1985 M1028 Gen 2 light

[moehos]

Hi all,

new to the forum, though ive been haunting the threads for a long time.

The other day when I started my CUCV, the Gen 2 light came on and stayed on for the long drive home.

Upon getting home, I tested the current at the back of gen 1(drivers side) and found 12-13volts and tested the current at the back of gen 2 (passenger side), 11 volts. Bad alternator, easy peasy. Ordered a new one from Romaine Electric, installed it and whoops, no generator light...

No gen 2 light when ignition in the on position, no light when running.

So, swapped old alternator back on, gen 2 light on all the time, running or not.

So the light works. Checked and cleaned the socket. Checked and cleaned the leads for the top plug terminal, checked fuses; No luck.

My preferred electrical shop is closed for the 4th of July weekend so I took both the new and the old alternator to Oreilly's and they very kindly tested both units. Results; old alternator is dead as a doornail, new alternator passed with flying colors. Not sure what all their whiz bang machine tests for, but the young man operating it certainly didnt have any insight.

Now, I am no electrical genius, but it seems to me that something must be wrong with the new alternator. Perhaps it is generating power but the function that sends power to the gen 2 light isnt working. I understand that there is a relay and diode under the dash for GEN 2, but it seems to me they must be working if the Gen 2 light comes on with the old alternator.

Thoughts?

Thanks in advance!

 

[Mg84648]

Just swap alternator positions. Put the functioning alt in position 2 and the replacement in position 1. Gen 2 needs to be an isolated ground alternator and as long as someone hasn't already swapped them out it should be stock.
Hope that helps.

 

[moehos]

Hi, the new alternator is the correct part number as the original stock acdelco alternator and has the
same terminals etc, the ground was not previously present on gen 2 only on gen 1. Everything was stock and functioning up till this point.

 

[Mg84648]

For the charging system to function properly, Gen 2 needs to be an isolated ground alternator. There will be no external differences on appearance, the isolated ground is just a phenolic bushing on the terminal. Check for continuity from case to the stud , you should not have it.
Just swap alternators and let us know.

 

[moehos]

Let me try to explain again. Gen 2, the passenger side alternator never had an isolated ground. The isolated ground is on gen 1, the driver's side alternator. The alternator that failed is Gen 2.

Everything worked marvelously for 2 years and 10k miles, so it was hooked up correctly.

 

[Mg84648]

Ok, hope you get it figured out, take care.

 

[moehos]

Ok, hope you get it figured out, take care.

I get the impression you dont believe me; I wouldn't believe me either with how often the wiring gets screwed up on these, that plus my disclaimer that I'm not electircian, but here is the schematic, and this is exactly what I am seeing in my engine bay. You'll note that there is no ground listed for Gen 2.

View attachment Charging Circuits Except M1010.pdf

 

[antennaclimber]

The ground on GEN2 is isolated, if it wasn't the sparks would fly. The terminal marked - on GEN2 is the ground.The GEN 2 ground is NOT the same as the ground on GEN1.

GEN1 Positive is connected to the GEN2 Ground terminal, this terminal is isolated from the GEN2 case ground. It is "floating above" the body/electrical ground.

If your original GEN2 was making the GEN2 light come on and the new one does not, then the issue is in the new unit or the connections to it associated with the GEN2 light.
With the new GEN2 in place, what is the voltage on the GEN2 + terminal and the electrical (body) ground with the truck running?

This may help explain it:
https://www.steelsoldiers.com/showt...ternator-Isolated-Ground-Therory-of-Operation

 

[moehos]

Thanks antennaclimber, I'm tracking somewhat on the set up of Gen 2 now thanks to that article.

I did test the voltage when I initially installed the new alternator, I cannot recall what the result was and the new Alt is packaged up and ready to return. When I get the new one, success or not I will test and share results.

 

[antennaclimber]

Can you get the original alternator rebuilt? It is a simple alternator and most any competent rebuild shop should be able to do it.
Many of the members here have rebuilt their own with parts that are available in the Classified section.

Here is more info on how to do that:

http://www.steelsoldiers.com/showthread.php?146703-CUCV-Alternator-Rebuild
https://www.steelsoldiers.com/showthread.php?60299-CUCV-Alternator-Rebuild-Question
https://www.steelsoldiers.com/showthread.php?136886-Alternator-and-starter-parts-amp-repair-kits
https://www.steelsoldiers.com/showt...ld-kits-for-alternator-p-n-1105500&highlight=

I have found that having a spare alternator is very helpful. Just swap out the bad one and then repair it as soon as you can.

 

[moehos]

Yeah, if the new one is good, I'm keeping the old one and will take it to out local rebuilder and have them do it when I can. I figure if one went out, the other probably isnt far behind.

 

[MarcusOReallyus]

Many of the members here have rebuilt their own with parts that are available in the Classified section.

It's not a difficult job, either. Takes me about 45 minutes, and I'm slow.

 

[moehos]

I forgot to update what was determined; I took the original to diesel doctor and they rebuilt it. Upon installation the Gen light in question lit when the ignition was turned on and went out when the vehicle is running. So far no issues and about 3k miles down the road.

So that Amazon sourced alternator was in fact, bad.

Thanks everyone for their input and support.