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MG MGB Technical - Sudden, total electrical failure heals itsel

I was turning onto the highway this morning, went across a patch, and the whole car went dead.

No headlights, no ignition, no gauges, no indicators, nothing. It's a 1972 mgb roadster, neg. ground.

I pull out the test light, clip on to the pos wire, and touch the neg terminal - good light. touch the rear deck - good light. Go to the engine bay, all the fuses are good, and seated. Clip on to the purple terminal on the fuse box, and I have good light to the engine, the radiator, eveything I touch. I wiggle the bullet connectors under the fuse box. Still no electricity anything - headlights, fan, etc.

A friend comes down with a strap, and we're towing back up the shoulder - a little dicey, but traffic isn't too heavy. I hit another bump, and everything comes back.

Drove it home and parked it as if nothing ever happened.

Anyone have any tips on what piece of the electrical system would do that? the only thing I can come up with is the ground-battery connection, but I had a bright test light (beautiful sunny morning to be watching traffic) everywhere I tried.

Thanks
Steve
Steve Aichele

That's what electrical gremlins do. If everything really was dead, and died while driving, then it really can only be the brown wires up from the solenoid to the ignition switch, fusebox, lighting switch etc. If it had been the battery or its connections the alternator would have continued to run the engine. The most likely location is where the brown wires connect to the solenoid, at some point it changed from iffy spades to a more robust bolt-through connector that fits onto the battery cable stud, but my 73 has that and it was the connection between the battery cable connector and the brown cable connector that went high resistance even though the nut clamping them all together was still tight. Removing that nut and cleaning up the connectors with emery then reassembling with copper-grease did the trick.
Paul Hunt 2010

Steve. I agree with Paul. The test light is a good trouble shooting tool, and fits easily into the tool box. However, it does not show what is happening under load, only that some current is present. But, it is not a true indication that there is sufficient current to run the engine and/or lights properly as you have found out.

As Paul notes, the alternator, if it is working properly, should handle the load even if the battery is disconnected. And, the battery should handle the load if the alternator suddenly dies. With a complete systems failure such as you demonstrated, the starter solenoid main terminal is, as far as I know, the only common connection between the two systems and would have to be the logical focal point for your investigations.

Les
Les Bengtson

That makes sense, and was the part of the wiring diagram that seemed most likely.

What's really throwing me is having power to the fuse box, despite no power anywhere else. There's a connection off the solenoid to the ignition, the fuse panel, and the headlight switch. The fuse box had juice, but not the headlights or ignition.

I wonder if they all have bolt-through connectors.

Oh well, more under the car time this weekend.

Thanks.
Steve Aichele

Seeing Les' comment, maybe that explains the successful test light at the fuse box.

I'll get down and clean up those connectors.

Steve
Steve Aichele

Something similar happened to me once ... I arrived home, went inside, came out again and everything was dead - even the clock was stopped. As luck would have it, there was an AA man working on a van across the road and he came and sorted my car.
Geoff Everitt

"Machines don't fix themselves".

10 points if you can tell me what film that's from :)

What happens if the battery is disconnected when the engine is running? Normally there is a warning label on the battery lid isn't there saying not to do that? Does it upset the alternator?
Simon Jansen

Steve,
The same thing happened to my '72 GT. I eventually discovered that the ground cable, which looked fine, fell apart when I checked the connection at the battery.
Joe
J. E. Guthrie

Actually I missed the bit about the purple fuse showing live. There is only one brown wire up from the solenoid feeding everything except the alternator so if that were really the case then the solenoid connection must be OK, and the only way lights, ignition, and hazards (?) could be dead is if the sealed five-way connection behind the dash was faulty, and that is very unlikely.

Whilst a test-lamp is not as good as a voltmeter at indicating what is happening under load, it is much better than trying to measure resistance. Just like a voltmeter will show a reducing voltage downstream of a high-resistance fault as current increases, so will an *incandescent* test-lamp by a reducing glow and in a relatively linear fashion. However an *LED* test-lamp will not do this until the voltage starts to drop very low indeed, i.e. it is very non-linear.

In answer to Simon's question the battery acts as a very large reservoir smoothing and swamping voltage spikes that are naturally output by the alternator when it is charging. In theory the alternator without a battery connected can develop very high voltage spikes which could damage itself or electronic circuits elsewhere on the car. I've seen claims that a battery becoming disconnected at night caused all the lights to blow, even though the ignition and lights were already acting as a rather large swamping 'resistor' across the alternator output. As mentioned on another thread I have battery cut-off switches on all my cars and on more than one occasion in the V8 have turned it off first when reaching behind at the same time as to the ignition switch. I have an electronic coolant level sensor and that flickers a bit brighter than normal when I have done that, but there has not yet been any lasting damage to that or anything else. I certainly wouldn't like to do that to my ZS with all its computers, but then that switch is under the bonnet. Also elsewhere there is talk of FIA master switches that cut off the alternator and the ignition as well as the battery, as that is the only certain sure way to cut the engine from the outside. There is a resistor on the alternator output lead as part of this. It can't be that low a resistance or there wouldn't be enough power left to run everything else including banks of spot lights on rally cars, and it would have to be physically massive to dissipate the heat. It is small enough and stays cool enough to be contained within the switch, but is obviously enough to protect the alternator itself.
Paul Hunt 2010

This thread was discussed between 25/06/2010 and 26/06/2010

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