M939a2 voltage gauge fluttering/ lights dimming issue

[dawico]

The TMs don't really seem to cover this issue but here goes.

My voltage gauge has always stayed toward the bottom of the green section no matter the temperature or speed I am traveling.

Yesterday I noticed the lights seemed to dim a little for a while then get bright again. It repeated this cycle. I also noticed when the lights were brighter the voltage gauge was up in the red area about half way. I reduced my speed thinking higher rpms may be the culprit but it didn't seem to change anything. At idle the gauge reads normal and the lights seem to be consistent.

I have a few ideas where to start diagnosing the issue but am also looking for ideas.

I have a similiar issue on another civilian vehicle and assumed it was the voltage regulator. It ended up being grounding issues. That is where I plan to start.

I am going to clean all the terminals on the alternator and check the engine ground.

I also plan on pulling the belt off and making sure the alternator spins freely.

I have already checked the battery wires to be sure none are rubbed through the insulation and grounding out. All seems to be well in that regard.

Per the TMs I checked the temperature of the alternator and it wasn't abnormally hot. After a long trip I was able to rest my hand on it.

Anyways, I am going to tackle anything I can do or test tomorrow.

Is there anything else I may be missing? Anyone had a similiar issue before?

Should I pull the batteries and get them tested or is that a waste at this point? I read that a bad cell in one can burn up an alternator.

I plan on trying to get the alternator tested if a local auto parts store can do that. Would adjusting the alternator help with this issue or is that unrelated?

Thanks in advance for any pointers in helping with this issue.

 

[MtnSnow]

I will venture an educated guess that the voltage regulator on the alternator is going. (common issue).

 

[tractors0130]

Check it with a digital VOM and see what your readings are. Load test and check each battery individually while still in the truck, no need to yank them out unless 1 is bad.

 

[Carlo]

Check it with a digital VOM and see what your readings are. Load test and check each battery individually while still in the truck, no need to yank them out unless 1 is bad.

This is his first thing to do. A volt meter will show the real volts coming off the alternator and arriving at the batteries.

 

[Monkeyboyarmy]

I would guess the voltage regulator too. But I am curious about bad connections or the protective control box. Doesn't the alternator circuit go through the PCB box?

Now, if we could only find out who bought the 3 lots of 600+ regulators out of Oklahoma last week....

 

[dawico]

Check it with a digital VOM and see what your readings are. Load test and check each battery individually while still in the truck, no need to yank them out unless 1 is bad.

This is his first thing to do. A volt meter will show the real volts coming off the alternator and arriving at the batteries.

I will do that but the issue is that while in idle the gauge stays consistent. It only fluctuates while driving. I will report what I find regardless.

 

[dawico]

I will also add that my truck only has two batteries that were put in new by the previous owner.

The only thing electrical that I have done is replacing a few dash lights.

These things are minor but thought I should add as much detail as possible.

 

[dawico]

Ok, so I checked all connections and they seem to be good. I checked the battery cables and there are no bare wire spots to cause a short.

I disconnected the batteries and removed the alternator to clean all the connections. I broke the quick connect plug on the small wire going into the cover and repaired it.

Before this the gauge read at the lower end of the green section. Now it reads about 2/3 up into the yellow. I guess this boggles my mind I would think it would read higher with better connections.

So the next step is to adjust the alternator as per TM 24 to get it back into the green area. Start engine, idle up, turn on lights and check alternator voltage. 24.4 volt and should be between 26 and 29. The adjustment screw does nothing. Turn it both ways and no change at all (yes, I took the cap off). Maybe whatever the screw turns is stuck or something, I don't know. Either way it still reads 24.4 volts and gauge is in the yellow.

Now I am stuck. Guess I will check all the connections again.

 

[MtnSnow]

With the adjustment screw not doing anything makes me lean even more to a shot/worn unit and needs to be replaced before it damages other items

 

[Monkeyboyarmy]

With the battery switch on, are you getting 24 volts to the small wire that you repaired?

 

[dawico]

With the battery switch on, are you getting 24 volts to the small wire that you repaired?

24.8 volts on both sides of the splice.

 

[Monkeyboyarmy]

I would guess bad alternator. Chances are that it is just the regulator but you would need a known good one to try on your alternator. Also the regulator for the US built alternators will not fit on the Greek built alternators....different diameters where they fit together.

 

[Andrmorr]

I would guess bad alternator. Chances are that it is just the regulator but you would need a known good one to try on your alternator. Also the regulator for the US built alternators will not fit on the Greek built alternators....different diameters where they fit together.

Sounds like a good excuse to do the 200 amp alternator upgrade...why spend money and time to get back to just O.K. when you could go big and bulletproof!

 

[smurph]

When I got my truck, the alternator was bad. It took me weeks to figure it out. The truck had no batteries, so I bought two big 1400 CCA batteries to put in there. The truck started if I just breathed on the start switch, so it took a while for the batteries to come off 28v. But as time went on, they certainly did.

Things that clued me in:
ABS would fail to initialize.
Falisafe warning module went nuts. Some lights and buzzers at times and none at others. Totally random.
Turn signal flasher quit working.
Dim lights. But I had a pop-eye, so I thought the dimness was because of that. So when I replaced the blow headlight, I still couldn't see that well at night.

The battery gauge was still in the green all this time! But when I put a DVOM on it, the batteries were at 22v. 20v with the lights on.

So I went about taking the alternator off to repair it. I used to be a mechanic and I have repaired 100s of alternators, so I wasn't scared. But I should have been. These alternators look bullet proof, but they are not. What happened to mine was one of the rectifier diodes blew up and became detached from the heat sink plate. This allowed the other wired end of the diode to contact the housing. After that happened, it melted the enamel on the stator wires and toasted it pretty well. Err... I mean charred it. Crispy crunchy stuff. It was a miracle the thing just didn't catch on fire and burn the whole truck down. When I opened up the back of the alternator to look at the regulator, a bunch of goo came out. That goo used to be the potting compound. The voltage regulator was probably the original fail point, if I had to guess. Or maybe it was caused by a bad shutdown procedure. The lesson learned was these alternators are fragile fickle things and you can't trust the volt gauge!!!

With the intent on keeping the truck original, I started sourcing parts to fix the thing. After adding it all up (VR, both rectifier plates, and a stator), I would have more in the repaired (and still vulnerable) alternator than I would just putting a 100 amp Delco unit on there. So the Delco went on there. There is no way the Delco could ever blow up like the original did. I still might fix the original alternator, but I would only put it back on there if I were to sell the truck and the new owner wanted it.

So I second the 200 amp upgrade (if they are better built than the original) or go for a Delco.

I got the Delco from Wolverine Tech. It was a bolt on op that took 15 minutes and only required new longer belts (needed them anyway). I got a spare 24v VR to go in the tool box. The rest of the pats in that alternator can be had at any auto parts store if the need to fix it in the field ever arises.

After a good battery charge, everything started working like new. The gremlins were gone.

 

[tractors0130]

When I got my truck, the alternator was bad. It took me weeks to figure it out. The truck had no batteries, so I bought two big 1400 CCA batteries to put in there. The truck started if I just breathed on the start switch, so it took a while for the batteries to come off 28v. But as time went on, they certainly did.

Things that clued me in:
ABS would fail to initialize.
Falisafe warning module went nuts. Some lights and buzzers at times and none at others. Totally random.
Turn signal flasher quit working.
Dim lights. But I had a pop-eye, so I thought the dimness was because of that. So when I replaced the blow headlight, I still couldn't see that well at night.

The battery gauge was still in the green all this time! But when I put a DVOM on it, the batteries were at 22v. 20v with the lights on.

So I went about taking the alternator off to repair it. I used to be a mechanic and I have repaired 100s of alternators, so I wasn't scared. But I should have been. These alternators look bullet proof, but they are not. What happened to mine was one of the rectifier diodes blew up and became detached from the heat sink plate. This allowed the other wired end of the diode to contact the housing. After that happened, it melted the enamel on the stator wires and toasted it pretty well. Err... I mean charred it. Crispy crunchy stuff. It was a miracle the thing just didn't catch on fire and burn the whole truck down. When I opened up the back of the alternator to look at the regulator, a bunch of goo came out. That goo used to be the potting compound. The voltage regulator was probably the original fail point, if I had to guess. Or maybe it was caused by a bad shutdown procedure. The lesson learned was these alternators are fragile fickle things and you can't trust the volt gauge!!!

With the intent on keeping the truck original, I started sourcing parts to fix the thing. After adding it all up (VR, both rectifier plates, and a stator), I would have more in the repaired (and still vulnerable) alternator than I would just putting a 100 amp Delco unit on there. So the Delco went on there. There is no way the Delco could ever blow up like the original did. I still might fix the original alternator, but I would only put it back on there if I were to sell the truck and the new owner wanted it.

So I second the 200 amp upgrade (if they are better built than the original) or go for a Delco.

I got the Delco from Wolverine Tech. It was a bolt on op that took 15 minutes and only required new longer belts (needed them anyway). I got a spare 24v VR to go in the tool box. The rest of the pats in that alternator can be had at any auto parts store if the need to fix it in the field ever arises.

After a good battery charge, everything started working like new. The gremlins were gone.

Ditto on the Delco alternator, this is what I did nearly 3 years ago on my M925A2 and was able to use the same length serpentine belt by changing the length of the lower bracket.

http://www.steelsoldiers.com/showth...Delco-Style-Alternator&highlight=m925a2+delco