Thermostatic Vacuum Switch (TVS)

Q: Thermostatic Vacuum Switch (TVS)

My 1968 400 Firebird has what I believe to be a vacuum switch on the intake manifold and there is at present nothing connected to it. Should there be? One would think there should. I have no idea what to hook up to it as all of my vacuum lines seem to be accounted for. Any helpful suggestions would be greatly appreciated.

A: Yes, it should be there. Should it be hooked up? If you want it totally stock yes. Is it necessary? Probably not, especially if someone has replaced the vacuum advance unit (regulating vacuum to the distributor) on your distributor and you don’t have a dual port unit anymore.

If you did want to hook it up, there should be five ports and they go something like this, I think:

Inputs: Carb Advance, Carb Retard, Manifold Vacuum
Outputs: Distributor Advance, Distributor Retard

If you want to hook it up and be sure, get the vacuum line kit from AMES and ask them for directions (if they don’t come with it).

A: It is what’s known as a thermovacuum switch.When the engine coolant temp is below the temperature rating of the switch certain ports are open to each other allowing vacuum to flow between them.Once the temp rating is exceeded the thermal pellet raises a rod inside & switches the vacuum to different port(s).This switch works very much like a thermostat.They are used for many different purposes on cars,in some cases they delay operation of some device (an EGR valve for example) until operating temperature is reached (to avoid stumbling & “sag”),other times they are used to operate a device only until operating temperature is reached (example- a vacuum operated heat-riser valve).

A: Here is the text of an article that AME’s FAX’d me. Looks like it was from Pontiac Enthusiast magazine (Vol. 1 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 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 1968 models, all distributor-advance units were the standard single-hose-connection style. All the ’68s use the 5-hose vacuum harness, while the 1969 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 1968 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 1968 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.

A:

  ________
 /        \
|  (1)     \
|           \
|        (4) |
|            |
| (2)        |
|            |
|        (5) |
|            /
| (3)       /
\_________ /

1. (1) To vacuum port on front of carb (steel routed line)

2. (2) To “Tee” at rear of carb

3. (3) To distributor advance

4. (4 and 5) are looped together at switch.

A:
From the 1968 Diagnostic Manual:
Fig 6D-9 Components of Controlled Combustion System
[pdfjs-viewer url=/files/FAQ/docs/6D-9.pdf viewer_height=800px fullscreen=true download=false print=true]
Fig 6D-10 Distributor Vacuum Layout V-8 2 Bbl.
[pdfjs-viewer url=/files/FAQ/docs/6D-10.pdf viewer_height=800px fullscreen=true download=false print=true]
Fig 6D-11 Distributor Vacuum Layout V-8 4 Bbl.
[pdfjs-viewer url=/files/FAQ/docs/6D-11.pdf viewer_height=800px fullscreen=true download=false print=true]
Fig 6D-12 Vacuum Routing Chart
[pdfjs-viewer url=/files/FAQ/docs/6D-12.pdf viewer_height=800px fullscreen=true download=false print=true]
Fig 6D-13 Vacuum Routing Chart
[pdfjs-viewer url=/files/FAQ/docs/6D-13.pdf viewer_height=800px fullscreen=true download=false print=true]
Fig 6D-14 Vacuum Routing Chart
[pdfjs-viewer url=/files/FAQ/docs/6D-14.pdf viewer_height=800px fullscreen=true download=false print=true]

A: Jim, a list member from the First Generation Firebird-L Mailing List, did research on this compairing all the documents available and put together his findings:
[pdfjs-viewer url=/files/FAQ/docs/TVS/68VacuumDialog.pdf viewer_height=800px fullscreen=true download=false print=true]

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