MG Car Repair: 79 MG Midget electrical problem

Question
When I measure my charging voltage at the battery with the engine running I get a fluctuating reading of anywhere from 14 to 16 volts. Most other cars I work on will have a steady 14+ volts under this condition. Is there a voltage regulator that could be bad or what might another problem be. I am familiar with meters and how to test circuits but I am not sure where to start on this type of problem.

Thanks for any help on this

Jerry Frawley

Answer
Hi Jerry,

I don't know of any particular problem with the Midget that causes a fluctuation in charge rate. Your Midget is a 1500cc Triumph Spitfire engine with an alternator with a built-in regualtor.

Any time you see a charge rate of over 15v it means the alternator is "Full Fielded" meaning it is producing everything the alternator can do.

The problem is there is no way to test the regulator. I guess it is possible for an intermittent short or open circuit in the system could drive the regulator crazy and make it do odd things.

The only thing I can think of to do, is to first run "Voltage Drop Tests" on all the circuits in the charging system. Your alternator is a Lucas 3 wire alternator and you must have 12v battery power all the time on the large and the small brown wires. Any loose or poor connection on either of these two wires can cause problems. This includes the battery cables both Positive and negative.

I am not sure it can cause an over charge but if it affects the regualtor it can. The 14 ga Brown w/yellow tracer wire is the "Exciter" wire that tells the regualtor to start charging. It is 12v from the "IGN" light (Charge light) and must be on all the time the key is "ON".

Any intermittent short or open circuit can drive the regulator crazy and no telling how it would react.

If I had the car I would do voltage drop tests on each of the three wires all the way, meaning the brown wires from the alternator to the battery Positive post and the case of the alternator to the negative post of the battery (battery Posts not the cable ends) I would run the voltage drop tests while the engine was running and at at least 1500 to 2000 RPM to be sure of a normal charge rate. I would not expect to see more then .5v and it should be steady.
The same is true for the brown w/yellow traceer at the alternator to the positive post of the battery. You may see a higher voltage drop due to the charge light filament being a resistor in this circuit but in your case you are looking more for a constant then the actual value. I have never done a "Voltage Drop Test" on this circuit before so I don't know how much voltage it will read.

All voltage drop tests must be done under load (running).

I worked for many years in BMC and BLM dealerships and only ran into one Alternator that charged so much that it overwhelmed the regulator and charged too muuch due to a shorted windings in the alternator.

Your 16 volt readings is a little on the high side of a "Full field" meaning a alternator should only have the capasity of about 15 to 16 volts even with no regulator and completely "Full fielded".  

So If you can't find any fault with the wiring or battery cables or connections, I would do a "Full Field test" too.

Look in the plastic cover of the alternator at the regulator. It is a small metal box mounted on the brush box of the alternator (usually at an angle) You can put the wires back on the altrnator with the back plastic cover off so you can easily have access to the regulator case.
Set your volt meter across the battery "POSTS" not cable ends. So you can monitor voltage at the battery and start the engine and set the RPM at 1500 to 2000 RPM and read battery voltage. (should read 13.8 to 14.5v) Then touch a ground test lead to the metal CASE of the regulator and that "Full fields" the alternator making it charge all that it is capable of. I would not expect to see anything over 16 volts. Do not leave it connected long, only enough to read the voltage.

The only other thing I ran into was one that was eratic in charging and several mechanics couldn't find the problem and I happened to connect a scope to the battery circuit and spotted a trace of AC current in the DC circuit and it turned out to be a leaking diod in the alternator pulsing a little AC current into the DC system. This can only be spotted with a scope and most people don't have a scope to check it. But some auto parts stores have alternator tests stand that can pick up a fault like that. Most test them for free but you have to take the alternator in to them.

Let me know the results of your tests.

Howard