Alternator output voltage, how much is too much?

Still trying to get my 68' RR on the road and figured out. I received my 3rd MSD 6A (2nd replacement unit) last week. Upon the advice of a mechanic friend I disconnected the the alternator output wire on in addition to replacing a suspected bad voltage regulator

Fired the car and it started right up, ran it a few minutes and shut it off. Started it again and got a voltage reading of 19-20 volts on the alternator post? I know that has to be high, and assume it would cause the previous MSD units to fail from over voltage had the regulator failed or not been able to correct that high of a voltage. Also, just after the reading the car died again. And again no spark at the MSD. This was with a new regulator and the alternator wire disconnected.

I then removed the alternator and took it to Auto Zone where it passed lamp/ diode, regulator and rectifier tests on their machine.

The alternator was on the car 4 years ago when I bought it, I recall the previous owner saying it was new and a high output 70 amp model.


Shouldn't the voltage output from the alternator be in the 14-15 volt range?

I suspect this is part of my problem but still not confident that this is the only issue.

13.5 - 13.8 would be ideal. Not sure your reading of 19-20 volts is a too high reading, considering you measured the alternator output with no load. It would be worth your while to recheck with output load attached. Kind of interesting.

It can not pass a regulator test as Mopar alternators do not have internal regulators. I would suspect the alternator has failed internally, stator or field windings.

65wgn440 said:

13.5 - 13.8 would be ideal. Not sure your reading of 19-20 volts is a too high reading, considering you measured the alternator output with no load. It would be worth your while to recheck with output load attached. Kind of interesting.

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Main problem with that is I can't get the car to run long enough to test possible culprits before the ignition quits again.

Waiting on a stock ignition from Jets to see how that performs.

Some basics -
If the big fat alternator output wire was disconnected, then it can't be responsible for toasting your latest MSD 6A. Something else is going on. What that could be, I have no idea.

Alternator should not really be spun up with output disconnected as it's output voltage can go considerably higher than 20V and cause internal damage. It is the fact that alternator is normally connected to a large reservoir (i.e. your battery) and has a functional regulator that prevents system voltage going above 14V.

Once you get car running on stock ignition, test alternator output with it connected to see if it and regulator are working as they should. Once confident they are OK, then move on to ignition woes...

If a two field wire alternator with electronic regulator like 1070 and up, check that the field brush holder that goes to the regulator has not melted and shorted to ground. If that field connection shorts to ground the alternator will put out max current/voltage.

62 Dart makes a good point (451 Mopar also, fyi). IF the alternator isn't hooked up, then it's not the alternator frying your MSD....it might be bad too, but that's not the issue. I suggest you take a VERY close look at your wiring to the MSD, bulkhead disconnect, engine harness wiring.... I mean, start from scratch & un-tape everything type of a close look. I personally think you have a short somewhere.

You could get a 19-20 volt reading on the alternator output, but how much do you measure on the battery terminals when the engine is running?
It might need to put out that high of a voltage to get 14 Volt at the battery due to very poor connections==>losses/heat build up.
If you do get only 14 Volt or so on the battery when the engine is running (and assuming you still have 19-20 Volt on the alternator) you are looking at going through your complete wiring harness and connectors (bulkhead connector especially) to check and clean the contact.

You should also check the voltage on the regulator, IGN side and see what you measure there, should be very close to whatever you measure on the battery terminals.
If there is a big difference, again the wiring and connectors are a high potential.
A new regulator might do the trick as well, the alternator only does what the regulator tells him, if the regulator is faulty or getting a poor (low) voltage signal he will command the alternator to go for max output while it is not required.
That high voltage might damage you MSD unit indeed.

Any options you can temporary run your ignition system from another 12 Volt source so you get a chance to troubleshoot the cars electrical system without any chance of burning the ignition module?

Still waiting on a shipment from Jegs to receive a stock ignition setup. I know the with the "big" wire disconnected from the alternator, it would not have sent high voltage to the MSD and kill it.

I have already untaped the wire harness from the bulkhead to the to the alternator and everything looks to be in good shape.

I have also previously removed each bulkhead connector, inspected and cleaned all terminals. Only a few had very slight corrosion.

I have also already bypassed the ammeter under the dash.

I just installed a new blue voltage regulator prior to the last MSD install, shouldn't even have mattered with the alternator charging wire disconnected.

Battery is in the trunk, didn't think to test voltage at that point, during the last brief start up. I will test it upon the next successful start.

Planning on ordering a new voltmeter to install in the gauge display to monitor voltage in the car.

About the only thing I haven't replaced in the engine compartment is the starter relay, and the alternator.


Probably going to upgrade the voltage regulator to the Wells model for 70 and up while I am it.


Open to any further suggestions...............

Mstone68440RR said:

Open to any further suggestions...............

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I have no more suggestions regarding alternator. Voltage test readings once you got it running need done first.

As to why MSD keeps frying... system voltage too high, short on coil wiring, faulty or wrong coil or ballast resistor. It is pretty simple wiring and MSD pretty robust, so something simple is seriously wrong.

How well was the battery relocation done? All it takes is for one high resistance joint in the wiring for system voltage to creep up high when engine running...

62 Dart Convertible said:

I have no more suggestions regarding alternator. Voltage test readings once you got it running need done first.

As to why MSD keeps frying... system voltage too high, short on coil wiring, faulty or wrong coil or ballast resistor. It is pretty simple wiring and MSD pretty robust, so something simple is seriously wrong.

How well was the battery relocation done? All it takes is for one high resistance joint in the wiring for system voltage to creep up high when engine running...

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I will have to crawl under the car and inspect the cable when I get home today. The relocation was done before I bought the car and it looks like it was done the right way. The car ran perfect for almost 4 years before current issues. I will say I could have gone about the initial trouble shooting more effectively and probably saved myself some headaches.