

Along with having separate signal and power grounds, there are two of each, for redundancy. That means if one wire is broken or has a rusty connection to the body sheet metal, there's a second one to do the job.

Next, you must have the voltage supply to run the computer. Here again there can be up to four of them. One gets 12 volts from the ignition switch to turn it on. That's called a "switched 12 volt" supply. A second one is on all the time, as long as the battery is connected. That is a very low current circuit that provides the "memory" power to keep the memory alive at all times. In that memory is stored any diagnostic fault codes that have been set, those sensor personalities I mentioned, and a huge lookup table of fuel trim numbers. There can be thousands of cells, each with a calculated value the computer has learned over time. One cell will include the results of calculating fuel needs for a specific set of conditions. Those will include things like engine speed, engine load, throttle position, throttle direction and rate of change, (think of accelerator pumps on older carbureted engines), coolant and air temperature, whether a charcoal canister purge valve has been commanded open, whether the EGR valve is open, and a lot more things like that. The 12 volt memory circuit keeps that data stored so it's ready for the next drive cycle. When the battery is disconnected or run dead, that memory is lost, but most of it gets rebuilt as soon as you start driving again.

The computer starts out with a set of default values programmed in at the factory. Once it calculates a better value for a cell, it puts that in there and uses that value. That is called the "short-term fuel trim" value, (STFT). If it sees it is always making the same correction over and over, it moves that value into the "long-term fuel trim" table. That table is what it starts out with each time you start driving. To say that a different way, if you observe those fuel trim numbers on a scanner, and you see a number other than "0" on the short-term table, the computer is making a change right now. If you see a number in the long-term table, it has been making the same short-term adjustment continuously, so it is using it as the starting point now. A positive number means it is adding a little additional fuel than what was programmed at the factory. Varnish in an injector and low fuel pressure are two common causes of insufficient fuel that the computer makes up for. A negative number in a cell means the computer is subtracting fuel. That is done by pulsing the injectors open for a little less time. On most car models we can't actually read the values in every cell in the lookup table. Instead, we are shown the overall average of short-term fuel trim data as a single value, and the same with the long-term. The best indication the engine is running right is when the short-term value is close to "0". You'll never find both values at exactly "0".

A third 12 volt supply brings in the current needed to run everything that is switched on and off by the computer. One of those things is the automatic shutdown, (ASD) relay on Chrysler products. On other brands, that can be called the "main relay", ignition relay", "ECM" relay, (Engine Control Module), or something similar. When the computer knows the engine is rotating, (cranking or running), it turns that relay on which sends current to the injectors, ignition coils, alternator field, oxygen sensor heaters, fuel pump or pump relay, and a few other places. One of those "other" places is right back to the computer on the fourth 12 volt supply. After the computer has commanded the ASD relay on, that fourth supply is used to verify it did indeed turn on. It is also used to measure the system voltage, then its internal voltage regulator adjusts how hard the alternator works, thereby holding system voltage within proper limits. Sometimes other circuits don't get switched on until the computer sees the 12 volts show up on that fourth terminal.

Next, every IC and every automotive computer needs inputs. Those include signal voltages from sensors, and on / off voltages from switches such as the brake light switch, cruise control switches, and the air conditioning system.

The fourth thing is the outputs. If we assume there is nothing wrong with the computer, and there are no defects in those output circuits, when you have the powers, grounds, and inputs, you will get the desired outputs. Those are the things the computer acts on, based on its decisions and calculations. If you're missing a ground or a power circuit, the computer can't operate. If you're missing an input or it is wrong, the computer can't be expected to make the right decision.

An interesting thing has been programmed in to most Engine Computers. First, they can figure out when a sensor signal is wrong or not to be believed. They will set, or memorize a diagnostic fault code to help you know which circuit needs further diagnosis. (They never ever say to replace a part). Next, if that defect might adversely affect emissions, it turns on the Check Engine light. There are well over 2,000 potential fault codes. Only about half of them turn the Check Engine light on, so you must still check for those codes even when the light isn't on. The interesting part of this story is in a lot of applications, when the computer knows it can't trust a sensor's readings, it disregards it, then it can "inject" its own value that it closely calculates, and run on that. Engine performance may not be at its best, but the engine will run.

To boil this down for diagnosing a problem, you must have powers, grounds, and inputs to get the desired outputs. When you aren't getting the desired outputs and you suspect the computer is defective, the first thing to do is check if you have the required 12 volt supplies. I use the connector views in the service manual because it shows the terminal layout, then there's a chart that lists the purpose, or function, of each of them. Just scan down the list to find every 12-volt supply.

Next, do the same thing with the grounds. The worst way to check ground wires is with an ohm meter. The only time this works is if a wire is completely broken or was accidentally left disconnected. Most problems stem from corrosion, then, all it takes is for one tiny strand to still be intact for the ohm meter to say that wire is okay. In reality, not enough current can get through that one strand. The connection has high resistance, so there will be a significant voltage drop across it when current is trying to flow. It's that voltage you want to measure. It may only be a few tenths of a volt, but while we usually can't measure the little resistance with any accuracy, we can measure the results of that resistance, which is that voltage. That has to be done with the computer turned on and trying to operate.

Very often, to save our customers' time and money, we'll skip checking the powers and grounds when we have a diagnostic fault code to get us started. They get specific enough to tell us we need to be looking for a break in a wire, a wire that's shorted to ground, both on an input or output circuit, when a sensor is reporting a wrong value, or when it observes an incorrect operating condition, such as running too lean or too rich. It's only when we rule out everything else and the computer is suspect that we check the powers and grounds before replacing that computer.

To answer the breakdown question, my first Caravan was an '88 model. Those were the last year to use a smaller version of their really tough 904 and 727 automatic transmissions. I used my van to drag a huge tandem axle enclosed trailer to an old car show swap meet for over 15 years. The trailer was bigger and heavier than the van. That transmission got one fluid and filter change in its life, only because the $3.50 side cover rusted out and the fluid had to be drained anyway. The brushes wore out in the alternator, which is common on every brand and model by around 150,000 to 200,000 miles. When that happened on my "94 model, I was able to replace the brushes without removing the alternator from the engine. The brush assembly used to cost around $10.00, far less expensive than replacing the entire alternator. The newer model also developed worn contacts inside the starter solenoid. That caused a very intermittent failure to crank that got progressively worse over many months. Most people just replace the entire starter, but I always relace the worn contacts. Cost is three dollars each, and it takes about an hour to remove the starter and do the repair. The same starter is used on Toyotas, and they develop the same problem with the same easy repair.

Had to replace the fuel pump once. Replaced the rusted gas tank at the same time. Also had to replace the pickup screen in the gas tank twice. Those are a very elusive solution to a stalling problem when coasting. The clue is the engine runs best under load and at high speeds when the most fuel is being used. Don't waste your time and money replacing a fuel filter on a Chrysler product. They commonly last the life of the vehicle unless they rust out and start leaking, like mine did. Unfortunately, where I live, they throw a pound of salt on an ounce of snow, and my '88 van got so rusty, the carpet was the only thing holding the front and rear together. I've been watching for a nice rust-free one from down south, but it has to have factory-installed 15" wheels. Those models had bigger brakes and easy-to-replace front wheel bearings. They also had a speedometer cable. No sensors or electronics involved in running the speedometer.