Check engine light on does not charge

Tiny
HUEY777
  • MEMBER
  • 1991 DODGE DAKOTA
  • 164,000 MILES
1991 dodge dakota 3.9 4speed manual - replaced alt that did not work light still on still does not charge does not charge battery alt is good replaced old one with new one still dose not charge battery
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Tuesday, March 13th, 2012 AT 3:41 AM

9 Replies

Tiny
ASEMASTER6371
  • EXPERT
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
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Tuesday, March 13th, 2012 AT 4:09 AM
Tiny
HUEY777
  • MEMBER
Were are the fuses for the alternator located
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Tuesday, March 13th, 2012 AT 4:12 AM
Tiny
ASEMASTER6371
  • EXPERT
Look in the underhood fuse panel. If not, then inside the car on the side of the dash.

Roy
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Tuesday, March 13th, 2012 AT 4:14 AM
Tiny
HUEY777
  • MEMBER
Checked fuses checked, all ground wires under hood code reads open field or short this truck is all stock this truck does not have power windows or locks has stock am fm radio I have looked at wires going from alt harness look good anyone run into this before please help
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Tuesday, March 13th, 2012 AT 10:18 PM
Tiny
CARADIODOC
  • EXPERT
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.
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Wednesday, March 14th, 2012 AT 12:21 AM
Tiny
HUEY777
  • MEMBER
Two small wires 11.34 and 11.32
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Sunday, March 18th, 2012 AT 12:21 AM
Tiny
CARADIODOC
  • EXPERT
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.
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Sunday, March 18th, 2012 AT 1:34 AM
Tiny
HUEY777
  • MEMBER
My computer is right beside the alt on pass side can I spice the wire a new one in from alt to computer if I knew which one is what or do I need a new computer thanks dodge put volt reg in computer an left old school coil whats up with that
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Sunday, March 18th, 2012 AT 7:55 PM
Tiny
CARADIODOC
  • EXPERT
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.
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Sunday, March 18th, 2012 AT 11:02 PM

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