QuestionQUESTION: Howard, the problem we were working on was the 1975 MGB that would stop
running after a while. I would lose spark from the coil so we believe the coil is
overheating. I had purchased a new coil and I had the same problem. We
started a trouble shooting process where we had to test the ohm's in the coil.
I was finally able to get a reading and found that the old coil read 1.5 ohms
and I had bought two new coils, one that required an external resistor and
one that did not require a resistor. They read 1.5 and 3.8 ohm's respectively.
The next step was to do the test of the coil to see if I was getting a good
spark. That was the test I did last night and the old coil had a good strong
spark.
I mentioned the starter relay because the first time the the car stalled the
starter would not crank from the key. I am suspect that the starter relay may
cause the coil to over heat because the two problems happened at the same
time but since I am not a pro at this I don't know if the two are related or just
a coincidence.
You told me I need a 12 volt test light and a low ohm meter to do the testing
and we needed to see if the coil was good as the first step.
I have a volt meter. Can I use that instead of a 12 volt test light or should I
go buy one?
Thanks
ANSWER: Yes, you can use the volt meter in place of the test light. All the tests you run must be made while the car is in a failed mode.
You said the coil was hot. If you are using the 1.5 ohm coil and it is hot to touch then it sounds like the the resistor wire that is suppose to be on that car has been removed or the ignition has been wired wrong. Here is the way it is suppose to be wired.
On the coil (-) side you should have Two white w/black tracer wires. One of these wires goes to the points in the distributor and the other goes to the tach. On the (+) side of the coil should be two white should/light green tracers and one of them should go to the resistor wire and then to a white wire to the fuse box. The other should go to the starter solenoid. The way it works is the wire that goes to the resistor and on to the fuse box is hot with the key on. The wire to the starter solenoid should be dead. When you turn the key to the start position the white w/light green tracer going to the solenoid gets hot as long as the solenoid is active.
The reason for this system is, when you activate a starter motor there is no longer 12v available for a coil so they install a 6 to 9 volt coil (1.5 ohm) and supply the power to the coil direct form the starter solenoid when in the start position since there is not 12 volts left from the starter being used. The 1.5 ohm coil then works ok. After the key is released back to Ign. then the power is supplied via the white wire from the fuse box to a resistor wire and on to a white w/light green wire to the (+) side of the coil. If direct 12 volts were supplied to the coil without the resistor wire the 6 volt coil would over heat and quit working. Plus it would probably cause the contact points in the distributor to burn and not make contact.
To test the resistor with a volt meter turn the engine until the contact points are closed. Then connect the red lead of your volt meter to the white wire at the fuse box and the black lead to the (+) post of the coil. Turn the switch to IGN. (not start) You should see from 3 to 6 volts on your meter. If you don't see that, the resistor wire may have been removed.
Let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: Howard, I did the test and it reads 3 volts when I turn the key to IGN. It also
looks like everything is wired the way you explain it.
What would be the next step?
ANSWER: With the other wire off of the (+) side of the coil confirm that it is not hot with the key on but is hot when you turn the key to the Start position.
---------- FOLLOW-UP ----------
QUESTION: Howard the other wire has no power in the on position or the crank position.
Next?
AnswerNo power in the crank position and no power in the Ign. position either, correct?
You need a wiring diagram to work on the electrical system. However the power comes from a small flat spade connector on the starter solenoid. There should be two flat spade connectors on the solenoid. One for the white and red start wire and the other for the bypass power to the coil in the start position.
