overcharging -17 volts

[jprather]

any help here ??

72 sat 440 - alternator went out about 2 months ago, quit charging, so i replaced the volt reg and went for a 45 amp to 65 amp alternator, all was well, yesterday, i disconnected the aftermarket elec water temp gauge and installed a mechanical gauge,went for a ride and the amp gauge was pegged, tested at battery, get 17 volts at idle now, changed volt reg, still getting 17 volts, checked all grounds,they are good. could the alt have gone bad ??

 

[Rockland71GTX]

What does it do with the VR disconnected? Have you tried putting the old one back?

 

[jprather]

it should not have anything to do with it, i took out the electric water temp gauge and replace it with a mechanical gauge. I changed the VR when it started to overcharge, but it did not help

 

[72RoadrunnerGTX]

Sounds like the alternator is in a full field state, disconnecting the vr will eliminate the vr as the cause if the full fielding continues. Also try disconnecting the vr and the green field lead at the alternator, check the green wire for continuity to ground, shouldn’t be any, may have pinched a wire.

 

[jprather]

I did disconnect the VR , and no change in charge, i assume the green wire runs to the VR ? so I could check at both ends? I have checked the grounding at the VR , it is good

 

[Rockland71GTX]

Don't have the diagram in front of me but IIRC... blue wire is 12v to the VR and alternator. VR senses voltage from this and varies the resistance to ground on the green wire. The field on the alt. is varied in this manner to regulate the output voltage. Check the green field wire at the alt for ground with the VR disconnected to see if it's shorted. May have disturbed the wiring when changing the gauge, because they are unrelated otherwise.

 

[72RoadrunnerGTX]

Ok, now disconnect the vr and the green wire from the alternator

Charging stops, the green wire is shorted to ground, locate the short, repair it.

Charging continues, the field brush/terminal is shorted to the case, look for missing insolating washers.

 

[jprather]

thanks guys,

i will pull green wire and unplug VR this afternoon and report back !

 

[jprather]

so, this is what i found when i went to check the green wire. it is not broken on the inside, just coating, I unplugged it and unplugged the VR, still pushing 17 + volts, so.. bad diod in alternator ?

 

[Rockland71GTX]

It should not charge without the green wire. It seems that the field circuit is grounded somehow. Pull the alt. and get it checked somewhere (AZ, Pepboys, etc.)

 

[72RoadrunnerGTX]

Disconnect both field connections at the alternator, check for continuity to the alternator case. Should not have any at all. Still full fielding with the green wire disconnected, the brush is grounded, likely assembled without insulating washers.

 

[jprather]

yea, it is only 2 months old, just talked to Advanced auto parts, still under warranty. gonna pull and take to the to test / replace

 

[jprather]

well, pulled the alt of took it to be tested, they could not even get a reading on it! they replaced, good to go !

 

[hammond472]

If it's an AGM battery you might want to switch it out to a lead acid battery until you get it sorted out. AGM batteries die easily when over charged.

 

[tonysrt]

If you were getting 17 volts out of the alternator how is it bad? Any answers to this?

 

[Wietse]

If you were getting 17 volts out of the alternator how is it bad?

14,5 volt is normally the upper limit, 17 volt will overcharge and boil the battery and it will start to throw out the acid/water.
It will be fatal in the end for the battery.

 

[72RoadrunnerGTX]

If you were getting 17 volts out of the alternator how is it bad? Any answers to this?

As I understand this question, it appears there was full (unregulated) alternator output even with the green field wire disconnected. Indicates that terminal/brush assembly was grounded to the case. There can be no continuity to ground from either field terminals on a dual isolated field ’70 and up alternator in a stock charging system. Fairly common with commercially rebuilt alts these days, that the needed insulating washers are omitted when the brush assemblies are installed. Brush/terminal mounting screws need to be insulated from the terminal. May also be intentional in some cases, grounding one terminal would allow the later alts (’70 & up) to be used on a ’69 and earlier stock applications.

 

[jprather]

got it all back together, charging fine, but, when i turn on A/C, amp meter needle " vibrates " whats up with that ??

 

[72RoadrunnerGTX]

Your factory sized alternator is not providing enough idle output current to fully cover your total idle loads it would appear.

 

[jprather]

does it even when driving