MG Car Repair: Starter motor, voltage spike, starter motor

Question
Hello I reciently rebuilt my engine. All is good but yeaterday I had problems with the starter motor. I have already replaced teh ignition switch and coil. I serviced the starter motor yeaterday. The car is now starting every time but when it reaches a certain rpm there is a voltage spike that causes teh starter motor to engage while teh engine is running. I have replaced the solinoit and tested the ignition switch. Don't know hwere the voltage spike is coming form but it causes the starter motor to engage causing a grindin sound?

Answer
Hi Darryn.  In order to answer your question I need to know the model and year of the MG that you are having trouble with (for example 1976 MGB).

I did have a problem like this on an MGB a few years ago.  The starter would kick in occasionally when the engine was already running.  The problem turned out to be a faulty earth wire on the starter RELAY (not the solenoid).  It is a small canister hanging down near the fusebox with 4 wires attached to it.  Two brown, one white with a brown stripe, one black.  If I am right, the black wire will not be a good earth, possibly due to corrosion of the push-on terminal.