Jaguar XJ-S. Manual - part 170

 
 

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plastic button.  At that point I threw in the towel, pried the case off, and formed some thin card to fit around the inside 
of the case and prevent shorting once and for all.  No problems since.” 

Another possibility, obviously, would be to replace the switch with something non-Lucas.  It is a very simple switch, so 
perhaps a suitable substitute can be found.  The contacts must be open when the button is released and connected to 
each other when the button is pushed.  Make sure neither terminal connects to ground under any circumstances. 

Another possibility would be to install a totally different kind of switch, perhaps in a different location.  A microswitch 
with a long integral lever could be used, which allows the large motion of the pedal to reliably operate the switch.  Just 
make up a little bracket to hold it where the pedal arm will hit it when it’s in the fully released position. 

Yet another idea would be to install a relay.  You can connect the YW wires to the C and NO contacts of the relay, and 
provide ignition-switched 12V to one side of the relay coil and connect the other side to one terminal of the pedal 
switch.  Connect a wire from the other terminal of the pedal switch to ground.  That way, a short to ground in the switch 
would look just like the switch is closed, enabling the system but not hurting any components.  Things would still work 
even if shorted when the pedal is pushed, because this is merely a safety feature; the primary means of entering disable 
mode when the brake pedal is pushed is via a 12V signal from the brake light circuit. 

Finally, you could simply omit the switch altogether and just connect the YW wires together; as mentioned above, the 
system will still work normally since this is only a safety device.  It’s a rather important safety device, however, so this 
method is not recommended. 

Once you have the switch problems corrected, you’ll need to fix the fried transistors in the ECU.  Segal says, “I’ve 
found that most of the time (except when you really need it) the cruise ECU’s can be repaired by replacing two of the 
transistors inside.  They are in the lower left hand corner of the board.  The board is arranged so that the wiring harness 
comes off the left side.  The transistors are labelled ZTX650 and ZTX750.  They can be replaced by Motorola MPS650 
and MPS750.  Very easy and very cheap.” 

“The ECU part number is DAC4293.  The older part number is DAC3672.  The DAC4293 definitely can be fixed, I 
don’t remember if the older one uses the same circuit.”  (Ed. note:  The ECU in the author’s ’83 is DAC3134 and 
contains these same transistors.) 

“The original transistors are Zetex ZTX650 and ZTX750.  Digi-Key (

http://www.digi-key.com

) sells Zetex so these 

parts should be available through them.  You can use ZTX750 and ZTX751 as well.” 

Walter Petermann adds, “I use ZTX753 for the power out to the yellow/white wire, and ZTX653 for the control 
signal on the yellow/black wire. They are rated for 2amps (6amps peak).  Digikey (800 344 4539) stocks them at 
$0.78 under the above part #.  They have a minimum order but I don't know what that might be.” 

 

ECU REPLACEMENT:  If your actuator and bellows are OK but your ECU is toast and you’re the kind of guy who 
tinkers with a soldering iron, “600 Low-Cost Electronic Circuits” by David M. Gauthier, ©1989, includes 
instructions for building a speed control unit using an RCA CA3228E IC plus four transistors and a handful of 
resistors and capacitors.  The actuator shown in the schematic has the same arrangement of vacuum solenoid valves 
as the Jaguar actuator so this project should provide a usable replacement ECU.  Presumably there are many other 
similar how-to books that include directions for making a cruise control unit. 

 

SURGING WITH TURN SIGNALS:  The author of this book may be the only owner who’s ever had this problem, but 
I’ll report on it anyway.  Whenever cruising with the cruise control set and a turn signal was first turned on, the car 
would surge forward briefly.  Then, after the EFI ECU was modified to provide better throttle response, the car would 
surge on each blink of the turn signals!  Once the turn signal was cancelled, the car would return to its original set speed 
-- a clear indication that the problem was not due to a crossconnection between the set switch and the turn signal switch, 
since a “set” signal would cause the car to increase speed and maintain the increased speed after the signal is removed. 

 
 

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The problem was eventually traced to a poor connection providing power to fuse 5 and on to the turn signals and also, 
via fuse 17, the G wires powering the cruise control system.  The high-resistance connection caused the voltage to 
fluctuate more than a volt at fuse 17 when the turn signal blinked on and off. 

In this particular case, the problem turned out to be within the ignition switch itself; it was pried open and the contacts 
cleaned and the problem was cured.  Theoretically, any bad connection along the way would have the same symptoms, 
such as corrosion at fuse 5. 

 

CRUISE CONTROL REPLACEMENT:  Larry Lee sends this description on how to replace the Jaguar cruise control 
system with a generic Dana unit:  “The particular unit I installed was purchased from Sears, Model 318.20309.  This 
model came with a turn signal lever that can replace an existing one (if no other switches are on it), but Model 
318.20308 included a clamp-on control switch.  Other than the control switch design, the two units are identical.  The 
manual for this unit does not say “Dana” anywhere in it, but I recognized the system as very similar to other Dana units 
(also purchased from Sears, but marked “Dana Electronic Speed Control Kit 250-1000”) I had installed on other cars as 
far back as 1978.  There have been some changes over the years -- such as a change from two driveshaft magnets to 
one, and deletion of an inertial deceleration switch -- but all of them that I have seen are installed essentially the same 
way.”  Note:  apparently Sears no longer carries these units, but they -- or something similar -- may be available 
elsewhere. 

“The installation manual is quite complete and easy to follow.  It includes an electrical schematic and a pictorial 
drawing that shows how the various components and cables connect.  The cable plugs are all different, so improper 
connections are difficult to make.  

Follow the manual instructions to install: 

  Driveshaft 

Magnet 

 

 

Road Speed Pick-up Coil 

  Servo 

 

 

Disengagement Switch & Valve Assembly (Brake Switch) 

“Some judgment must be exercised when mounting the various components.” 

The Disengagement Switch & Valve Assembly mounts on the brake pedal.  When the brake is applied, it breaks an 
electrical connection to ground, killing the cruise.  Besides that, it also vents vacuum to the system, making doubly sure 
it is off.  Considering the fact that the stock XJ-S already has a switch mounted on the brake pedal housing for disabling 
the cruise control, it may be tempting to discard the Dana unit, plug the vacuum dump line, and just connect the wire to 
the stock Jaguar switch.  Note, however, that the Jaguar switch works the wrong way; it breaks contact when the brakes 
are released, and connects to ground when brakes are applied.  Therefore, a relay would be required to use this switch.  
Considering the effort required as well as the reduction in safety due to lacking the vacuum dump, it’s probably better 
to simply use the Dana parts. 

The driveshaft pickup is normally mounted right behind the transmission.  However, considering how difficult it is to 
get around the transmission mount on the XJ-S, it may be preferable to mount it at the rear end of the driveshaft 
adjacent to the differential.  If possible, the pickup should be attached directly to either the transmission or differential 
in order to move with them on their soft mountings; if mounted on the surrounding bodywork, the transmission or diff -
- and hence the drive shaft with magnets on it -- may move around relative to the pickup, possibly interfering with the 
signal. 

The Dana kit Lee describes attaches only one magnet to the drive shaft.  While he reports no noticeable balance effects 
from attaching the small magnet to one side, those who are concerned may easily add a dummy weight to the opposite 
side when installing. 

Note that any cruise control servo should be attached to the throttle linkage in a similar manner to the original.  On the 
early XJ-S, it may be helpful to unbolt the throttle pedal assembly from the firewall and lift it out to look at it.  If the 

 
 

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cable connection is relocated to the turntable at the top center of the engine, operation of the cruise control may cause 
the throttle cable to come undone or jam; however, later cars were designed to permit attachment here. 

Locating the servo unit itself may be challenging.  The original mounted just forward of the distributor -- and you are 
replacing it, presumably because it didn’t survive the heat in this area.  If the cable is long enough, the best place is 
probably out in front of the radiator.  You could get creative and make up your own extra-long cable, or find a way to 
connect the end of the Dana cable to the end of the Jaguar cable and use both! 

Whichever Dana kit is purchased, the control switch assembly won’t look all that great in the XJ-S interior.  Worse yet, 
the original set switch (on the turn signal stalk) and ON/OFF/RESUME switch (on the console) will no longer be used, 
leaving either unused switches or gaping holes where they once were.  The following is based on Lee’s method for 
using the original Jaguar set switch and ON/OFF/RESUME switch in place of the control switch that came with the kit. 
 This will maintain the appearance of the Jaguar interior, but it requires the purchase of a relay.  Note: the illustration 
shows the terminal layout for an automotive relay, but any 12V SPDT relay will work.  If the an automotive relay is 
used, make sure it has the real 87a connector (normally closed contact).  Many standard automotive relays, notably 
driving light relays, actually have two 87 connections and no 87a connections, which won’t work.  See the notes on the 
radiator fan relay on page 224. 

Remove the 4-wire plug from the Dana control switch cable, and connect the Jaguar switches to the plug as shown in 
Figure 39.  This sketch shows only the wiring that differs from the Dana (Sears) schematic in the area of the control 
switch; basically, the section of the Dana schematic from the four-connector plug onward should be replaced with this 
sketch. 

87A

87

85

86

30/51

2

4

6

7

8

1

3

5

SET

SWI

TCH

RELAY

ON/OFF/RESUME

SWI

TCH

DANA
PLUG

1

2

3

4

1

2

3

4

R

N

G

Y

W

GY

B

YO

BS

 

Figure 39 - Wiring for Dana Aftermarket Cruise Control

 

“How the Jaguar switch functions replace the Dana switch functions should be clear from the Dana schematic.  The 
Jaguar switches are physically located in two places, but that makes no electrical difference.  A good ground for the Set 
Speed switch is essential (as is a good ground for the Servo), but it can be located just about anywhere.”  

For clarity, Lee describes the operation of the Dana control switch that is being replaced:  “It is a single piece that 
contains two separate switches, but they share a couple of wires.  (For convenience, I’ll refer to the wires by the 
numbers shown on the connector.)  One is a momentary push button that is NC across wires 2 and 3.  When pressed 
(SET/COAST), it opens between 2 and 3 and closes between 2 and 4.  Actually, releasing this button is what engages 
the control and determines the speed setpoint.  The second switch is a slide switch that has three positions.  In OFF, no 
connections are closed.  In ON, a detented position, wires 1 and 2 are closed.  This same switch can be pushed 

 
 

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momentarily (spring return) into RESUME/ACCEL position, which maintains the 1-2 closure and also closes between 
1 and 4.” 

For those whose parts don’t look exactly the same as those in the illustration, a description of the contacts made may be 
helpful.  First, the relay:  85 and 86 are the coil connections, 30/51 is the common contact, 87 is the normally open 
contact, and 87a is the normally closed contact.  On the ON/OFF/RESUME switch, placing the switch in the ON 
position connects contacts 3 and 7.  Pressing the RESUME maintains the 3-7 connection and momentarily connects 
contacts 6 and 8. 

“Once the unit is installed, the check-out procedure described in the manual should be followed without any changes.  
Use the electrical schematic to sort out any errors in the new wiring.  Hopefully, this will not be necessary. 

“The pictorial drawing in my manual showed one unlabeled adjustment screw on the Regulator.”  Note: the adjustment 
screw referred to here is on the side of the regulator box, to the left of the “Centering Adjust” screw.  “It should be 
labeled ‘Minimum Speed Adjust.’  The manual describes how to make all set-up adjustments.  I set the Minimum speed 
at 30 mph, which enables the unit to maintain a set speed anywhere between about 30 mph and 90 mph.  Others may 
prefer a different range. 

“Once everything has been tested and adjusted, push the Regulator and excess wiring up behind the underscuttle panel 
below the steering wheel.  I have not found it necessary to fasten the Regulator to anything. 

“One should now be able to enjoy miles of foot-off driving!” 

Note that while this installation maintains the interior appearance of the Jag, the Dana cruise control does not work 
exactly the same; it has more features.  First, the stock Jaguar set switch is just that, but with this Dana system it 
becomes a set/coast switch; if pressed and held when the system is engaged, the car will slow down, and a new cruise 
speed will be set when the button is released.  Second, the resume position on the control switch becomes a 
resume/accel; if held down, the car will speed up. 

One other issue:  The original Jaguar cruise control system includes a switch on the shifter to ensure that the car is in D 
before engaging the cruise control.  The Dana aftermarket cruise control lacks such a safety provision, probably 
because there is a built-in electronic control to prevent the engine from racing if the cruise control is engaged while in 
neutral; there is a step in the instructions for testing the regulator to ensure engine racing does not occur.  The system 
also supposedly will not permit engagement below an established minimum speed, so engaging in neutral would 
evidently require attaining highway speed and then moving the shifter to neutral. 

If the safety features that are good enough for Dana are good enough for you, the YP wire from the shifter switch may 
be simply abandoned.  However, this same switch also ensures that the car is in D before sending power to the 
kickdown switch on the GM400 transmission.  Therefore, the switch itself and the other two wires connected to it -- a 
LGW wire to the kickdown switch and a BW wire to the solenoid within the transmission -- must be left in place for the 
kickdown to function properly. 

If, on the other hand, you insist upon restoring this safety lockout, another relay will be required -- a DPDT this time.  
A 12V power supply should be fed through the shifter switch to operate the relay.  When in D, one pair of contacts on 
the relay should provide the same connection between the kickdown switch and the transmission solenoid that was 
provided before.  When in any position other than D, a totally separate set of contacts should be wired in series with the 
disengagement switch on the Dana system.