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MG MGB Technical - Puzzling electrical problem

I have, over the last 30 years, been able to sort out electrical problems in my various MGB's, but after removing the engine, harness, etc for painting the engine compartment on my '73 I am at a loss in how to get things to work again. All other wiring remained untouched and everything worked before. The horn, turn signals work and the engine turns over only once, then stops. The head lights don't work at all and most alarming is my test light indicats power in the head light ground (black wire). The reassembly was straight forward and I have triple checked all connections, wire colors, etc.

My review of the schematics in the manual indicate their could be 3 multiple ground locations similar to the one located in the boot - license plate holder. I can't locate the other 2 if in fact there are more. Can anyone verify this? And give advise?

The car looks great, but???

Stan Arendts
Irvine, CA
Stan Arendts

Hi.

My late model UK spec car had a chassis connection in a difficult to see place, but I can't quite remember where.. I think it may have been behind the brake master cylinder / servo (RHD, remember).

HTH.. Don
Don

Is the braided engine ground wire clean ? MAY be a cause of the turn over once.

The black wire at the headlights as you know is just the ground --- what happens when the switch is turned on and volatge check between black and blue red/white wires?

From what you write you've got a heck of a problem, but MUST be any strage combination of bad grounds or voltage carrying wires.

With the experience you have, it'd just be a matter of going through the electrics circut by circut.

best of luck on tracking it down
glg

Stan, Just a thought. Did you attach the engine to chassis ground after reinstaling the engine?
Dan H.
Dan Hanson

If you have 12v on a black, which should be bolted to ground somewhere, then it isn't, or is but hasn't cut through the paint. As well as in the boot/trunk there should be one high up on the firewall behind the dash, on the RHS on an RHD at least. I think the third is in the engine compartment somewhere, possibly near the fusebox and starter relay, but it should be easy to spot as a couple of short blacks coming out of the harness terminating in a crimped bolt-through connection. The headlights ground is shared with the starter relay ground, and this could be the cause of the cranking once but no more. The relay would ground via the main beam filaments and the tell-tale on the dash instead of directly. Initially the filaments are cold and relatively low resistance, perhaps just enough to allow the relay to operate. As you pass the relay current through them they heat up and increase in resistance, which perhaps causes the relay to drop out or fail to operate a second time. Well, it's a theory, and would cause the engine not to crank at all if the main beams were switched on.
Paul Hunt

Thanks for the input. It is my conviction that the problem is the ground. Although I have a new braided ground strap on the engine, I will put another one on the tranny, just to make sure.

Paul, would I hurt anything if I completely bypassed the starter relay? (for test purposes) How about just grounding the black relay wire?

SCA
Stan Arendts

Stan - the black relay wire should be grounded anyway. If it isn't i.e. shows 12v with the headlights on then that is what is wrong with the headlights if not the starting. Adding a second ground strap won't do any harm, may do some good for the throttle, choke and heater cables if the existing one is making poor contact, but won't do anything for the headlights in any event. 12v on the headlights black wire means the headlight ground is missing, pure and simple, and this is entirely seperate from any engine or transmission ground.
Paul Hunt

Paul,

On the engine ground strap, I was thinking more on engine turns over only once -- a result of some kind of over load or under load.

All the other electrics not working I'd guess is bad grounds or bad connections or, one or two wires mis-matched.

<;+}
glg

This thread was discussed between 20/10/2003 and 23/10/2003

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