Gen 2 Bulb

[Red Dragon]

Hey, I know this has been hashed out over and over but I pretty much know specifically what my problem is. My truck is a 1986 m1008. I just had my alternators rebuilt, with the key on I get 24v to the brown wire on the 2 plug on alt 2, 24v relay works, I get 24v to the bulb but no ground to the bulb so the bulb will not light. I can't actually start the truck because I'm waiting for a new harmonic balancer to arrive. So my question is which wire supplies the ground to the ALT 2 bulb? Can I just jump a wire to chassis ground to get it lit or does it have to be the isolated ground?

Thanks to all of your help! This forum has helped me a lot.

 

[Warthog]

The ground for the alternator bulbs is inside the alternator. It is part of the voltage regulator.

I don't have the GM manual at this moment but I can post it tomorrow.

 

[Red Dragon]

So alt 2 gets it's "ground" +12v on isolated negative terminal (Red orange stripe). The positive terminal is +24 volt (Red also on this terminal there is an orange wire) 2 wire plug is Red +24v all the time, brown +24v key on. I'm not seeing a ground that would light the bulb here? When I put my volt meter on the lower ribbon on the bulb holder I get +24v to chassis ground, but no continuity to chassis ground on the upper ribbon strip on bulb holder.

 

[Red Dragon]

After thinking about what you said about the bulb getting it's ground from the rectifier, that would imply that there is a path for the brown wire with white stripe to take to ground via rectifier and the alternator housing correct? That would light up the bulb at the other end until the engine starts then that path would be broken. I remember the guy at the alternator shop telling me that everything in the alternator had to be isolated from ground, otherwise it would short out and mess up the trucks wiring. M​aybe he isolated it too much? Also there appears to be a capacitor with a strap around it which I took off because I read somewhere that it was a suppressor and not needed, is this indeed the case?

Thanks for all the help.

 

[cucvrus]

Are you sure that it is not in the instrument cluster. Many times it is just a matter of cleaning contacts on the bulb inserts and the printed circuit board. Have you checked that? It is just a thought. I don't know enough to get so deep into this diagnostic conversation. So I will share the simplicity of what I know.

 

[Red Dragon]

Thanks for the reply cucvrus. Since I'm getting all the right voltage on the correct wires at the alternator then it's got to be an open ground inside the alternator itself. Maybe when they rebuilt this alternator they used the negative post as the ground for the rectifier not realizing it's actually a 12v positive wire? Not sure until I can get my voltmeter on it, maybe this afternoon. Thanks.

 

[Warthog]

Here is a Delco Remy Service Bulletin for the 27SI type 100 alternator. While it isn't the "isolated ground" units we have on the CUCVs, they are very similar.

View attachment Alternator 27SI.pdf

I misspoke early and said it was the rectifier. It is actually the voltage regulator.

On the "isolated ground" units it is indeed the ground terminal where the bulb" grounding will take place. On the passenger side GEN2 it is the 12v positive feed that is providing the ground for the bulb.

If you are getting 24v on the small brown wire with the key on the the bulb and wiring in the cab should be okay. You should also be seeing 24v on the small red wire at all times.

While it doesn't fully explain the current path this thread may help shed some light on how the current flows on the CUCV charging system

http://www.steelsoldiers.com/showth...ternator-Isolated-Ground-Therory-of-Operation

 

[Red Dragon]

Thanks to you I now fully understand the Gen 2 charging circuit, I will tackle this later and post the results.

Thanks again warthog.

 

[Red Dragon]

O.K. so i don't think my cucv is wired correctly. On Alt 2 the red wire with white stripe now reads 21-22 volts, I could have sworn it was 12.5 before also it shows continuity to the red wire which is reading 24v which it should not be doing (short?) also I just noticed that the wire going to the large resistor for the glowplug relay is coming off the negative terminal of the rear battery (12v) shouldn't it be coming off the + side of that battery (24v) Hmmm, where does that red wire with white stripe go from the alternator? I read the diagrams but in the truck it gets buried in all that split conduit.

 

[Red Dragon]

Found it! blue fusible link on 12v terminal to red wire white stripe totally fried. I must be getting the 21-22 volt reading through the alternator winding, very confusing. anyway just need a new fusible link and I should be good to go.

Thanks for all the help.

I found out why the fusable link blew, that post with the red and white wire made contact with the alternator bracket chaffed the rubber boot and shorted the wire. I can see some molten metal on the bracket. So I will get the proper size belt on that.

 

[Red Dragon]

I guess my alternator is not very "exciting". After hooking up the white wire with white stripe to the 12v buss still no light, also no 24v to the brown wire w/ white stripe on the 2 wire plug at alternator even though I had it before... I checked continuity from the alternator plug to the diode plug and it's good. Continuity from diode plug to relay plug is good, I get 24v to the voltmeter plug but not the diode plug. I get 12v which activates relay but no voltage on feed 24v wire. does 24v pass through the bulb before it gets to the relay plug or does the relay feed the bulb?

 

[Red Dragon]

So I did some more troubleshooting, here is what I found for anybody having this issue-

The Gen 2 bulb does feed the 24 volt wire to the relay, it is solid brown. I get 24v to the bottom ribbon strip on bulb holder and continuity from upper ribbon strip on bulb holder to the relay plug brown wire. the reason why I'm not getting 24v at plug is because the upper ribbon is not making good contact to the bulb. I think I'm really close to getting this solved.

 

[Red Dragon]

Solved it, I'm not a fan of jury rigging stuff but this was the only way I know how to solve this problem short of replacing the cluster. The copper ribbon on the upper contact of the bulb holder was in bad shape. Sometimes I would get a voltage reading sometimes not, lot of resistance. Also there seems to be a point of failure in the cluster plug as well. There are many possible points of failure on this circuit, pretty bad. so I bypassed some of that mess by soldering a 16 gauge wire to the underside of the upper bulb contact and cut the brown feed wire about 6 inches from the 24v relay plug. I butt connected this wire to the new wire going directly to the bulb. Now the light works Until I hit a bump then well, who knows.

 

[Warthog]

Glad you solved the problem.

Did you happen to read these threads in the CUCV Helpful Threads sticky?

http://www.steelsoldiers.com/showthread.php?52821-Instrument-Cluster-Cleanup
http://www.steelsoldiers.com/showthread.php?43351-CUCV-Instrument-Cluster-Repair
http://www.steelsoldiers.com/showthread.php?46054-CUCV-Instrument-Cluster-Decoded

There are replacement circuit boards available

 

[Red Dragon]

Thanks warthog! I will read over this and add it to my list, would love to get that cleaned up, lots of corrosion. But for now I need my rig ready for winter snow plowing. My driveway is 3/4 mile long.