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MG Car Repair/75 Midget blinking alternator light


QUESTION: My 75 MG Midget alternator light blinks.  Tested the battery at the terminals at 13 volts.  Started the engine and raised rpm to 1500 and the battery voltage read 14-14.2 volts.  When I start the engine the light is steady at first for about 10-15 seconds and then it begins to blink.  As I raise the RPM the blinking becomes slower.

ANSWER: hi Patrick,

Your charge rate is good but you need the volt meter on the battery at the time of the light blinking to see if you can spot any fluctuation in the charge rate too.

The charge light is powered by the battery and grounded by the regulator when it is not charging, thus the light is on. When the alternator starts charging the connection in the regulator becomes positive and since the light is also positive the light goes out. Any intermittent failure for the regulator to be positive will cause the light to come on.

One other thing can cause a intermittent flash of the charge light and that is for there to be a intermittent short to be anywhere on that charge light wire from the light to the alternator.

Also a poor connection of the power wire (brown) from the starter solenoid or the battery can cause a intermittent failure to charge. This is true of the grounds also so check the ground strap from the engine to the frame and the battery ground to the body. Check the power cable to the starter solenoid and on to the alternator. check the charge light wire from the light to the alternator for possible intermittent shorts.

If all that checks out good, then you most likely have a alternator/regulator problem.


---------- FOLLOW-UP ----------

QUESTION: Hi Howard,

Not a follow up question, but what I have found.  Checked all the wiring continuity and ground connections and found all okay.  Then I pulled the light socket out of the dash and removed the bulb.  Using my volt meter I checked the voltage inside and outside the socket with the engine at 2000 rpm.  The voltage from the starter side was 14 V and from the alternator side (inside the socket) was 20 volts.  When I bumped the engine speed the 20 volts increased.  No wonder the light was on.  My determination is that the voltage regulator is shot.  Because of the age of the alternator and internal wear an tear, brushes, etc. I have opted to buy a new alternator.

Thanks.  Patrick

Wait Patrick, before you buy anything do this, put the bulb back in and put your volt meter across the battery posts and read battery voltage. (12v +) and then start the engine up and read battery voltage again at a fast idle (1500 RPM) and you should see 13.8v to 14.5 at the battery.

The problem with your test of the charge light socket without a bulb is that the current is going the other way to the regulator to tell the regulator to start charging. This circuit is called the "Trigger Circuit" and it is suppose to power the regulator. With no trigger power there is no telling what the charging system will try to do when spinning and no signal from the battery through the bulb. Running it without this trigger power can even damage a regulator. Jaguar even put a resistor shunt across the bulb so that if the bulb burned out it would not allow the charging system to not have a trigger circuit to fall back on.

Alternators and even generators can produce excess voltages but it is usually caused by shorted winding in the unit itself. Even if you bypass a regulator and do all that the regulator does anyway is to FULL field the alternator or generator and that will usually only give you about 15v or so at 2000 RPM. I found some failed generators and alternators that produced in excess of 85v and much higher but a regulator can't make a good alternator do that.
I even had one that could jump a spark and would shock you like a plug wire.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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