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MG MGB Technical - Anti Run On Valve

I have a barn-find 1974 MGBGT that I am trying to get back on the road. I had the car running this afternoon. I shut it down and did some tinkering on the right side of the engine. Now, when I try to restart, the ignition light comes on and the fuel pump starts running. Turn the switch to start and the ignition light continues to glow bright red and the fuel pump continues to run but nothing from the starter. Not even a click. Fifteen minutes earlier it was working fine. This happened once before and I had knocked a wire off of the starter relay. Looking for a loose wire, the only one I find is the slate gray with a pink tracer to the anti run on valve. The terminal is broken off of the valve. I don't see any way to repair the connection; and since I will probably de-smog the car eventually, I am not going to spend $65 to $70 for a new valve.

My first question is: Will this broken wire cause my problem?

Second question is: How do I fix it. According to Paul Hunts excellent wiring schematics, this wire only runs from the ignition switch, through the anti run on valve to an oil pressure switch and to ground.

What would happen if I removed the valve and tied the two wires togather?. The circuit would then run from the ignition switch to the oil pressure switch to ground.


A Sneed

Mine is gone, and it looks like the PO just clipped the wires.
Greg Bowman

Mack, I have had a rash of bad solenoids coming thru lately. Did you check the wires down at the starter solenoid for connection. Check the white with green tracer wire for poor connection. I believe that is the color of the wire there attaching to the solenoid.


Mack, I don't think the problem is in the anti run-on valve. Check for 12 volts on yellow/pink at the start relay with the ignition in the start position. If you have 12 volts check for 12volt on the brown wire on the relay. If bothe voltages are there the problem is in the relay or the witing going from the relay to the starter. If you don't have 12 volts on the YK wire you probably have a problem in the sequential seat belt system control unit. Try running a jumper wire from the terminal with the YK wire on the relay to the white/red wire at the ignition switch. If it starts OK with the jumper, find the control unit under the dash, as I recall it's on the firewall behind the console, you may have to remove the console screws and move the console out to access the area. You need to find the WR wire on terminal 1 of the plug and the YK wire on terminal 1. Tie those two wires together unless someone has already tied them. It should now start.

Clifton Gordon

Mack, The wire from the relay to the solenoid is white/Brown on the 74. I wish we had an edit capability, I really butchered my spelling in my previous post.

Clifton Gordon

As Clifton says the anti-runon wiring problem is nothing to do with the non-cranking. If you bypass the valve you will simply blow the fuse when you turn the ignition off with a running engine! Not that you will notice, it is an in-line fuse just for that circuit. Whilst the anti-runon valve is actually needed to stop then engine in 77 and later cars due to a wiring design error, it isn't for earlier cars, although you may get running-on of course. The change to unleaded in the UK, even on higher octane versions, made the running-on in my 73 roadster so bad I actually fabricated a system very like the North American system which stopped it completely. I tried an after-market valve but plumbed as suggested did nothing. En route to developing my own system I bought a new American valve just to see how the American system worked. I still have it unused in my garage ...

There are some drivers-seat diagnostics you can do with non-cranking. Normal cranking will drown out the solenoid clonking in, which will drown out the starter relay clicking in. So if you hear the solenoid clonk you know the problem is with its contacts, the motor brushes etc., or the connections between them. If you hear the relay click you know the problem is the relay contacts, the solenoid winding, or the contacts between them. No relay click is the ignition switch contacts, relay winding, or again connections between them, which on a US 74 includes the Sequential Seat Belt system aka Bum Detector that Clifton mentions.
Paul Hunt 2

Yes, I'd chime in on the relay, just front of the fuse box on the 74 I believe. Relatively common. Also, eyeball the solonoid white/brown and/or white/green spade connex: It could've come loose with your fiddling above. Reg. Vem
vem myers

Surprise, Surprise!! I went out to the car this morning; and eventhough they appeared to be firmly attached, I fiddled with the wires on the starter relay again. Hit the starter, and it fired right up. Last night, nothing, not even a click, and fiddling with the same wires had no effect.

You are all right. The anti run on valve was not the problem. I couldn't see how it could be but the broken connection was the only visible problem that occurred as a result of my fiddling in that area.

Again, thanks for the advice. And Paul, your wiring diagrams are excellent. Thanks for providing them.

A Sneed

This thread was discussed between 28/02/2006 and 01/03/2006

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