OLD SKL 69

Hi all. Been going over my 71 Cutlass Supreme with a 350. I was not getting any vacuum advance so I started troubleshooting. Found the vacuum cannister was fine but it was not receiving a signal from a switch:

Located right behind the oil fill tube:
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I know it was put there for emissions purposes. Since I do not have a service manual yet for the car, can anyone tell me when this switch is supposed to let vacuum go to the advance cannister? Certain rpm or when it reaches normal operating temperature? Thanks.

svnt442

It's a temp control switch. It allows vacuum to flow when the engine warms up enough. You can bypass it if you'd like or just replace it.

OLD SKL 69

Thanks for the reply. I am not sure if a replacement is available, so I am just going to bypass it for the time being.

Lady72nRob71

The ol' transmission controlled spark switch...
A search on this will pull several good posts.

The general purpose was to control amount of advance when transmission was in top gear for emission control.
Normally the source was ported vacuum from the carb. The switch also provided full manifold vac to the disty when coolant temps exceed 210 or so.
The electrical connection is switched by hydraulic pressure from the tranny.

Mine was bypassed when I bought the car. The disty was connected right to manifold vacuum and the ports on that switch were capped.

joe_padavano

What he said. TCS was an early emissions control system to reduce NOx. You won't see any vacuum advance unless 1) the trans is in third gear or 2) the engine is overheating. So far, it appears to be working correctly.

One way to test the thermal vacuum switch is to ground the wire that goes to the trans. This makes the TVS think you are in third gear and should allow vacuum at the distributor.

OLD SKL 69

Thanks for the explanation guys. I will check it to see if works when I ground the wire. However, to me it feels like the car needs more vacuum advance at idle and off idle. Yes I can advance initial timing, but it is already at 14. I am going to hook it up directly and bypass the tcs to see if that makes a big difference or not.

Now... if I could just get the weather to improve so I can check it out.