MG Car Repair: Coil weak spark stops running, resistor circuit, starter solenoid

Question
QUESTION: Howard, sorry that I am clogging up your AllExperts emails.

I really am trying to understand this as well as fix the car. It has been 30
years since I was a working mechanic so my skills are very rusty but they are
there deep down in me somewhere!!

I think this will get us on the same page.

I sent you a message this morning as follows just below:

"Howard, this is a new message thread so a quick refresher.

This is the 1975 MGB. You thought it might be a 1976 because there were
two spades on the solenoid.

I ran the car with both the 1.5 Ohm coil and the 3.0 ohm coil.

The 1.5 ohm coil crapped out after about 30 minutes. The coil was hot but I
could touch it.

The 3.0 Ohm coil lasted about two hours but it too crapped out. I could not
touch the coil it was so hot.

Your last suggested test to me was:

"I suspect your test of the resistor circuit was faulty if your coil is getting hot.
Run this test exactly. Turn the engine until the contact points are closed.
Then turn the ignition to "ON" (not start) place your black lead of your volt
meter to the case of the coil and the red lead of the volt meter to the (+)
post and read the voltage. Tell me what the meter reads".

I did that test, black lead to the coil case and red lead to the + term of the
coil. When the points are closed I get an 8.5 volt reading. (When the points
are open I get a full 12 volts).

I really do think I am starting to understand what is going on here. There are
two leads to the positive side of the coil because when you start a car the
starter drains so much power that there isn't enough juice to spark the coil
from the "always hot" wire that goes to the coil. The second wire is from the
starter solenoid which will switch from the solenoid proper voltage to the
plus side of the coil until the engine starts, once started the starter is
released and enough power can then make its way to the "always hot" wire
and keep the coil with enough power to run.

I understand that too much power to the coil will overheat it.

I don't know why both the 1.5 and the 3.0 coils both overheated.

Let mw know what you think.

Thanks!!!

ANSWER: As a coil is used it will warm up but should not get hot. The 8.5 volt reading indicates there is a resistor wire in place. The drop of voltage is due to the resistor and the load of the coil. If the 3 ohm coil is also getting hot you should check voltage at the battery when the engine is running at about 1500 RPM to see if the charge rate is correct. It should be 13.8 to 14.5 but not higher. You could have a bad 3 ohm coil. Also try replacing the condenser to see if that stops the failure. Also check for a good engine to frame ground.
Howard

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

QUESTION: Hello Howard, I just checked the voltage on the battery with the car running
at 1500 RPM. The voltage across the terminal was 15 volts. My meter is not
the greatest so if you think that .5 volt from 14.5 to 15 is critical I will go buy
a better volt meter. When I put the engine and tranny back in the car after the
engine rebuild, I made sure the strap at the tranny and cross brace was well
connected. Is there a good way to test of the ground is sufficient?

I assume the condenser is the condenser under the distributor cap. Correct?

I have three coils that fail, the original 1.5 ohm, a new 1.5 ohm and a new 3.0
ohm. I agree with one of your earlier assessments that the likelihood of three
bad coils is slim.

I don't object to buying another new one but lets try a couple more tests?

I am going out to buy a new condenser but will wait for your next
recommended step before I do anything.

Thanks

ANSWER: If you really have 15 volts you need to correct that right away as that is what is called a "Full Fielded" alternator, meaning the regulator has totally failed or you have a short inside the alternator. 15 volts will fry the ignition and should start burning all the light bulbs. Also it will fry a battery.
Howard


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

QUESTION: Howard, this one is a dopey question.

Where is the regulator in a 1975 MGB?

Is there are test to see which is broken?  

Answer
The regulator is inside the alternator and the "Full Field" is a test also to isolate a charging system failure between an alternator or regulator failure. If you run the test and you get 15 volts at 2000 RPM it indicates a failed regulator. However an alternator that already puts out 15 volts without full fielding it could be several things (all internal) Most any starter/generator shop can test it for you and make repairs.
Howard