MG Car Repair: MGB Starter circuit, starter solenoid, jumper wire

Question
I have a "78 B.  I have recently had difficulty with starter switch and solenoid.  It all started when the car would no longer respond to the switch, which was old.  I replaced the switch to no avail.  I checked the solenoid by jumping across the 2 poles and getting a response from the starter.  I was then able to get the car to start using the new starter switch, all worked but the new switch and wire harness became very hot after 2-5 minutes of the car running.  I decided that the solenoid was likely the shorting culprit and have replaced it.  The switch, but mostly the plug from the switch to the dash wiring is very warm to slightly hot.  I have traced the white/red wire that contains the diode from the resistor.  The diode had gone bad for me about 10 years ago, and I installed a new diode.  That connection appears to be good.  I am not sure how to proceed.  Do I suspect a bad or dirty connection plug, or is the new ignition switch faulty?

Thanks for any input, I do have a wiring diagram and have studied it frequently.  I know it usually helps to test circuits before replacing parts, but  don't have that capability in my garage.

Answer
Hi Brian,

BLM and/or Lucas Made a monkey puzzle out of the starter circuit on the late MGBs. At this point it is a waste of money, tossing parts at the problem so you need to find out what is wrong first then if you need to purchase a part you will at least know which part to buy.

First disconnect both white w/red tracer wires from the starter solenoid relay (W-1 terminal). Then go to the ignition switch and disconnect the white w/red tracer wire from there too. Now take a long jumper wire and connect the Ignition switch (where the white w/red wire was) and the other end to the W-1 terminal on the relay.

Doing this bypasses all the junk that can cause a short or load that is heating up the wires. Now it is a basic starting system and it should start and run normally and no wire should heat up. When the car is running, the starter solenoid is NOT active so it can not cause any wire to heat up. (unless you have some wires connected up to the wrong place.) Most likely it was one of many added items tied in to that white w/red tracer wire circuit.

If every thing works ok with the jumper in place and the white w/red circuits out of the system then to locate the actual problem you will need to lay a wiring diagram in front of you and start tracing each of the white w/ red circuits in the diagram with a test light and a powered up short test lead to that circuit.

If you don't have a diagram let me know and I will put one up on my web site for you to copy.

Howard