I have a 1990 GMC K1500 with an intermittent charging problem. It will charge fine while cold but after the engine and cab warm up it will charge for a while then discharge then charge, etc.

I’ve tired 3 alternators, tried a different battery, I’ve jumped between all the wires and makes no difference. The one thing that I have found is while it is charging property if I pull the brown wire, at the alternator, from the instrument cluster and check the voltage it is about 12.5 volts. When it is not charging the voltage reads .2 to .5 volts, almost nothing.

I checked the voltage of the brown wire while it was charging properly and while it was connected to the alternator and it would read about 13.9 volts, at the battery it was reading 14.5 to 14.7 volts. Is there something in the instrument cluster that limits the voltage to just under 14 volts? I tried jumping between the battery and the brown wire once and for a short time it charged fine, then it started charging at 18.5 volts then did not charge. I had that alternator bench tested and it was toast. So another alternator.

Is it possible that something in the instrument cluster is going hay wire and dropping the voltage to the brown wire and causing my problem? I might point out the in dash volt meter and idiot light are working properly.


Thanks for your help

Steve

that little wire is supposed to take switched power through the light and turn on the alt. its has to be between 35 and 350 ohms. if you have a bad connection or the light socket is getting corroded your going to have problems blowing your internal reg in the alt. so if you dont have proper voltage there all the time your going to have to check your wire back and find out whats going on

Thanks for the tip JWfirebird!

Do you know if it'* possible to just run a different wire to the alternator if I'd put in a resistor? I did take out the instrument cluster to see if there was anything visibly wrong. Didn't see anything burnt etc. I did pull the bulb out and spread the wire attachments a bit, hoping against hope that it might just be a bad connection but no luck. It still did it'* charge/discharge thing.

I should have been a little clearer in my question. Is it possible to run a wire from the fuse box that would be hot on start and run with a resistor in it to the alternator and simply avoid the wire coming from the instrument cluster? Is the 30 to 350 ohm resistance in the wire or the circuitry board of the instrument cluster?
Thanks again
Steve

thats supposed to be from the bulb and normal connections
ive got my firebird alt wired that way, seems like you already know there are some that are keyed but not hot in start.

did you check the resistance with a meter or just look? alot of times you cant see the problem, loose connections, worn through wires in the insulation you wouldnt notice without checking it.

i wired my car that way because its got aftermarket gauges and stuff, you wount have a light or voltage reading if you wire around it.

I just looked at the circuit board, I didn't check any resistance on the wire. I have looked at the harness and it all looked fine. And yea you are right, looks can be deceiving. I've had this problem since last November. I live in ND, so running the truck and doing tests outside gets to be a little limited this time of year.

You did hit on something there JWFirebird. You said you put in after market gauges. I'm thinking that would be a far easier and cheaper way to do this than chase a bad connection somewhere. Can you tell me how you wired your volt meter in? What about that 35 to 350 ohm resistance that you mentioned?

Thanks for your help. I feel I may be getting closer to solving this irritating mess.
Steve

the voltmeter just connects to any 12v. the alt is just connected to key switched 12v. i didnt use any resistors.

Thanks for the info JWFirebird. I may just put in an underdash volt meter and run another wire. I think that would be quicker and probably save a lot of frustration.

A couple of weeks ago, I thought of doing this and rather than go through all the work, I thought I'd run a little test. I ran a wire from the battery to the alternator to excite the alternator to see what would happen. I had a digital volt meter to check the battery to see if it was charging. I'd connect the wire and it would charge, heck even the in dash volt meter read properly. I disconnected the wire from the battery lost the excite and it would discharge, and the in dash volt meter showed discharge and the light came on. I thought man this will be an easy fix. I did it a couple more times and the next thing is it was charging at 18.5 volts, the in dash meter was pegged at 18.5. I thought this isn't good. I even disconnected the excite wire and it still charged at 18.5 volts. I put the original wire back on the alternator and called it a day. The next morning I started the truck and nothing. No matter what I did it would not charge. I took the alternator to NAPA and it was toast. Good thing I have a life time replacement. So that has made me a little nervous to try that one again. That'* why I asked about putting resistors in the wire. But seems like you're AOK with out any resistor. May be just a bad set of coincidences on my side.

i found there are different types of cs alts.,

There are two different flavors of the CS series alts. you will either have terminals P L F * or P L I * so dependeding on what you have

P= not used (optional tach or other device)
L= ignition through resistor, indicator light or both in parallel
F= not used (optional fault indicator)
*= not used (optional voltage sense)
or
P= not used (optional tach or other device)
L= ignition through resistor, indicator light or both in parallel (optional when I is used)
I= ignition with or with out resistor (internal resistor)
*= not used (optional voltage sense)

do you know witch one you have? if you already have blown one up you probably going to have to get a 3w 35-100 ohm resistor and use that.