The 1966 Coupe deVille is a very well equipped car with lots of options.
Too bad that almost nothing worked when I got the car.
Of course I do not accept any non working things on my cars.
So with the help of another Cadillac aficionado who likes to work with anything electric - we took on the big challenge to repair the “Perfect Circle Cruise Control“ of the 66.
We had no idea how it worked and only had the shop manual as a reference.
The speed reminder function did kind of work, but the cruise control function was completely inoperative.

It took us a couple of days to fix it properly and now it works like the day the car left the factory!

We encountered a multitude of problems:

• 1) The dash mounted switch was defective - the plastic gear to set the desired speed was broken.
• 2) Most of the contacts inside the unit were oxidized
• 3) We had to adjust the contacts inside the unit properly and calibrate the system to the speed set on the dial
• 4) As it turned out later the brake switch was not properly adjusted (we should have checked this first and read the shop manual properly...)

After a lot of trial and error we eventually learned how the system works and we finally could adjust it correctly. It's a very nice system when it works properly, but you really have to calibrate it correctly. It's a fascinating option and there is some great engineering behind it. As it is rather complicated to repair you will see a lot of 60s car without a working cruise control.
Luckily its pretty well described how to adjust the contact points in the shop manual and I should have read it more thoroughly before we started to work on it - it would have saved a lot of time troubleshooting.

UPDATE: Here is the chapter about the Cruise Control from the 1967 Cadillac Shop Manual as a downloadable pdf file. I do no longer have the 1966 manual - thats why I have this document for 67 only. Its very similar to 1966 and differs mainly from the set switch.

inside the system - still on the car

the “perfect circle“ unit on the bench

on the bench

the dash mounted control switch disassembled

the broken plastic gear before we repaired it

the rebuilt and perfectly cleaned unit - back in the car

Tags:Cruise Control, 1966 Cadillac Repair Tips, Restoration, Perfect Circle, 1966 Cadillac Coupe deVille