MG Car Repair: Coil weak spark stops running, golden anniversary, shoregroup

Question
QUESTION: Howard, correct, no power in the crank or the ignition position.

I looked at the starter and in addition to the main power lead there are two
wires on spades. A white/green and a white/brown.

What next?



ANSWER: Yes Robert, your right and I was wrong, my white with red only goes as far as the relay and then it goes to the starter as a white w/brown. However, I show that the white w/light green is the coil bypass wire that should get hot when the starter is activated. Also I see that your car should be a 76 car as the 75s didn't have that extra flat spade connector on the solenoid nor a bypass wire. Confirm that the spade on the solenoid does get hot when you activate the starter. Then trace and test the wire to see that it does go to the (+) terminal on the coil. Let me know if you don't have a diagram and I will e-mail one to you.
Howard

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

QUESTION: Howard, this is fun for me, I hope this isn't so basic for you that you are
bored to tears!!

A couple things, this is a 1975 MGB I bought when it was three years old. It
even has a 1975 Golden Anniversary Emblem on it however, as you know
75/76 is the year that there was some mid year transitions.

Anyway, it does have the two spades on it.

I put the volt meter on the spade of the solenoid that the white/green wire
goes to and turned the key to the crank position and it showed zero power.

I do have continuity on the white/green wire from the spade on the solenoid
and the coil.

Remember, I think I have a faulty starter relay and not sure if that plays into
this at all.

Let me know the next step.

Thanks

PS if you could send my a wiring diagram that would be great as I was able to
find something on the web but the years they are associated with are hard to
understand becasue they mix eurpean models with US models.
My email is robert@shoregroup.com.

Thanks!




ANSWER: If the solenoid works, the relay is working and is not a factor in operating the bypass wire to the coil. You would normally have to replace the solenoid to correct the problem but you can do a test the see if that has anything to do with the weak spark when trying to start. Do this,
Take a test wire and attach it to a hot terminal (or the battery post) and have someone try to start the engine and just as they start to spin the engine with the starter you touch the test lead to the (+) terminal of the coil. If it starts quickly remove the test lead or you will overheat the coil. You are taking the place of what the dead flat spade terminal was suppose to do.

No, I am not bored, I use to do this for a living as a hot line answerman for BeckArnley /WorldParts Corp and Echlin Corp and for Gabrial Shocks. I am retired now and only do this e-mail help as a hobby and only on MG, Triumph and Jaguar up to the Series III. I also have a website with tech tips you may find useful. http://mg-tri-jag.net

Let me know how the test goes,
Howard

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

QUESTION: Howard, I can't do that test. When I turn the key to crank the engine does not
crank any more. It used to work intermittently but will not work at all now.

If I jump the starter with the key in the on (not crank position) the engine
cranks and starts. It runs progressively worse as the engine warms up. The
coil goes from cool to the touch to warm to hot but not quite too hot to
touch. The problem is that I do not know how warm a coil would normally
get.

Eventually the engine stops and it seem the spark is intermittent when I test
the spark.

I also don't know that if a coil overheats if it goes bad or just needs to cool
down.

Based on what we have done so far it seems the wiring is OK but I don't think
we tested if the resistor wire is working.

Also, I think I need to replace the starter relay unless you think this problem
with cranking from the key might be something else.

Help

Answer
A coil should get warm when in use but not too hot to keep your hand on it.

The starter relay only operates the starter solenoid and has nothing to do with the ignition. The solenoid is suppose to connect the battery to the starter motor and connect a direct power to the (+) side of the coil ONLY while in the start position. When the ignition switch is released back to the ign position the relay drops the connection to the starter motor and drops the direct connection to the (+) side of the coil and at that time the 1.5 ohm coil is suppose to be powered by a circuit with a resistor wire in it so as to supply only 6 to 9 volts to the coil. If 12 volts is supplied to a 1.5 v coil it will get too hot to touch and finally quit working.

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.

Howard