1990 Dodge Dakota altenator not charging battery

Tiny
IVANHGGS
  • MEMBER
  • 1990 DODGE DAKOTA
  • 3.9L
  • V6
  • 2WD
  • MANUAL
  • 160,000 MILES
My check engine light comes on. I check the fault codes and 33 and 41 come up. Since I dont have a/c in my truck, that eliminates code 33. Code 41 says alternator field control circuit open or shorted.

i have traced my wires. I have a brand new alternator installed. My battery terminal was corroded so I replaced the battery cable connector. I cleaned my ground wire. I think I looked at all the wires. But my mechanical gage still shows that the battery is not being charged.

I have read the the power module and the logic module could be bad. I dont know what these are or where to look for them. Where can I go to find a picture of them? I also would like to know what you think could be the problem. Where is the voltage regulator circuit and how do I check that?

thank you
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Friday, May 22nd, 2015 AT 4:54 PM

6 Replies

Tiny
CARADIODOC
  • EXPERT
The voltage regulator is inside the Engine Computer which is next to the battery. That circuit has a very low failure rate.

To start the diagnosis, measure the voltages on the large output wire on the alternator with the engine not running, then on that one and the two small terminal nuts with the engine running. List those four readings on your reply.
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Friday, May 22nd, 2015 AT 5:41 PM
Tiny
IVANHGGS
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It is to late this evening. I will respond first thing in the morning
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Saturday, May 23rd, 2015 AT 12:38 AM
Tiny
IVANHGGS
  • MEMBER
Here is what I have

Off I have 12.56
On I have 14.65 on the large post
The small posts I receive a 14.04 on one & and the other one fluctuates between 11.23 - 11.34.

Right now my check engine light is not on. Usually first thing in the morning it is not. But once I drive it for awhile them it will come on. I am going to drive it for a little bit and see when it will come on. I will report back with the same test after the alternator goes code 41.
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Saturday, May 23rd, 2015 AT 9:10 AM
Tiny
IVANHGGS
  • MEMBER
I just noticed the part about the voltage regulator. My truck does not have one. I have the SMEC box on the right hand side of the truck.

I took the truck out and after a bit the check engine light came on. I went and checked and here are the results with the alternator not charging.

Off--- I get 12.66
On--- I get 11.88
Smaller posts I get 11.58 and 11.34

This is where I am stumped. It works usually first thing in the morning, but them isn't doing is job after awhile.
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Saturday, May 23rd, 2015 AT 10:14 AM
Tiny
CARADIODOC
  • EXPERT
Yup. My memory is fuzzy on the location, but the voltage regulator is in the SMEC, (single module engine controller).

The second set of readings does indeed suggest the voltage regulator could be intermittently open, but a bad connection in a wiring connector would be more common. Also, you're one step ahead of the diagnosis by finding one of the two smaller alternator terminals having a lower voltage than the other. If you look at the first set of readings where you had 14.04 volts, that's the feed. That one you leave alone from now on. The other one, being lower, is going to the voltage regulator and is the one to do the testing on.

With a break in the wire going to the regulator, you will normally see exactly the same voltage on both small terminals. I have a trick to figure out which one is connected to the regulator. You can't tell by the wire colors because they both go through a small black plastic block that the terminals are molded to. There's no easy way to know which wire goes to which terminal, ... Unless you know my wondrous trick.

What you want to see is a difference in voltage between those two small terminals. The greater the voltage difference, the stronger electromagnetic field the field winding is making and the more output you'll get. In your first readings that difference is 2.7 volts which is about typical for running the fuel pump, ignition system, and radio. Maybe even the head lights. In the second set of readings there's only a quarter volt difference. The terminal with 11.58 volts is down from 14.04 because the system voltage is down. At issue here is both terminals are essentially the same. That means almost no magnetic field is being developed, so no output will be developed.

Also, we know the brushes inside the alternator are okay. The most common failure is worn brushes making intermittent contact. That can go on for months before the failure becomes permanent. When that happens you'll find 0.00 volts on the second small terminal.

To prove everything else in the output circuit is working, use a piece of wire or a clip lead to ground that second small terminal that had the 11.34 volts. Do that just long enough to verify everything works. If you monitor battery voltage or turn on the head lights, you'll see the charging voltage go way up to more than 15.0 volts. Don't raise engine speed because that can send the voltage to more than 20.0 volts and cause damage. And PLEASE do not ever disconnect the battery while an engine is running. I did that every year on the alternator test bench to prove to my students what will happen. The voltage regulator needs the battery in the circuit to help it do its job. It is real easy without the battery, and by raising engine speed, to hit well over 30.0 volts. That will damage computers, the radio, and any bulbs that are turned on.

Once you momentarily ground that terminal and see system voltage go up, we'll know everything else is working. You'll hear the alternator strain too, and the head lights will get bright. The next step is to ground that same circuit in a connector further down the line, but I'm going to have to find a wiring diagram to see where that is. That's going to tell us if that piece of wire is okay. This was usually a dark green wire but I'll be back shortly with more information.
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Saturday, May 23rd, 2015 AT 4:29 PM
Tiny
IVANHGGS
  • MEMBER
I went out and picked up a new smec. I'll install it and see what happens. I had another issue come up that took priority. The water pump decided to crap out so I am replacing it. But I just broke a bolt off on the damn water pump. I didn't even get it to 30lbs torque. Maybe 20. I will see if it holds but if not o will have to try to extract the bolt. It may be a few days before I can hey back to the electric issue. But I well keep you updated. Thank you so far
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Saturday, May 23rd, 2015 AT 7:43 PM

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