MG Car Repair: 1977 mgb, mgb car, starter solenoid

Question
car was running fine then died. will try to start by holding key in start position but dies when the key returns to normal. Is it the coil or starter relay?

Answer
Hi William,

Your symptom narrows the problem down to only a hundred possible causes. You need to run some tests to narrow it down a little further.

It is a waste of time trying to guess what it might be so the first test is to turn the key to Ignition (not start) and test the (+) side of the coil for power.

The ignition system has a low voltage coil that is powered through a resistor wire to drop the voltage so as not to over heat the coil. However, in the "Start" position the starter draws a lot of current and a full 12v is not available so the reduced voltage due to the starter draw is routed direct, bypassing the resistor wire. This direct voltage comes from a small connection on the starter solenoid.

Since this car runs ok while in the "Start" position, that circuit is probably good. So you need to look at the Ignition key switch to resistor wire to coil (+) circuit to see if power is being supplied. The ignition switch could be bad, the wiring or the resistor wire could be bad (If you have no power to the (+) side of the coil in the run position of the key.)
The most common failure is the resistor wire but you need to test each part of the full circuit.

Howard