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MG MGB Technical - Intermittent Electrical Failure

I'm in the process of making my newly purchased '74 B a reliable car by taking care of some overdue maintenance items. (fluids, hoses, belts, U joints, etc).

I was taking a test drive around the block and the electrical (everything) died. I've only had it do this while driving once (after it afterfired out the tailpipe - I replaced the vac advance and have that problem fixed). The engine died, but I'm not sure if it was due to the afterfire or the electrical cutout.

By everything, I mean starter, lights, idiot light, everything. I let it sit for 30 seconds and it came back to life.

It happened another time while I was just turning the engine on and off while adjusting the dwell/timing.

My first checks are going to be the battery and engine grounding along with the battery cable from the battery to the starter.

The PO did install a homemade battery kill switch that is make of a couple wing nuts a a piece of flat stock. One of the holes in the flat stock has a U shaped hole that allows the flat stock to rotate.

In the stock config was the neg cable just grounded to the body near the battery?

The problem seems to fix itself without doing anything, so it's difficult to duplicate.

Are there other common failure modes I should be aware of as I troubleshoot?

Many thanks!

Paul
PR Peterson

Paul. Oh no! Another Paul. You are now, officially, Paul P. Our best electrical expert, being a qualified electro-mechanical engineer with many years of MG experience is Paul Hunt. His website, the Pages of Bee and Vee, has some excellent mechanical tech articles and some outstanding electrical related articles. He can be contacted, via e-mail through the site, if he does not pop in on this question.

Yes, the negative terminal of the battery grounds to the upright on the battery box near the battery. Cleaning the contact surfaces and ensuring there is good contact is an excellent idea. Also cleaning the battery terminals and cable clamps and making sure they are all tight is a good idea.

It is possible, but not desirable, to run the car with the battery disconnected. It is also possible to run the car, on just the battery, with the alternator unplugged. (Also not a good idea, but I am not an engineer, so cannot speak from personal knowledge, only what I have been told or have read.) There is one point where both systems come together and, if that point were to become disconnected or was not connected properly (loose), it might well cause the problem you describe--complete loss of all electrical function. This point is the starter connection with the read wire from the positive terminal of the battery and the brown wires from the alternator and to the fuse box. If this turns out to be an exercise in faulty logic, contact Paul Hunt for additional ideas. Les
Les Bengtson

The consistent problem i have heard (and experienced - albeit thankfully <touch wood> only in a small sense) with MG electrics is poor earth.

Many systems are connected to an earth wire, rather than seperate earth connections. Finding the black wire and checking for breaks/poor connections along its length can often be the solver of all problems electrical - that of course assumes power is getting there in the first place....!

Good luck ~PHIL
Phil

I had something similar to this at the beginning of summer. My initial problem was a funky wire connector located on the starter. Essentially the wire came a little loose and the car completely died. Of course that seemed to spread to other wires until it stopped after infecting the multi-wire connector on the alternator.

I found the wiring diagrams and electrical troubleshooting section on the top of this BBS section really helpful.
Jason Coolbaugh

Complete death can be caused by a bad connection at the battery or its ground, or at the starter solenoid, engine grounding won't affect lights, ignition warning light etc, only cranking. I had this problem on my roadster earlier this year during its MOT, embarrassingly. The mechanic claimed that he had checked the solenoid connection but when I measured the voltage on each lug and the bolt I was able to show him that whilst the battery cable lug showed 12v under load the brown didn't, so I reckon we ended up pretty even. When I got to the garage he was trying to jump-start it with another battery connected to my battery and wondering why it still didn't start.
Paul Hunt

Paul P.:
if your PO doctored the electrical system with a homebrew cutout switch, is it possible he did as some others have and played with the fuse block, possibly replacing it?? I just wonder because what you describe sounds similar to the action of a circuit breaker. There are automotive circuit breakers available and some in very large ampacities. I would wonder if the DPO didn't wire one in somewhere. This is just a thought, but perhaps a possibility. FWIW.
Bob Muenchausen

If you're trying to increase the car's reliablitly, you might consider wholesale replacement of the car's bullet connectors -- those little rubber sleeves that join the wires. I found that on my '69 roadster, nearly all of them *looked* good. No visible corrosion, etc. Disconnect one though, and you could crush it in your fingers. The metal just crumbled. I've since gone through the car and replaced every one I could find. Some people suggest cutting the wires and either soldering the wires together or else replacing with a different style connector, but simply replacement is easier, and if the new ones go 30 years like the old ones, I'm happy.

Note that I doubt these connectors are the cause of your current problem (my vote is the spade connector on the starter that Paul Hunt and others mentioned. I've been bitten by that one as well.... ;-) But it might save you some future headaches.

Hope this helps!
Rob Edwards

"... your current problem ..."

Or lack of current ...
Paul Hunt

Thanks everyone!

I'll check the battery ground and starter connections first and go from there. I'm hopeful fixing my total failure problem will help correct my less than satisfactory normal operation (The turn signals don't flash if I've got the headlights and brakes lights on at the same time, unless I rev the engine over 1500 rpm or so).

If I can get this problem fixed and my driveshaft reinstalled (new U joints), I'll be able to enjoy some of this beautiful fall weather we've had in the Pacific NW.

Best Regards - PaulP
PR Peterson

The turn signal thing is not uncommon but seems quite severe in your case. Can be caused by tired bulbs or flasher unit or bad connections anywhere in the brown - ignition switch - white - fusebox - green - hazard switch - green - turn switch etc. circuit. I have also heard that there is a bulb available in the USA that fits but has the wrong wattage flasher filament, and as they only seem to be available by reference number unlike wattage as in the UK this is not easy to spot. Flasher filament should be 21W. Could also be a weak battery and low output from the alt. Mine doesn't flash with headlights and brake lights on with the engine *not* running (they do so slowly with headlights only), but it does so at near normal rate with the engine idling after having been revved to 900 or so to start the alt charging.
Paul Hunt

On the turn signals, I had an interesting one recently. On my B the right had indicators always flicked faster than the left, and the MOT people weren't happy. But all the lamps seemed to work OK. Checked all the indicators and found one all rusted up on the front left side so did good clean up job and hey presto they both work the same now. I suppose bad connection equals lower current equals slower flicking.
T Green

Exactly so. The standard turn flasher is very susceptible to even small reductions in current. This is its way of warning you when one of the bulbs has failed, which is why it is unsafe to fit a hazard flasher in place of a turn flasher (which can flash one to four bulbs equally well). The hazard flasher also only comes on after a delay unlike instantly like a turn flasher. Many modern OE flashers flash at double-speed when one bulb is out and don't have the same susceptibilty to bad connections, but some after-market items are just hazard flashers in disguise and should not be used. If using a non-standard flasher always check that there is a clear indication from the driving seat when one of the bulbs is out.
Paul Hunt

This thread was discussed between 14/10/2002 and 18/10/2002

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