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Thermal Vacuum Switch 1972 Cutlass 350 4bbl.

Old June 11th, 2019, 05:42 AM
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Thermal Vacuum Switch 1972 Cutlass 350 4bbl.

Does this switch need to be hooked up. It has the two connections on the top of the can. When hooked up car shifts hard and idles poorly. Will it do any harm to the coolant temperature if left off? Thanks
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Old June 11th, 2019, 06:12 AM
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That switches between ported and manual vacuum to the distributor vacuum advance. The OEMs switch to ported vacuum for improved idle emissions in the early 70's/late 60's, but that could aggravate overheating. The switch throws back to manifold vacuum in certain situations (high temp being the most important) to help cool down the engine. The earlier switches were temp only - three vacuum ports on a temp sensor. The later ones like you have add a switch input from the trans. That switch - in good condition and working order - can be worth a fair bit to folks doing concours restorations. A lot of them got thrown away (including mine!) a long time ago and they're not reproduced.

You can experiment to see whether your car prefers manifold or ported vacuum. Just run the vacuum line from the distributor to one, then the other, and disconnect all the other stuff in that set. You don't specifically need that switch.

Just search around for "ported vs manifold vacuum advance". There's tons of info. Most of it is just opinions. Some of it is even correct.
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Old June 11th, 2019, 09:06 AM
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You should be fine bypassing that switch and connecting the distributor vacuum advance to either ported vacuum or manifold vacuum. I did the same as oddball and removed the switch, but I tossed it into my tool box, where it's been since the 1980s.

As said, it is an early emissions control device: The electrical connections go to the trans and allow vacuum advance only when in third gear. The thermal portion of the switch allows ported vacuum advance only after the engine is at operating temp, then during an overheating event it switches to manifold vacuum to assist cooling the engine.
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