[GoreMaker]

Can someone please explain to me how vacuum is routed through this particular vacuum switch? And does anyone know what temperature it switches at?

https://www.classicindustries.com/pr...rts/PY375.html

[25stevem]

First off is the distributor your running a stock one the the vacuum hose nipple on each side of the can?

[Schurkey]

Originally Posted by GoreMaker

Can someone please explain to me how vacuum is routed through this particular vacuum switch? And does anyone know what temperature it switches at?

https://www.classicindustries.com/pr...rts/PY375.html

Grab a service manual for the appropriate application, and look up the vacuum-hose routing.

[gto4evr]

no idea on the temp, I'd think somewhere in the 130-150 range it would be open.

there were bulletins later after 68 that had the dealers disconnect the retard portion of the distributor advance and block the ports on the valve. unless you're going for originality, this valve is pretty much useless, it's an emissions thing. it also has no where to go if you don't have the dual port vac advance/retard unit on your distributor.

they discontinued the retard side because it was causing drivability issues. backfires on hard decelerations if I recall.

[OG68]

Originally Posted by GoreMaker

Can someone please explain to me how vacuum is routed through this particular vacuum switch? And does anyone know what temperature it switches at?

https://www.classicindustries.com/pr...rts/PY375.html

The valve switches from a ported vacuum source to a manifold vacuum source to the distributor advance when the water temperature reaches 230*. This speeds up the idle speed increasing air flow through the radiator.

From an unknown source;

Here is the text of an article that AMES FAX'D me. Looks like it was from Pontiac Enthusiast magazine (Vol. I No. 2) and was written by Peter Serio:

Back when emissions systems first appeared on cars, it was almost an instant reaction to open the hood and say, "Who needs this stuff to slow my car down? Let's rip it out, plug the holes, and go faster!" Years later, it may become mandatory for vehicles manufactured in the early days of emissions systems to have fully functional systems in place. In addition, in concours competition, a deciding factor can be a detailed engine compartment with a correct and operational set of emissions parts. In addition, your car's drivability could depend on an understanding of how the emissions gear works. In the previous issue of Pontiac Enthusiast, we looked at the 1968-69 manual transmission vacuum advance valve. In the next issue, I will cover the transmission-controlled spark systems used from 1970-72. For now, it's time to turn to the TVS, or thermostatic vacuum switch.

The TVS-GM#3016754 is used on all 1968 V8's and on 1969 V8's with automatic transmissions. Also, several early production 1969 Ram Air III cars with manual transmissions were built using the TVS and the vacuum advance valve. The 1971 455 HO engine also used the TVS with either transmission.

In all applications, the TVS serves as a safety device to help prevent overheating. The switch is located at the front of the intake manifold, threaded into the coolant passage. There are positions inside the TVS related to coolant temperature. Standard vacuum flow is ported vacuum from the carburetor to the distributor vacuum advance. Whenever the engine coolant temperature rises above 230 degrees F, the TVS switches the distributor advance from ported to full manifold vacuum. This advances the timing about 20 degrees at idle, allowing the engine to cool down to normal operating temperature. After the engine cools, the system returns to ported vacuum.

If your car does not have the system hooked up properly, it could overheat on a hot day when you're stuck in traffic, which could reduce the life of your engine. In 1968 the fan shroud become standard for the GTO, whereas in earlier years it was an option on non-air cars. With the idle retarded in 1968 to reduce idle speed emissions, the switch's purpose was to advance the timing when necessary to allow the motor to cool down, to prevent pinging.

A manifold connector in the vacuum hose harness to the TVS is used to prevent the hoses from being installed improperly. Note that some of the vacuum hoses in the harness have color-coded stripes running on the supply lines to the TVS. The red-striped hose is manifold vacuum, and the ported vacuum is routed through a small steel pipe forward of the carburetor. The yellow hose (used in 1968 only) is the retard-at-idle-speed vacuum supply.

There were two different hose harness assemblies used, depending on the year of the car. In 1968 only, with the dual-port vacuum-advance unit attached to the distributor, the idle speed timing is retarded 10 degrees to reduce emissions. After the '68 models, all distributor-advance units were the standard single- hose-connection style. All the 68s use the 5-hose vacuum harness, while the 69 V8's and 71 455 HO make use of the 3-hose type. The two extra hoses on the 68- only harness are the idle-speed retard feature.

Apparently it did not take long for dealers to receive complaints that some of the '68 cars idled poorly, and Service Bulletins 68-T-2 (dated 10/16/67) and 68- T-2A (dated 1/4/68) were released. Models affected were the 68 Firebird, full- size, and Tempest/LeMans/GTO with the 2-barrel carb and automatic trans, plus full-size automatics with the 4-barrel. The complaint was that the second-to- first downshift could clunk badly due to the retarded timing. Manual- transmission cars were unaffected, since the idle speed was higher and you shift your own gears. The procedure outlined in the bulletins basically involves removing the two vacuum hoses that retard the timing at idle; shortening one and connecting it to the two switch holes in a U to keep dirt out of the holes; and readjusting the idle speed. If you have a 1968 2-barrel automatic or full-size 4- barrel automatic, these Service Bulletins would be a nice item to search for.

[GoreMaker]

Thank you! That's exactly what I needed to know.

Wow, 230f is way too high. I was looking for a PVS that switches from ported to manifold vacuum AND a pair of ports that go from closed to open at about 130f. Mostly so I can switch the distributor from ported to manifold vacuum once the engine's warmed up and still have a switched vacuum source in the same fitting. I've run out of water crossover ports

[Joe's Garage]

If you're going for the ultimate 'factory-correct' look, just run the vacuum hoses as in the diagram and insert a small 'bb' into each hose on one end.

Looks right and doesn't make your car run poorly!

[GoreMaker]

Nah, I'm just looking for a vacuum switch that does what I need it to. There's a surprising dearth of specs for different vacuum switches out there. Different colors that represent different switching temperatures depending on different numbers of ports... short of buying them all and testing each one, I just can't find specifically what I need

[Joe's Garage]

Maybe you could find one that switches or grounds at the temp you need and then wire it into a solenoid like the TCS solenoid to turn vacuum on or off ?

[GoreMaker]

That's a very good idea! But it's also part of the issue I've been having, figuring out what temperature they switch at. When looking on Summit Racing, Advance Auto or the SMP web sites, almost none of the PVSes they list include any specs at all.

However I just found that the NAPA web site DOES list the switching temperature of all their PVSes. Some of them appear to have 2 different switching temperatures for separate circuits on the same unit. They still don't list what each port does, but at least now I've got half the information I need. I found this switch:

https://www.napaonline.com/en/p/CRB229050

which appears to have one circuit that switches at 120f and another at 165f. That sounds just about perfect for my needs, so I ordered one and I'll confirm whether each circuit does what I need it to.

[Kenth]

Originally Posted by GoreMaker

Thank you! That's exactly what I needed to know.

Wow, 230f is way too high. I was looking for a PVS that switches from ported to manifold vacuum AND a pair of ports that go from closed to open at about 130f. Mostly so I can switch the distributor from ported to manifold vacuum once the engine's warmed up and still have a switched vacuum source in the same fitting. I've run out of water crossover ports

If the TVS is connected according to the manual, it operates only when idling. While driving, the ported source on carb acts just like, well, a ported source.

SMP PVS24 opens @ 125°F and SMP PVS51 opens @ 154°F

If you can use 3/8-18 threads SMP PVS7 opens @ 133°F.

FWIW