Front Battery not charging, Alternator tests good

[wtn6auva]

I've got an 84 M1009 that I've owned for 9 years now and have been able to fix most of the problems that have arisen over the years but I'm not the best with electrical so I could use some help from those of you that are experts on the 24v system.

My problem started back in the winter after a seemingly normal start up. I had a squeal from the alternator which was normal because it always squealed for a little while after start up but this time it would not stop.I revved the engine a few times because that usually makes the squealing stop but this time it didn't work. I drove off very slowly hoping the squealing would stop and after about 30 seconds slow driving I heard a pop and the squealing abruptly stopped.I looked at my voltage gauge (it's aftermarket, replaced stock gauge) and it was reading 12v rather than 14v as it normally does.

So, the front battery is not charging.I’ve read up on the alternator threads and I’ve done the following to try and diagnose the problem:

1. Took the alternator off and had it tested by Advance Auto. The alternator tested good at 14.x volts.
2. I got a slightly smaller belt for the Gen 1 which allowed me to tighten the belt to remedy the squeal.
3. I checked the fuses in the fuse block and found one broken 25 amp fuse at the very top of the fuse block. I replaced the fuse but it didn’t change anything.
4. Voltage is as follows: Small Red Wire (Key Off): 12.75, Small Brown Wire (Key Off): 0
   Small Red Wire (Key On): 12.3, Small Brown Wire (Key On): 0
   
   Front Battery (Key off): 12.81, (Truck Running): 12.5
   Rear Battery (Key off): 12.34, (Truck Running): 13.79

It seems the problem is with the small brown wire which is what I need help with.The truck starts and runs fine, it’s just not charging the front battery.Also, the bulb for the GEN 1 light wasn’t working (GEN 2 light does work when key is turned and then goes off after starting).I changed the bulb and it still isn’t working but I’m not sure if it ever worked.

If anyone can help me out I’d greatly appreciate it!

 

[m38inmaine]

You need to fix the gen 1 light first. You will need to pull the instrument panel and clean the gen 1 bulb contacts. I'm pretty sure if gen1 light doesn't come on you are not completing the circuit that "turns on" gen 1. If I remember correctly thats why you are not seeing voltage on the brown wire with the key on. The brown wire is what tells the alternator to turn on and that circuit runs through the gen 1 bulb, no bulb no charge.

 

[Warthog]

Check the very top Heater fuse. It controls the brown exciter wire for GEN1. Can't find it in the wiring diagrams? It isn't listed. However it is listed the the GEN sticky (I know because I put it there )

 

[stationjj]

Oddly, I have the exact same issue. No voltage on brown wire with truck on, and Gen 1 doesn't come on. I was leaning towards a bad Alternator.

Thanks so much warthog! Your post was right on. I checked my heater fuse. I found it had melted. Also can you please verify? Is the heater switch also tied to Gen 1. So if your heater switch goes out it can cause issues with Gen 1? Is that correct?

Thanks,

stationjj