John Swift

I installed a inside the car vacuum gauge in my 77. Can I tee off the vacuum line going to the headlight pull down switch, or should I pull vacuum directly from the manifold? I was thinking..vacuum is vacuum, but there I go thinking again! I know there are valves etc in the system, so I would like to do it right the first time.
Thanks

HeadsU.P.

Those light switches can be finicky about vacuum and working properly. I would stay away from that and maybe find a manifold vac source elsewhere. If you have an automatic there is a vac line going to the tranny you could "T" off of. All the other ports are likely being used for priority systems like brakes, distributor and PCV.

derekderek

Light switch is behind a one-way vacuum valve. It'll only tell you how much vac is in the reservoir, not what throttle setting, load, etc.

John Swift

Ahhh!! Thank you guy's. That won't do me much good. What about teeing off that same line from the manifold before it gets to the first valve? From the manifold it goes to a valve, and then splits into two lines, one going to relay's and the other to the light switch. I will have to go through the firewall, but that's okay if it will work. Or, should I avoid that whole light system all together??? The line to the gauge is VERY small in diameter compared the the large lines for lighting system, AND has a fitting inside the tee that reduces the inside diameter even more to about the size of a pin!
Ok..and I DO have a automatic...th400.

cardo0

Well thats the convenience of a vacuum gauge. You can monitor whatever system/process you want. But be careful of crankcase pressure as often the crankcase has positive pressure that could allow gasses to enter the passenger compartment - for those gauges you would want to mount them outside and more likely use a combination vac/press gauge. But that will show you the effectiveness of your PCV sys.

Normally for engine diagnosis you want to connect to the manifold vacuum. With this you can see the smallest of engine loads and can observe a bad/burnt valve in real time.

But you can connect on the downstream side of the little check valve to monitor accessory vacuum (head lights).

Hope this helps.

John Swift

Yes, It helps a lot. I was able to get my hands on all vacuum line diagrams for this car, and decided to tap right off the port on the manifold that runs to the transmission. I then went through the plug / grommet where all the headlight vacuum lines go through the firewall. Pretty easy actually. I have the vacuum gauge mounted inside at the moment because I want to monitor manifold vacuum on different loads etc. One issue down, now on to the next!!
Thank you guys!!

bobs77vet

I love vacuum gauges for test purposes it tells alot about an engine

derekderek

Also helps maximize fuel mileage at cruise.

John Swift

Yes Bob and Derick, I have found it to be extremely useful for a few different things. That's why I decided to install a full sweep one inside.
When I first got this beast I was getting 9 mpg, It needed a lot of tuning. Last I checked it was 17 and now waiting for the weather to improve to see what I am now getting.
As someone told me, "mileage.. who cares, it's a vette", and I agree to an extent, but why throw money out the window for nothing! With every MPG I get a little bit better performance as well! Thx