Hi all ! I have a 1990 XJS V12 and the Cruise Control does not work. Other than the fuse (which I have not tested yet - if I could ever find out which one it is ?).

Any thoughts on what else to check?

 

Fuses are #5 (10A) on the drivers side panel ( under the steering wheel) and #6 (3A) on the passengers side fuse panel.

Does your trip computer and speedometer work? If yes, we can rule out the speed signal.

 

After you have checked the fuses I would test vacuum actuator. Use a syringe to pull vacuum on the actuator and ensure it holds.

After that check the connectors and wiring. Then either the CC module or the speed interface unit in the boot.

The speed interface unit has 2 outputs 1 for the speedo/CC and another for the TC as they use different pule rates, even if the speedo is working the speed interface unit could still be the cause of the problem, check the connectors on this as well. It is located on the left inside the boot up under the buttress.

 

On my XJR there are two micro switches on the bake pedal, one kicks out the CC and the other applies the bake lights, if these play up it will kill the CC, could be that is something to look at? Caveat, I know nothing about the XJS wiring.

 

Does it look like this ???

 

Nothing under the hood looks like that. The speedometer and tachometer work fine. I will check out the other items as suggested, after the holidays.

Thanks so much - you all are GREAT !

 

Malc4D, is that a replacement bellows or is that what they look like on the facelift XJS

Mine doesn't function because the bellows leak, its a pretty meh design.

 

That's the facelift design. The later cruise is completely different from the earlier system, the ECU's are the same as the later XJ40 cars.

 

Does it look like this?

The Cruise Control Bellows on a 1990 Pre-Facelift V12



The Cruise Control Bellows on a 1990
Pre-Facelift V12

 

Yest that is it. In the space between the two banks.

 

The bellows like to leak over time, as the rubber dies out...

 

Hi

Well I won't pretend that I know how to Fix it as I don't but at least we now know what you've got

So next I will put up a Photo of where to find the Fuse and then let Jagboi or someone else take it from there

You'd be surprised how many times an iffy fuse can easily turn out to be the problem, as in my own experience

Some of those 'Aftermarket' ones sometimes have much Thinner Blades that don't make very good Electrical Contact

Even though they seem to be a Tight Fit in the Hole



The Dropdown Fuse Box Panel on the Drivers Side of a 1990 XJS V12 which should be about level with your knees

(5) 10A @Jagboi Should be the one you're looking for although for some strange reason its not marked on the Panel

 

Can I test this by using smoke or some other aerosol fluid?

 

Lots of good reading here

Cruise Control


Cheers
DD

 

Thanks so much. I think that should get me going and testing after the Holidays.

 

I know that this won't be of any help but my cruise control is stuck in 'go faster all the time' mode.
I think that it is connected to the red line somewhere.

 

Mine is not working either but I have eliminated several potential culprits by doing the following:

Take the unit out and remove the control unit out of the front side of the bellows. Bench test it to see if the little plungers are working. If not try some JB Blaster or other lubricant. If they still won't work you'll need a replacement.

As for the bellows use some high temp silicon to seal both end plates into the bellows grooves to ensure you have a tight vacuum seal.

 

On a side note, there have been a few occasions where the cable has pulled out of the retainer and the retainer falls out. If you add a washer under the screw then the retainer can not fall out of the bellows.

 

I have had several reasons why mine didn't work and they were all to do with the bellows. check the wiring that goes through the end plate the heat and years will have taken there toll on them brittle and break at the connectors. and bellows sealing is the biggest cause of failure.