Bad Alternator?

[chrismoore701]

I have a 1979 C10, I have the engine bay wired on my truck. I have a new painless wire harness hooked up. I have three (3) wires on the back of my alternator. I disconnected the battery while running and it died. I measured the voltage off the post while running and had just under 12 volts, i.e. battery voltage. I did paint the alternator case so I am wondering if I don’t have a good ground. I had it tested and they said it worked. I didn’t think to ask them what voltage I was getting, but it should be somewhere north of 13 volts correct?

Thanks
Chris

[ray_mcavoy]

Yes, if you're measuring just under 12V at the alternator's output post with the engine running, the alternator isn't working.

The painted alternator case interfering with the ground is definitely a possibility.

Also, for future testing, you should avoid disconnecting the battery with the engine running. That can cause voltage spikes which can potentially damage the alternator's diodes, voltage regulator, and other electronics in the vehicle.

[chrismoore701]

I just put the alternator back on after cleaning all ground surfaces. Same thing. I did check the ground and it is grounded. I just hate to buy a new one and have same problem. Had it checked twice and they said it is good. Do these guys not know how to test. Is there another test I can do?

[v30crewcab]

Do you have a light or gauge in your truck? a bad dash light can cause it not to charge.

[ray_mcavoy]

v30crewcab ... good call on the light!

Along those same lines, a bad connection anywhere in that "exciter"circuit (brown wire) can prevent the alternator from charging as well. Similarly, a truck that has been swapped from warning lights to a gauge cluster without anything to address the removal of the battery/gen light could have the same results.

A quick way to test is to unplug the 2-wire connector from the alternator and use your meter to check for voltage on the brown wire with the key on.

[chrismoore701]

I have not installed the gauge cluster yet. Someone else mentioned that last night. I would not think that could cause it. I am going to finish the total wiring job job before I make a final check of the alternator. Thanks guys

[northeimcustom]

I am having a similiar issue, no voltage on that brown wire with the key on. I dont think i had a battery dash light that i can recall ever being on. Where does that brown wire go??

[ray_mcavoy]

Originally Posted by northeimcustom

I am having a similiar issue, no voltage on that brown wire with the key on. I dont think i had a battery dash light that i can recall ever being on. Where does that brown wire go??

The brown wire from the alternator runs back to the bulkhead connector on the firewall (driver side below the windshield wiper motor). Once the circuit passes through that connector, the wiring inside the cab depends on the style of gauge cluster.

Trucks with the basic "warning light" style cluster have the brown wire connected to pin #7 of the instrument cluster connector. A trace on the cluster's printed circuit then takes it to one side of the "GEN" / battery warning light. The other side of the light is connected to a battery voltage source that is switched on/off via the ignition.

Trucks with the factory gauge clusters (that included an ammeter for 73-75 or a voltmeter for 76-up) do not have a gen/battery light. So instead, the factory wiring includes a special resistance wire (brown with white stripe) that connects the brown wire (from the engine side of the bulkhead connector) to a terminal on the ignition switch.

[northeimcustom]

Originally Posted by ray_mcavoy

The brown wire from the alternator runs back to the bulkhead connector on the firewall.

Trucks with the factory gauge clusters (that included an ammeter for 73-75 or a voltmeter for 76-up) do not have a gen/battery light. So instead, the factory wiring includes a special resistance wire (brown with white stripe) that connects the brown wire (from the engine side of the bulkhead connector) to a terminal on the ignition switch.

I have an 83. So if I am understanding this correctly, the resistance wire (brown with white stripe) is under the dash on the steering column/ignition wiring that would burn up?

If i just connect that brown wire from the alternator to a 12v ign power source i have to use a resistor of some sort? this seems easier than chasing wires.....

[ray_mcavoy]

Originally Posted by northeimcustom

I have an 83. So if I am understanding this correctly, the resistance wire (brown with white stripe) is under the dash on the steering column/ignition wiring that would burn up?

Yes, that resistance wire is included with all of the other wires in the under-dash harness and runs from the bulkhead connector (behind the fuse box) up to the ignition switch. It's pretty rare for that wire to burn up but a poor connection (either at the ignition switch or at the bulkhead connector) is possible.

Originally Posted by northeimcustom

If i just connect that brown wire from the alternator to a 12v ign power source i have to use a resistor of some sort? this seems easier than chasing wires.....

Yes, if you want to bypass the original wiring and add your own, you will need to include a 10Ω resistor inline with the wire. I believe the stock resistance wire was rated somewhere around 6.25 Watts so you'll want a resistor that can handle the same (or higher) wattage. A 10Ω, 10 Watt resistor is a fairly common value (the most common in this size would be a wirewound style resistor in a ceramic case ... they look sort of like the ballast resistors that were used on some ignition systems). Note, the 10Ω value assumes you are running a stock type SI alternator. If you have changed to a newer CS series alternator, those require at least 35Ω.

[northeimcustom]

So i connected the brown exciter wire to a 12v ignition power with a 10 ohm 10 watt resistor. And yes i have a si type alternator. It still wont charge. But atleast i have power to that exciter wire now. Origanally i thought i was my cheap chrome ebay alternator, so i thru on a different one i had laying around that didnt work as well. This is when i realized i didnt have power to the exciter wire. I think i am going to have my chrome alternator tested at the parts store before i put it back on.