Why is the Gen light on?

[jayoldschool]

Taking care of the last few things on buddy's 63 C20. We bought it with a 350/350 combo, but we pulled and swapped in a fantastic low mile (30k!) 235/SM420 from the 62 C30 that we got in a package deal.

Looked like it had been converted to a later three wire internal alt. We converted everything back to stock following the FSM diagrams. Horn relay, regulator, correct ext reg alt, wiring. Gen light comes on when started. Ordered another regulator, no change.

Just looking for musings or experience that will point me in a new direction.

[chevylookin]

I hate to ask but you put the wire from the horn relay to battery on, right?

[jayoldschool]

Yes. I had my 65 parked next to it to double check wiring. Ran it from the batt, out through the factory grommet, over to the horn relay.

[Pappyx3!]

Check back of the alternator, locate the R terminal in the 2 wire plug. Check to see if you voltage from there back to regulator #2 on the connector.

This is a signal wire to the reg. when it sees approx. half battery voltage it removes the ground inside the reg. to turn the light out.

[jayoldschool]

Thanks, I will check that tomorrow. I'll bring my 65 to work to park beside the 63 to compare the tests at the same time (mine works fine). I checked the fuses in the 63 today, everything's fine. Wiring underhood is correct, went through it all again today.

With the engine running, should there be voltage on both the R and the F terminals on the alt?

[Brad54]

Could be that the wiring and light is working as it's supposed to, and it's telling you the generator is going bad.

It doesn't happen often with our old cars, but every once in a while, the simplest answer is the right one!

-Brad

[jayoldschool]

It's a brand new alternator, the correct external reg model. But yes, we're running out of options. As soon as I get the meter on the F and R terminals on my 65, I'll know if the new alt is bad on the 63.

[jayoldschool]

Update time. A comparison between the two, both running:

65:
- close to 15V on the pos stud of the alt
- 7.5V and 9V on the R and F terminals (can't tell which is which)

63:
- batt voltage on the pos stud of the alt
- R/F: voltage on one terminal, no voltage on the other

We already tried another regulator (both are brand new). I'll try another alternator, it sure seems like this one isn't working. When we spoke to the parts place, found out that they don't deal with this supplier anymore. Wonder why, lol...

[chevylookin]

Jut for poops and wiggles, why dont you swap out your alt from the 65 into the 63 setup and see if you get a different result before you go through the hassle of potentially returning a good alternator. I have overlooked so many lil silly things when dealing with these dreaded electrical problems. Its usually so simple but then again, on the other hand, quality control aint what it used to be in China anymore....

[chevylookin]

Oh yeah, how is the battery grounded? Just to the frame? Or to the engine? I have had problems before in the past especially with the old 6V systems. I double ground things, ie: battery to engine and engine to frame and this has helped me in the past.

[jayoldschool]

I had considered that, but... if there is something wonky with the wiring on the 63, I really don't want to blow my good alternator. It's not a problem to get another one for the 63, the parts place delivers to us a few times a day (I work at a high school, my buddy is the auto shop teacher).

[chevylookin]

Ok, well, I cant wait to hear the end of this gremlin. Let us know. If after the next alternator and the same result, you will have to start from square one and go through every stinking wire from the battery to the starter to the ignition switch. Everything. The regulator is mounted properly with the plastic firewall insulators and everything. No second guesses?

[jayoldschool]

Yes, everything is brand new under the hood. I followed the FSM diagrams, and, parked my truck next to the 63 to triple check. I've checked fuses too.

I'll update when the new alternator is on.

A pic of mine and the 63 last time they were together in the shop. Just cause everybody likes pics.

[Pappyx3!]

Originally Posted by jayoldschool

Update time. A comparison between the two, both running:

65:
- close to 15V on the pos stud of the alt
- 7.5V and 9V on the R and F terminals (can't tell which is which)

63:
- batt voltage on the pos stud of the alt
- R/F: voltage on one terminal, no voltage on the other

We already tried another regulator (both are brand new). I'll try another alternator, it sure seems like this one isn't working. When we spoke to the parts place, found out that they don't deal with this supplier anymore. Wonder why, lol...

Jay,

Looking at the back of the alternator, the R terminal is on the left, F terminal is on the right. Back probe the "R" terminal right at the alternator if no voltage, swap you working alt. or replace the alternator. Has to have voltage signal back to the reg. to turn the light off.