the needle goes back and forth. high and low. Maybe twice a second...what would be the problem? would it likely be the voltage regulator or alternator? let me know which one you would try first. I dont want to buy both unless I must.

 

oh yea..its a 78 f150 with a 351m

 

I'm having a similar problem. I just posted a question about a hour ago about my problem.

Check your voltage at the starter solenoid "I" terminal. My truck is reading 6volts while idling on that terminal.

 

Ok I will check it and I will read your post

 

My 76 had a bad voltage regulator it just ran the truck at 20+ volts until I replaced it it didn't bounce around but I had an aftermarket one my thoughts is maybe one of the wires might be disconnected to the alternator

 

I've been having the same problem for most of a year, overcharging, good thing the truck hardly gets used. Have changed alternater, solenoid and voltage regulater with ones I know work, problem still there. Now I'm chasing grounds. 75 390.

 

The voltage regulator needs to have an accurate reading of the battery voltage in order to put more or less voltage into the field. In many cases this is called the "sense" wire. If the sense wire has a bad connection or oxidation in a connection along the way, it will show the battery as having a lower voltage than it really does, and the regulator will put more voltage into the field, resulting in overcharging. The sense circuit includes the ground path back from the regulator to the battery Negative post.

IF you want to test the system, run a temporary wire from the regulator ground to the Negative post, and a temporary wire from the sense terminal on the regulator to the positive terminal. By "to the terminal" I mean connected DIRECTLY to the terminal, not connected to the battery cable clamp. I either drill a hole and use a sheet metal screw or else clean off the terminal and put the fresh bare wire between the terminal and the clamp.

This should straighten out the charging problem, unless the problem is in the regulator. A mechanical regulator doesn't change the field voltage gradually, it has steps. So with a mechanical regulator the needle may jump when the regulator changes steps. That's what the little resistors are doing on the back of the regulator, and why there are relays inside the regulator. A solid state regulator is able to vary the voltage continuously and IMHO is better because of it.

One other problem may happen and that's when you lose a diode. The alternator is a 3-phase AC generator and the diodes convert to DC. So with the engine running check voltage across the battery in the AC mode. It should be way less than one volt. If it is a volt or more, you have a bad diode in the alternator.

R.