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MG MGB Technical - Electrical Problem at start-up

When I try to start my 74 B all I get is a buzzing sound from the starter(the engine does not turn over) and the rev counter needle moves up the guage. The battery is fully charged, I have cleaned all of the ground points and battery cables. The lights, signal lights, everything electrical seems to work just fine.
Any idea what could be causing this ? I have to assume that it is short circut somewhere as it was running just fine yesterday. Help please as I am at the end of wire on this one
Ken Janes

Hi Ken

The only possibility that I can think of is that the starter motor solenoid is oscillating (ie pulling in and out rapidly). This would account for the buzzing, and possibly the tacho reading too.

The sort of thing I am thinking is this : starter solenoid engages, connects starter, voltage drops due to current drawn by starter, solenoid drops out due to reduced voltage, voltage rises again......

This could be due to a high resistance connection somewhere in the following loop :

Battery positive, starter motor (including the internals), engine earthing braid, battery earthing strap.

A high resistance in the batter / ignition switch / solenoid circuit is also a possibility.

If you have the headlights on while attempting to start, do they go dim ?.

Good luck... Don
Don

I'd agree with Don, a bad connection is allowing the load of the starter to cause the voltage to switch rapidly between 12v and close to zero, which is simulating the normal input to the tach. Put your voltmeter on the battery posts (not connectors), connectors, hot connector and a body ground, solenoid stud and body ground, and brown at the fusebox and body ground. If you see a much higher voltage at the battery posts than at the fusebox then testing at the other points should help you to isolate the location. Use an analogue meter as a digital may not settle and give a steady reading. I had a bad connection between the browns and the battery cable on the solenoid even though the bolt was tight which gave a similar symptom.
Paul Hunt

This thread was discussed between 29/06/2003 and 01/07/2003

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