View Full Version : help with solenoid under carbon canister

Rob DeScherer
04-13-2009, 07:23 AM
I need some help understanding the function of the solenoid under the carbon canister in my 1974 TR6. Is it supposed to be open or closed at start up, and what should it be once the car is at running temperature? Can it be circumvented?

Thanks for your help in advance.


04-13-2009, 09:53 AM
That thing is an anti run-on valve (ARV).
What it is supposed to do is open when the ignition key is turned to the "OFF" position. When it opens, the vacuum that remains in the intake manifold "passes" thru the TBV, into the carbon canister, thru the hose that leads to the float chamber vents (on the left side of your carbs near the air filter housing). When that vacuum hits the float chamber it is supposed to prevent any fuel from continuing to be drawn up the jet that may cause run-on.
The electrical circuitry is a little complicated, too.
There is an electromagnet that opens the valve and as long as the key is in the "OFF" there is a hot wire going to it. BUT it is the oil pressure sensor sending unit that supplies the ground for that circuit.
Picture this. When the engine is running, the key is in the "ON" position and you have oil pressure, so the TBV circuit has a ground but no hot.
Then you turn the engine "OFF". Now you have a hot and you will have a ground only as long as the oil pressure sensor, senses residual oil pressure. When that dops, the ground is lost, however one of the wires on the TBV will remain hot as long as the key is in the "OFF" position.
Many people, including myself, have found no need for an ARV and removed it.
If you decide to, you will need to cap the nipple of the banjo fitting on your intake manifold that held the vacuum line to the ARV.
But, DO NOT cap the nipples on the carb. In addition to being used as part of the ARV plumbing, it is a vent for the float chamber when the engine is idling.
It's ok just to remove the hose and leave the nipples exposed and open to the atmosphere.

04-13-2009, 10:26 AM
:iagree: Just wanted to emphasize the point that the ARS should only be activated during the short period between when the key is turned off, and the engine stops turning.

It was added because engines with vacuum retard tend to "run on", that is, keep firing after the key is turned off. In addition to being disconcerting and a bit dangerous, run-on is bad for the engine. The ARS function can easily be replaced by developing the habit of leaving it in gear and letting out the clutch just as the engine should die (keeping your foot firmly on the brake at the same time, in case it does try to runon). Or if you've removed or disabled the vacuum retard, then the ARS should no longer be needed.

Rob DeScherer
04-13-2009, 05:50 PM
Randall and poolboy --

Thanks for your help. I finally know what it is and how to deal with it.