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MG MGB Technical - Won't Stop Running

Hello My Friends -

We recently awakened a 1977 MGB after a 16-year nap. It starts and runs well, but when the ignition is turned off, the engine keeps on running!! Is there a circuit that needs attention, or is this caused by a faulty ignition switch??

Please Help!!

Thanks,

Tim Moorhead
Dallas, TX
T. L. Moorhead

Could be caused by the alternator, the anti-run on system not working, fuse box installed upside down, bad ignition switch, an improperly connected wire to the fuse box, if fitted, a sticking ignition relay.
Kimberly

So, you basically have no idea, and you're hazarding several guesses??

T. L. Moorhead

Basically, it could be anything.

You're not giving us much to go on.
Dave O'Neill 2

Tim,
Common problem with '77's. This is caused by just enough current running from your alternator signal line, through your dash board ingition light, and into the ignition relay to keep the relay on. It doesn't take much. I have my own personal fix that is not favored on this board, but it seems logical and it works for me on 2 cars so far. I cut the white wire going from the inertia switch to the ignition relay and connect a green wire to the ingition relay instead. This way when the ignition is turned off, the relay has to go off too.

Mark R.
Mark Rotsky

Tim. No, Kimberly is giving you a list of areas which you have to check out. None of us are going to be able to do it for you--we can only offer things which might demonstrate the symptoms you describe. It is up to you to check out the areas and, if you are not successful in finding and correcting the problem, telling us what you have found so that we can offer additional information.

You note that Kimberly's first suggestion is an alternator problem. Mark follows up with the information that the 77B was, in most cases, wired so that the alternator can feed back through the ignition circuit, causing the engine not to want to shut off. The archives contain several fixes for this problem.

Les
Les Bengtson

Quote from T.L. "So, you basically have no idea, and you're hazarding several guesses??"

No, I am not hazarding guesses. The list of possible causes that I gave you is based on experience. A failure can be caused by several things. You check each one to eliminate possibilities. Using a test light you can easily verify if the anti-runon valve is getting power, the fuse box is properly connected, the ignition switch is opening, and/or if the ignition relay (if fitted) is sticking.

If it is the charging circuit that is causing the problem, install a diode in the brown/yellow wire. Diodes are one-way valves for electrons and will stop feed back in the circuit.


Kimberly

It's a design problem in the wiring of the ignition relay which was covered up by the anti-runon system - until that system goes faulty. Basically any blocked or ruptured hose in the emissions plumbing can cause it, or the valve itself not operating.

Really you should find and fix the problem in the emissions plumbing or anti-runon valve. If you don't want to do that, don't install a diode, it is a frig, and I hate frigs. Do the proper mod that was done on UK cars. As wired from the factory the white from the ignition key goes to the ignition warning light and to the relay winding. The coil is powered from the white/brown on the fusebox. The factory mod was to swap the ignition warning light and coil wires over, although I reckon either would have done the trick. The easiest is to locate the white/brown feeding the coil and move that from the fusebox to the white terminal on the relay.
Paul Hunt 2

Mark-

I'm confused about your use of the green wire. For cars fitted with ignition relays, green wires don't have power unless pin W1 on the relay is energized allowing the brown wire to connect to the white/brown wire which supplies power to the green circuit. What is the power source of the green wire you used?
Kimberly

Paul Hunt-

I don't understand the difference between the switched power source from the relay and the switched power source from the ignition switch.
With the U.S. wiring, the white wire from the ignition switch energizes the winding in the relay, closing a switch in the relay, allowing power to the white/brown wire from the brown wire to power the coil.
The U.K. version you described above has the coil powered by the white wire from the ignition switch directly, causing a higher power draw through the ignition switch. How does the U.K. version stop the electrons from flowing in reverse? Is it that the U.S. version requires less of a power draw on the white wire circuit to operate the relay, allowing the brown and white/brown to stay connected rather than having the white wire supply power to the coil?
Kimberly

Kimberly,
Slight correction. I wrote the last message without my diagram in from of me. The whie wire that should be cut is the one between the ignition switch and the inertia switch. Feed the green (Any green available)to the inertia switch side of the wire you just cut. This way there is no direct feed from the alternator to the ignition relay. The green takes care of powering the fuel pump, the OD circuit, and provides the current needed to balance the Alt Warning Light which will still work properly. You can also use the other side of the cut white as a switched power for something else, but I would keep that a low current item as the power has to come through the ingition switch.

As far as Paul's methode, I would not have gone that way, but looking into it further I now understand where he is coming from, both in the US and UK schematics, and it certainly appears to be a viable option.

Mark R.
Mark Rotsky

Kimberley - It's not the reverse current that's the issue, but the components involved. All MGBs exhibit reverse current when the engine is switched off, probably all Lucas systems, and probably many others as well. It's what causes the ignition warning light to glow, flicker then die after you have turned off the ignition and as the engine is slowing down to a stop.

Cars without an ignition relay also have current flowing in reverse through the light when the ignition switch is turned off but the engine is still spinning. This flows thought the coil and all the other components hanging off the white circuit, but because of the resistance of the light is insufficient to maintain an HT spark. With North American ignition relay cars this reverse current only flows through the relay as all the other loads are powered from the relay contacts, and the current is sufficient to keep the relay operated.

When the coil is connected in parallel with the relay it restores things to virtually where they were beforehand, i.e. you still get the reverse current, but the low resistance of the coil has the effect of shunting nearly all the available current away from the relay so it releases as it should.

The alternative is to move the ignition warning light from the relay and switch junction, to the output side of the relay, and that removes all current from the relay when the switch is turned off which puts things *exactly* back to where they were on cars without a relay. As I say either should be sufficient to release the relay, but for some reason the factory did both.

Whilst the diode frig will work it removes one of the diagnostic functions of the warning light. If you are driving along and suddenly lose power first you should look at the tach and if that is still registering you know you have an HT or fuel problem. If the tach has dropped to zero you look at the warning light. If that is glowing you know you have lost the 12 supply from the ignition switch. But if the tach drops and the light remains extinguished you know you have a break in the ignition LT circuit i.e. coil 12v feed, coil, points, or any connections between them, and before the car has even come to a halt you know where to start looking. That is why I don't like the diode frig.
Paul Hunt 2

Kimberly,

Had the same problem. A "sneak" current develops throught the ignition lamp circuit. Cut the brown/yellow wire from the alternator and install a blocking diode in the circuit.

This will solve the problem.

Mark.
M Mallaby

It's not a 'sneak' current and it doesn't develop, it's there all the time on all systems of this type.
Paul Hunt 2

you have a green wire (the second green wire at the fuse block) feeding the white and red wire to the tach, the diode is probably shot, (the diod is cylndirical,and about an inch long located beside and just under the glove box) it is 100w 15?amp you can get one at radio shack. as a safety i also installed a single pole switch between the diod and the green wire.
j daniels

*That* diode when short circuit causes the engine to *crank* continuously, unless you have the presence of mind to drop the handbrake. Different problem. And it connects green/orange to the white/red, and that wires goes to the starter relay (hence the cranking), not the tach.
Paul Hunt 2

PS. And the pukka diodes are still available from Pektron http://www.pektron.com/
Paul Hunt 2

This thread was discussed between 22/07/2007 and 02/08/2007

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