Can anyone tell me why a 78 needs a vacuums canister? Why can't I just run the line from the headlight switch and "t" it into the 2 yellow striped hoses that would normaly attach to the canister?

 

having a reserve of vacuum prevents any rapid changes in engine vacuum during a period of demand,(lights raised or lowered).

 

What would that cause? Would the engine run bad?

 

If you didn't have a vacuum reserve, you would not be able to open the headlights while driving uphill... that would be a bit of a safety issue. You would also not be able to close the headlights after the engine is shut off.

Lars

 

Ahhhhh... Now that I understand! Well, my name is bubba and I am not scared! And it is not my car.
The thing is,,
I can make the lights work when hooked direct, will not work when hooked to the tank. The tank does build a vacuum , just will not operate the lights. Very weird. More beer and pontification tonight!!

 

I will have the fittings to bypass the tank tomorrow. We replaced the actuators and regulators so not looking to go electric yet. We have 15 inches of vacuum at the hose from the manifold, before going to the tank.

 

Let's say the light are electric converted, then vacuum tank is completely useless?

 

Yes. And so is the $1000 you spent to make them work electrically.

 

 

If you do this, you will loose all vacuum for the headlight actuators when you pull out the headlight switch, and your headlights will not extend up.

When your headlight switch is in the "off" position, vacuum is routed from the intake manifold, through the filter and the check valve, through the headlight switch, through the backup "pulldown" switch below the steering column, and to the tops of the headlight relays. Vacuum pulls the valve in each relay upward and vacuum is routed from the tank to the rear side of each headlight actuator. Thus vacuum holds each headlight down (retracted) when your headlights are "off".

When your headlight switch is in the "on" position, manifold vacuum is blocked at the headlight switch and atmospheric pressure enters there, proceeds through the "pulldown" switch, and to the tops of the headlight relays. Without the vacuum, the valve in each relay is pushed downward by a spring and vacuum is routed from the tank to the front side of each headlight actuator. Thus vacuum holds each headlight up (extended) when your headlights are "on".

If you really do want to bypass the vacuum tank, remove all 3 hoses (1 unmarked and 2 with yellow stripes) from the vacuum tank. Take the unmarked hose that comes directly from the check valve (near the intake manifold), and connect it (using a "T") to the 2 yellow-striped hoses that connect to the middle ports of each relay.

However, if your vacuum tank does not leak, bypassing it will not solve your problem. Did the beer and pontification help?

 

I connected the 2 yellow hoses to a T and then hooked the line that would have been hooked to the tank for the vacuum to these lines. Then I used the regular set up for the other hoses. All i did was just bypass the tank. It works like a champ. If I am going up hill when it gets dark I will have to pull off to the shoulder and put it in neutral till the lights come up... At least that is what we decided after several beers and much thought on the subject. I will also pull the canister this summer and see if we can get it right.

 

Excellent thread.


I have a solid 18 vacuum on the lines.

When the passenger headlight comes up (it raises quickly and strong) it empties the vacuum tank in the frame.

It then takes about 8 seconds for the vacuum to build back up to 18 in the tank, and then the driver side headlight pops up as expected.



1. Why does the passenger headlight empty the tank?


(I, too.... am pondering a bypass of the tank.)

 

I will try and remember to update this when we get a little road time with this set up...

 

81_vette!:

Rather than the passenger headlight emptying the tank, it's more likely the driver's side headlight vacuum circuit is doing it.

Suggest you first check the driver's side actuator using this procedure:

http://www.corvette-101.com/vacuum.htm#actuator

I like to run this procedure by disconnecting the red and green striped hoses from the relay and connecting the vacuum pump to those hoses. That way you check the actuator and the hoses.

If the actuator passes that check, then test the relay using this procedure:

http://www.corvette-101.com/vacuum.htm#relay

Remember to test the yellow-striped hose connecting the vacuum tank to the relay.

The the relay test in the above link includes a link to a "Headlight Relay Repair" procedure that you may want to consider. Here's a link to another relay repair that worked well for me:

http://forums.corvetteforum.com/c3-t...ay-valves.html

Good luck!
Jerry

 

My conversion didn't cost me more than 150$ and some elbow grease.

 

I don't want to hijak this thread but I'd like to know how many of us can get their headlights to open just once after the engine is shut off. Once I turn off the key in my '72 I'm done. I know there must be a substantial leak somewhere but I can't hear any "hissing" anywhere.

 

My 1970 will... no leaks.

 

A car I work on passed its performance test this last summer and it would do 1 1/2 -2 openings

 

So what was your conversion and what parts were used?