Gen 2 light / Alternator trouble fixed the hard way

The GEN lights, both GEN1 and GEN2, are used by the alternator exciter circuits. If power isn't flowing thru the bulb the alternator will not excite.

For what ever reason GM wired the GEN1 exciter circuit and the heater to use the same fuse. When the fan is blowing it is robbing some juice from the GEN1 exciter circuit. Clean all the contacts and maybe add an additional ground to the fan.

I am in a tough spot. I left my Son use my M1009 Crown of Thorns. After 20 + years of trouble free use. I have NO Gen 2 light. It charges perfect and I have no ill side effects. Any clue what could be wrong? I managed to fix the stereo while looking at it today but still no Gen 2. I switched the socket and bulb with Gen 1 and the bulb and socket are clean and electrical is my weak spot. The Gen 2 bulb and socket shone bright in the Gen 1 hole. It is probably something simple. Any guesses? I will find it. I am a sleuth on anything. I eventually figure it out. Could possibly be the alternator.

Check to see if you have any jolts at the socket. Also check the traces where the socket connects.

Volts not jolts? I could get jolted if I up the voltage. And I am fortunate enough to have a new Snap On test tight that you do not need to ground. It is just a small probe and it uses the ground thru your body contact to complete the circuit. It is handy little thing. I bought 2 and have 1 in my Crown of Thorns CUCV M1009. When I get to that vehicle again I will give it a try.

I have a CUCV that was sold from a fire department in Mississippi. Someone wired the 2nd alternator (passenger side) reverse polarity and blew up the battery and really messed up the wiring. I replaced the alternator with a new one (not reman). I replaced all the bad fuseable links and any burned wires. I replaced the 2 relays under the dash. I cleaned up the bulb mount for gen 2 and still no light and no charging. Could the reversed wires have blown the Diode in the circuit? I checked all the appropriate wires to make sure they are good. So I wonder if it is the Diode that is keeping the alternator from charging? Can someone help me with this issue? i have spent a lot of time on it and do not know where the Diode is located and what it looks like.

goldneagle said:

Could the reversed wires have blown the Diode in the circuit?

Click to expand...

Yes, that is quite possible.

There are very detailed troubleshooting steps in the manual (TM 9-2320-289-34. Section 4.2, paragraph 11) that will walk you through the process of testing the diodes. They are inside the alternator, and yes, that means you must remove and disassemble it. There are some alternator tests that require special equipment, but testing the diode trio only requires an ohmeter. Also check your rectifier bridge (paragraphs 12-14). It's a series of diodes, too.


Replacements can be purchased from a member here (whose name escapes me, but he's probably in the Classifieds), or ASPWholesale. It's an easy rebuild. While you're in there, you might as well rebuild the whole thing. It's not difficult at all (there are two threads on it in the Helpful Threads sticky), and since you have it apart, you've already done the labor. Might as well put new parts back in.

There is also the gen 2 diode in dash harness, over the heater control, as per Warthog's CUCV diodes and relays sticky, picture #7. While I have never to replace, from the sound of the abuse, that could be bad also.

I have hit my diagnostic/parts changing lid and could really use some guidance. M1009 50K miles, unmolested mechanically, mine for a couple weeks now. Problem....No Gen2 light on ever. I have done my SS Homework and logged a lot of seat time in front of this lap top and still no success. The truck runs great, volt meter appears to be functioning, and shows its charging right on the line between yellow and green. The needle moves as the glow pugs cycle on and off but never gets into the green.

I have thus far, cleaned every electrical connection I run across, replaced both batteries (fully Charged them) replaced the Gen2 light in dash (old bulb did not appear to be bad), replaced both small square relays under dash, checked all fuses in fuse block, checked voltage on the plug for gen2 (key on and key off both contacts were showing voltage at the correct time)....took a gamble and ordered a new Generator. Installed new generator this evening and still no GEN2 light and no change in volt meter. Still charges at the line between yellow and green.

With vehicle running the voltage at front battery is 14.6 and the voltage at the rear battery is 12.6. While vehicle is running the lug on the alternator shows 12.8 volts. This leads me to believe the rear battery is not being charged.

Any additional information would be appreciated.....additionally the replacement Generator did not have the "thing" bolted to the rear of it that looks like a "capacitor" unsure what it is but I did have to use the one off of the old Generator.

Again, I have read up on this and am not just trying dump on here and expect someone else to do my homework, I have zero electrical background.

Thank You
James

Just found the diode above heater controls, it looks intact but was wrapped in duct tape....also just checked voltage on printed circuit board. 12 volts at gen1 socket, 0 volts at gen2 light socket. key on

Hello. I had the same problem. The printed circuit "board" on the back of my instrument cluster was bad, so power was not getting to Gen 2 bulb. LMC truck has them, I ordered one, it is perfect fit. Part #38-3086. There is also diode in the Gen 2 circuit that may be bad. If you do take your instrument cluster out, I HIGHLY recommend removing the dash pad, makes the job much easier.

acthomp781 said:

Hello. I had the same problem. The printed circuit "board" on the back of my instrument cluster was bad, so power was not getting to Gen 2 bulb. LMC truck has them, I ordered one, it is perfect fit. Part #38-3086. There is also diode in the Gen 2 circuit that may be bad. If you do take your instrument cluster out, I HIGHLY recommend removing the dash pad, makes the job much easier.

Click to expand...

Thanks for the reply.... after hours of problems solving I finally got the GEN2 Light on and the meter is charging on the line inside the green. The contact between the wiring harness plug in the back of the printed circuit board was the problem. The orange wire that gives voltage to the GEN2 bulb needed to be cleaned...just that one spot on that one wire. I can't remember the last time I was that excited!!!! no one else in the world cared but when that GEN2 light was shining bright I felt like a big deal for a second. This site and the people who contribute to it are awesome. Thanks again.

I am new to the CUCV (despite being on this site lurking for nine years) and I just found out that the entire operation of the #2 alternator (and all that goes with it) is dependent on a crappy GM bulb socket and/or crappy GM printed circuit board.

If that doesn't define these trucks, I don't know what does.

Prospect62 said:

I am new to the CUCV (despite being on this site lurking for nine years) and I just found out that the entire operation of the #2 alternator (and all that goes with it) is dependent on a crappy GM bulb socket and/or crappy GM printed circuit board.

If that doesn't define these trucks, I don't know what does.

Click to expand...

Well, the bulbs are pretty easy to check and replace. If it makes you feel any better, the alt in my '83 Mercedes 300D diesel is exited the same way, never had a fail. I agree the printed instrument cluster array is not the best, but the usually works OK until a mouse craps on it, or someone decides to take it out and 'detail' it.

Of course, it is pretty much a '84 K-30, pretty easy to set up electrical system differently if you want to.

dependable said:

Well, the bulbs are pretty easy to check and replace. If it makes you feel any better, the alt in my '83 Mercedes 300D diesel is exited the same way, never had a fail. I agree the printed instrument cluster array is not the best, but the usually works OK until a mouse craps on it, or someone decides to take it out and 'detail' it.

Of course, it is pretty much a '84 K-30, pretty easy to set up electrical system differently if you want to.

Click to expand...

Oh yeah! I'm in no way trying to "talk trash". I just find it amusing, probably because I've owned my share of civilian square-bodies and these issues make me laugh now because if I don't laugh it turns into aggression.