# Thermo controlled vacuum switch and vacuum advance



## TxGTO69 (Dec 2, 2020)

Wondering if someone can shed some insight on the thermo vacuum switch, I am trying to determine if mine is faulty. I installed electronic ignition (new distributor and coil) and I was setting initial timing (with vacuum line disconnected from the distributor vacuum advance of course). Got my timing set, but when I connected the vacuum line, I am not getting any vacuum advance? I only have three lines for the thermo switch, one to the back of the carb, one to the front of the carb, and one to the distributor vacuum advance. Im not getting any vacuum advance when the engine is cold or when the engine is up to temp. I have to connect to vacuum advance canister directly to the back of the carb to get any advance. Any insight would be helpful. This is on a stock YS 400 with a quadrajet carb.


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## O52 (Jan 27, 2019)

Two sources of vacuum, ported and manifold. The engine under the 69 emissions system is tuned to operate on ported vacuum. When an overheat condition exists (230* and above) the thermo valve switches to manifold vacuum which advances the timing, increasing engine and fan speed when at idle. 

The Ported vacuum source on an original 69 carb is on the middle right front of the carb, that line then connects to the middle left nipple of the thermo switch. 
The Manifold vacuum source comes from the rear carb baseplate fitting that provides vacuum to the power brakes and auto trans modulator; that line connects to the lower left nipple of the switch.
The lower right nipple of the switch provides the sourced vacuum to the distributor advance.


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## TxGTO69 (Dec 2, 2020)

O52 said:


> Two sources of vacuum, ported and manifold. The engine under the 69 emissions system is tuned to operate on ported vacuum. When an overheat condition exists (230* and above) the thermo valve switches to manifold vacuum which advances the timing, increasing engine and fan speed when at idle.
> 
> The Ported vacuum source on an original 69 carb is on the middle right front of the carb, that line then connects to the middle left nipple of the thermo switch.
> The Manifold vacuum source comes from the rear carb baseplate fitting that provides vacuum to the power brakes and auto trans modulator; that line connects to the lower left nipple of the switch.
> ...


Ahhhhhh! So if I am understanding correctly, there wont be any vacuum advance until temp reaches 230*? Thats interesting 
Thank you for that, I did not find that information anywhere.


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## O52 (Jan 27, 2019)

There will be advance off idle when the ported vacuum comes into play.


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## O52 (Jan 27, 2019)

The service manual shows you how to replace stuff. The Diagnosis Manual (hard to find) explains the theory of operation and troubleshooting (diagnosis).


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## TxGTO69 (Dec 2, 2020)

O52 said:


> The service manual shows you how to replace stuff. The Diagnosis Manual (hard to find) explains the theory of operation and troubleshooting (diagnosis).


Gotcha. I could not find that explanation. Thanks again.


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## TxGTO69 (Dec 2, 2020)

O52 said:


> The service manual shows you how to replace stuff. The Diagnosis Manual (hard to find) explains the theory of operation and troubleshooting (diagnosis).



Is this what I would be looking for?









1968-1969 Pontiac Diagnosis Manual Tempest LeMans GTO Trans Am Firebird Shop | eBay


If you've got the shop manual for your 1968 or 1969 Pontiac, you've only got half the story. This book was written by GM as a companion to the shop manual and has many instructions you'd expect to find in the shop manual but don't. For each part of the car listed, you'll see a description of how...



www.ebay.com


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## O52 (Jan 27, 2019)

Nice find. I wasn't aware they're selling them as a reproduction. 
I have a couple originals for 68 only.


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## O52 (Jan 27, 2019)

Something else thats nice to have are all the service bulletins for your year. Issued when needed to the dealers they include maintenance tips, running changes in production, and corrections of misprints or errors in the service/diagnosis manuals. There were also the 'Craftsman Service News' issued quarterly highlighting the service bulletins and other news from corporate.


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## TxGTO69 (Dec 2, 2020)

O52 said:


> Something else thats nice to have are all the service bulletins for your year. Issued when needed to the dealers they include maintenance tips, running changes in production, and corrections of misprints or errors in the service/diagnosis manuals. There were also the 'Craftsman Service News' issued quarterly highlighting the service bulletins and other news from corporate.


Thats awesome!! My 69 is an early production I believe, Ill see if I can find those. You guys are awesome.


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