I’m lost on this one and need help. I have a 1989 pickup with 22R that has a Aisin carburetor and manual transmission. I recently (2 months ago) installed a rebuilt alternator from auto zone as the old one wasn’t charging the battery. Installed alternator and everything was fine for about a month, then charging and battery light stayed on after starting one day. Also the automatic choke didn’t pull off as it wasn’t getting voltage. I checked the voltage across the battery while running and got 14.4 volts, then I disconnected the three wire plug in the back of the alternator and checked that the white wire was getting consistent battery voltage (12.4) always even with truck off. The red wire gets energized with battery voltage when the ignition is turned on ( not started). Not running the yellow wire has no voltage, but when running it has 7.2 volts when measured at the 7.5 amp charging fuse in the engine compartment fuse panel. So I figured the alternator was bad, but when I brought it to auto zone for testing it tested good. If I pull the charging fuse, one side (yellow wire directly to alternator) has 7.2 volts but nothing on the other side of fuse. That wire ( I pulled fuse panel off inner fender to trace) goes to a plug on the panel and into the wiring harness. I have no idea where it goes from there. What am I missing? I need to fix this as the carb won’t pull off the choke. I have checked wiring diagrams and can’t figure out the issue, please help me.

Alternator test result

 

i don't know much about your model, but if you have 14.4volts at the battery, it sure doesn't sound like a charging issue.

First, as I think you've figured out, the yellow wire is grounded (not "no voltage") when the alternator is not producing output, and as it produces output the wire rises to some higher voltage (your 7.2v). The charge light is connected to battery voltage on one side and the yellow wire on the other, so it illuminates when the yellow wire is grounded. Since you determined the yellow wire at the alternator is not grounded (when disconnected), my first guess is your yellow wire is shorting to ground somewhere between the alternator and the charge light.

Next, what about your choke pull-off? You've confirmed you have 14.4v at the battery, but "no" (there it is again; there's no such thing as no voltage) voltage at the pull-off. What does that tell you? Somehow, you're missing the connection between the battery and the choke.

 

Thanks for the help, after reading your response I went back out and did a little more testing with my meter. I attached the wiring diagram I have been using. With the plug removed from the alternator and the 7.5 amp fuse (charging fuse under hood behind battery). I ohm’ed out the yellow wire from the plug to the one side of fuse and got 1 ohm so I don’t have a short in that wire. Continuity test also proved no short on that wire. However on the other side of that fuse, I have continuity to the negative terminal on the battery. When I disconnect the negative terminal and ohm out that same side of the fuse terminal to ground wire I got 6 ohms.

with the battery connected charge light is on when ignition is turned to “on”. Unplug the 7.5 amp fuse in the panel inside (driver kick panel) and charge light on dash goes out. Disconnect battery and ohm’ed from one side of each 7.5 amp fuses (under hood and inside kick panel) and got 72 ohms.

the yellow alternator wire runs through the 7.5 fuse and then through a plug as a yellow with red dashes as seen in photo. But I can’t find that same wire under the dash anywhere.

does this additional information help ?

I could just change out the alternator but I hat being a “parts changer” as I want to know for sure the issue and the alternator tested good.

thanks in advance.


Yellow wire goes where?

Ohm reading on yellow wire from alternators plug and 7.5 amp fuse

 

I'm having a little trouble following your description, but at least you're doing a careful diagnosis.

Remember, the Charge indicator comes on when there is a current path to ground on the charge-fuse side to the alternator. When the alternator is not turning, it provides that ground. If you disconnect the 3-pin plug from the alternator, does the Charge indicator come on with key-on? (it shouldn't) If you remove the charge fuse, does the Charge indicator stay on (it shouldn't). If you remove the charge fuse, with key-off you should have battery voltage on one pin (the Charge indicator side) and ground on the other. (To detect "ground" with a voltmeter, finding "zero" volts to ground isn't enough. You need to measure from battery + to the terminal; if the terminal is at ground you'll get battery voltage on the meter). You should be able to start and run with the charge fuse out; then you should find battery to ground on one side, and about 13.9-15.1v to ground on the alternator side of the fuse.

Jimkola knows a lot more about trucks than I do, but with 14.4v at the battery it's hard to imagine that the alternator is bad.

 

Thanks for the advice.

 

I recently had my charge/brake light coming on, voltage when running was high... Turned out to be a battery cable that seemed tight but wasn't making a good connection, I think there was a bit of crud between the terminal and the clamp.

 

Thanks everyone for your input, I finally figured it out the issue and diagnosed a bad alternator.

So the wiring diagram isn’t totally clear. There are two sources of DC positive voltage to the battery and brake light indicator. One from the ignition switch, one from the alternator, each has a 7.5 amp fuse. The indicator light has a ground connection not clearly shown on the wiring diagram. If either source (alternator or ignition) is lower than 12 volts the light comes on.

The 7.5 amp fuse in the drivers side kick panel is protecting the battery voltage through the ignition switch and the 7.5 amp fuse in the engine compartment protects the alternator voltage. My light was staying on so I ran a jumper wire from the positive battery terminal to the 7.5 amp fuse in the engine compartment. The light went out so it HAD TO BE the alternator. I exchanged the alternator at auto zone and the new one works perfectly, charge light is now out.

the plug that goes into the alternator has three wires (yellow, red &white) the white has 12 v from the battery at all times, the red has 12 v when the ignition is turned to on. And the yellow produces 12 volts only when the alternator is running. If you remember I had 7.5 volts on my yellow wire. It threw me for a loop because AutoZone said the alternator tested fine but in reality it wasn’t. Hope this helps someone in the future.

thanks again!

 

It’s so lame how many threads are from aftermarket or reman parts, especially alternators.

Glad you got it sorted