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MG MGB Technical - Electrical Problem 74 MGB

Hi

I am having an electrical problem I can’t chase down.

1974 mgb roadster. All electrical from the engine bay has been moved behind the glove box with relays added etc. I left the fuse configuration pretty well as stock. This set up has more or less worked fine for a few years now

About 1000 miles ago the car just stopped. No power, no alternator light, no solenoid, no turn signals nothing. I got out to check to see if a wire had come off the coil. Returning to the car I noticed the alternator light was on. the car started right up. 20 minutes later, same problem, same solution. I then drove it over 1000 miles back home without a problem and it has sat for 2 weeks.

Today while chasing down a bad battery cell (I hope to replace this tomorrow or Monday) the car was idling and then just stopped. Within a couple of minutes the alternator light was on and it started right up. About 15 minutes later same thing. It just died.

When this happens the alternator light is out, the electric fuel pump is not working, the solenoid doesn’t click and the instruments (up-graded VDO) don’t work as well as the turn signals. The electric cooling fan works and so do the lights and horn.I wiggled all the wires behind the glove box and under the steering wheel and was unable to get the alternator light to come back on. So I am unable to reproduce the problem.

As I look at the electrical schematics (Autowire 1975 W/Sequential Seat Belts) I can figure out I have power through the big brown wire cluster and through the brown wire to the purple wire fuse as the horns work. This big brown wire cluster also feeds my headlight/fan and horn relays so all is fine there. The only other thing I can see is a brown wire which comes up from the solenoid (again passing through the big brown cluster) and supplies power to the ignition switch and the white wire which runs through the fuse to the green wires.

Is it possible there is a short in the ignition switch itself?

Any ideas gratefully accepted.
Got to find a place to start from.

Thanks

Bruce
Bruce Mills

Bruce - You are loosing power to the white wire. Use a meter or a test light to probe the white wire, starting at the ignition switch on to the fuse block. The switch may be the cause of the problem or any of the connections right up to the fuel block. Cheers - Dave
David DuBois

Hi Bruce
I had a something of a similar problem last year with my 79 'B'. It took a little while to track it down but it turned out to be an arcing rotor arm. Hope your problem is as easy to rectify
Jan
M.J. Egan

Check the Brown wire, downstairs, at the Starter Solenoid.

It employs a simple Push On Tab connector and it often wriggles loose due to engine vibration, oil, etc..

As it loosens most all of the electricals will sputter and wink off
and on. When finally it disconnects, everything then conks out.
Daniel Wong

"All electrical from the engine bay has been moved behind the glove box with relays added etc"

Do we need to know exactly what wiring configuration is in place on this car? Clearly, it is not a factory design.

Regards
Roger
Roger T

Thanks everyone. I checked the distributor cap/rotor and found no sign of arching but used the dremel to cleanup the cap contacts while I was there.
I checked the wire on the starter as Daniel mentioned and although it is making good contact I removed and turned the ignition on, the alt light came on.
I then pulled the white wire apart closer to the fuse block than the ignition switch and the alt light does not come on but the car will turn over.
This is leading me back to the ignition switch isn't it?

Bruce
Bruce Mills

I bought a new battery which once installed and the ignition turned on read 12.38 volts. Chasing the white wire from the ignition switch I checked every connection over tot he fuse. All the readings at the connections were 12.36-12.38 with the exception of the ignition relay which is installed as per Advanced Auto wire 1977, which read 12.30. This was the lowest reading I could find. Replacing the relay didn't change the reading.
I stopped at this point but a couple of hours later checked the battery and it read 12.49

Thoughts?
Bruce Mills

Bruce,

If I were you, I would go back to Advance Autowire. They do have tech support and they should be able to guide you to a solution.

Mack


M Sneed

Bruce

sounds like you have a connecter that is connected very loosely (i.e. a spade terminal) that when the car moves and vibrates it becomes loose.

have a look at the terminals on the starter motor solenoid,(as Daniel suggests)if these pull off easy then they need crimping tighter,also check the earth cable from the battery to the body,i had a problem with mine and when i took out the battery the earth lead fell to the floor but the connecter was still connected to the body,replaced with new cable and all electrics are now fine.
phil
phil mcneil

Thanks All

Mack. I didn't buy anything from Advance Autowire. I just used their schematics.

Besides replacing the battery and checking the stater wires I went through the white wires and checked all connectors. Tighten a couple in the wiring itself and redid 2 which went to the relay. Ran the car for 1/2 an hour and all seems fine. Checking with a multitester the readings are now much closer.
I'll cross my fingers and hope I have solved it.
Bruce Mills

It will have been the brown feeding the ignition switch, through that onto the white, and wherever that goes before it starts splitting off to things like fuel pump and ignition fuse(s).
Paul Hunt

Thanks Paul

I still have the panels off under the dash and will give the brown wire a good checking out this evening.

Bruce
Bruce Mills

This thread was discussed between 23/08/2009 and 26/08/2009

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