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MG MGB Technical - Installed ignition relay, engine will not stop

I installed an ignition relay in my 74 B to bypass a small voltage drop in the white electrical circuit. I used a generic Bosch type four terminal relay. It worked fine until I turned the ignition off and the engine kept running.

I have done some testing and determined the alternator charging brown/yellow circuit is preventing the relay coil from de-energizing. It feeds about 7 volts to the to the relay coil. If I disconnect the brown/yellow wire at the alternator the engine shuts down normally.

I thought the alternator may be causing the problem and replaced it with my spare. Engine continued to run when ignition was turned off. One alternator is a Lucas 17ACR, the other a 18ACR. With engine running on a 40 degree day both alternators have around 14.10 volts on the two wide spades. Both alternators have around 14.23 volts on the small spade. All measurements made with the B/Y wire connected. Both alternators keep the battery charged.

The relay coil measures 72.9 ohms between terminals 85 and 86. I measured two new spare 2.2 watt bulbs, one was around 8 ohms the other around 11 ohms. I assume the one in the dash is similar, I don't want to remove it unless necessary to solve a problem.

I have a second fuse box in my car. The white ignition feeds through one fuse. The other three fuses have brown wires from the starter solenoid feeding them, one fuse feeds lights through relays, the other feeds an electric cooling fan through another relay. This fuse box and associated wiring has been in the car several years and everything works as intended.

On the ignition relay, I removed the ignition white wire from the second fuse box and connected it to terminal 85. Terminal 86 is grounded. I connected a brown wire from the second fuse box to relay terminal 30. Terminal 87 was hooked to the second fuse box to replace the white wire removed earlier. Wiring diagram of relay is shown.

I removed the relay and the car runs fine and I don't have to use an ignition relay but I don't understand why it doesn't work, it's wired about the same as the ignition relay in later cars. One difference is later cars do not run the fuel pump from the relay, I did but I don't see why that would change anything.

Clifton




Clifton Gordon

Clifton. Paul Hunt may be the best one to answer this question. I do not know a lot about the subject. I do know, however, that the 77 model had the exact problem you describe and that Paul has posted a modification to the system to correct it. Should be in the archives.

My daughter's 77 has a diode in the brown/yellow line, which I found when I replaced the alternator wiring spade push on connectors with the proper Lucas push on plug. When I removed the diode, the engine continued to run, just as your's does. Replaced the diode and the problem went away.

Les
Les Bengtson

I believe that Paul has the very thing Les is talking about in his web site in the electrical section. Paul's sit is Pages of Bee and Vee at: http://www.mgb-stuff.org.uk/ Click on Spanners once there and then on electrical. Cheers - Dave
David DuBois

Congratulations, you have re-invented the original design defect devised by Abingdon on 77 and later cars :o)

You will either have to move the white coming from the ignition warning light from being on the relay winding side to the relay contact side, or move the white going to the coil from the contact side to the winding side. Abingdon did both on the UK cars (where unlike the American cars with the anti-runon valve it was imemdiately discovered) whereas in theory only one of those changes is needed. Physically moving the coil wire should be easier as the warning light wire presently joins the ignition white by the steering column, although it is trickier to identify which of the two whites.

I give you two choices above, but there is actually a third which involves putting a diode in the warning light circuit such that it allows current to flow from the ignition switch to the warning light but not back the other way. But that removes one of the diagnostic functions of the light and is really tacky. IMO.
Paul Hunt 2

Hi Paul, that puts me only about 20 years behind.
I'll work with your suggested wiring chances but will not even consider the diode, the warning has helped me previously and I wouldn't want to chane it's operation. Have good day and thanks for helping straighten out the old man.

Clifton
Clifton Gordon

I need to start my day over. Above comments should have read;
I'll work with your suggested wiring changes but will not even consider the diode, the warning light has helped me previously and I wouldn't want to change it's operation"

Clifton
Clifton Gordon

Paul,

I am no clear by what you mean about the diode removing 'one of the diagnostic functions'. What is the added effct of putting a diode in if this solves the problem of the warning light circuit latching the relay to on? The ignition relay is a task I am looking to do but have also seen reference in 'MGB Electrical Systems' to the need for a diode but without any mention of a disadvantage to that. I'd welcome clarification on this.

Thanks,

David Booker.
David Booker

David, Here is Paul's web site comment on installing a diode in the charging circuit.

"For heavens sake don't do what someone said and fit a diode to 'correct' i.e. hide this problem. If you do you may well have stopped the warning light from glowing dimly at night, but you have also stopped it telling you of complete charge failure. If you want to do that you might just as well unscrew and throw away the warning light bulb and save the hassle of fitting the diode!"

More information is here.
http://www.mgb-stuff.org.uk/electricsframe.htm

Clifton

Clifton Gordon

In the past I've seen two recommendations to fit a diode in the warning light circuit, and one is to prevent it glowing dimly at night as Clifton mentions. But in order to stop the ignition relay latching the diode must be inserted the other way round. This doesn't stop the warning light glowing when the charge has failed, but it does block the less important but still significant (in my view) diagnostic that causes it to glow when you have lost voltage to the ignition.

If the engine suddenly stops producing power when you are driving along, the first thing you should do is look at the tach, and the second is to look at the warning light. That is before dipping the clutch, knocking it out of gear, switching off the ignition, and before coming to rest. If the tach is still registering you know the problem is HT or fuel related. If it isn't then you know it is ignition. If the warning light is glowing as well you know you have lost the 12v supply from the ignition switch. If it isn't glowing you know it is something to do with the coil or points. In other words before the car even comes to a standstill you know the likely area of the problem. Fitting a diode to prevent the ignition relay latching removes this latter diagnostic.

There are lots of ways of preventing the ignition relay latching, simply removing the warning light bulb is one, but that would not be a sensible way of going about it. Much better to do what the factory did on UK cars and alter the wiring to remove the problem at source and still be left with the diagnostics.
Paul Hunt

Thanks for that - now I understand! Thanks by the way for the great 'Bee & Vee' website. It has helped with several issues.

David B
David Booker

My pleasure :o)
Paul Hunt 2

This thread was discussed between 21/01/2008 and 26/01/2008

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