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MG MGB Technical - Run-on control valve

Can anyone tell me how the run-on control valve works on a 1979 MGB. Questions.
1. Is it normally energized while the car is running or does the valve energize when the oil pressure drops upon turning off the engine?
2. There are two connections at the top of the motor or solenoid. Does it make any difference which is + or _ ?
3. I presume that while the engine is running the lines between the cannisters and carb are lined up to take off fumes. Correct or not?

Dick Field

The Anti-Run-On Valve (BMC Part # 12H 4295, Moss Motors Part # 367-110) is fitted on the 1973 and later models. It is a solenoid-operated low-pressure 3-way air valve. This normally vents air from the absorption canister (side hose) to atmosphere (bottom hose) at all times except when actuated for the Anti-run-on mode momentarily after ignition switch-off. It also has a vacuum supply hose which is connected to the intake manifold. During Anti-run-on activation, the valve closes the atmospheric vent to induce a vacuum inside of the absorption canister. Vacuum in the canister then acts through the vent lines on the carburetor float chamber(s) to induce such a strong vacuum above the fuel inside of the float bowls that the fuel cannot exit the fuel jets when the ignition is switched off, thus preventing the car from running on. When the ignition is turned off, the ignition switch energizes both this solenoid-actuated Anti-Run-On valve in order to close it, and the oil pressure switch (BMC Part # BHA 5197, Moss Motors Part # 141-715) releases it after the engine has stopped and oil pressure has fallen. The Anti-Run-On valve is open when the engine is running, allowing fresh air to pass through and be pulled through the adsorption canister, clearing it of the vapors that have expanded into it from both the fuel tank and the carburetor float bowl chambers, then through the rocker arm cover and the tappet chest into the fuel induction system to be consumed inside of the combustion chambers.
Stephen Strange

The oil pressure switch is closed, supplying an earth to the valve, all the time the engine is running, and for short time after it stops (in most cases), and then it opens when the oil pressure drops towards zero. This works the other way round to those used to light the oil pressure warning light when there is no oil pressure. The ignition switch supplies 12v to the other side of the valve in any but the 'run' and 'start' positions. So turning the ignition off on a running engine operates the valve, and oil pressure dying away releases it. You can usually hear the valve click open a couple of seconds after the engine stops.

When you next turn on the ignition or start the engine 12v is removed from the valve, so when it starts, build oil pressure, and closes the oil pressure switch the valve can't operate.

All the hoses have to be connected and not blocked, and the canister not blocked, for the anti-runon system to work correctly. When the starter relay was added in 1977 there was a design error made in the wiring, which meant that when turning off the ignition switch you did *not* disconnect power from the ignition or fuel pump, it was the anti-runon valve that actually stopped the engine. If you get pretty-well any fault in the anti-runon system, the engine will continue to run normally for maybe several seconds or more, even though the valve may have operated.

UK cars had the relay but didn't have the anti-runon system, the fault was discovered, but only corrected on those cars. The modification was to move the white wire feeding the ignition warning light from the ignition switch/relay coil circuit onto the ignition relay contacts, as it was power being fed from the alternator though the light and keeping the ignition relay operated that was causing the problem. Subsequently they moved the ignition coil, fuel pump and various other circuits from the relay back to the ignition switch, but that was due to the relays tending to stick on from a completely separate mechanical problem.
PaulH Solihull

Stephen & Paul,

Thanks, this helps. I'm in the process of rewiring the car and putting the mechanical components back in after a complete teardown.

Dick Field

Glad to help. Since the OE hoses are now ancient, replace all of them with new fuel-resistant ones. They are inexpensive and readily available at any auto parts store, and replacing them now is well worth the small effort that is involved. Just be sure that all of your hose connections are tight and well-sealed. Any leakage (or kinkage) can cause the system to malfunction.
Stephen Strange


Thanks. Will do.

Dick Field

This thread was discussed between 11/11/2012 and 13/11/2012

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