Unable to cancel speed control

[landscapesofwales559]

Hi everyone

I have an issue with the speed control on my 1990 V12 convertible.

With it set to the ‘off’ position, I get up to speed, move the switch to the ‘on’ position, then press the ‘set’ option. The car immediately starts to accelerate going way past 70mph and keeps going !! The same happens if I press the ‘Resume’ button on the end of the stalk. Touching the brake or switching to ‘off’ cancels it.

Before I go delving into the system or spending a fortune having a specialist look at it, does anyone here have any ideas or suggestions as to where I may start? Many thanks.

Stephen

[SimonR]

Hi Stephen,

I remember a friend had this issue with their '89 XJR-S and i seem to remember it was a simple case of adjusting a potentiometer on the speed control unit.

I think he got the information from here: https://www.jag-lovers.org/books/xj-s/16-Cruise.html

Specifically:

"CRUISE CONTROL ADJUSTMENT: If the cruise control is adjusted correctly, when the set switch is pressed the system will maintain the speed the car was doing at the instant the button was pressed. If adjusted incorrectly, it will maintain either a higher or lower speed. Hence, the test procedure is drive the car on a straight and level road and press the set switch, allow the speed to settle about ten seconds, and press it again. If adjusted properly, it can be set over and over and still maintain the same speed. If incorrectly, the repeated sets will result in gradually higher or lower speeds.

If adjusted incorrectly, adjust the speed control unit until correct; it is a simple matter to have the speed control unit hanging under the dash (it is located above the passenger side footwell) and the necessary tools along for the test drive, and possibly even an assistant driving. Although the repair manual describes a method of adjusting the cruise control, some of these units have no obvious adjustments. If you pry the box open, however, there are two adjustable pots on the circuit board. The one to adjust is the one in the corner, farthest from where the wires enter the box. NOTE: The adjuster is very sensitive; it is difficult to move it a small enough increment."

Hope that helps - well it'll give you a starting point anyway!

[landscapesofwales559]

Hi Simon

Thanks for the article. I will read it through and take a look at the problem this week.
Regards
Stephen.

[SimonR]

No worries Stephen - I hope it's an easy fix.

Another thing I've just remembered is that if you set the speed control so that it maintains 1mph above where it's set, you can use the set button to increase your cruise speed by that amount whilst it's in operation.

Not sure if that's intended in the design, but it's a handy feature!

[PaulGover]

...
... if you set the speed control so that it maintains 1mph above where it's set, you can use the set button to increase your cruise speed by that amount whilst it's in operation. ...

As far as I know, that's a standard feature covered in the driver's handbook. There's no need to "set the speed control so that it maintains 1mph above"; it's just part of the design.

[landscapesofwales559]

Hello everyone.

I’ve done a few checks on the speed control system, and spoken to one or two experts, and it turns out that I need to buy a new speed control ECU. The Jaguar parts catalogue only lists one unit (No DBC1169). However, this is listed under the parts for the 3.6 straight six. If I look under the 5.3 V12 convertible index a speed control ECU is not listed. I’ve been in touch with SNG Barrett and they tell me that DBC1169 is not suitable for the V12.

So my question is, does anybody happen to know the part number for the speed control unit ECU for a 1990 V 12 convertible? Thanks.
Regards
Stephen.