1979-1993 & 94 Conv [Subscribe to Daily Digest] |
Hi Bob,
My pleasure! Auto body is my weakness... we all have different strengths--it's what makes life interesting!
The VDO-manufactured cruise system in our cars is a closed vacuum system. The cruise brain gets a speed signal from the sender in the speedo. When you switch the system on and hit the set button, the brain powers up the vacuum pump which is the funny-shaped black box mounted behind the front driver's side turn signal, which sucks on a hose, which sucks on a vacuum canister (mounted to the behind-the-dash crossmember) which pulls on a chain which pulls on the top of the accelerator pedal assembly right where the throttle cable passes through the firewall. Bit Rube Goldberg, huh? The brain tries to keep the vehicle speed consistent with what it was "seeing" when you pressed the "set" button. The brain does this by modulating the vacuum canister by either powering the motor in the vacuum pump, or a release valve (also in the black pump thing -- it's the unused vacuum connection on the unit which seems to confuse many people) which lets air back into the system. Again, this is a completely closed vacuum system, and has NOTHING to do with engine vacuum, or any accessories attached to the engine! There are two vacuum tees above the driver's feet which go the clutch and brake pedal switches, which, when depressed, allow air into the system immediately, causing the vacuum canister to re-inflate and turning off the cruise.
The cruise system does not interact with the Jetronic ECU, or any other system in the car, in any way whatsoever. In '89/up cars, the only thing that it has in common with the Jetronic ECU is that it shares the same speed signal generated by the sending unit inside the speedometer.
OK, I sort of lied -- there is one strange interaction with APC -- the vacuum line coming out of the pump tees off to a little switch a few inches away -- this switch, when vacuum is applied to it (i.e. the cruise system is active) cuts power to the APC solenoid so that whenever you're on cruise, the car is limited to basic boost as adjusted by the turbocharger actuator rod, rather than the rod being "held" closed by the APC brain (black or red box also in fender well near cruise vacuum pump) via the APC solenoid on the rad support, but I digress...
The upshot of this is that moving stuff around between the two cars won't break anything else...
I've just done some googling on this and have found that the cruise brain box is common between '88 and '91, suggesting that the older brain/system should be perfectly happy receiving a newer-style speedo signal! How one would wire this up, and why does the newer sender have 3 pins while the older has 2, I'm just not sure... and my Bentley manual isn't at all handy to try to figure this out... perhaps someone else knows...?
Sorry I can't be more helpful!
Bests,
--jeff
posted by 96.49.71...
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