I've notice a couple of threads lately refering to the good 'ole " my headlights/wiper door don't/doesn't work. And in every instance, someone refered the poster to the many excellent troubleshooting guides available. The question I have is what's the minimum intake manifold vacuum that the headlight and wiper doors should operate correctly at. Unless I've overlooked it (very, very possible. I seem to have "Old Timer's itis" lately ), I've not seen this number refered to in the various guides. And i assume the best place to measure this is right at the fitting on the intake...........

 

Ah...good question...which is what is the minimum vacuum required to overcome the spring tension in the headlights right? I think it's safe to say that it's around 10psi judgeing on what the readings are of the people having problems. But, it also depends on the condition of the headlamp units...some units gets loose and are easier to move, thus require less vacuum, while other units get a lot of friction in them due to the lack of lube and require more vacuum.

 

You have a buffer in the vacuum accessory system so, in that regard, there isn't a minimum vacuum since the engine produces different vacuum levels for different operating conditions. If the tank is empty, you start the car and never let it get above idle with a real lumpy cam, it may take a few moments for the tank to pump down to manifold to supply the system. Given the size of the vacuum motors, I'd guess that it wouldn't take more than 6-8" to operate. They may work a bit slow, but they should still work at that level. Now, given that vacuum isn't static, there are times when it's going to be less (WOT) and times when it will be much higher than at idle (decel). The vacuum reserve will pull down to the lowest vacuum the engine achieves and hold there until you start to deplete it, where it will pull up again, and again, and again.

 

Thanks for the replies. It all makes a tad bit more sense now. Ya' just gotta' understand the theory of operation before you can start troubleshooting the system. Got a feeling I'm in for some "fun" getting the wiper door and headlight doors to work properly on my recently aquired '68 BB as the cam in it makes it sound like a pro-stocker.

 

I'm only getting abbout 10 inches of vacuum at idle and this is enough on a good system to operate the headlights. Check to see what vacuum you are getting and if it does not operate the headlights give it a little rpm to increase the vacuum and see if it works. You might have a bad actuater or other problems.

 

my first (and abbreviated, as mother nature decided to throw a nasty little storm at us which shut down the power for a bit) troubleshooting attempt on my non functioning headlight and wiper doors. First thing I discovered is that the vacuum resevoir tank only has 2 hose attachments, rather than 3 as is shown on the diagrams for 1968 - 1972 I've looked at in the various troubleshooting guides. Also the car has a single, rather than 2 relay valves for the headlight actuators. It is mounted directly in the middle of the nose of the car between the horns which are located next to each of the actuators. The 2nd and 4th port are tee'd, which allows the single relay to signal both actuators simutaniously. Both the resevoir tank and relay appear to be the originals, due to physical appearance and condition. Also, the diagrams for '68 thru '72 cars show a relay valve at the firewall on the drivers side. I still ain't found that sucker??? The VIN identifies this car a being built in December of 1967 and as the headlight and wiper door override switches are mounted to the far right of the drivers side dash I assume that this would be considered an "early" car? Any present or past "68 owners discover only having a single headlight relay or 2 ported vacuum resevoir, as my car does?

The headlight doors would not open via the headlight switch or override switch, with or without the engine running. I was then able to open the doors by blowing (creating a possitive pressure) on the lines going into the front (engine side) of the actuators. The headlight doors would not close by either blowing or appling vacuum (used the good 'ole shop vac to create a vacuum) to the ports on the opposite side of the actuators. I then started the car with the headlight switch in the off position and after about 3 seconds, with the engine at about 1000 rpm (lumpy cam) both doors literally slammed shut.
I did find some hoses in questionable condition, and a couple had been replaced by a previous owner with what appears to be rubber gas line, of which the I.D. is slightly too large and not making a good seal on the attachments. I'm gonna' go ahead and order a new hose set fron Doc Rebuild and head down to NAPA after work and buy a hand operated vacuum pump. This is gonna' turn into a real challenge.

Comments or suggestions?

Jerry

 

I just replaced all the vacuum lines in my 70 Vette and if you have an air compressor take a look at the vacuum pumps on Ebay for about $15.00 they work great and give about 15" of vacuum and you wont have to start the car either to test the system. My cars headlights and wiper door work great now after replacing the lines and Doc rebuild gives you a schematic and the lines are color coded for ease of installation.start out with the easier lines first and use a little ky jelly for pulling the lines through the grommets, its easier that way. You will have to pull the dash though to finish the job, but its worth it. good luck with it.

 

OK, answered one of my own questions. Well actually, Doc Rebuild did: Per his catalog 68 & 69 cars used a single headlight relay valve, while 70 thru 82 used 2. That would logically explain the vacuum resevoir only having 2 hose attachments rather than 3, as the tanks with 3 attachments have one vacuum hose going directly to one headlight relay and the second one tee'd, with lines going to the other headlight relay and the wiper door system. The single line on mine is tee'd, with one line going to the single headlight relay and the other going to the wiper door system. The more I look and read, the more I learn. This '68 is gonna be a whole different creature compared to my '73, although it's nice to have the '73 to look at for reference.

 

I hooked up a vacuum pump directly to a headlight actuator once and tested how much vacuum is needed to open the light. It was less than 2" of vacuum.

 

Thanks, Zwede. Gives me a figure to reference from. I picked up a MiteyVac hand operated vacuum pump at NAPA on the way home tonight. No sense in attacking the problem without the right tools. Now if just weren't so friggin' hot and humid..........95 degrees at 8:30 pm in west central Indiana. This is nuts.

 

Markus is right. It's just slow w/ very low vac. in my test.