Alternator #1 no light

[2deuce]

I've looked here at the sticky's and the TM but can't find any answers so I'm asking for help. I think my alternator is good, it charges the battery. I can't get a light at start up on alt #1. There is power to the #1 alt with the key off. There is no power to the exciter wire key on or off. There is no power to the plug for the light behind the dash either. I figure that the next place to check would be upstream from the plug, but where? The #2 alt works as it should with the dash light. The #1 alternator will come on without the light if I rev the engine high enough. Thanks

 

[2deuce]

I'm still stuck on what to do with this alternator. The alternator lights that comes on when the key is first turned on before the engine turns over... what controls the light coming on? When the alternator is energized by the power flowing thru the light and as the alternator begins charging something turns off the light as that energizing is no longer needed. How is this accomplished? Thanks

 

[Warthog]

Check the heater fuse. It supplies power to the GEN1 excitor circuit.

 

[2deuce]

Which one covers the heater? I couldn't find one labeled for the heater, but I checked them all and they appear good. The heater fan doesn't work. There are open slots for a fuse.... possibly I'm missing that fuse. I'm going to look for a wiring diagram, but it really is hard to search on this site without being able to define the search. Thanks

 

[ThunderCracker]

Maybe this will help

 

[ThunderCracker]

 

[cpf240]

The TMs are available on this site, and they have the wiring diagrams in them.

 

[2deuce]

Thanks Guys, I found the heater fuse and it was burnt. I replaced it and the light comes on and goes out when the engine starts. I haven't checked the alt output yet but I think it's fixed.

 

[2deuce]

Update...I'm now back to the No.1 alternator. The alternator light started to come on dimly and eventually it stayed on all the time and before I took it off it would get brighter if I reved up the engine. I figured that the alternator was bad and found 2 takeoffs for sale. I put one of them on and got no light at no.1. I checked the bulb it is good, the fuse is still good. So I put on the 2nd takeoff alternator I bought with the same result. Then I put the original alternator back on and now no light with it either. I can see why people want to change to 12V. Next I'm going to try to figure out where the exciter connects to the alternator and what turns it out. Any help is appreciated.

 

[doghead]

"Converting" to 12 volt, in this case would do absolutely nothing for you. Your dealing with the 12 volt alternator.

Next time, repair not replace(with used) parts.

Every single step and troubleshooting step is listed in the TMs. All you need to do is read and follow the step by step instructions.

 

[rtk]

Throwing parts at your MV is a crap shoot at best , as they say take the time to read the TM's . It is not a difficult system to diagnosis once you get a understanding of it . Those GM alternators are some of the easiest to rebuild and parts are available . Just get yourself a decent VOM and you are in business , good luck .

 

[Skinny]

I would rebuild both Gens anyway...good precaution. I would buy a third kit just as a spare.

 

[dependable]

Probably the 'take offs' you bought needed to be 'rebuilt', just like your original. Putting in new diode pack likely to solve problem, you can do the whole 're build' to be thorough, but make sure you use good components. It is a mistake to take out good working original parts and replace them with 3rd world junk. There are a lot of existing threads on fixing alternators and where to get parts.

If you pull off your heat blower motor and you can see mouse nest (likely made of firewall lining) then it is time to check resistor and blower motor for shorts caused by mouse piss or chewed components. Warthog has mentioned that the heater switch can also be a problem.

The wiring diagram in the TM is not that good in showing this particular circuit, in my opinion.

 

[2deuce]

I had the thought that the takeoffs were bad too, but wouldn't the dash light just stay on if that were the case? I don't understand what turns the light on and off. If the alternator is charging the light must go out, but it also must come on with the key. I looked at the TM and I couldn't understand it well enough to help me. I couldn't even find the gen 1 circuit to get an idea what is going on. I probably have a bad connection somewhere whether the alternators I bought are good or not. The way I understand it the alternators don't turn dash lights on, only off....somehow.

 

[dependable]

The symtoms you described in post number 9 would call for an alternator rebuild or replace/test with a known good one. Does you heater blower work?

 

[Warthog]

READ the first few pages of the Troubleshooting Section of the TM 9-2320-289-34 Tech Manual. It explains how to read the wiring diagrams.

Sometimes the TM wiring diagrams don't post all the circuits. The GEN1 circuit is one of those.

Going back to the 3rd post, the GEN1 circuit is controlled by the heater fuse. Why GM built it this way is anyone's guess.

Follow along as I trace the GEN1 circuit in a stock system:

1. From the Negative terminal of the rear battery (same as the positive terminal of the front battery) to the 12v Engine Wiring Harness Block (12v Bus) on the firewall - wire RED 2A - Diagram E-1



2. From the 12v Bus to the firewall wiring block - wire RED 2E - Diagram E-3



3. Thru the firewall plug to the inside half of the plug and continued to the next diagram - wire RED 2A - Diagram E-3



4. From Diagram 3 thru the wiring harness to the ignition switch - wire RED 2C. Thru the switch and to the heater fuse - wire ORN 300 - Diagram E-4



Now this is where the diagrams are not marked. I have physically traced the wires and have verified the circuit.

The Heater fuse supplies power to both the heater circuit and PIN#5 of the instrument cluster. PIN #5 supplies power to the GEN1 bulb, the 4x4 Indicator Lamp and the Oil Pressure circuit.

I have added the wire to the Diagram

5. From the Heater fuse to PIN #5 of the cluster plug, thru the circuit board and GEN1 bulb back to PIN #8. From PIN #8 of the cluster plug to the interior firewall plug - wire BRN 25 - Diagram E-9



6. Thru the firewall and to the GEN1 excitor plug - wire BRN 25 - Diagram E-9



The brown wire at the GEN1 exciter circuit plug has to read 12v when ever the key is on. The red wire will read 12v at all times


So to sum up. The Heater fuse controls many things: the heater, the GEN1 excitor circuit, oil preasure circuit and the 4x4 lamp circuit. An issue with any of these will cause a low voltage condition and possibly interfer with the GEN1 operation.

Did you also notice the black "goo" in the firewall plug? GM used some type of grease that transforms into this black goo and DOES cause issues with circuits. Taking the plug apart and cleaning the contacts does wonders.

 

[Warthog]

Here is a Service Bulletin that help explain how an alternator works. It is not for the 'Isolated Ground" units we use but covers how they work.

The "lamp" will be on when no power is being generated and "excites" the windings. The "lamp" will go out indicating that the alternator is producing voltage