Overcharging Blue Field Wire Grounded

[Ronjon]

Tonight my 72 Dart started overcharging when I took it for a short ride. The ammeter would peg when I stepped on the throttle, at idle it would be closer to center of the gage. The voltage regulator was smokin hot to the touch.

I read through some threads here and tested the alternator field terminals to the case. With both green and blue wires connected, both terminals had continuity to ground. When I removed the wires and rechecked, both terminals to ground were open, so I think that rules out the alternator.

I found that the blue wire when disconnected from the alternator had continuity to ground. I then disconnected the blue wire from the alternator, VR, ballast resistor and disconnected the electronic ignition box connector and the blue wire still has continuity to ground.

Looking at the factory wiring diagram I see that the blue wire goes to the ignition switch and 12v feed to the warning lights and gauges in the dash PCB.

I’m guessing I should disconnect the ign switch connector at the steering column and see which side is shorted? Looking for some troubleshooting guidance. Could it be a bad ignition switch or pcb? I did notice when I turned the car off and removed the key the brake light in the dash stayed lit, it would only go out when I disconnected the battery ground. Thx in advance for any insight.

 

[Dana67Dart]

On a isolated field alternator...

The voltage regulator veries the ground to regulate the alternator.


You should have 12v (nominal - battery voltage) on one field wire. (Blue IIRC,)

The other field wire (green IIRC) should not be grounded. You might not be able test the VR with an ohm meter. But you can test the wires. Disconnect at both ends and test one end to ground. Should be open. If it has continuity to ground find the short.

If it is grounded the alternator will over charge

 

[1WildRT]

If the blue wire truly has continuity to ground there should be smoke seeping out of the wiring everywhere... Continuity to ground on a main power wire is referred to as a dead short... Either your test is flawed or your wiring should be melting down...

Plug everything back together, Unplug the voltage regulator, Set your DVOM to DC volts, connect the black wire of the meter to ground, key on, is there full battery voltage on both connections at the voltage regulator plug? Leave the voltage regulator unplugged.. Start the car.. What's the ammeter say? Plug in the voltage regulator.. Is it overcharging now? If so.... Replace the regulator..

 

[67Dart273]

I think you are overthinking, and you cannot make such continuity checks on wiring. They are meaningless

IF you are certain that the alternator does not have a shorted/ grounded field, and you can double check that by:

Connect the blue wire, leave the green unconnected. Turn the key to "run." Measure voltage and both field terminals should show full battery voltage. Now take a jumper and ground the second field terminal--the one not connected. Should see a small spark

Confirm, now, by running engine. With blue only connected to either field, and second field terminal bare, alternator should not charge. Again, field terminals, both, should show full battery

Now jumper the unconnected field terminal to ground, and alternator should charge "full" depending on RPM

I think you have a bad VR, or VR is not grounded

 

[67Dart273]

If the blue wire truly has continuity to ground there should be smoke seeping out of the wiring everywhere... Continuity to ground on a main power wire is referred to as a dead short... Either your test is flawed or your wiring should be melting down...

Plug everything back together, Unplug the voltage regulator, Set your DVOM to DC volts, connect the black wire of the meter to ground, key on, is there full battery voltage on both connections at the voltage regulator plug? Leave the voltage regulator unplugged.. Start the car.. What's the ammeter say? Plug in the voltage regulator.. Is it overcharging now? If so.... Replace the regulator..

Sorry my friend, not true. A switched wire such as that WILL show continuity to ground, because of the other branched loads on that circuit, and depending on car and year, that might be--VR, ignition system, electric choke, idle solenoid, and a couple of smog doo dads. Again this varies.

 

[1WildRT]

Sorry my friend, not true. A switched wire such as that WILL show continuity to ground, because of the other branched loads on that circuit, and depending on car and year, that might be--VR, ignition system, electric choke, idle solenoid, and a couple of smog doo dads. Again this varies.

Yeah, I agree, I didn't think it through... Since I never try continuity tests on a powered circuit I failed to consider the loads..

 

[Ronjon]

I think you are overthinking, and you cannot make such continuity checks on wiring. They are meaningless

IF you are certain that the alternator does not have a shorted/ grounded field, and you can double check that by:

Connect the blue wire, leave the green unconnected. Turn the key to "run." Measure voltage and both field terminals should show full battery voltage. Now take a jumper and ground the second field terminal--the one not connected. Should see a small spark

Confirm, now, by running engine. With blue only connected to either field, and second field terminal bare, alternator should not charge. Again, field terminals, both, should show full battery

Now jumper the unconnected field terminal to ground, and alternator should charge "full" depending on RPM

I think you have a bad VR, or VR is not grounded

Thank you everyone for the responses. 67Dart273, I followed your instructions, everything checked out as you described. I ended up replacing the VR and it fixed the overcharging issue. I did check the case to ground on the old VR first and it checked out good, so I replaced it. Thanks again for all your help.

 

[67Dart273]

You're welcome