Schooling needed - Thermal Vacuum Switch

[chalet2506]

I'm trying to clean some things up on the 400sb on my 76 k5. Right now I'm replacing vacuum lines and capping off the ports I don't need(cruise control for example).

Looking at the switch on the t-stat, I don't understand how it works or exactly what it does. It has 3 ports.

1. Line to the vacuum advance on the dizzy.
2. Line from the front driver side of the carb.
3. Line from the passenger side of the carb that is tee'd with a vac line to the air cleaner housing.

What would make sense to me is that the #2 line supplies vacuum to the thermo switch, as the motor heats up the switch opens and supplies vacuum to the dizzy(advancing the ignition, not sure why) and the whatever that is in the air cleaner housing(emissions related I guess). I can't understand the T in the #3 line. If its t'd to a line on the carb, in effect there's always vacuum to the air cleaner, so whats the point of connecting it to the thermal switch?

Can somebody explain this so I can put it back together right? Might be better to ignore my ramblings and start from scratch. thanks.

 

[dyeager535]

T in the #3 line doesn't make sense to me either.

Does that truck have THERMAC? (valve on the air cleaner for hot air) If it does, the piece in the air cleaner should be the temp switch for that, which should have vacuum all the time, and the switch simply closes/opens based on engine temp. And if that's how yours is setup, goes right back to why would they use a "dead end" T'd into the TVS?

The supply line is manifold vacuum I assume? Very odd how GM played with emissions and timing. Low initial timing helps emissions, but hurts idle, so perhaps that was an early way around that issue, no vac advance during cold engine operations, once it warmed up then it saw higher timing at idle.

 

[Big Ray]

I cleaned up my 86. I wired my thermac valve open (valve in the air cleaner). And I only have the dizzy advance hooked up.

I took a bunch of hose and "dead ended" everything under the air cleaner, strictly for visual inspection... That way it at least "looks" right.

Everything else is capped or plugged.

 

[RaisedK5]

If it works like the one on my old buick, its full manifold vacume to the air cleaner, and on eside of the thermo switch. the line off the front of the carb should be ported vacume. In normal operation ported vacume goes to the Vac advance, and full vacume goes only to the aircleaner. When the engine gets "too" hot, the switch opens and gives full manifold vac to the vac advance on the dist. This in effect raises idle and spins the fan faster in an effort to cool the engine. Thats if it works the same way.

 

[chalet2506]

It does have a thermac. Found this thread on a hotrod forum and am trying to digest it.

http://www.hotrodders.com/forum/port-full-time-vacuum-23169.html

A p.o. converted my engine from HEI back to a points dizzy. I've got a new HEI I'm going to install, basically reconverting it. Reading through it appears the different ignitions used different vacuum. Hmmmm.

 

[chalet2506]

If it works like the one on my old buick, its full manifold vacume to the air cleaner, and on eside of the thermo switch. the line off the front of the carb should be ported vacume. In normal operation ported vacume goes to the Vac advance, and full vacume goes only to the aircleaner. When the engine gets "too" hot, the switch opens and gives full manifold vac to the vac advance on the dist. This in effect raises idle and spins the fan faster in an effort to cool the engine. Thats if it works the same way.

So if I'm reading this right, the switch has 2 inlets, one outlet and I should connect it like this.

1. Ported vac inlet off the front of the carb to the switch.

2.manifold vac off the rear tee out of the intake manifold to the switch.

3.one vac hose from the switch to the dizzy.

4. Ported vac from the side of the carb direct to the THERMAC.


Would it be better maybe to just bypass the thermo switch and run a vac line directly from the t off the intake manifold direct to the dizzy, one off the carb to the thermac, and cap everything else?

Edit. Apparently not, Checked the instructions on the HEI I got from Summit (part# 850001R) and they say to hook to a ported vac.

 

[dyeager535]

If you decide to go that route (bypassing things), hook it up both ways, and see which your vehicle likes, it's that simple.

I have yet to see an engine that prefers less initial advance over more, based on idle quality.

All that stuff is for emissions. I'm not a fan of removing things GM put on, but the TVS and everything else that switches, etc., if not working right, is a real waste of resources for little "green" gain when looked at per the individual vehicle.

 

[chalet2506]

Although this is new to me, I can find no reason to use the thermo switch. If I run the thermac to a constant vac port, it should stay open all the time. I'd think cooler air to the carb would be better since I'm in Texas. Also, no switch to go bad or cause problems between the timed port and the dizzy sounds like one less thing to go wrong.

Thanks for the help guys. Somebody link this thread if I can't figure out why the truck won't idle in January.

 

[dyeager535]

If you've got the switch in the air cleaner housing (my 80's rigs did) then you could run manifold vacuum to it, just in case it ever did get cold, or went somewhere it was. Certainly wouldn't hurt, and they seem to be pretty reliable.