Hey All,
I've got a problem with my cruise control and I cant find a circuit diagram. my car is Nov 89 built and has the bellows type actuator and has the SET function on the master switch on the console, not on the stalk like most diagrams I can find seem to show. The Resume switch is on the stalk. Does anyone have the diagram for mine?

 

It will be exactly same as the circuit with the set on the stalk, except the set and resume wires will be swapped.

 

Thanks Warren, Looks like the fuse number/locations are different too

 

What is the problem with the CC?

 

not working at all. But I did have the brakes off so im going to check the thingy on the Diff first up. all fuses are good and connection to the solenoids look ok. switch is new.

 

If the speed sensor on the diff is bad the speedo would not work.

Does the bellows hold vacuum?

 

I've done all the checks outlined in the XJS book bar the bench testing of the control unit, everything looks OK,I've pulled out the bellows an resealed the rubber to the end plates. I lived up the solenoids. When vacuum testing I did get the bellows to move but just a little. I'm thinking I might try the bellows under motor vacuum and apply 12v to the solenoids to see if it pulls up the revs.

 

Lubed up the solenoids. Autocorrect!
So I reinstalled the bellows attached the vacuum line started her up and applied 12v it worked fine so I'll try again with everything plugged in tomorrow road test.

 

My CC has not worked for some time either. during my reinvigoration project this past winter I did get a new control unit and lubed up the little servo motors. Installed the nit on the bellows and sealed both ends with Permatex. I haven't had a chance to test it yet. The speed sensor on the differential is something I hadn't thought of. I'll go there if my road test doesn't work. Thanks!

 

The speed sensor drives both the cruise and speedometer, so if the speedometer works, then the speed sensor is good.

 

OK, so I can eliminate that as the reason the CC doesn't (hasn't) worked. Hopefully my replacement control unit and sealed bellows will fix the issue. thx.

 

There is a separate switch for the cruise disconnect, in addition to the brake switch. On one of my Series III sedans that switch stuck, so the signal to the cruise ECU was the brakes was on, thus the cruise wouldn't engage. Check that switch electrically and make sure it's behaving as it should. I think it's off the brake pedal, but I'm not sure about your year.

 

There are 2 brake switches.

One is normally open, this one also activates the brake lights and disengages the CC.

The other one is normally closed and opens when the brake pedal is depressed. This switch breaks the Yellow/White wire connection to the cruise bellows.

 

according to 'the book' the brake switches arent an issue on the XJS, and can be checked via a simple test at the ECU connector, mine checked OK. One of the first things I did was pull the hose off the unload solenoid -blue front right of motor - and busted the spiggot right off the solenoid...
Looks like US$150 min. I googled the Denso number off the solenoid and found similar units used on Toyotas/Lexus for 1/4 price. ATM i've run the vacuum line direct to the manifold to diagnose the CC issue.