1999 Dodge Caravan alternator

AGGGGHHHH!

DO NOT DISCONNECT ANY CABLE WITH THE ENGINE RUNNING!

Every year I did a demonstration on the alternator test bench for my students to show what can happen when you do that. It was real easy for the voltage to reach over 35 volts. That WILL destroy any computer on the vehicle, the alternator's internal diodes, the voltage regulator, and any light bulbs that are turned on.

The thinking is that if you disconnect either cable and the engine stays running, the alternator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good alternator appear to be bad so that test is not valid.

If a mechanic is caught pulling this stunt he will typically get one verbal warning. For the second offense he will be fired. It's that big a deal.

Some alternators respond to the high points in the ripple. That momentary higher voltage goes right back to the field winding and creates a stronger magnetic field. That stronger electromagnet creates a higher output voltage which again creates a stronger electromagnet. It's a vicious circle and voltage can keep on rising until something gives out. The main thing that smoothes out that ripple so it doesn't affect the voltage regulator or the alternator is the battery.

Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, ... As long as you don't raise engine speed.

One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the alternator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.

Another common alternator problem is one defective diode out of the six. You will lose exactly two thirds of the alternator's capacity but system voltage will remain normal or it could even be just a little high from the voltage regulator responding to the greatly increased dips in the ripple voltage.

It's always a good idea to wear safety glasses when working around car batteries, but if you still insist on removing a cable while the engine is running, a face shield makes more sense, and have plenty of water on hand to wash any acid off the vehicle's paint.

Ford used to have a really nice generator design that allowed testing right on the back of the unit. Only Chrysler alternators are easier to diagnose. Unfortunately the engineers don't really care about ease of service on GMs and many other brands.

The way you tell if the charging system is working is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. There still could be a bad diode though. You need a professional load tester to test for that. Ripple will be very high and the most output current you will get will be one third of the generator's design value. That is not enough to meet the demands of the electrical system under all conditions so the battery will have to make up the difference, until it runs down.

What problem are you having or what are the symptoms that you replaced the alternator for?

Sorry that I overlooked your reply. The majority of people at auto parts stores never were mechanics. They're salespeople. In this case you got very bad advice even though the person may have been trying to be helpful by giving you a test procedure that wouldn't cost anything.

The best test is the one I mentioned about measuring the battery voltage with the engine running. You stated the battery will be dead in three days, but you didn't say if that's sitting for three days or driving the van for three days. If the van is just sitting, a good battery will have enough charge to crank the engine for starting after sitting for three weeks. In practice you'll find it's longer than that, but that's the industry standard. The relatively short time is due to the constant drain of the memory circuits of the many computers on the van. If your battery is fairly old, its capacity is going to be a lot lower and you won't get three weeks out of it without driving the van often to keep it charged.

If you're driving the van daily and the battery only lasts three days, you either are doing a lot of short-trip driving or the alternator has a bad diode. With one bad diode of the six, you will lose exactly two thirds of its capacity. 30 amps from the common 90 amp alternator isn't enough to meet the demands of the electrical system under most conditions. The battery will have to make up the difference until it slowly runs down. That could take three days. If the alternator has completely failed, you'll only get about one hour of driving time from the battery.

If you find around 12.6 volts or less on the battery with the engine running, we have to diagnose the charging system. If you find between 13.75 and 14.75 volts, it's working, but the next half of the test requires a professional load tester. All repair shops have one. That will show how much current the alternator is capable of producing, and it will show "ripple" voltage. Those two things will tell us if you have a problem with the charging system or if we have to go looking for a drain.

Please start a new question specific to your van. That will help others researching the same problem. Also, this was a conversation between just two people. As a result, the other experts won't get to see your question or have a chance to reply. That won't always get you the help you need. If you want to copy and paste your question to a new one, here's the link for that:

https://www.2carpros.com/questions/new

First of all, the red warning light means the charging system is not recharging the battery after it has cranked the engine to start it. The alternator could be the cause, but there are a number of other possibilities. If the problem is intermittent, it is most commonly due to worn brushes inside the alternator. Most people just replace the entire alternator, but you can buy a replacement brush assembly for around ten dollars from a starter / generator rebuilding company, (most larger cities have at least one of them), and often from an auto parts store. You'll have to remove the alternator to do this job, but on many Chrysler products, the brush assembly can replaced with the alternator still on the engine.

To verify this is the cause, you must take two voltage readings on the two smaller wires on the back of the alternator, but that must be done while the engine is running and while the problem is occurring. You'll find 12 volts on one of them. The secret to diagnosing this is the voltage on the other one. It is supposed to be less than 12 volts, but more than 0 volts. Typically it will be around 4 - 11 volts. If you find 0 volts, the brushes are worn and must be replaced.

For the Check Engine light, you'll need to check the diagnostic fault codes. You can do that yourself by cycling the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then watch the code numbers appear in the odometer display. Here's the list of fault codes:

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

I can interpret them for you too if necessary. If you get a code for "generator field circuit not switching properly", that is in response to worn brushes too in the alternator. The Check Engine light turns on for any fault code related to something that could have an adverse effect on emissions. With low system voltage, the injectors might not operate properly, and that could increase emissions.