I have a 3G alternator on my 72 F250 390...question is:

With the positive cable disconnected from the battery, and with the positive lead of the DMM touching the positive terminal of the battery, and the negative lead of the DMM touching the battery post on the alternator, should I be measuring ~12V?

I ask because I am...and my alternator is not charging.

 

uh...hoping i get this right. (someone will yell at me if i don't.)
to answer your question- yes, that test would show 12v.
remember- the voltmeter is measuring the *difference* in the voltage between the two points you are testing with the leads.

your battery + terminal has 12v. if your alternator is not charging- then it is a ground. hence the 12v difference you are seeing.

a much better test would be to hook the battery cable back up, and then just measure across the battery terminals.

if its still reading 12v, i'd check the alternator brushes.

 

Matt,Rusty is correct in that you should put the positive battery cable back on as you may cause damage to the alternator (frying the diodes) or (frying sensitive electronic equipment) by running it without being hooked to the battery. Remember that an alternator needs an outside 12 volt power input to get it to energize and produce power. Never remove the battery cable or the alternator wire when the engine is running. Information on alternator systems found at, ratwell.com/technical/ChargingSystem.html#testalt

"bowties in the rearview mirror"

 

Pretty sure he means without it running/key off. Check across battery not running, key off, 12.5+v, engine running, 13.5-14.5v if it's charging.

 

question- did you just put the 3g in, and are trying to finish the swap, or has this alternator been in place and running properly for a while?

 

I certainly wasn't doing this with the engine running!!

I was trying to finish a 3G swap, but looks like the alternator is just not working. Too bad, Autozone says it tested out ok...hmmm.

I put in another 100amp 1 wire that I had on the shelf and that one is charging just fine. Think I'm gonna leave that one in there.

 

wait- did you have the small yellow & green wires hooked up?

the yellow wire is the 'sensing' wire, and tells the alternator how to adjust its output
the green wire is the 'trigger' wire, and the alt won't charge unless its hooked up (to a keyed ignition source, usually.)

 

Yep, I had everything hooked up correctly.

The test I was trying to do earlier was a "parasitic draw" test, should have been measuring amps and not volts.

I tried a stock wiring harness and also temp wiring and no matter what, the thing would not charge. So I took it back to Autozone and they could not tell me how many volts it was putting out.

 

Got the 3G working! I exchanged the alt for a new one and got new wiring...and it's working great!

Also in my search for parts locally, I found a company that rebuilds alternators, motors, starters...basically anything electrical that rotates. They are called Romaine Electric, I got my alt connector from them...very nice people!

 

right on! was the first 3g (the non-working one) new or used?

 

I was a new (reman) from autozone.