!977 MGB Inignition Switch

[mcapple]

I have a 1977 MGB, when I start it it runs fine until I release the ignition switch. The engine then cuts off. I can easily restart it but when I release the switch it cuts off again. Can anyone offer any suggestions? Do I need a new switch?

Thanks!

Robert

[Flash75]

Robert: Does the fuel punp run when the switch is in the run position? If it does the switch is probably ok. Do the windshield wipers work when the switch is in the run position. If they do you ignition relay is probably ok. With the switch in the run position check for a voltage at the + coil terminal, it should have around 6 volts. If it isn't there and the wipers don't work it's probably the ignition relay. If the wipers work and there is no voltage at the coil there is probably a wiring problem check the white/brown wiring spades at the fuse box. Insure the connection aren't corroded. I hope this information gets you started. I assume you do have a Haynes repair manual. Good luck, Clifton

[KimMG]

Sounds like the electrical part of the ignition switch has failed. The switch is easy to replace. It should take less than an hour

[jimmy wade]

I have a problen maybe opposite, mine won't stop.I have to keep moving the key until the INNITION light comes one. Then is a few seconds, to engine will stop. It seems to depend on how long I have bbeen driving as to how long it takes to turn off.

[Flash75]

Jimmy, What year car? If it is a rubber bumper car look at the wiring diagram in Advance Auto Wire at;
http://www.advanceautowire.com/
On the left click on stock schematics,
next click on MGB, MGC, etc and find the correct wiring diagram for your car. If your car is a 76 or newer it should have a diode in a service reminder circuit and another in the handbrake circuit. I believe the 75 has the service reminder but not the handbrake diode. These diodes have been known to cause the engine to continue running when the ignition has been turned off. The temporary solution is to disconnect the circuit and see if the engine stops. If it does the diode needs replacing. You will have to find a substitute diode from Radio Shack or some electronics store.

Clifton

[jimmy wade]

Originally Posted by Flash75

Jimmy, What year car? If it is a rubber bumper car look at the wiring diagram in Advance Auto Wire at;
http://www.advanceautowire.com/
On the left click on stock schematics,
next click on MGB, MGC, etc and find the correct wiring diagram for your car. If your car is a 76 or newer it should have a diode in a service reminder circuit and another in the handbrake circuit. I believe the 75 has the service reminder but not the handbrake diode. These diodes have been known to cause the engine to continue running when the ignition has been turned off. The temporary solution is to disconnect the circuit and see if the engine stops. If it does the diode needs replacing. You will have to find a substitute diode from Radio Shack or some electronics store.

Clifton

Thank you so much. I will see what I can find. It is a 1978 model.

[jimmy wade]

I found the two Diodes and took them out. The car still ran and still had the problem. I tried the local Radio Shack but they did not have a replacement. I think one of them is bad.

[jimmy wade]

Problem solved! Put a diode between the alternator and the ignition light.

[Flash75]

Jimmy, Thanks for letting us know what fixed the problem.

Clifton