Check engine light on does not charge

Make sure the big wire at the alternator is hot with 12 volts. Check the fuses for the alt as well. Have the code pulled and get back to us with the code number.

Roy

Look in the underhood fuse panel. If not, then inside the car on the side of the dash.

Roy

Hi guys. No fuses for the alternator. Measure the two voltages on the two small wires on the back and holler back. You must make those measurements with the engine running.

Dandy. We know the brushes inside the alternator are okay. You have either a problem with the voltage regulator inside the Engine Computer, (possible but very rare), or a broken / corroded wire going to it.

As a test you can "full-field" the alternator to verify it is okay but this is going to get tricky. You have to ground one of those small wires but if you ground the wrong one, hopefully you'll blow a fuse and the engine will stall, but it is also possible to damage the computer. I might have a fix for a damaged computer but my experience is from that happening on a '97 Dakota that my students were experimenting on. That is a different design computer.

Normally I would tell you the wire to ground is the one with the lower voltage but .02 volts isn't enough to be sure which wire is which. You'll also notice those wires go through a black plastic block so you can't tell which wire goes to which bolt. Rather than retype the whole story, take a look at this web page to see how to be sure you have the right wire:

http://randysrepairshop.net/which-field-terminal-do-i-ground.html

Don't worry about most of the "story" It's written for students, but the last paragraph explains what to do. When you ground the control terminal, you will expect to see battery voltage rise, head lights get bright, and the alternator will make a noticeable singing noise and load the engine down a little. Don't raise the engine speed during this test because voltage can go way too high and damage the Engine Computer and burn out any bulbs that are turned on.

If that works, the next step is to follow that wire to a connector and back-probe it there and ground it. We'll be working our way to the voltage regulator, all the while looking for a break in the wire or a corroded or stretched terminal in a connector. To bypass all those steps you can ground the control wire right at the computer but it takes some work to move the battery out of the way and get the plastic cover off the connector, and I don't have the right service manual in front of me right now so I don't know for sure which wire it is. On the cars it is the rear-most dark green wire in the top row of wires, but in that time period many vehicles had an additional 14-pin plug where the alternator wire went to.

Yer 'puter is on the passenger inner fender? You sure you have a '91 model?

I'm not sure what you're asking about the wire. We never splice in a new wire unless the old one is melted or severely corroded. We always want to see where and why the old wire broke so the wires next to it in the harness don't do the same thing. Rubbing on hot exhaust parts and sharp metal brackets are things to look for along with corroded connector terminals.

As for putting the regulator inside the Engine Computer, at first that does sound stupid but those regulators have proven to be extremely reliable. Chrysler had almost no Engine Computer trouble through the mid '90s and those that did need to be replaced rarely needed that due to a regulator failure. The nice old firewall-mounted regulators of the '70s had temperature compensation built in but that's all. By putting the regulator in the computer, the charge rate can be modified for varying battery temperature, but the alternator can also be turned off completely to gain up to 5 horsepower at wide-open-throttle or other high load conditions. The circuitry adds almost no weight but eliminating the external regulator saves a little weight and there's less wiring.