Battery voltage bouncing

[buddy]

Hi guys,
The last couple times out, I notice on my scanmaster that my battery voltage fluctuates from 11.1 to 14.8 volts bouncing around pretty rapidly. I'm assuming my alternator is going. Sound normal for a bad internal voltage regulator?
Thanks.
Rich.

 

[Ttype6]

Hi guys,
The last couple times out, I notice on my scanmaster that my battery voltage fluctuates from 11.1 to 14.8 volts bouncing around pretty rapidly. I'm assuming my alternator is going. Sound normal for a bad internal voltage regulator?
Thanks.
Rich.

Does it bounce even at an idle?
Does the dome light flicker when it does this?
If yes,I would suspect the capacitor inside the generator.

 

[buddy]

It does bounce at an idle. Dome light doesn't work but headlights do intermittently dim while driving.

 

[mikestertwo]

Could be an intermittent connection going to the volt lamp. Possibly at the back of the gauge panel.

 

[Ttype6]

It does bounce at an idle. Dome light doesn't work but headlights do intermittently dim while driving.

Dash lights while it's dark outside?

 

[Nick Micale]

Just replaced the alt on one of my cars, went from 12.8-12.9v to 7.0v for a second or 2?

Failed every thing on the test machine, so my lifetime warranty worked again!

 

[earlbrown]

I don't know if the scanmaster reports voltage from the orange ECM wire or the power wire that powers the unit.


I'd verify the voltage drop is actually at the battery or back of the ALT. It might be a false reading on the inside from a crusty ignition switch.


And it NEVER hurts to clean those damn voltage idiot light contacts. That's the only warning system I've ever heard of that will take out the system when it fails.

 

[davelostmissing]

Scanmaster reports the value sent in the ALDL stream from the ECM register.

 

[SCOOBY DOO]

Like Earl says. Throw a voltmeter on the back of the alternator.

 

[bpman1234]

Yeah my scanmaster tells me 11-12 volts at idle but at the back of the alternator and battery 14.49 or so.i think I am loosing the at the starter.or maybe ignition switch..

 

[TurboBuRick]

Take as much aftermarket load off fuse panel that you can.

 

[Chuck Leeper]

^^^^
As in add a rely to handle the key switch load.

 

[bpman1234]

Gonna try that see what happens.

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[Nick Micale]

Yeah my scanmaster tells me 11-12 volts at idle but at the back of the alternator and battery 14.49 or so.i think I am loosing the at the starter.or maybe ignition switch..

The Scanmaster is not a good source for measuring accurate battery voltage unless your ground wire is at the battery or the alternator.

Grounding the SM to the dash is not a good place when depending on an accurate voltage reading.

The accessory power feed from the ignition switch is also an issue, and I have seen a 2v. increase inside the car by replacing a 30+ year old corroded switch.

 

[bpman1234]

Thanks gonna do that also .

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[earlbrown]

Scanmaster reports the value sent in the ALDL stream from the ECM register.

Well that's 1/2 the circuit.


A quick and dirty test is to look at the voltage, then run a temporary ground lead from the ECM case to a known good ground. (the metal part of the door pin switch is a good one if your dome light works correctly). If you see a voltage jump on the scanmaster, then you just ID'd a weak ground path. If the voltage stays put, odds are the ground path is OK.

That being said, if there is no change, there's still a chance the orange wire is compromised. It has been exposed to harsh underhood conditions for 1/3 or a century.

I'm still curious if your inside voltage matches the underhood voltage.


Another quick and dirty test is to measure the voltage at the cigarette lighter and compare it to battery/ALT voltage. Also compare a switched ignition wire (yellow) to a constant hot like the cig/dome (orange).

 

[bpman1234]

Well guess better get down and dirty.thanks.