1970 headlamp malfunction

[cwd]

Was wondering if any one could explain as to why all of a sudden the headlamps on my 70 big block would decide to go up by themselves when under hard acceleration and then go back down after deceleration. They also decide to come up by themselves as soon as the car is shut off.

The overide switch works fine and when I turn the headlamps on at night they stay up and on until I turn them off.

Any suggestions.

[Jack H.]

Re: 1970 headlamp malfunction

'Winking' headlights and 'dancing' wiper door is a common malady that results from one of two things:

(1) Lack of appropriate maintenance.

(2) Aging of vac lines resulting in minor leaks.

Each vac controlled item (headlight doors and wiper door) is serviced by TWO separate vac lines: (a) the servo line (vac that actually makes the door open/close) and (b) the control line (vac that tells the servo which way to go--open or closed). If there are leak(s) in the system (generally when there's a leak due to age there's also more than one to track down and kill!), when you're under low native vac conditions (initial engine startup, hard accel), the reservoir tank that's in the system to supply continued vac lacks the capacity to feed vac and the 'circuit bias' (stay open or closed) 'droops' causing the fluxuation of the movable item.

Notorious 'culprits' causing such are:

(I) Random leaks in hose lines, generally at this/that nipple connection.

(II) A faulty check valve intended to maintain vac in the storage reservoir during low intake vac periods.

(III) A plugged vac filter denying air to move quickly enough throughout the system to 'recover' from this/that transient episode.

You can get a book from most of the parts catalog houses to explain the vac system layout and give you a grounding in how to troubleshoot (VERY important knowledge for these cars!). The check valve is often the culprit and you can remove and test it by blowing/sucking on the nipples to see that air passes in one direction only.

If there's a system leak (or 2, 3,....), you can expect foreign matter has invaded the hoses and is migrating toward the vac source (intake manifold). This can/does cause the check valve to gum up and not close fully. Sometimes, spritzing the check valve with carb cleaner and exercising it wil effect recovery IF it's just a matter of 'gook' inside.

Last, between the check valve and the intake manifold nipple, there's a filter. This is a white plastic disk arrangement. When these cars were new, this component was intended to be changed (discard and replace) at regular intervals (at least once a year). It's a whooping $4 cost item still available from most of the Corvette catalog parts houses and it's amazing how few 'weekend warriors' drive their cars without ever doing this maintenance chore!

Now, if/when the filter element becomes plugged with 'gook', the rate of air flow through the filter drops dramatically and this can result in all kinds of 'funny' behaviour, especially under vacuum transient conditions. PLUS, if the system IS found to be leaking and you repair the individual leaks, bet your bottom dollar there's foreign matter migrating through the vac lines that WILL wend it's way back and into the vac filter!!!!

When the filter element clogs substantially, the element is going to rip free. In that case, it's going to travel in the direction of vacuum--into your intake manifold, rattle around, and eventually go through a cylinder. It's only a reinforced cotton element, but valves, spark plugs, piston/rings are known to 'not appreciate' foreign material....

Bottom line, whatever you decide to do, I highly recommend buying at least two vac filters, one to use right away and another to use a few months after you've found and fixed your problem(s)....

[Wayne W.]

Re: 1970 headlamp malfunction

Most likely the check valve.

[cwd]

Re: 1970 headlamp malfunction

Thanks guys. Your input is always appreciated as is the value of this message board.

Chris