Excessive voltage at 1500 rpm

[jvonhaden]

In June took possession of a well maintained 1976 Palm Beach with the 80 amp alternator/455 Olds.
First two 100+ mile trips alternator worked as it should. On the third trip volts would jump to over 16 and dummy light lit when I went over 55 (2000 rpm. So my short term solution was to drive at 50. Unfortunately after another 100 miles, it would exceed 16 volts when I got engine to 1500 rpm. I attached my voltmeter and it showed a sudden jump from normal to 16.3 volts at 1500 rpm, then drop to about 16 volts at 3000 rpm. At 1400 rpm voltage would suddenly drop down to the normal 14-14.5.
I replaced internal regulator. Exact same failure. Then took alternator into Autozone and alternator tests fine. 
Thoughts?

 

[Tom Lins]

> In June took possession of a well maintained 1976 Palm Beach with the 80 amp alternator/455 Olds.
> First two 100+ mile trips alternator worked as it should. On the third trip volts would jump to over 16 and dummy light lit when I went over 55
> (2000 rpm. So my short term solution was to drive at 50. Unfortunately after another 100 miles, it would exceed 16 volts when I got engine to 1500
> rpm. I attached my voltmeter and it showed a sudden jump from normal to 16.3 volts at 1500 rpm, then drop to about 16 volts at 3000 rpm. At 1400 rpm
> voltage would suddenly drop down to the normal 14-14.5.
> I replaced internal regulator. Exact same failure. Then took alternator into Autozone and alternator tests fine. 
> Thoughts?
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It is not sensing the battery possibly.
Check the isolator is good.
--
Tom Lins
St Augustine, FL
77 GM Rear Twin, Dry Bath, 455, FI-Tech EFI, Aluminum Radiator Quad-Bag Suspension Solar Panel
Manuals on DVD
GMC Dealer Training Tapes
http://www.bdub.net/tomlins/

 

[Ken H.]

You've probably got a bad/intermittent connection somewhere in the voltage
sense lead to the alternator. When that lead, which carries chassis
battery voltage to the alternator is open, the alternator's regulator
increases the output in an attempt to sense that the battery is receiving
the desired charge voltage. It will continue that process until the output
reaches the internally limited voltage. That should be 18 VDC, but might
be lower, and can even be higher -- I've seen 24 VDC output several times.

Ken H.

On Thu, Sep 5, 2019 at 4:06 PM Jeff Von Haden via Gmclist <

> In June took possession of a well maintained 1976 Palm Beach with the 80
> amp alternator/455 Olds.
> First two 100+ mile trips alternator worked as it should. On the third
> trip volts would jump to over 16 and dummy light lit when I went over 55
> (2000 rpm. So my short term solution was to drive at 50. Unfortunately
> after another 100 miles, it would exceed 16 volts when I got engine to 1500
> rpm. I attached my voltmeter and it showed a sudden jump from normal to
> 16.3 volts at 1500 rpm, then drop to about 16 volts at 3000 rpm. At 1400
> rpm voltage would suddenly drop down to the normal 14-14.5.
> I replaced internal regulator. Exact same failure. Then took alternator
> into Autozone and alternator tests fine.
> Thoughts?
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Ken B]

I endorse Colonel Ken's posting 100%. I stated this here before. On the way to a rally in Georgia or Alabama I had your exact same symptoms. When I
got there I pulled and replaced the alternator under free exchange and had the same symptoms. Problem turned out to be the plugin connector to the
alternator. The voltage sense line was not making a good connection. A little squeeze with a pair of pliers on the female terminal fixed the problem
permanently.

If you need to prove it to yourself, slide a very thin piece of wire into the connection between the wire terminal and alternator terminal. Run it up
into the cab and monitor the voltage from there under failure conditions. For the ground side I would use the ground side on a different wire
connected to something on the engine (Not something inside on the dash).
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[johnny]

Only problem is, a wire in the connector is likely to make it connect.

--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

 

[john r. lebetski]

Check connections at stud on horn relay and tug on fusible wire feeding it. If it separates or is soft, you found problem.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[Ken B]

> Only problem is, a wire in the connector is likely to make it connect.
>
> --johnny

That is part of what I was hoping for. If the voltage problem went away we could assume that the added wire did just what you suggest and that and
the connection was bad. I was hoping that he would use two meters. One where he is metering now and a second to the added wire.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[chris tyler]

You probably already did this, but check with another gauge to verify it.
Otherwise, what Ken said
--
76 Glenbrook

 

[jvonhaden]

Thanks for all the advice on solving my issue with excessive voltage...
Backstory... replaced regulator and still had 16-16.5 volts when over 1500rpm. Alternator tested fine at auto parts store.
I then took the ideas sent and explored. 
- Cleaned and tightened the connectors on the alternator, including the voltage sensor wire. No difference.- Disassembled and cleaned the connections at the battery switches, starter solenoid, battery isolator. No difference.- Disconnected the aux battery from the battery isolator. Seemed to solve the problem... I reattached the aux battery to isolator and excessive voltage came back. Disconnected and excessive voltage went away. At that moment I'm thinking the isolator or something in the house wiring is my issue.
With the house battery disconnected from the battery isolator I took it on a test drive at dusk. Was working fine until I turned on the headlights. With the lights on, at 1500 rpm the voltage would pulse from 14 to 16+. At 2000 rpm alternator light was on and if I remember correctly my meter was reading 18 volts (higher than the 16 volts I had seen with house battery attached). When I turned off the lights the voltage went back down to 14, even at 2500 rpm.
I'm not sure what an isolator will, or should do, when you only have the starter battery attached...
I used my meter to test the battery isolator diodes and if I tested correctly, there is no flow in either direction. Is that a possible failure mode?
So my inclination is to replace the isolator and see what happens, but was hoping for your thoughts before I became a parts changer...
And if you think replacing the isolator will help, what are your thoughts on going to a solid state isolator rather than the standard diode isolator?
Thanks for any advice you can give,
Jeff Von Haden76 Palm Beach

 

[johnny]

First, the isolator IS a solid state device. Second how's to put your meter on the sense wire to the alternator, fire it up, and read the voltage.
Then see if that voltage changes when you turn the headlights on. If it doesn't, but the battery voltage goes up, there's a problem in the regulator
regardless what Advance had to say. (I am assuming you removed the contacts from the two wire plug on the alternator, shined them, and checked
connection before replacing. If you didn't pull the contacts out of the plug, do so. You cannot clean them very well when they're in the plastic
shell. Use a jeweler's screwdriver to release the tab holding the contact in and pull the contact out while holding the tab down. I say this because
your description describes an intermittent sense wire. If the sense wire feed has been mistakenly moved to the house battery side of the isolator the
system would act as you describe when the house battery is disconnected.

--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

 

[Ken B]

If you think that you have a flakey diode in the isolator, simply switch the top and bottom leads on the isolator and you have switched the diodes
between house and the engine systems.

If the symptoms do not change forget the isolator and go back to what Johnny suggested.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana