Jaguar XJ-S. Manual - part 152

 
 

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Bob Colson of the Jaguar Club of Southern Arizona points out that the window lift switches can be taken apart.  First, 
remove them from the panel -- easiest to do by first removing the panel so you can push them out from behind.  Then, 
by spreading the housing slightly, the rocker itself can be popped out.  Then the parts can be cleaned up and repaired as 
needed.  The two rocking contact plates are symmetrical but only one end of each gets worn, so the plates can be 
reversed to extend their life.  The cruise control on/off/resume switch is constructed similarly. 

Phil Patton adds:  “Usually it seems the plastic things on the ends of the springs wear unevenly for some reason.  When 
cleaning the switch replace the one from the left side to the right and vis-a-vis.” 

Another solution is to replace the switches with generic double pole/double throw self-centering rocker switches with 
better contact ratings.  The difficulty here, obviously, is getting them to look right.  Phil Patton sends this tip:  “I have 
found a switch which is less expensive, IMHO looks much better, and I am positive will last much, much longer.  This 
part fits the existing hole perfectly and has a small, coloured illuminated strip across it, making it easy to find in the 
dark.  It is rated at 20 amps @ 12 volts and is (unlike the Jag switch) completely sealed so that dirt cannot contaminate 
the contacts.  The part is GC number 35-3565 (green light) or 35-3570 (red light).  They should be available from any 
decent size electronics parts house.  The only modification necessary to use this part is to cut off the plug on the wiring 
harness and replace it with push-on lugs on each wire.  If you don’t like the light then just don’t connect it.” 

Stephen Wood says, “I replaced the power window and power lock switches in my ’76 XJ-S with ’82-92 
Camaro/Firebird units, $10 ea. new and they work great.  I had to make a wiring jumper and a sub plate for mounting, 
but they function better than the Jag ones ever did.” 

A preventative measure may be to install a pair of zener diodes at each motor.  See the treatise on controlling inductive 
loads starting on page 557. 

Perhaps the best solution, and one that maintains the original appearance, is to install relays to operate the windows and 
operate the relays with the stock rocker switch.  See below. 

 

WINDOW CONTROL RELAY INSTALLATION:  Both the dragging windows and the burned switch contacts are 
symptoms of the same problem: The switches are inadequate for the load.  The high resistance, due to marginal, 
overloaded, or charred contacts, results in less than ideal power to the motor and causes the contacts themselves to fail 
often.  This author measured the voltage at the window motor with the window trying to close, and it was less than 8 
volts -- and dropping below 7 volts as the motor struggled.  This operating voltage means the motor is only developing 
about 45% as much power as it would at 12V. 

A set of window control relays will provide full battery voltage to the window motors, eliminating the losses in the 
wiring harnesses and switches.  See Figure 31.  Two SPDT relays will be needed for each window, an “up” relay and a 
“down” relay, and each will need to have serious contacts -- at least 10-amp rating.  Since the current needed to operate 
the relays is minimal, the rocker switches should last forever -- even if they’ve already been abused and cleaned up a 
couple times. 

The XJ-S comes with a relay referred to in the Jaguar literature as a “window lift relay”; all this relay does is turn off 
power to the windows when the ignition is off.  I will continue to use that term when referring to that relay, and will call 
the new relays being added in the following scheme “window control relays”.  Try not to get confused. 

 
 

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RU

RG

GU

GR

(L) 

(R) 

(L) 

(R) 

RU

(L) 

(R) 

RG

GU

(L) 

(R) 

GR

B

NU

12V 

REVERSE THE 
POWER AND GROUND 
SUPPLIES TO THE
ROCKER SWITCHES

N 

CIRCUIT BREAKER

WK 

2A 

IGN 

ROCKER 
SWITCH 

WINDOW 
CONTROL 
RELAYS 

WINDOW 
MOTOR 

WINDOW LIFT 
RELAY (DISCARD) 

 

Figure 31 - Window Control Relay Wiring 

Since window motors are permanent magnet motors, the direction they run depends on the direction of current flow.  
The wiring within the motor itself is not grounded.  To run one direction, the switch grounds one motor lead and 
applies 12V to the other.  To run the other direction, the same switch grounds the second lead and applies 12V to the 
first.  The window control relays should be wired to do the exact same thing.  In the wiring scheme shown, the 
normally-closed contacts on the relays ground both leads of the motor when the relays are idle, and each relay switches 
one lead to 12V power when energized. 

A massive power wire -- 12 gauge or so -- must be routed to the window control relays directly from some heavy-duty 
source (any big, fat brown wire).  You can toss in an inline fuse for safety, but it’s probably a better idea to use the 
original thermal circuit breaker; it will do a better job of protecting the motor from overheating if the power is left on 
(somebody sets a book on the switch). 

For the passenger’s side window, it is quite convenient to use the existing 12V power source and circuit breaker for the 
windows, leaving the breaker mounted right where it is; just connect a new wire with a 1/4” spade terminal to the 
output side of the breaker and route it to the new relays. 

For the driver’s side, it may be preferable to just buy a new circuit breaker and power it from a lead to the fusebox; this 
would eliminate the necessity to run a heavy wire from the existing breaker on the passenger’s side across the car to the 
driver’s side.  The original “Otter” circuit breakers are inordinately expensive and of unknown rating (apparently big 
enough to allow both windows to run at once, probably too big to properly protect a single window motor), but generic 
circuit breakers are available in auto parts stores in 10A, 15A, 20A, 25A and 30A ratings for around $5 each.  The 10A 

 
 

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should work for one window motor; this author used one and it has never tripped.  All of these generic automotive 
circuit breakers are “thermal” type, as opposed to the electromechanical circuit breakers typically used in homes. 

For the relays, 12V DPDT relays with 15-amp contacts from Radio Shack, cat. no. 275-218, will serve nicely.  Since 
SPDT relays will do the job, wire both contacts together; two 15-amp contacts working together definitely should 
handle this job! 

The sockets that Radio Shack sells for these relays are not recommended; buy a handful of 3/16” spade terminals 
instead.  Radio Shack seems to only offer 3/16” spade terminals in a box with a lot of other connectors, and since you 
need 16 of them you’ll be buying a lot of boxes.  If you go to an auto parts store, though, you may be able to buy a box 
with the correct “crimp-on” terminals alone.  Even if the terminals come with plastic insulating collars, it is suggested 
you rip the plastic collars off and throw them away, solder the wires to the terminal after crimping, and insulate them 
with 3/16” and 1/8” heat-shrink tubing. 

Since SPDT relays will work, automotive relays are another option -- if you can find some with an 87a (NC) terminal.  
Most of the relays sold in auto parts stores (usually for controlling driving lights) have two 87 (NO) terminals and no 
NC terminal, which won’t work.  Suitable relays include the one used on the XJ-S electric radiator fan, SRB411, and all 
of its substitutes listed on page 224. 

Another option would be to use two normal driving light relays (no NC contacts) instead of each SPDC relay.  Rocking 
the button should operate both relays, and one should make the power connection while the other makes the ground.  
This wouldn’t make any sense if relays were priced the way they should be, but the fact is that you might find two 
driving light relays are cheaper and/or easier to find than one SPDT relay. 

Automotive relays require 1/4” spade terminals, which are a good deal easier to find than the 3/16” terminals needed for 
the Radio Shack relays.  Again, insulate them with heat-shrink tubing, 1/4” and 1/8” this time. 

The relays can be located anywhere between the switch and the motor; simply break into the RG and GR (right side) or 
RU and GU (left side) wires from the rocker switch to the motor and wire in the relays as shown.  Within the door itself 
is one possible location; in this case, a massive ground wire should be routed back into the car -- relying on ground 
contact through a door hinge is not recommended.  Also, before closing the door up, it’d be a good idea to fasten the 
relays down (possibly with foam tape) and tie the wires down, and run the window up and down and operate all the 
latches to make sure the wires aren’t in the way of moving parts. 

Another possible location is adjacent to the footwells; you can intercept the wiring near the door hinge by removing the 
kick panel on the side of the footwell just forward of the door (2 screws).  There is a pocket that looks like it was made 
for these relays behind the kick panel.  There is a door lock relay in the same space, but it isn’t taking up all the room.  
The wires from the door all go through this space, and there are a lot of them -- stereo speaker, electric mirrors, puddle 
light, door lock, and window.  They are also plenty long enough, making it a simple matter to cut into the two window 
wires and install the relays.  This location makes it unnecessary to have to route heavy 12V and ground wires into the 
door, or even to take the door panel off. 

The relays can also go within the console if preferred, although that reuses a lot of the original wiring to the motors and 
reduces the potential benefits of installing really heavy power wiring. 

It doesn’t make good electrical sense to use the existing circuit breaker to protect the wiring to the rocker switches and 
new relays, since that’s a heavy duty breaker and they are now a low-power circuit.  Instead, provide an inline fuse to 
the rocker switches; a 2A or 3A fuse should be plenty to power the relays.  Physically, this means that the NU line from 
the window lift relay to the rocker switches is disconnected from the relay and connected to a fuse instead. 

Remove and discard the window lift relay, it won’t be needed any more; the window control relays themselves will 
accomplish its task.  Unfortunately, the window lift relay doesn’t have a NC connection, so it cannot be reused as one 
of the four window control relays. 

Where the power to the other side of the fuse comes from is a matter of preference.  If you wish the system to work the 
way it originally did (windows won’t move unless the ignition is on), then connect the WK wire that originally 
controlled the window lift relay to the fuse, as shown in Figure 31.  If, on the other hand, you’d rather the windows 

 
 

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worked whenever you hit a button regardless of ignition, just connect the fuse to a constant 12V power supply -- and 
there happens to be one right nearby, connected to the input side of the circuit breaker. 

A minor complication:  One might assume that the window rocker switches are a center-off, DPDT operation, and the 
wiring diagram for the window circuits in the ©1982 Supplement is obtuse enough to let you go on thinking that -- but 
they are not.  Each rocker switch is actually two individual SPDT switches, neither of which is center-off; both have 
NC and NO contacts.  With the rocker in the center position, the NC contacts on both switches happen to be connected 
in such a way as to send 12V to both leads to the motor.  Since 12V at both leads results in no current flow, the motor 
doesn’t move.  When you rock the switch, one of the two internal switches disconnects the 12V source and connects 
that motor lead to ground, while the other internal switch doesn’t move -- hence, the motor runs.  Figure 31 shows the 
switch schematically correctly, although it doesn’t really indicate the actual physical operation properly. 

All of this is normally just fine, except that all those leads are hot all the time.  Something to keep in mind while 
working in the area, especially if you have bypassed the window lift relay so the windows are operational at all times 
with or without ignition (see page 600). 

With the window control relay installation, however, these hot leads cause the relays to be energized when the windows 
are not moving.  Rocking the switch causes one relay to unenergize, and the system will in fact work just fine; the 
relays will just get warm.  However, if you have wired the system so that the windows can be run with or without 
ignition (see above), the relay coils would be draining the battery when the car is parked.  Ungood. 

The solution is to exchange the 12V and ground connections to the rocker switch so that both motor leads are 
connected to ground when the rocker is centered -- just like you’d have expected Jaguar to do in the first place.  You 
can make this change by cutting and splicing wires, but an easier way is to move the connectors around on the block 
connector that plugs onto the bottom of the rocker switch, putting the NU wires back where the B wires were and vice 
versa.  If you have the switches with six round posts on the bottom, it is a simple matter to pop the connectors out of the 
block connector with a piece of 3/16" OD brass tubing (available at hobby shops); just insert the tubing into the round 
hole around the outside of each tubular connector, and it will compress the tangs on the sides of the connector itself and 
allow the connector to be pulled out the back side.  Then they can merely be pushed back into the proper holes until 
they snap into place.  If you have the later style switches you should be able to do something similar.  Note that, for 
operating window control relays, the ground wires could simply be removed since they are not used. 

Exchanging the power and ground wires may even be desirable without installing window control relays, since the 
windows will still run exactly the same way; you just won’t have hot leads when working on the car any more. 

When fiddling with all this wiring, rewiring, relay installation, etc., etc., trying to keep track of which circuit will move 
the window up versus which one will move the window down will drive you nuts.  Instead, just forget about it.  When 
you’re all done and everything is working fine, if the button must be rocked the wrong way to move the windows, just 
turn the connector around on the bottom of the rocker switch.  If you prefer, you can reassemble the console but leave 
the rocker switches hanging out of it by the wires; when you’re ready to test it, just hold one rocker switch in your hand 
and push it one way or the other and note which way the window moves.  Then turn the switch whichever way is 
correct and snap it into the panel. 

This author has installed window control relays in my ’83 XJ-S.  I can report that the modification is a resounding 
success; the windows now zip up and down like they should.  It is now clear to me that this is yet another modification 
that should be done by all XJ-S owners ASAP, before their stock window switches bite the dust from overload. 

 

ELECTRIC WINDOWS -- EARLY CIRCUITRY:  According to the ©1975 ROM, “Selections can be made on one 
switch at a time, the driver’s switch over-riding the other panel switch”, possibly to avoid overloading the circuit 
breaker.  However, the ©1982 Supplement shows no such override and both windows on this author’s ’83 run at the 
same time.  Randy Wilson says, “The SII XJ6, and early XJ-S, were set up in the manner your book describes.  The 
switches are double pole, triple throw (down, pass-through, up).  With this early system, you could only run one 
window at a time.  And, if a switch failed such that the pass-through didn’t work, all windows downstream also quit.  
Since the most used window, the L/F, is also first in the daisy-chain, it’s not uncommon to see a SII with no working