Do check the brake switch (both of them) The cruise cancel switch is a metal cased switch and mine had an internal short that cause the CC to think the brakes were on all the time so wouldn't engage the CC. Should be a yellow/white wire down by the brake pedal.

 

Thanks

Looking in the drivers footwell I see only one switch. I have just finished reading the pages on speed control in the ROM (86-13) and they also mention 2 switches. The second switch must be placed higher and out of sight. Which one is the brake light switch and which one the CC cancel switch ?
i have checked brake lights work and had therefore concluded switch was ok until I recognized there was another switch...

Leo

 

The brake light switch will have a capacitor across it. It's usually a lever arm operated switch on the brake pedal and has a green/purple wire. The cruise inhibit switch is a plunger mount and operates off a bracket. At least it does on Series III cars, I'm not familiar with early XJS, but it should be similar.

EDIT: Looks like the XJS uses a different switch on the pedal housing casting, I think in the engine bay.

There is also an inhibit switch on the shifter, so cruise will not engage i other than D.

 

Fine. The visible switch has a capacitor across, so that settles which is which. I will remove the knee bolster tomorrow to look for the other switch

Leo

 

The brake switch is in series with the YW wire and needs to be closed for the CC to operate, Check continuity between YW wire at the CC bellows and CC unit push brake pedal and it should go open circuit. The brake light switch also disengages the CC but also operates the brake lights if the lights are working then the this switch is probably ok.

 

Warren - the last couple of pictures are the finished article, all soldered in.
Now just waiting for the ran to stop for a test drive... cant wait!

 

Update:

Took the car for a spin and no CC, so went home and fitted my test lights.

Once above about 40km/hr I turn on the CC one light came on, so good so far.
Pushed the set button and the other light came on, very good.... as I accelerated the car the second light got dimmer and dimmer and went out.
As the car slowed down the light came on again slowly until it was fully lit.

So Warren's fix has worked, the ECU is operating as it should... big thanks to Warren.
Next I bi-passed the safety vacuum thing, which was fitted during a re-call - and went for another drive.
Still no CC.

So I am getting closer, the bellows is new and sealed, I can JUST suck it in some with my mouth. So its sealed.
I manually checked the solenoids and they clicked and I could blow and suck through them when open and they sealed shut, so at this point I think they are OK.

Not tested is the vacuum... is there away of testing this?
Also the cable maybe too loose and not pulling the cable enough, if I tighten it it upsets the idle as it hangs open.

any thoughts?

 

Another thing to check is the bell crank on the throttle pedal. Mine is worn and does not pull on the pedal very well. I have not fixed as I will changing to the 6.0L capstan with direct CC mount.

 

Looks like I have the identical Econocruise unit on my 1989 XJ-S V12. Been through the stages you have detailed. The unit works except that when cc is set, the care continues to accelerate without foot on the throttle. I see 2 apparently adjustable pots on my control board which I can see on your photos. Do you know which one I adjust and which way?

 

Hi - sorry no, have not got this far yet.
I tested the vacuum in the car in neutral - with a manual switch and the engine will rev. So the vacuum side is working.
The bellows will suck in and rev the engine.
My next plan is to try this driving, have a manual switch to turn on the CC and see if the car will accelerate like your does.
If so then I need to see why it does not work automatically.

With luck Warren will answer you as he is the man on electronic things.
let me know as I am interested as mine is not running yet.

I have the steering rack out at the moment so will not be testing for a week or two depending on the rain!

 

If you are this far, you are "nearly there". The pot to adjust is the one furthest away from where the wires enter the box. It's intuitive, counter clockwise slows it down. The easiest way to do it is get a helper in the passenger seat and a bit of open flat road that you can traverse repeatedly until you get it right. When we did mine I had my wife turn it down until it did nothing and then gradually increase until it stayed put..did it on the highway in traffic just to increase the fun factor but I don't really recommend that. The other pot either increases the rate of correction to the solenoids or sets the speed range that the cruise works in, depending who you talk to. I fiddled with it until I got the lowest working speed around 65kmh. Seems to work fine at highway speeds too so I left it at that.

 

Appears that my control unit is defective. The adjuster on the PCB (the one nearest the end remote from the wiring harness) does not have any effect. When cc kicks in we just head for the stars until road or courage runs out.

 

Thank you. That is most helpful and I have done as suggested. Unfortunately it seems to demonstrate that my control unit is defunct. Turning the adjuster has no or very little effect. I have been told that the capacitors on the PCB need replacing. Do you have any view on that?

 

I don't but if caps need replacing it'll likely be the electrolytics only. The metal tin cans, the others usually stay fine forever-ish. Transistors could go bad and they are discussed early in the thread.

 

It is unlikely that replacing the caps will fix the problem, I would however suspect the driver transistors, if one of those is open or short circuit then it could cause the vacuum solenoid to be energised. It could also be the speed detection circuit. Without looking at the unit it is hard to say.

I posted earlier in this thread how to test the transistors, I would start here.

 

HI - Been a while but my cruise control is now working - BUT it only accelerates...... and does not stop or maintain the set speed.
When I plug in my test lights one light comes on and stay on when I turn on the switch, the other light goes on and off depending on how fast I am going compared to the set speed.
BUT not with the bellows plugged in... zooom we are off

Any idea Warren?
cheers

 

Sounds like the problem is in the bellows. Either a leak or bad solenoid.

Pull a vacuum on the bellows with a syringe and see if it holds, then check resistance of solenoids.

 

The little pads on solonoid plungers can displace and if it happens one end of that solonoid it can prevent valve opening at other end.
Been a while but l think one solonoid has pad one end only and the other has pads both ends.
End of life for the adhesive used after all these years.

 

I had similar symptoms although I was not using a test light set up. I had checked out the bellows and solenoids etc. and when I hit the cruise control switch we headed for the stars. The problem is most probably in the circuit board hidden under the cover panel under the glove box. It is in a black plastic box the size of a pack of playing cards and as I recall there is a 9pin and a 1pin connector which came apart easily enough. They don't make them any more but I got mine refurbished by a man called Chris Knowles at Home - XJS & XJ from KWE Cars and I was able to post the unit off to him.

Good luck
Chris