How to test Cruise Control Vacuum Servo Unit for '85 911?
 

Hi all,
Does anybody know how to test Cruise Control Servo units. I have performed the test procedures outlined in the Bentley manual and it has indicated faulty clutch switch/faulty servo unit. The clutch switch has tested OK so that only leaves the servo unit. The problem that I find though is how to actually test the unit. Hence the question.
I guess the problem could also be the main control unit but best to tackle one issue at a time.
Looking forward to any info on he subject.
Dave

apply vacuum to the port then 12 volts to the terminals. Check the resistance between the terminals I think it should be 12ohm. When the cruise is not working it's almost always the the control unit. Do a search you'll find lots of posts.

Hi Kodioneill,
Thanks for your reply. Can I check with you one more issue?

Obviously the the minor diameter vacuum hose is the one that comes off the 'T' Piece but my larger diameter hose that comes off the servo unit is attached to nothing and just goes to atmosphere. Is this correct? or is it attached to some other vacuum point to create a two way pulling system for the unit.

I was afraid to check the servo with 12volts as I didn't think that it would in fact be 12volts. I thought it may have been a lesser voltage as it was all happening through the control brain but you have put me straight and for this I am grateful.
Dave

The larger diameter hose is a exhaust /vent to dump the vacuum. I don't want to go out on a limb but I'm almost certain that the servo operates on 12v. I know the rest of the components switch,relay etc... operate on 12v. Wait to see more replies to the voltage question.

See if this helps

http://forums.pelicanparts.com/porsche-911-technical-forum/394659-understanding-vdo-cruise-control-amp.html

Urghh!! had a look at the link, methinks that side of the electronics is a little beyond my tiny brain.
R. Dave