Electrical gremlin....help

UNBROKEN

UNBROKEN

Ezekiel 25:17
Joined
Apr 21, 2006
Messages
17,158
Cliff notes:
Truck sat for a month...no issues prior
Started slow today but cranked up fine.
Drove about 70 miles, digital voltmeter off the batteries reading 9.8 volts
Check all battery wiring and grounds...all good
Remove and test alternator....says internal diode bad????
Replace alternator with reman from Auto Zone...only one available in town.
Charge off truck with 300 amp alternator to about 12.8 volts before starting...held that for a few minutes then started dropping while driving.
I got stopped for the night voltmeter at 10.4 volts and steadily dropping.

Gonna have David Lott pick it up in the AM since I'm close to his shop and I'll have a place to work on it. Any ideas?
Gonna remove and test "new" alternator first thing...not sure what to do after that.
Hints? Tips? Advice?
 
You have a wire that has either been chewed by an animal, or chafed by something. Time to start chasing. Or at least that is my guess.
 
FWIW the charge wire from the alt to the batts is 0/1 gauge and is in perfect shape.

Who knows what about how the alternator works on these things?
Like...how it gets it's signal to excite. I know after a phone call that it's through the ECM but where is it and how to I trace/check it ?

Did I mention I hate electrical crap? Tomorrow is gonna be filled with cuss words. LOL
 
Stock Dodge. Everything is like a Dodge truck underhood down to the trans and how it's controlled.
The only things not there are the heater grid and fan clutch.
 
UNBROKEN said:
Stock Dodge. Everything is like a Dodge truck underhood down to the trans and how it's controlled.
The only things not there are the heater grid and fan clutch.
Click to expand...

Well i know the clutch fan is not ur problem. lol That alt should have 3 wire if i remeber correctly one from the battery (big one) and a lil connector with 2 wires in it one should have 12v when the key is in run or motor runnin position i think the other is a ground not 100% on that though. Dont know if that 12v lil wire recieves power from the ecm or not. Tellin it that the motor is running or not i would need to look at a tech book and we dont have one in the gulf. Maybe that will help some.
 
Yeah, might want to grab a spare ECM, as Roachie stated. Dodge has run the charging system through the computer for ages. Used to always go bad in the 80s and 90s. Vehicle would run but wouldn't charge.
 
I hope not...wouldn't surprise me though. I'll get it started to
tomorrow and check for 12v at that plug. It's a black wire/green tracer I believe.
I just need a DMM to work with.
 
If the truck is OFF and you are still seeing a drop, you have a short somewhere...I know, welcome to the gig Mr. Obvious. However, I beleive this would rule out the alternator. They way I would find this is to:
1) Disconnect the negative from the battery(ies).
2) Put DMM in AMP mode on the highest setting (probably 10 amp) and move red lead to AMP input (should be marked).
3) Put black lead on neg of one battery.
4) Put red lead on the neg battery terminal on the cable coming to the battery

You should now read how much amperage is leaking off. Hopefully it is not higher than 10 amps. If so, your meter should be fused and at worst case, you have to replace the fuse. It will spark if a load exists when you complete the connection so don't jump and hit your head on the hood you 6'3' SOB. LOL

Now, you can start unhooking things or shaking wires until the drain is very small. You should normally see around 30ma - 70ma drain depending on your central timer and other crap that stays on when the iginition is off (memory wire on radio, etc also).

If both batteries are connected on the positive side then it shouldn't matter which side you use...use the easiest to get to.

Another thing, you should probably start by taking all terminals off both batteries (red and black) and reading voltage on just the battery itself. You may have a dead cell since the long set time of the truck and one bad cell can kill both batteries and sap power.

Wish I was closer, I'd come help ya. :D

Good luck. Hope this helps you.
 
I'll start with that basics when I get to Lotts shop in a bit....then I'll start on your list, Jory. Thanks
 
Gonna check voltage out of the ECM to the exciter wire first. If that's bad it's getting converted to a single wire alternator on the spot. lol
If that's good and the alternator is putting out then batteries are next.
I really don't wanna buy 400 bucks worth of Optimas today but I REALLY don't wanna buy a new ECM. LOL
 
I hear ya. I'm on the Optima band wagon now....but MAN does it hurt the back pocket.
 
Yea, but their worth their weight in gold! We've had two of them in a tractor for over 7 years. Have had terminals break off before battery runs down.
 
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