Gen 1 light will not come on

[1008Newbie]

Hi Everyone,

I bought a M1008 two weeks ago, but the Gen 1 light does not come on. No charge on front battery. Back battery charge good. I have replaced the passenger side alternator (tested bad), but I didn't replace the drivers side because it was tested on the stand and they said it was good.

I just took out my instrument panel last night and removed 25 years of oxidation on all the light bulb sockets and plastic circuit sheet, and put in new bulbs, dialectric greased contacts, resistance readings on everything to make sure it was all good, and fixed my shift indicator as well.

Now I can see my speedo at night, have cabin lights, but still no Gen 1 light.

I have 12V to the Gen 1 bulb, and I have 12V to Alt #1 brown wire (exciter?)with key turned in ignition.

Any thoughts? Think I should go ahead and replace Alt #1 just to be sure? TM says to trace circuits, then replace Alternator, but Alt #1 tested good.

I do have an unknown Blue and pink/black wire hanging suspiciously next to the firewall on the drivers side (photo shown) I think the blue wire has 6 volts, and pink/black has 12V, not sure if that was intentional by previous owner after replacing relay, or if that could have something to do with charging problem.

The wiriing diagram isn't very clear to me. I think the ignition switch supplies power to Gen 1 light, then that power goes to brown lead on alternator, and not sure what happens after that, even after reading the posts.

Thanks for any advice you may be able to offer.

Eric

 

[Wolf.Dose]

To my understanding the blue one is the plus of Gen 1 to the connecting point at the dash (ENG.Wrg. Harness Block) or the minus of Gen 2 from the dash. For your Gen 1 control light does not go off, I guess, it is the plus of Gen 1.
For this refer to chart F-9 of TM 9-2320-289-20.
The black white than is the feeding of the counter voltage of your gen 1 control light, even the manual says it should be brown.....
Six volts you will not find in these trucks!
Hope this helps.
Wolf

 

[1008Newbie]

thanks

Thanks for the help.

Another person said the blue was the ground of the glow plug relay. I'm thinking those two wires were for the old relay and are now dangling because they replaced it and put the glow plugs on a toggle on the dash.

The problem with my alternator / Gen 1 light not coming on, is that the front battery has no charge while running, and the Gen 1 light won't even come on with the key turned on (before starting). Everyone says if the light doesn't come on the alternator won't charge, and its not charging.

I've had the alternator tested once and they said it was ok, so I'm at a loss as to what it might be except maybe some fusible link somewhere. I've redone the instrument panel in hopes it was a bad connection. I'm pretty close to trying to change it to 12V to eliminate some wiring. My driving is limited to daylight hours only because the headlights will drain the batteries.

 

[doghead]

It might be better to cover your issues in one thread. I just answered in the other one, and I see you are dealing with it here also.

 

[1008Newbie]

Ahh, I see what you are saying, there are blue wires going to the 12Volt junction as well. I think this blue wire is smaller than those blue wires, although I sure wish that were the case since I can't get the charging system to work and a loose wire would be a quick fix.

 

[1008Newbie]

Yeah, sorry about that. Since nobody replied to the other one for a while I made it a simple 2 wire question without all the charging system info.

 

[Warthog]

The two wire plug is the excitor system.

The brown wire on the drivers side is a 12v feed from the ignition switch. It feeds thru the Gen1 dash bulb.m Since it is showing 12v the bulb is good.

The red wire is a continuous 12v from the front battery. It feed thru the 12v terminal block on the firewall. Check for 12v. If not check the fusable link.

The big red wire is also a continuous 12v from the same terminal block.

If the alternator is an original isolated ground it must ground to the engine from the Ground terminal on the alternator.

There are two additional small wires for the ICE/STE diagnostic system.

No charge on front battery. Back battery charge good. I have replaced the passenger side alternator (tested bad), but I didn't replace the drivers side because it was tested on the stand and they said it was good.


Any thoughts? Think I should go ahead and replace Alt #1 just to be sure? TM says to trace circuits, then replace Alternator, but Alt #1 tested good.

On a stock system the driver side alternator charges the front battery and the passenger alt charges the back battery. So I don't understand how the battery is charging good but the alt tested bad.

 

[1008Newbie]

I replaced the passenger side alternator because it tested bad. The back battery has 14.5 volts across it when running, so it should be good. I'm not so sure the guy grounded to the terminal instead of the case when he tested it, but either way it is working.

The driver side alternator is good as well. In fact I just took it to a parts store and watched them check it again because I have thrown a few sparks while troubleshooting so I thought I might have cooked it. I nearly burned off my multimeter lead once.

So both alternators are good, but the front battery doesn't charge, and the gen 1 light does not come on. That sounds like the wiring you mentioned. I will double check that there is ignition voltage on the brown exciter wire, and 12V from the firewall terminal on the red wire of Gen 1.

And once again, thanks, I appreciate the help.

 

[1008Newbie]

Warthog, because of your post I rechecked the voltages on the wires while the alternator was out.

12V on Big Red Wire
12V on little brown exciter wire
12V on little red wire

Then I checked resistance from ground connector to engine and radiator....well there was connectivity, but there was also more resistance than there should be for the big fat black wire.

I took off the ground sanded the nut and connector, and the connection bolt, applied dialectric grease to everything and reconnnected it. Turned on the key and WHOOHOO! There was my Gen 1 light for the first time since I've had the truck.

WHOOHOO! MY LIGHTS ARE BRIGHT! MY VOLTMETER IS IN THE GREEN! THANK YOU ALL SO MUCH!

Now I apologize for the rookie mistake of not thoroughly checking that before. Feel free to flame me for that. These trucks are 25+ years old, every electrical connection needs thoroughly cleaned.

I drove it from Savannah, Georgia to Indiana (with a convoy behind me the whole way )and the batteries tanked in South Carolina when i turned the lights on. But now I have a killer truck!! I feel a night time mission to the beer store coming on!!

 

[Warthog]

No flames from me.....

Glad you got it fixed. Since that alternators use the "ground terminal" to complete the circuit, if they aren't clean/tight it makes for a pain to troubleshoot.

Good Job