Hi , noob here, got my truck a couple of weeks ago. Ive been soaking up the wealth of info on these trucks you guys have provided- priceless.
91.5 w250 auto
so I got the thing fired up the other night, first time it ran in about 3 years. I checked what was working, air blows cold, heat blows warm, trans goes into gear, (cant drive it yet, tires, registration) gauges, lights etc. voltage gauge is reading high. I checked it with my vom 15.5-15.75 volts. I probably wouldn't have paid the gauge that much attention if I hadn't been reading here. probably saved my $130 battery! I checked the field wire at the alternator (green?) I've got 10 ohms to ground. I disconnected the 4 terminal connector for the alt. to see which side was grounded. when i check each side of the conn. with it unplugged both sides are open. today I decided to throw a VR at it. I've got the same thing (almost) 6 ohms to ground, 15v at idle, voltage climbs when I turned on lights / blower.
does this make sense to anyone? what am I missing?

 

Take an AC reading directly off the alternator. You may have a bad diode.

 

Yes it makes sense to me. With the alternator unplugged in the harness it becomes disconnected from the regulator. The terminals on the alternator will not and should not show any continuity to ground since the rotor in the alternator is not grounded. It should show a resistance between the two terminals which is the resistance of the coil which is in effect a wire wound resistor. Now measure the continuity between the regulator case and the alternator case and it should be close to 0 or as high as 3 ohms which is the sum of the resistance of the looped path between the alternator, the body and the leads of the meter. Maybe time to add a ground lead between the regulator and the alternator or add a grounding lead between the engine and the cab. Always use either an internal or external star type lock washer between the VR mounting screw and the body of the regulator. The same applies to the alternator end or a connection between the engine and the cab or body. A dab of grease on the star washers and on the wire ends generally aids in preventing any future issues of corrosion.

 

Checked the a/c output of alt, (2 studs on back of alt?) between the 2 I've got 6-7 vac. From one side to ground, 6-7 vac, from the other to ground, less than 1vac.

Sounds like Cougar is right? 1 legged alternator? Why would that cause overcharge?

 

Because the ac causes the regulator to see a lower voltage. The negative swing and the RMS value.

 

You said you threw a voltage regulator at it? I was under the impression that 91.5 was controlled by the Power Control Module (PCM). However, I heard 91.5 was an odd baller too.

Was this an internal, or external voltage regulator? If you have 2 wires coming OUT from inside your alternator to the 2 fields ( orange and green I believe) along with a separate 12v wire (probably blue) going to one of the fields: it's internally regulated. This was a retrofit to the alternator.

An issue might come about from having both an internal and external voltage reg. hooked up.

My 93 (PCM controlled VR) also had an external (but unplugged) on the fender, and an internal inside it! My high voltage issue I just cleared up came from a chaffed wire off the internal regulator INSIDE the back cover of the alternator (I believe it was the short, brown one). Anyway, I put a new one in, and I bet I could splice my old one and have a spare. I'm constant 13.7VDC now instead of varying 12.5-17VDC like before.

 

From what I've read around here on this kinda problem, I think I'm supposed to have an external reg on this. All the wiring looks factory.
earlier I said I checked the ac output of the alt, I realize now that I was checking the field terminals. I had almost 7vac.
I shouldn't see any, correct?
In the meantime, pulled the alt, had it tested= no good.
gonna have to wait a bit to replace it. does anyone know other vehicles in the junkyard that I could find this alternator on? To digress, when I first got this truck, I cruised to the local pick-n-pull, a few hundred trucks out there, TWO Dodges, a 76 and 86. Made me happy and sad at the same time.

I guess the 91.5 is an oddball. 92 engine with 91 drivetrain controls? Can I start calling it a hybrid?

 

All 91s have an external regulator. You should see no ac voltage if the diodes are working correctly. The diodes turn ac into an overlapping pulsing dc and the battery take out the remaining ripple.

 

The alternator output is measured between the battery connection post, the big one usually marked "bat" and the case ground. If you measured the rotor connection terminals it has no diagnostic value.

My 91.5 has the external voltage regulator. Model year 92 changed the wiring harness to include a crank pos sensor & eliminated the voltage regulator circuit and switched that function to the control box on the lf fender. Check the door sticker or the VIN for the model year that you have.

Now 1 more time, did you run a jumper from the engine to the voltage regulator. A flickering ground connection between the engine and the cab will cause erratic voltage regulation because the sensing portion of the regulator will be receiving voltage spikes.

 

ok put the alt back in. check for ac between alt output and case- .1xx vac
system charging @ 14.9v
checked resistance from VR case and alt case. O ohms. I ran a jumper between them anyway
I also ran a # 6 wire between the alt output and the bat, and disconnected the factory lead, because the fuse link had cooked itself out of it's insulation. But it also showed no resistance.
Charging at 14.7-14.8

So today I went to the pick-n-pull and found the 86 D250 I saw earlier and it had a mopar VR in it.
Next I removed the jumpers and put back the factory wiring, ran it again with the VR I got from autozone, just to get back to square 1.
charging at 14.8
Changed the VR to my junkyard mopar unit, now it charges at 14.3 at idle. If I turn on the lights / blower, I read 14.7- 14.8

Am I getting close, is the voltage supposed to climb under accessory load?

I know I'm gonna have to clean up that fuse link mess on the fender.

 

14.7 -14.8 sounds like normal reading to me. A +/- .1VDC on an inexpensive parts store digi-volt/ohm meter is within allowance.

The alternator replaces used current loads placed on the battery. As you increase the load the voltage and current increases to replace what is being consumed.

You need to replace the fusible link that you bypassed. That may have been the root of your problem.

 

yeah, I'm planning on replacing all of them with fuses.

I really appreciate all the feedback on this issue. I could have- fried my new battery.
-bought a new alternator

thanks to you guys I'm only out 13 bucks for the bad VR from auto zone. Do you think I can get my money back?
they also told me my alt was bad. So did O'Rieley's. Parts stores ain't what they used to be