Voltage on alt case??

[nutz4spd]

I've been fighting an electrical gremlin for a while now. Just got the car to maintain a somewhat consistent charge of 14.1 to 14.5v. All of the lights are still fluctuating and I can't figure out why. The VR has been replaced with a VR706. That helped the over charging I was having. The alternator is a reman square back style. I've grounded one of the brushed back to the case. Today I was checking voltage at the alt terminal and it was consistent with the battery reading. However, and I don't know what made me so this, I touched the alternator case with the probe a got a voltage reading. Not a consistent one, it was all over the place, but still voltage. I tried grounding the second field directly to the negative side of the battery. Same thing, voltage on the case. When the car is off there is no reading. What should I look for inside the alternator to cause this? Could this be a cause of the fluctuating lights?

 

[66Satellite47]

It sure sounds like a grounding problem somewhere. Chased mine for a long time on y '70 Barracuda for a long time. The dealers failed miserably
Could be the alt. The diodes can do lots of odd things.
There are lots of things inside the alt can do weird things. The brushes and the shaft contact may not be clean and constant, and maybe diode quality.

 

[sam dupont]

I've been fighting an electrical gremlin for a while now. Just got the car to maintain a somewhat consistent charge of 14.1 to 14.5v. All of the lights are still fluctuating and I can't figure out why. The VR has been replaced with a VR706. That helped the over charging I was having. The alternator is a reman square back style. I've grounded one of the brushed back to the case. Today I was checking voltage at the alt terminal and it was consistent with the battery reading. However, and I don't know what made me so this, I touched the alternator case with the probe a got a voltage reading. Not a consistent one, it was all over the place, but still voltage. I tried grounding the second field directly to the negative side of the battery. Same thing, voltage on the case. When the car is off there is no reading. What should I look for inside the alternator to cause this? Could this be a cause of the fluctuating lights?

A diode?

 

[pnora]

Feed back from electronic components, electric fans, Stereo amps, Ect. As said grounds. Anything else on the car not working correctly?

 

[493 Mike]

The square backs are easy to disassemble and test the components.
Mike

 

[pnora]

A diode?

One bad alternator diode can do some funny things.

 

[nutz4spd]

Feed back from electronic components, electric fans, Stereo amps, Ect. As said grounds. Anything else on the car not working correctly?

There's nothing else. Factory radio that doesn't work... it does have an electronic distributor and MSD box. Oh and a FiTech Command Center and throttle body. That's it. As far as I know everything works as it should.

 

[Geoff 2]

If the starter cranks the engine ok, then the ground connection from battery to block is fine.
Your problem as described, if there is a problem, has nothing to do with VR.
Maybe how/what you were measuring?

The alt body is grounded via bolts/brackets to the engine. If there is paint/corrosion on these, it could cause a problem, but most unlikely because of the number of points of contact.

You can do a simple test. Engine running, use the DC volts scale on a DVM. Place one probe on the alt body, the other on batt [-] terminal. You should read 0.2v or less.

 

[RJRENTON]

A diode?

There are six (6) diodes in the alternator. There are three (3) positive diodes and three (3) negative diodes. The system functions as a three phase full wave bridge rectifier circuit. If any diode becomes open, the output of thealternator will be half or 50% of the rated output. A square back unit will have the positive diodes on a common connection tied to the output stud. The negative diodes are on a common connection tied to the case thru two small screws. The round back alternator has individually pressed in diodes and soldered to the stator leads. The alternator makes AC voltage and current in the stator windings which is changed to DC Voltage and current by the diode assembly.
BOB RENTON

 

[451Mopar]

If you were measuring from the alternator case to the battery negative terminal, you are seeing the resistance of the ground path times the current in the path = voltage.
It is an indicator that the ground path is not very good.
If you measure from Battery negative to block and see the same voltage, or a better test measure while cranking the engine (higher current through the ground battery cable), then replace the battery cable. If you see the voltage from alternator case to engine block (alternator providing current), the alternator case is not grounding the the engine block.