[jpg69bird]

I am trying to find the so called "Distributor Vacuum Advance Hot Override Protection" switch. The parts I find when searching look nothing like the part. This is the one installed in the passenger head.

Does anyone have the real part number or a better search term to find the right part? All that shows up on my search is the distributor vacuum advance or a later model electronic version.

It's for a 76 455 4 Speed.

[Kenth]

Try this:

https://www.rockauto.com/en/moreinfo...t=4604&jsn=475

HTH

[Schurkey]

Distributor thermovacuum switch.

Most folks disable the thing, and re-route the vacuum hose so that the vacuum advance gets manifold vacuum all the time, instead of only when the engine is overly warm.

[Kenth]

You will have vacuum to the vacuum advance using the TVS while driving (off-idle) at normal engine temperatures.

[Schurkey]

Yes, I could have been more clear. The OEM vacuum supply provides vacuum to the vacuum advance above idle and off-idle--i.e., ported vacuum. When the engine is overly-warm, it also provides vacuum to the vacuum advance at idle.

Re-routing the hose from manifold vacuum direct to the vacuum advance, bypassing the Thermo-Vacuum Switch (TVS) would give you operation of the vacuum advance all the time. Probably have to reduce the throttle opening to maintain an appropriate idle speed.

Bypassing the TVS is more important when the vacuum advance is also limited by the Transmission Controlled Spark vacuum solenoid, where you'd only have vacuum advance in high gear, and with the throttle part-way open. No vacuum advance in P, R, N, 2, or 1; and no advance at idle or just off-idle.

[Kenth]

I have found vehicles using the TCS do not have or need a TVS since they uses an electric temperature sensor as a "hot temperature overide device" controlling the ignition vacuum advance.

Of course the TCS is often bypassed too....

[Cliff R]

I've lost count of how many times I've had 1970 Q-jets sent here to add ported vacuum to them. Folks dealing with TSC are just worn slam out trying to get all that stuff to work and find the parts in those systems in useable condition. I remember back in those days most folks amputated all that stuff off those engines before they were two months old, good bad or otherwise.

HIC isn't needed much these days either, at least the way most of these vehicles are driven. The vast majority of these cars are nice weather only, half a dozen or so blasts around the block during the warm weather, and an ocassional car show. Sitting in a traffic jam on your way to work or stuck in stopped traffic in the middle of a hot August day just seldom happens so the devices intalled by the factory for those scenarios just aren't needed much.

Personally I like to see ALL the emission and other engine function related devices the factory installed in place. Looks a LOT better than a bunch of vacuum caps or even worse small rubber vacuum lines with screws, bolts or golf tees in them. When I work on those cars here I will typically re-run all the vacuum hoses but block most of them off internally with steel check balls. I only leave the ones working that need to be, and on some of these sort of set-ups very carefully bypass or re-route to provide vacuum to the items that need it for best function, like PCV, power brakes, vacuum modulator, inside vacuum controlled vents, and the distributor VA...........

[jpg69bird]

Thanks for all of the help. I am also one who doesn't like seeing vacuum caps all over the engine bay. Always looks like something is missing, and makes you question whether the engine is running properly.

I'll be picking up the Rockauto part and hopefully all gets working again.

[25stevem]

Nothing could be more informative to having a low compression motor not over heat at low speed / light throttle or idle conditions then the factory engineers seeing a need to activate the vacuum advance with this switch!

[mgarblik]

Originally Posted by Cliff R

I've lost count of how many times I've had 1970 Q-jets sent here to add ported vacuum to them. Folks dealing with TSC are just worn slam out trying to get all that stuff to work and find the parts in those systems in useable condition. I remember back in those days most folks amputated all that stuff off those engines before they were two months old, good bad or otherwise.

HIC isn't needed much these days either, at least the way most of these vehicles are driven. The vast majority of these cars are nice weather only, half a dozen or so blasts around the block during the warm weather, and an ocassional car show. Sitting in a traffic jam on your way to work or stuck in stopped traffic in the middle of a hot August day just seldom happens so the devices intalled by the factory for those scenarios just aren't needed much.

Personally I like to see ALL the emission and other engine function related devices the factory installed in place. Looks a LOT better than a bunch of vacuum caps or even worse small rubber vacuum lines with screws, bolts or golf tees in them. When I work on those cars here I will typically re-run all the vacuum hoses but block most of them off internally with steel check balls. I only leave the ones working that need to be, and on some of these sort of set-ups very carefully bypass or re-route to provide vacuum to the items that need it for best function, like PCV, power brakes, vacuum modulator, inside vacuum controlled vents, and the distributor VA...........

Absolutely agree Cliff. I worked at an independent garage and a Pontiac dealer during the height of the vacuum control and electro-vacuum control era. I made it my mission to understand all the systems, how they worked, when and why. Then the next step was to experiment and use logic to make the cars run better. Early EGR systems had the most negative effect on how the cars ran and felt, light and moderate throttle. At the shop and dealer we were in the job of satisfying customers. That usually meant, disabling or modifying early systems with BB's inside hoses, restrictor plates under EGR valves, and other little tweaks to make the cars run and drive better. We always kept 100% stock appearance and gave a hand written piece of paper to the owner of what was done. Made allot of people happy improving those mid 70"s to late 80's lousy running engines.