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MG MGB Technical - electrical stuff


Just over a week ago I took the V8 roadster out, parked it.......engine wouldn't switch off! Pulled the + feed off the coil to stop it.
When I got home I had to do this again.
Presumably ignition circuit issue.
1. Changed the switch............no improvement.
2. Went over all the "white" wiring.plus the green circuits........nothing changed.
Then I noticed that the ignition light stayed on........., and stayed on till revved.
If I turned the key off whilst the ig light was on, it turned the engine off but it wouldn't turn off when the light was out!!
Checked the light wiring and the bulb holder..............nothing.
3. Took the alternator off and checked the brushes and cleaned the slip rings..........no improvement.
This has exhausted my generator knowledge!!!
But still suspect there is an internal fault on the alternator. Leaking current, via the warning light circuit, to the ignition circuit is my guess.

Diode pack??????? Heard it mentioned, but that's all!!
Gave in and a local auto electrical firm has it for a check up. I've used them before, they're good lads.
UPDATE............They came up with nothing!!?? So reinstalled, this time put a switch on the ignition light feed.
Same problem BUT switch light feed from alternator off and engine stops, as it should.

Allan Reeling

This happens to North American cars with the ignition relay when a fault with the emissions system develops because of a wiring error, but presumably yours started doing it without making any changes yourself?

What year is/was it? How far from the factory wiring have you taken it? Do you have an ignition relay? Does that power the ignition?

There should not be enough current flowing through the ignition warning light to power the ignition with the key off but the engine still running, all MGBs were wired like that until the 77 model year. If the warning light is short-circuit it can happen, but then the light wouldn't glow. Is is incandescent or LED? When you switch off a running engine the warning light will come on, then flicker and die as the engine spins down.

"the ignition light stayed on........., and stayed on till revved." At what rpm did the light go off, and did it come back on when you released the throttle pedal? It should go off at about 900rpm or so and not come back on until the revs drop to 600rpm or so.

"If I turned the key off whilst the ig light was on, it turned the engine off but it wouldn't turn off when the light was out!! " Ordinarily the light would go off immediately after starting the engine and giving it a bit of a rev, then stay off until you switch off. So when you come to a halt and let it idle, is your light on or off?

paulh4

Nice one
I'd try a new globe in the ign. light, it 'could' have a partial short in the globe allowing feedback
During normal operation if the engine is going and the light is out there is 12v coming from the alternator back to the globe and usually the globe value is low enough not to feed anything back through the ign when it gets turned off
You could fit a diode in the line between the globe and the alternator, that would stop it happening but doesn't solve the problem really
It's an unusual one as you have to rev it to get the light out, usually that points to a regulator problem where the alternator isn't getting 'excited' properly
Is this a two wire alt with built in reg or an external reg----?
willy
William Revit

Another thought triggered by the above is to measure the voltage on the brown and the brown/yellow at the alternator under the various conditions you describe i.e. light on, light off, idling, revving and so on.

Once the alternator is charging both should be at the same voltage, which should theoretically be the same as throughout the brown, white and green circuits. However in practice you will always get a slightly lower voltage at the fusebox brown, a bit less at the white and a bit less than that at the green with certain things switched on but only tenths. Unless there is a very heavy electrical load from somewhere the brown and brown/yellow at the alternator should only be slightly above that, and in the order of 14v to 14.5v. The brown/yellow higher or lower than the brown indicates alternator problems, as does an excessively higher or lower voltage at the brown.
paulh4

Solved, well at least partially!!!
Thanks Willy, I did try the new bulb, to no avail. I think this alternator has always been slow to excite..............a bit like me rally.
Paul, I gave your web site a working out which pointed to at least a solution.
I fancy the original problem was indeed the ignition switch, but my attempt to take some of the load of the new one, by introducing a relay, led me to the '77 "issue", i.e., the ignition light output holding the relay contacts closed!!! Took the relay out and back to proper functioning. I'll explore taking the green loads off the switch when my patience has returned!!!
Being too clever eh!!!
Thanks to you both.
Allan Reeling

White loads off the switch, not green, or they will be unfused :o)

But if you have used a cube relay you shouldn't have any problems with it sticking on (which I suspect was the reason the factory moved some of the loads from the relay back to the switch in 78). So moving the ignition warning light white from the ignition switch to the relay contact will also solve the problem.
paulh4

Surely Paul, with ignition on, the green circuits are activated through the connecting fuse. Hence they are drawing current through the switch. The contacts weren't sticking, the output on the ignition light circuit was energising the relay electro magnet. I put a switch on that wire which killed the engine when switch was off.
I'll re-route as you suggest tomorrow.
Allan Reeling

Hmm
OR
A diode in the line between the alt. and the globe would fix that by stopping power coming back through the globe to the relay--yes-no-?
The globe would still work, the alt would still get energised but it would stop feedback to the relay circuit when the ign is turned off---sounds good to me, but then.
William Revit

Willy,
The diode is a good idea, but since the problem seems to be alt light output feeding back into the ignition/white, circuit (it doesn't take much current to hold a relay closed), maybe spliced into the ignition switch/white side of the light?
Worth a try i think.
Allan Reeling

Green circuits are powered through the fuse from the white circuit, so it's a white wire you would need to move from the relay back to the ignition switch. Just making sure you didn't end up moving the wrong wires.

Diode is a bit Mickey Mouse when the problem is a self-introduced wiring error, nicer to do it the way the factory did on RHD cars.

Inserting a diode also removes one of the diagnostics that the ignition warning light offers. If the engine cuts out while driving the first thing you do - while the momentum of the car is still spinning the engine - is look at the tach. If that's dropped to zero the problem is ignition LT. The second thing to do is look at the ignition warning light. If that is glowing you have lost the 12v supply from the ignition switch, but if it isn't glowing then the problem is with the ignition system itself. Adding a diode means the warning light will never glow under those conditions, only when you first turn on the ignition. Personal choice.
paulh4

Yes, get that Paul, a diode in that position would also stop the "excitement" of the alternator!!
"Self introduced", I hold my hands up to that...............idle hands eh. But in my defence BL made the same mistake, I just followed their '77 diagram!!
Anyway a touch of re-routing today and the re-introduction of the ignition relay. At least I learned some more AND had the alternator checked out for free!!!
Allan Reeling

All good but the diode wouldn't stop the excitement of the alt--power wouls still flow to the alt but not back from it
Anyway better to wire up the relay properly as Paul notes
William Revit

As Willy ... unless you get the diode the wrong way round :o)

BL only made that error on North American cars with the full emissions kit (probably all LHD by that time), not RHD on the 1977 diagram I have. They didn't realise it at the time as the anti-runon system was so effective. That sucks the fuel out of the carb jets which stops the engine as quickly as if the power had been cut to the ignition. They would have realised immediately on the first RHD car wired like that, so moved the ignition warning light wire from the relay winding to its contact as well as having a second fuse for some of the green circuits. A year or so later for RHD cars they moved several ignition circuits back from the relay to the ignition switch as well as adding a third green circuit fuse, possibly because of problems with sticking relays. I've not seen a diagram for later LHD cars, but have been told by Kelvin Dodd of Moss USA that a similar correction was made to those cars, possibly at the same time as the second change to RHD cars.
paulh4

Re-routing of ignition light white to relay contact, and all good!
Thanks to you both for the "lessons".
Allan Reeling

This thread was discussed between 01/10/2019 and 02/10/2019

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