[xj-s] Cruise control

Thanks for input. The unit is black box under passenger side dash - AE Group
ECONOCRUISE,
Part No. DAC 4293, AE Part no. 3,5392.
I did take it apart and it was seal unbroken before I opened it. The only
adjustment I could see was 2 rotary, mini potentiometer like things that I
could turn with a screwdriver. Turning them had no effect except panic from
my wife driving the car as it got faster & faster!
Any chance of me finding a bad transistor, I looked for bad joints but all
looked OK? I have the Jag repair manual, but it shows a special test unit
required.

Thanks for input. The unit is black box under passenger side dash - AE Group
ECONOCRUISE,
Part No. DAC 4293, AE Part no. 3,5392.
I have the Jag repair manual, but it shows a special test unit
required.

Why don’t you try the special test unit described in the XJ-S help
book? Involves a coupla flashlight bulbs and a resistor or two.

Fact is, your problem MIGHT be in the actuator, not the control unit.
As I recall, the light bulb tester shows that one solenoid moves to
an engaged position, and the other one modulates to control speed.
If the second one has failed in the position that corresponds to more
speed, then as soon as the cc is engaged and the first solenoid
moves, the car will be off to the races.

– Kirbert | Palm’s Postulate:
| If anything is to be accomplished,
| some rules must be broken.
| – Kirby Palm, 1979From: “Chris Carley” ccarley@shawaero.com

In the absence of the ability to refer to the archives, I’m going to seek
some help on what I am sure is a recurring subject (sorry). I have been
trying to fix my 91 Marelli XJ-S cruise control without luck. The original
problem involved the cruise dropping out after it was set, and then
remembering to switch back on again, normally after I’d set it to 65 on the
freeway and was by then driving around a residential area!!

I have already switched out the speed sensor cable that bolts into the bell
housing behind the catalytic converter and I stripped down the bellows
unit, checking the solenoids and replacing/resealing the bellows at the
same time. Still no joy.

Bearing in mind the symptoms, where else should I be looking and what
should I be checking/measuring?? (Resume stalk switch, Speed Control
switch, Cruise Control Unit etc??)

Regards

Chris Yewdall
91 XJ-S Classic Conv

Sounds like an electrical problem. From looking at the Electrical Diagrams for
the '91 XJ-S, it appears that the speed interface unit (in the trunk, left
side, US) may be sending incorrect signals to the speed control ECU (under the
right side dash - US), either because it is intermittent, or because it is
getting bad signals from the speed transducer mounted on the rear suspension
cradle next to the propshaft. Since your unit occasionally remembers the speed
setting, the ECU must be getting power all the time, otherwise it would
“forget” the set speed when powered off. Of course, the ECU itself may be
bad. Another possibility is that the signals from any one or more of the above
are not getting to where they are supposed to due to an intermittent open or
short circuit. The white wire with yellow striping from the speed interface
unit goes from the trunk to a connector that pairs it up with a green wire
going into the speed control ECU located above the passenger’s feet. Another
maybe: the brake light switch sends a signal to the ECU to override the speed
control, and the brake pedal switch cuts off power to the ECU bellows.
Stepping on the brakes requires hitting the resume switch before the ECU will
command the speed control to its former setting. Does the ECU only reprogram
itself only after one of the above switches is activated? I don’t know. But
if so, maybe you are getting spurious signals from the other switch, which
kills the speed control, but does not reprogram the ECU to require a “resume”
because you did not actually hit the brakes and activate both switches. With
your '91, you should have the speed control mod that is supposed to assure that
you don’t get spurious speed control operation. Well, since you are, do you
have that mod? It does not make a bad speed control work correctly, but it is
supposed to keep it from succeeding in making uncommanded throttle movements,
as I understand it. Good luck.

Wally Magathan
'91 XJ-S Coupe

Chris Yewdall wrote:> In the absence of the ability to refer to the archives, I’m going to seek

some help on what I am sure is a recurring subject (sorry). I have been
trying to fix my 91 Marelli XJ-S cruise control without luck. The original
problem involved the cruise dropping out after it was set, and then
remembering to switch back on again, normally after I’d set it to 65 on the
freeway and was by then driving around a residential area!!

I have already switched out the speed sensor cable that bolts into the bell
housing behind the catalytic converter and I stripped down the bellows
unit, checking the solenoids and replacing/resealing the bellows at the
same time. Still no joy.

Bearing in mind the symptoms, where else should I be looking and what
should I be checking/measuring?? (Resume stalk switch, Speed Control
switch, Cruise Control Unit etc??)

Regards

Chris Yewdall
91 XJ-S Classic Conv

I bypassed the vacuum to the blue recall thingy directly from manifold to
actuator. Is the way to check the thingy to see if it is causing my lack of
C/C. Thanks

Byrnal Haley

I may have licked it , but I still need help. Little rubber seal on vent
solenoid missing, but I found it. The solenoids seem to be peened in the
body. Any suggestions on how to get them out without destroying the solenoid
in the process? I tried prying with a screw driver under the lip of the
solenoid but nothing moved. I think I’m asking, should I put it in a vise
and really put some pressure on it?

Byrnal Haley

It works, probably better than before. Vent solenoid seal now glued back.
Many thanks to the list for your help and especially The Book and Kirbert
“what have you got to lose” Palm. Thanks again.

Byrnal Haley
89 XJ-S Conv

I’ve just replaced the bellows - it works more or less = but
only keeps to about +/- 5 mph around the set point and
sometimes doesn’t kick in = suspect air could be leaking
between the plates and the bellows = any views gratefully
received. BTW I didn’t use any sealant when fitting!–
Rolph Muller
Alicante, Spain
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Hello- I have a 95 XJS convertible 2+2 with the 6.0 litre
V12. The cruise control doesnt work. Is there anything I can
check easily, or is there a chronic problem with these that
someone will know about? Thanks.–
flapper
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I have a similar car. The CC is unusual on that it is operated by an electric vacuum pump located at the front LH corner of the engine bay.
I am not aware of any endemic problem.
Suggest a systematic review of the system checking fuse,switches, pump operation and vacuum lines and bellows.
Rowen Baker------Original Message------
From: flapper
To: xj-s@jag-lovers.org
Sent: 23 Apr 2009 20:37
Subject: [xj-s] Cruise control

Hello- I have a 95 XJS convertible 2+2 with the 6.0 litre
V12. The cruise control doesnt work. Is there anything I can
check easily, or is there a chronic problem with these that
someone will know about? Thanks.

flapper
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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Sent from my BlackBerry device on the Rogers Wireless Network

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In reply to a message from Rowen Baker sent Thu 23 Apr 2009:

The brake light switch on the newer facelift cars is famous for
sticking in the ‘‘on’’ position when you release the pedal. When that
happens, the cruise control thinks you have hit the brakes and
shuts off. Hit the pedal a couple of times from the side with your
foot, and see if the cruise control comes back. This is so simple
to do you need to try it before addressing other possibilities.
I’ll keep my fingers crossed that this is your problem.–
The original message included these comments:

V12. The cruise control doesnt work. Is there anything I can
check easily, or is there a chronic problem with these that
someone will know about? Thanks.


1995 4.0 Roadster
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Snoopy D wrote:

The brake light switch on the newer facelift cars is famous for
sticking in the ‘‘on’’ position when you release the pedal. When that
happens, the cruise control thinks you have hit the brakes and shuts
off.

Hah! Mid-80’s Hondas – of which I owned two – have a little rubber
button on the brake pedal so when released it makes a soft “thump” as
it come up against the stop. The brake light switch is a plunger
type right in the middle of that stop, so it’s actually the little
rubber bumper that pushes it. I was driving down the road one day
and the aftermarket cruise control that I had added to the car
wouldn’t work. A quickie investigation found the brake lights were
always on and a badly dried out and crumbling rubber button was
sitting on the driver’s side floor mat. The plunger on the switch
fits very neatly right through the hole that the rubber button was
popped into.

A quick stop at an auto parts store and fifty cents later and the
problem was fixed for good.

Then, over the next few years, several of my friends who own Hondas
came to me and said “My brake lights won’t go out! Can you help me?”
I’d show up with a rubber button in hand and fix their car in about
20 seconds. They thought I was some sort of genius.

– Kirbert

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1985 XJ S HE Could someone please advise which vacuum port on the
inlet manifold is used to provide vacuum to the cruise control
bellows? My car seems to surge when the cruise control is active
and I am not sure if the vacuum hose pickup could be the problem,
regards Rodney Spratt–
Rodney Spratt
Gladstone /Queensland, Australia
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In reply to a message from RGS46 sent Mon 24 Feb 2014:

Any vacuum port at the front or rear of the manifold will do
just fine.
Do not use the ones at the butterfly housing.

Best,
Aristides–
The original message included these comments:

1985 XJ S HE Could someone please advise which vacuum port on the
inlet manifold is used to provide vacuum to the cruise control
bellows? My car seems to surge when the cruise control is active
and I am not sure if the vacuum hose pickup could be the problem,
regards Rodney Spratt


Aristides Balanos, 1986 XJ V12,VDP - www.e-psychi.com/jaguar
Roussillon, Provence, France
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