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MG MGB Technical - Engine dies - electrics?

Hi

Grateful for any help... driving along in my 77 roadster yesterday. Out of the blue (or maybe linked to going over a bumpy bit of road) the engine died. While coasting to a halt, I noticed the red alternator warning light glowing brightly and also a complete failure of the hazard warning lights. The headlights, however, continued to function.

Cranking the ignition made it turn over and sound like it wanted to fire, and possible did once or twice.

After about 5 minutes and around 10 attempts it finally started up (kept revs quite high (not ideal as I'm running in a new engine)) for the 500 yards home and made it no problem (and the hazard lights were working again).

I now feel I can't trust it! - not least of all because rather amusingly, last week the fuel pump failed and while being towed home the front of the car got pulled off and I got to watch it go bouncing down the road in front of me. I'm keen to avoid needing tows for a while!

Any ideas - I've tried to make sense of the Haynes wiring diagram as to what electrics the hazards and ignition system might share but nothing stood out.

I do seem to be having a bad time of it at the moment, since jacking up the rear to attend to the fuel pump an odd clonk has developed (I've checked the wheel bolts are tight!). Anyone else found a few jacks up and down (on springs/axle rather than body) was enough to mess up the diff thrust washers?

Sorry for the long post!

Darren

D Lewis

The bumps could have jostled the connections at the starter. Check to see that they are clean and tight. Also, check the alternator plug and coil leads and wiring.

Some of the hardest problems to figure are the intermittant electrical ones. This one is just probably a loose connection somewhere.

Good luck
Luigi

Hi Darren.

A very useful set of diagrams are availible by following the 'electrical help' link from the MGB Technical home page.

I agree with Luigi that you probably have a dodgy connection, probably in the battery (brown) wiring.
Take a look (and a waggle) at the fusebox connections.

Did the headlamps work on flash, or dip / main or both ?.

HTH.. Don
Don

Its pretty common to have a loose fitting at the starter solenoid as Luigi commented. Let the car run at idle with the parking brake on and just start wiggling wires. If there is a wiring problem, it may be related to removing and installing the engine, so start with the starter and alternator and go on from there. Did you remember to install a ground strap from the engine to the chassis? You'll know when you find the culprit. Otherwise, the problem could be in your distributor, but that's another set of problems that's been discussed in length the last few weeks.
Jeff Schlemmer

If the warning light was glowing when the forward motion of the car was still spinning the engine then that is telling you that the 12v was missing from the ignition relay on the white/brown. Although this doesn't feed the coil (which comes direct off the ignition switch) it does feed the pump. So if the relay had failed the warning light would come on some time before the engine stopped because of lack of fuel. But if you had pushed in the clutch or knocked it out of gear and the engine was stationary when the light was on then that tells you the ignition supply was still there, and something else caused it to stop.

The hazards are probably another problem altogether (when did you last use them?) as my diagram shows the supply for them as coming from the starter relay, and if the supply to that had failed it wouldn't have cranked. They have their own in-line fuse joining two browns somewhere. But I note that they were working again when you got the car running. Did you leave them switched on when they weren't working?

If the headlights still worked that indicates the main feed up from the solenoid was OK.

Intermittent problems are always the worst to track down, in the past I have hooked up a voltmeter to various circuits so I could see what the voltage was immediately the problem occured and tracked down problems that way. The next time it happens leave it in gear with the forward motion of the car spinning the engine and look at the tach as well as the ignition warning light. If the warning light is off but the tach drops to zero there is some disconnection in the ignition LT through the ballast resistor, coil, points and distributor ground. If the tach is still registering it is probably either HT or fuel.
Paul Hunt

Hi

Thanks for all the comments - thought I'd share the answer (or at least what looks like the answer, I've not yet replaced the part so I suppose I might just have come across another problem!)

It seems the ignition relay was misbehaving. The confusing behaviour of hazards etc was due to some PO creative wiring not forseen in the Haynes wiring diagram!

PS I'm assuming a 40A replacement is the ticket?

Cheers
Darren
D Lewis

This thread was discussed between 05/05/2004 and 08/05/2004

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