Charge light strange problem

[82C10Burb]

Hello all,
I'm working on an 82 C10 Suburban, 350/700r4 with non gauge cluster (idiot lights) and getting an intermittent charge/batt idiot light. I'm stumped, I went through the light test in the manual and the light does not come on with the key on engine stopped - never comes on, as if a bad bulb.

So I checked the fuses and cleaned the connector at the alternator (alt is new and working, charges fine), and can't figure it out. Has anyone seen this, I'm guessing a bad connector/dirty but have yet to find it. Kind of annoying, since the light only comes on erratic once it's warmed up. Any ideas? Grounding terminal 1 at the alt did not light it either, I did not figure out what fuse feeds the light in the diagrams.

Thanks

 

[82C10Burb]

Update: The light only comes on when I turn on the fan blower, the fan does not run. I went and checked the fan relay near the motor and it seemed flakey, motor would power on just wiggling it. Cleaned all contacts and even swapped it out with one from the boneyard and the fan will not start, and the charge light pops on. Will check the manual for testing the relay, I only get power to the motor on high setting. May be the fan control going out? if I unplug the fan and plug in it runs, but won't run from inside.

 

[chengny]

The light only comes on when I turn on the fan blower, the fan does not run. Do you mean this happens with the alternator running?

The charge indicator lamp is connected on one side to terminal 1 at the alternator. That terminal is connected to ground when the alternator is not charging. When the alternator is charging, that terminal 1 is at 12 VDC.

The other side of the charge indicating lamp is connected to a representative point in one of the ignition switched circuits. For whatever reason, GM decided that a good spot to connect this side of the lamp was between the load side of the heater blower fuse and the speed select switch.

The lamp needs to see a voltage differential across it's pins in order to light up (alternator not charging) and if the voltage on both sides is equal it remains unlit (alternator charging).

If the alternator isn't charging (and terminal 1 is grounded) and the ignition switch is moved to RUN, the lamp then sees 12 VDC on the cab side and a ground on the alternator side. It lights up.

Then, if the engine is started, the alternator develops voltage at terminal 1. The ignition switch is still supplying the other side of the lamp with 12VDC. When the alternator voltage equals the voltage supplied from the ignition switch (battery voltage), the differential is lost and the warning light goes out.

With the alternator running and putting out 12 VDC, only when voltage in the power feed to the blower is less than the alternator output voltage should the lamp illuminate. So, I think you are close. Something is causing a voltage drop in the blower motor circuit . Try working backwards. Unplug the lead at the motor and see if that causes the charge indicator to work normally. If not, unplug the harness connector from the resistor stack. If that doesn't make the charge light work right, unplug the harness from the blower speed select switch.

Eventually you'll find where the problem is in the blower circuit. I am going to guess that the blower motor windings are grounded. Once you solve that, charge indicator lamp should work normally. As an initial test, try unplugging the power to the blower speed switch from the fuse block/charge indicator lamp feed:

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[82C10Burb]

" GM decided that a good spot to connect this side of the lamp was between the load side of the heater blower fuse and the speed select switch."

Right you are! So apparently I was fighting multiple problems. The switch that is inside the vacuum switch 'box' was shorting out somehow, popping the fuse (caused the light to not work with key on/engine off). So I took the switch apart, even the fan speed switch to the left of the controls, cleaned all the contacts, cleaned everything up and put it all back together, popped in a fresh fuse. Now it is happy, I have the light with the key to on, then it is off when running. Thanks for the info, I had no idea it was fed that way for power, the diagram is not the most intuitive! Thanks for the info sir!