I put a GM alternator on my old Ford and now I can't get my amp guage to work. The alternator is bolted to the engine and I have it wired to the battery. I'm getting 13.98 volts on my volt meter at the battery with the engine at idle and all the accessories running.
The alternator has 4 wires. There is a black wire that is bolted on by it self that I ve grounded to the block. There is a big red wire on a stud the is connected to the battery side of the starter solenoid. There is a plastic clip in the side of the alternator that has a red wire that is spliced in to the first red wire and connected to the battery side of the solenoid. The other wire on the plastic plug is a brown wire. I don't have it connected at all.
Any one have any bright ideas on this one?
Doc

 

Did you switch from 6V positive ground to 12V negative ground? If so you need to run your wire the opposite way through the loop on the back of the ammeter.

 

Doc,
JD has a good suggestion, if you are using the original harness.
For the GM alternator, the brown wire goes to the idiot light for the battery/alternator/generator or whatever your lense might say. You have the large red wire in the correct place. The small red wire should route to where all your loads are connected. If this is the battery side of the solenoid, then that would be a good place for it. There is a good explanation of how the GM alternator is to be wired at www.madelectrical.com

Good Luck,
Kent

 

Guys, I'm working on a 73 model F100 so it's already wired for 12 volts. Nobody over at the 73 to 79 model forrums seem to know, and I know some of you guys are running the GM alternator. So I thought I might ask yall how to hook up the amp guage.
Doc

 

Doc,
If you really want the amp gauge to work, you will have to run the power wire from the alternator, through the cab, to the gauge, back out of the cab and to the battery side of the solenoid. An amp gauge measures current flow and has to be "in line" with the main charging wire.
Another alternative may be to add a voltmeter and hook it to the battery side of the solenoid.
Good Luck,
Kent

 

Kent, the newer trucks don't use the old "total amperage output" gages that the stock trucks in the 48-60 years used. I got confused about that a couple times and finally figured it out. You are right-on with the wiring, though.

Doc, I'm gonna give you Kent's wiring in different words to see if it helps.

First, make sure the contacts are clean on all connections. The black ground is fine where you have it.

The alternator puts out it's voltage (and current) through the big red wire. This should be connected to the battery positive terminal, or another terminal that has a big wire to the battery positive terminal - like the battery side of the starter solenoid.

The two wires in the small connector serve two different purposes. The #1 terminal (marked with a "1" on the case) is used to connect to the dash warning light. In this circuit, one side of the warning lamp is wired to the system 12V supply that is on with the key in the ON position. The other side is connected to the #1 terminal on the alternator. During normal operation the lamp stays off because the #1 terminal supplies 12V . If the regulator is damaged, the #1 terminal goes to zero volts, and the warning lamp will light - unless it's burned out (dumb design).

The #2 terminal (marked with a "2" on the case) is used to excite the alternator into operation. It is connected to the battery positive. For simplicity you can connect the #2 connector wire directly to the "batt" terminal on the alternator.

Hope this helps a little.

 

So...I'm assuming to make my factory amp guage work, I'm going to need to take my alternator charge wire, run it through the amp guage, then to the battery. Correct?
Doc

 

You shouldn't have to if you have stock 72 gages. By that time, I don't think they were still running all that current into the cab. I think all you need to do is follow Kent's suggestion (or my rewording) and it should work.

 

the factory amp gauge is really just a fancy volt gauge it senses highs and lows. the shunt wire should be listed in a shop manual but i'm pretty sure it was part of the stock voltage regulator setup.