73 coupe my headlights stay in the open position with the headlamp light lit, i just got the 73 that has been off road for 10yrs, i got the lights up by pushing up, what do i need to check, the 73 runs off vacuum.
Roy said:The vacuum supply line to your headlight switch is routed from the intake manifold tap (behind the carb) to a plastic filter disc, then to a metalic disc with one intake and two outputs. This metal disc also acts as a "one way" control valve that opens to receive vacuum from the manifold and closes to stop vacuum from escaping in low vacuum or no vacuum situations, such as shuting off the engine.
One of these outputs from the metalic disc connects to a plastic "T" fitting that feeds the heater control vacuum system and the other feeds the vacuum resevoir (long tubular cylinder)located under the headlights. The other vacuum hose from the metalic disc goes to the headlight switch. When the headlight switch is pulled, vacuum is fed to another "T" connection in front of the radiator, which sends it to the two small diaphrams between the headlights, which in turn actuates the two large vacuum supply lines connected directly to the top of the vacuum resevoir, which in turn provides a strong supply of vacuum to the two large diaphrams that raise the headlights.
Your problem with the headlights and heater control valves is that they are competing for vacuum. If your vacuum system is tight and doesn't leak, there is no problem. However, the problem you describe indicates inadequate vacuum to operate both systems at the same time. The headlights will normally win this competetion for vacuum because the vacuum hoses that supply the headlight system are larger in diameter and closer to the vacuum resevoir. The heater control vacuum hoses start out small at the firewall and become very small after they pass through the firewall.
Without sufficient vacuum to keep the heater control diaphrams operating, the heater and vent doors that are kept open with vacuum, close. Simple as that. Turn the headlights off, vacuum in the system increases and there is enough vaccum to operate the heater control diaphrams and open the heat and vent doors.
Recommendation: 1) Check the hose from the metalic disc to the heater controls. Not easy or fun, so skip to 2) Check the hose from the first "T" connector to the vacuum resevoir. Chances are it is loose or cracked and allowing vacuum to leak from the resevoir, leaving only the intake manifold vacuum to operate your system. An easy way to check this is to run your engine for a minute or so, shut the engine off with the heater controls in "off" position, then see how many times you can raise and lower the headlights. Take your time and if you only get 1 or 2 up and down cycles, your vacuum resevoir, or the vacuum lines connected to it are leaking. If you get 3 or 4 cycles, your vacuum system and hoses are fine. 3) Disconnect the rubber hose feeding the metallic disc with vacuum from the manifold. If you can suck air from the single inlet side, it's fine. If you can suck and blow air through the disc, the control valve is shot and needs to be replaced. 4) Check and replace all vacuum hoses in the headlight and heater control system, which would include Item 1 above.
Actually, any leaking vacuum hose anywhere in your vacuum system will cause the other vacuum components to falter and the engine to be less responsive. If your Corvette has 20 years or 100,000 miles under it's belt, it's time to replace all vacuum hoses. Your car will run better and in some cases, much better.
Hope this helps.
Roy in Portland, Oregon
Roy said:It would be nice if I thoroughly read the question before posting such a lenghty response. Please accept my apologies.
To answer the "heat only comes on when the headlights are on" is easy and simple.
Your heater control vacuum hose is incorrectly connected to the headlight vacuum line running from the headlight switch to the "T" fitting that feeds the small diaphrams in front of the radiator. When the headlights are turned on, this hose has vacuum. When the headlights are off, this hose is static and has no vacuum to operate the heater control vacuum system.
To correct the problem, connect the heater control vacuum hose to the "T" connection with the vacuum hose that goes from the metallic control disc to the vacuum resevoir under the headlights. This way, your heater controls will always have vacuum available, with or without the headlights on.
My other posting should be helpfull to anyone having a problem with the vacuum system on their Corvette.
Bad Reader Roy from Portland, Oregon