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MGB Ignition Light and Fuel Pump Won't Quite!

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Silas Brown

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I am struggling with a 1979 MGB that has not been driven for at least 8 years. I replaced the ignition switch due to a wiring fire within 3 minutes of buying the car! I also replaced a wire that had melted its way across the width of the wiring loom. After doing that, everything except the gas gauge seemed to work as it should. After a few hours of short runs and city driving, it refused to start. In an effort to eliminate causes of the problem, I checked the connections to all of the wiring connections under the driver's side of the dash. Nothing changed. While i was seated in the driver's seat, my 35 year old son began wiggling wires on the right, inner fender. Something changed! I tracked down a loose connection on the Ignition Relay and replaced the connection. Now, the gas gauge even works.

With this small success, came a big puzzle! The car starts as it should. The "Ignition" light (and "Brake" light, if brake is set) both come on when the key is turned on. They both go out once the engine starts. Things seem great until I turn the key to the off position. In the "OFF" position, the engine stops as expected but the "Ignition" light AND THE FUEL PUMP continue to operate! They will stop if I pull either the black or double with lead from the Ignition Relay. Does this indicate the relay is stuck or does this indicate that power is getting into the circuit from some place it should not?

Looking forward to your help!

Silas
 
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Could be a stuck relay, or something is still shorted as a result of the fire. I think you need to do two things: take apart the harness where the wire melted, and be sure nothing else is damaged and leaving two wires touching where they shouldn't. If that doesn't fix it, the best thing is to use a voltmeter or test lamp, along with a wiring diagram, to trace through the ignition circuit and isolate the problem. That will tell you if the switch is really turning off, for example, or if the relay is sticking.
 
I used my VOM to check the relay and am not sure what I learned. Testing the brown and white/w/brown leads I learned that they show continuity when connected to the relay and that each shows 12+ volts when disconnected but tested with ground. I tested the sides of the relay switch/break and found that the connection was open when there was no power and connected when power was applied via turning on the key. None of this addressed my problem of the "Ignition" light staying lit and the fuel pump active after the key was turned off. I did find that if I disconnected any of the four wires on the relay, the pump and light quit! Currently, I have left the ground lead off of the relay and everything seems to function as it should. "Ignition" light and fuel pump come on with the key. Light goes off after starting. Fuel pump pumps fuel. Both stop, along with the engine, when the key is turned off. It seems to me that I am running the car without an ignition relay. Should this work?
 
that shouldn't work - though it may be grounding elsewhere. what number on the relay do you have what wires hooked up to? and what colour are they?
 
86 Blk
30/51 Brn
85 Wht
87 WB

If all 4 are connected the Ignition light and fuel pump don't turn off with the key. If I pull any wire they stop. Right now I have the WB off and the car starts and tuns off a it is supposed to!

I did open the harness completely and replaced the one wire that was fried (Wht with green from ignition). New ignition switch installed.

I tested the relay by connecting my VOM set on ohms to posts 30/51 & 87. I then turned the key and it played "connected tone" until I turned the key off! Seems like what I would expect from a good relay
 
86 and 85 are the coil terminals of the relay. Check to be sure you are getting voltage at those terminals when you turn on the ign switch and it goes away when you turn it off.

If that doesn't happen, your relay is likely OK and the problem is miswiring elsewhere.

If that's OK, disconnect the other two terminals and make a resistance measurement at those terminals. You should get zero ohms when the power to the coil is on and infinite when it's off. You should hear the relay clicking, too, when you turn the ignition on and off. If you have zero ohms in both states, the relay is sticking.

That should get you started.
 
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