68 headlight door vacuum question

[amiller]

I've searched the forums and all my maintenance and restoration books, but cannot find detailed information regarding the headlight vacuum system.  Specifically, what is the high/low range for the vacuum pressure coming from the block? ie. What is the normal range of pressure created by the engine, what is the minimum pressure required for the actuators to function, and what is the high end pressure that would begin to cause the actuators to break?

[Dans 68]

There are no values listed in the FSM (factory service manual) so I'm thinking that those values don't exist. The FSM simply states that if no vacuum or extremely weak vacuum exists at the actuator top tube connector when the engine is running then install a new actuator.

Dan

[Ghoste]

And as for a high end pressure that could break it, I don't think any of our engines will ever be drawing enough vacuum to worry about that.

[8WHEELER]

I run an electric vacuum pump for my brakes, it pulls 23-25 lbs at all times. The way I
had it hooked up the first time, it was running the headlight pods, as well as the brakes
with no problems. I soon hooked it back to manifold vacuum for for the headlight pods,
and the doors still open just fine, I run about 9 lbs at idle, that is why I got the pump
for the brakes.

Dan

[Dans 68]

Good info, Dan! Now we have a working upper limit. 

Dan

[amiller]

Thank you for all the feedback.

Also, for 8wheeler Dan, does your vacuum pump make much noise?  I assume it is inaudible with the engine running.

[Dans 68]

Oh, and welcome to the site, amiller! 

Dan

[daves68]

Amiller; welcome, and best of luck with the Charger. Let's clear up the vacuum thing. Vacuum, in an auto application is simply the absence of pressure. Stay with me. Atmospheric pressure, the "air" around you is approx 14 psi (pounds per square inch)-always. That's the pressure exerted on everything-all of the time. Look closely at a pressure gauge. It may SAY 0 but it has a built in correction of 14 psi. The corrected term is psig, or pounds per square inch/gauge. That means it has been calibrated to read start at 0 so you don't have to subtract 14 to get the desired reading. Vacuum is not read in pressure but rather in inches of mercury. Here's how it works. In a lab, vacuum applied to a glass column with mercury in it will lift the mercury a number of inches up the column depending on the strength of the vacuum. Max on the surface of the earth is about 25 in. Look at a gauge for the words in/Hg. Hg is the chemical symbol for mercury. Fortunately for all of us, gauges are  made without actual mercury in them. They are just calibrated to the mercury standard. SO,how does this relate to vacuum in your car??? Vacuum is anything less than atmospheric pressure. When the air is drawn into the engine, the physics are that a drop in pressure is produced- we call that vacuum. Pass it through the narrow venturi in the carb throat and the vacuum will increase. It's called the venturi effect. That is where you will find the tiny port holes in the carb wall to tap into that vacuum. The vacuum advance tube to your distributor gets it's vac from this source. When the engine runs, the entire intake manifold will be under vacuum because of the air being drawn through it. Bear in mind that real "rumpy" cams will affect the vac- meaning they produce less of it. Sometimes not enough to run the necessities. That's why some guys resort to a separate electrically operated vacuum pump to help out the system. There is probably a fitting on the back part of the intake with 2 or more branches, all with hoses attached to operate whatever accessories on your car that operate with vacuum. Heater controls, headlight doors, power brakes all source from here. In a perfect world, it works fine. But, on our beloved older cars, things fail. Vacuum actuators get old, diaprhagms and hoses get dry and hard, plastic system parts get brittle and break and metal parts rust out. All of them can create a vac leak. When that happens, no matter how much your engine is sucking, there isn't enough vac to go around and stuff quits working. Here's some tips I learned the hard way on my '68. The heater control switch may be leaking, listen for a hissing sound, same with the headlight door vac switch. It's coupled with the electric one in the dash. Replacements are available, but costly. The last one, and this drove me nuts for a while, is the vacuum canister. It's located under the battery tray and is about the size of a tomato juice can. It's location under the battery for 40 years and the fact it is made of very thin gauge steel makes it a #1 candidate for rust through. Mine rusted all around the seam- tiny vac leaking pin holes that you can't hear because of the location. Take it out and inspect it closely. I sandblasted (carefully) it and appiled JB Weld. Everything works fine now.  A little long but I hope this helps you out. Hang in there.  Dave

[8WHEELER]

Thank you for all the feedback.

Also, for 8wheeler Dan, does your vacuum pump make much noise?  I assume it is inaudible with the engine running.

I mounted my pump between the battery, and the headlights, bolted to the front side of the rad support.
It is tight as hell. But MPB suggested I mount it in the trunk, and I sure did not want to do that.

It is actually pretty load, sounds similar to an electric fuel pump, but in the engine bay, not out back.

Dan