Vacuum operated headlight adjustment

[tail_heavy]

Does anybody have any experience of getting the vacuum headlight adjustment to work? Car is a 87 UK spec 3.2. I'm not sure how common this fitment is on UK cars, but it does not appear to have been an option in the US, so it doesn't feature in the Bentley manual. From what I can make out, it's 100% vacuum operated with no electrical control. A connection to the main vacuum service pipe for the brake servo runs to the (sideways on) rotary control next to the headlight pull switch which appears to vary the vacuum that is then passed to two diaphragms, one on each headlight. These then adjust the vertical position of the headlight via a little pushrod acting against a spring. There is also a little vacuum reservoir that presumably smooths out fluctuations in the (negative) pressure. Thing is, a) mine doesn't work and b) I can't really understand how it could work. Surely, if the rotary control is just a valve controlling the vacuum, the full vacuum would eventually build no matter which position it was in. Also, unusually compared to systems in modern front engine cars, the control gives the option to raise the headlights (by one notch) as well as lower them - presumably to offset the effect of a heavy weight in the boot.

This seems like a typical 80s bizarre Porsche solution to a simple problem. Wouldn't it have been better to have some nice, heavy, electrical stepper motors in the headlamp shells?!

This is the least critical of my 3 requests for help this evening, but I'd like to know more about this strange system.

 

[Peter Aston]

Hi, there is an interesting thread dealing with your issues ref the vertical height adjustment issues on the impact bumpers forum here:
http://www.impactbumpers.com/forum/index.php?showtopic=589

Peter

 

[tail_heavy]

Peter,

Thanks for the tip. Link followed and found to be very helpful.

Like you, I like to have everything working (eventually), but it's hard to see how a system like this can be particularly accurate, but I guess I should have trust in the Porsche engineers, if it was good enought for the TUV etc. etc.

I looked at one of the replacement units I bought (with the vacuum adjustment) and found that vacuum applied results in the reflector unit moving forward at the bottom, where the diaphragm is, so applied vacuum moves the beam up. This suggests that, at rest (or without vacuum due to non-functioning system), the beams are in the lowest position, so they would have to have vacuum applied to rise to the normal position. This would be a kind of failsafe (for other motorists, not the 911 driver trying to see their way down a twisty road on a dark night!).

The follow on to this is that garages probably adjust the beams with the engine off (although I guess a healthy system might retain sufficient vacuum in the accumulator tank to keep the correct 'zero' position).

When / if I get it working I'll post back.