That previous answer is probably the best one I've seen in weeks except it pertains to older rear wheel drive vehicles. Your voltage regulator is located in the engine computer and takes advantage of all the things the computer knows. My daily driver is an '88 Grand Caravan. I also have an '89 and a '95 with the same system but different alternators.
It is very rare for the voltage regulator in the engine computer to give trouble, but if it does, you can install the electronic regulator shown in the previous repair description. It has temperature compensation built in, but it can't modify the charge rate for things like wide-open-throttle and engine overheating. Also, the computer monitors the voltage regulator for proper operation. If there is no current flow, it will set a fault code for "Field Circuit Not Switching Properly" and will turn on the "Check Engine" light.
The perceived charging problem is expected to cause low system voltage which affects the fuel pump, injectors, and possibly some sensor readings. Any of these things, or anything else that could affect emissions must turn the Check Engine light on. Although the external regulator will run your alternator just fine, you will always have the Check Engine light on, so you'll never know if a different problem shows up.
To verify the engine computer is the problem, have your mechanic look at the dark green wire in the corner of the 14-pin computer plug. The best is to back-probe the terminal with a paper clip. If the connector is molded around the wires, some people will pierce the insulation to take readings. This is never an acceptable practice, but if it is done, the hole must be sealed to prevent corrosion. If the voltage regulator is indeed defective, grounding the dark green wire will cause the alternator to go to full charge. The headlights can be used to verify system voltage has gone up. Do not raise engine speed higher than idle to prevent system voltage from going so high it burns out light bulbs and other parts of the computer.
If the system voltage does not go up when the dark green wire is grounded, the problem is before the regulator, most likely worn brushes in the alternator or corroded pins in a connector. For help diagnosing this system, go to my web site:
and use the pages for:
Testing the Chrysler / Nippendenso '87-'02 Alternator
You might have a large Bosch alternator, but the testing procedures are the same as for the smaller Nippendenso unit. The web site is still under construction and there's a glitch preventing me from uploading my own pictures, but the text describes how to test these systems.
I have a spare computer for my van, but the original one is still in there after 206,500 miles. These are also readily available in the salvage yards. If the regulator is defective, I'd recommend replacing the computer because it's easier and faster than modifying the system.
Saturday, March 21st, 2009 AT 4:05 AM