Low voltage problem

-
hotrod swinger

hotrod swinger

Well-Known Member
Joined
Mar 6, 2020
Messages
661
Reaction score
478
Location
Sooke
1966 Plymouth Valiant
Older style VR

Suddenly noticed voltage is 12.4 when car is running, verified with multimeter.

All connections looked good.

Based on another thread I first measured voltage at alternator with engine off, same voltage as battery.

Next, I connected a jumper from the field connection at the alternator to the battery positive, started the car and monitored voltage, revved up the engine, no increase in voltage.

I removed the alternator planning a rebuild thinking I had isolated the alternator as the problem.

Roundback alternator with one field wire.

However, I didn’t disconnect the field wire from the alternator before performing the jumper test.

Did failing to disconnect the field wire invalidate the result?
 
Last edited:
hotrod swinger said:
Next, I connected a jumper from the field connection at the alternator, started the car and monitored voltage, revved up the engine, no increase in voltage
Click to expand...
She'd more light on the jumper.

From where to where?
 
From the field connector to the battery positive. I’ll edit original post.
 
Last edited:
hotrod swinger said:
However, I didn’t disconnect the field wire from the alternator before performing the jumper test.

Did failing to disconnect the field wire invalidate the result?
Click to expand...
Most likely not. Essentially you applied 12v to both sides of the VR. So no current flow.

If it was an electronic version of the older style VR then who knows.


Just to be on the same page.


Your alternator is a single field wire type, regardless of square back or round back?
 
From what I can gather you jumpered this correctly. The field is a simple electromagnet. The early cars have one terminal grounded (grounded brush) and the VR applies a varying amount of power to the insulated brush to obtain output and more or less. Don't discount that the green wire itself could break. Make certain the VR is grounded, and make certain it is getting power to the VR "IGN" terminal.

Replacements nowadays all seem to be insulated field, and you may or may not have to ground one brush. CHECK the new one (if you buy one) to make CERTAIN that both brushes are open to ground, then ground one or the other. Many rebuilt/ new alternators with TWO insulated brushes come with one grounded, intentional or not
 
I’m going to have my alternator rebuilt by my local shop. They did a great job with my starter.

From now on that’s what I’m doing. I’ve had too many problems with parts store refurbs, loose brushes due to hogged out bolt holes to name one example. Everything goes to the auto electric shop from now on.
 
Yup, the brushes are like those in your elec drill or angle grinder. They do not last forever......
 
68gtxman said:
I went through 3 rebuilt alternators before I got one that worked. Good luck!
Click to expand...
OP has a local rebuilder.

That is pathetic to have to go through 3 rebuilt alternators.


Maybe a good plan is to have any alt we purchase tested by the store selling it before we leave the store.

I had no issues with the last rebuilt I bought except it was advertised as a round back and came in as a square back wired for the 1 field wire VR.


I had them send it back and get another one but it was a square back too.

I called the rebuilder directly (US based rebuilder) and they said the round back cores are non existent.


I went ahead and put the square back on till I found a round back. Worked fine but it had the almost 3" pulley so idle charging was not good.


The replacement round back had the 2.5 pulley and idle charging was back to expected.


It sounds like the OP has the know how to check and replace the brushes, and a reliable rebuilder nearby.
 
The first time I saw a defective out of the box rebuilt alternator was way back in the late 70's. Guy came in with a Dodge pu, he'd bought a rebuilt "and it never quit charging." One brush was grounded, and it was only the 50-50 chance that he'd hooked the blue power wire to the non grounded brush, if he'd done that the harness would have burned up.
 
I am sure lots of perfectly good alternators went into trash cans when all they need were new brushes......
 
You must log in or register to reply here.
-

Back
Top