Engine Battery No Charge

[afastchief]

I was returning from a short trip to burn off fuel so I could drop tanks and replace vent lines. It was getting dark and by time I got home the
headlights had set with the sun(Very Dim). I looked at voltmeter and it was showing around 10v. I charged engine battery and checked voltage at
isolator center post and reading was erratic but mostly around 8-10v. I removed alternator and took it to local alt/starter rebuild shop and a week
later I have a rebuilt 80A alt that was run on the bench for 20 minutes and met all GM specs.
I installed alternator and fired it up with dash voltmeter still showing 10v and when I turned the lights on it dropped to 8v. The dash voltmeter did
not follow engine rpm at the slightest, rock steady. I checked voltage at isolator with the following results:
Top: 12.02 steady
Center: erratic, i.e.… 0/2/4/0/18/0/2 etc….
Bottom: 12.45 steady

I removed all wires on isolator and cleaned and treated with CAIG Deoxit, reinstalled, started engine and rechecked numbers; no change?
I am looking for suggestions or a good T/S link.

Thank you,
Alan Edmondson
King, NC
1978 Royale

 

[RF_Burns]

The two pin plug on the back of the alternator should have a white wire and a smaller brown wire (unless modified by a PO).

The white wire is the battery voltage feedback wire and should have battery voltage on it at all times.

The smaller Brown wire is the field exciter used to supply field current when the engine is started. It should show the following voltages:

- Key OFF zero volts
- Key ON, engine not running Approximately 3-4 volts
- Key ON engine running About 14 volts (same as the alternator output voltage)

Have you checked the output voltage directly at the alternator output stud?

Does the GEN light come ON? I'm thinking not since you have not mentioned that. If the GEN light does not come ON with the Key ON and engine not
running, that would indicate there is no current in the exciter wire (Brown wire). If there is 12V (battery voltage) on the brown wire with key ON
and engine not running, that shows there is not exciter current flowing into the alternator. Check the 2 pin connector for corrosion. If you have
zero volts on the brown wire, that indicates a break in the exciter wire circuit.

Hope this helps.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

 

[johnny]

You are describing bad connection of the sense and/or turn on wire plug on the alternator. Take it of, carefully remove the contacts, clean and bend
for good contact and put them back - in the correct places. Consider an alternator protection cable is you don't already have one fitted. Then
accomplish Burns' test if it doesn't operate correctly after cleaning and re-tensioning.
(The alternator was likely OK, but now it's in new condition for another 40 years or so.)

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell

 

[afastchief]

I really appreciate your advice. I should be able to T/S this weekend and will repost. Thanks again...Alan

 

[Ken B]

I have read the above recommendations / instructions and agree with both Bruce and Johnny. My initial reaction was that the connection at the
alternator, or one of the two wires going to it, are open or partially sightly open. I had a similar problem once and when I later found the problem
it was with that connection on the alternator.

MY GUESS is your problem is with the Start line and the alternator failing to start. There are several other things in that start circuit that could
be the issue if cleaning and adding tension to the connectors does not fix this.

If it were starting, and the sense line or connector failed, the alternator would go over voltage. Without looking at the diagram IIRC the gen or
alternator light and the dreaded nichrome wire are in that circuit too. If it fails after following Bruce and Johnny's recommendations I will
research this start circuit some more for you.

One last Comment. When cleaning these connectors do not over look the blade part of the connector INSIDE the alternator. Check it for electrical
cleanliness and make sure is is not loose inside.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana