Jaguar XJ-S. Manual - part 151

 
 

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SPEED SIGNAL:  See the section on the speedometer signal on page 590. 

 

TRIP COMPUTER FUEL MILEAGE:  The CATALOGUE reports that erratic fuel mileage readings can be caused by 
a poor connection at the fuel injector resistor pack. 

 

TRIP COMPUTER FUEL CALCULATIONS:  Craig Sawyers:  “One of the niggly things that didn't work was the fuel 
related aspects of the trip computer.  Mileage, fine; clock, fine; fuel, nada. 

“The trip computer gets its fuel signal from an "interface unit" mounted near the ECU.  It is a plastic case, with four 
connections.  Power (green) and ground (black), a line from the fuel injector feed for the resistor pack (yellow/green to 
pin 29 of the ECU) on the B bank, and a line that goes to the trip computer (orange).  When you get the lid off this box 
(drill out four copper rivets), you find a more complex circuit than you expect, based around a quad op-amp and eight 
transistors.  (Op-amp is short for operational amplifier.  The "quad" bit just means that there are four of them in one 14-
pin package.)” 

“To cut a long story short, including tracing out the arcane circuit and rigging up a pulse generator to mimic the injector 
signal, the op-amp was history.  The op-amp is a LM2902N, and is a generic part (the one that came out was Texas, the 
one that went back was Fairchild).  I just bought mine from Farnell.” 

“Cost of op-amp, £0.29UK.  Result, source of joy!  I don't know what Jag would have charged me for a new one, but it 
certainly wouldn't have been within two orders of magnitude of 50 cents.” 

Is there a way those without pulse generators, oscilloscopes and the like could determine that the op-amp or even the 
entire interface unit is in fact their problem?  “Not easy.  The input signal is taken from the ECU, and is the injector 
drive to the resistor pack.  This goes from 12V to 0V for the duration of the injector pulse.  The output of the interface 
unit looks like a pulse-width modulated signal.  Maybe the interface unit counts the pulses, and uses that to estimate the 
fuel consumption.  I haven't gone into that bit of it at too much depth; working is working. 

“Assuming that the other functions of the trip computer are OK, that power is present, and that a connector hasn't fallen 
off, I'd suspect the interface unit.  Also given the low cost of the chip, and that everything else in there looks pretty 
bullet proof, I think I'd advise stuffing a new chip in (the right way round!) and see what happens.” 

The op-amp is soldered to the board.  “The easiest way to get a chip out if you don't have a solder sucker is to use a 
small pair of cutters to snip each pin at the chip package itself.  Then pull each pin out of the board separately with a 
small pair of pliers while heating the pad with a soldering iron.” 

Edwin Hyatt tried this fix.  “Thanks to Craig, I now have a fully functional trip computer, having recently fixed the fuel 
consumption part of the display.  The IC in the interface unit was the culprit, and at a cost of $1.29 at Radio Shack, it is 
by far the lowest cost repair to date.  If the fuel related part of your trip computer is not working, this more than likely is 
the cause.” 

 

EXTRA CONNECTORS:  The author’s ’83 has two 3-conductor connectors behind the dash in the vicinity of the top 
left corner of the glovebox, with blank-off caps on them.  One is red with Y, O, and two G wires, and the other is black 
with P, RU, and two B wires; the P wire is from an inline fuse connected to an N wire.  These connectors are apparently 
for the trip computer -- although this car doesn’t have a trip computer.  There are also connectors near the ECU in the 
trunk for connecting to the fuel interface unit, which likewise doesn’t exist. 

 

 

Electric Windows

 

 

 
 

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There are reportedly three different types of window motor used in the XJ-S coupe:  A Delco unit up to 1987, a 
different Delco unit 1988-89, and a Bosch from 1990 on.  The early Delco motor has a large cylindrical housing, while 
the 1988-89 Delco unit has a smaller, more rectangular housing -- perhaps better described as a cylinder with two flat 
sides.  Both of these are held together with a pair of long bolts. 

The convertibles, of course, have another motor operating the rear windows, and apparently have different units in the 
doors as well. 

Peter Cohen provides some part numbers:  “The January 1987 - January 1989 parts book lists the non-convertible 
window motors as:  Left: JLM975, Right: JLM974, and the regulators as Left: BCC5775 and Right: BCC5774.  
Convertible is listed as Motor: Left: JLM1496, Right: JLM1495, with the convertible regulator listed as Left: 
BDC5079, Right: BDC5079. 

“The right side Delco motor that I removed had a paper label with the number 20060098 above the numbers 121 and 
30P8 (the left side label was illegible). 

Unfortunately, problems are reported with all types, even though none of them are Lucas.  Some of the problems 
discussed below apply to all, and some apply only to particular units as noted. 

 

ELECTRIC WINDOWS -- KEYLESS OPERATION:  Some of us prefer the electric windows to be operable 
whenever we’re in the car, not just when the ignition is on.  If you share this preference, there is a “window lift relay” 
under the passenger’s side of the dashboard that provides power to the windows whenever the ignition is on.  All you 
have to do is remove this relay and connect the power wire directly to the wire to the windows, and the windows will 
operate whenever the buttons are pressed.  Since the buttons are inside the car anyway, it’s not exactly a security risk. 

 

SLOW-MOVING WINDOWS:  The early Delco windows always move slowly.  This tip is from Leonard Berk:  His 
windows operated very slowly, so he sprayed WD-40 down the frames without even dismantling the doors.  The 
windows operate like new.  Perhaps WD-40 isn’t the ideal substance since the odor may be objectionable to some 
people, but it is worth noting that lubrication may be in order.  Victor Naumann says, “Try not to use oil in the channel, 
it destroys the rubber.  Silicone spray or graphite works OK.  WD-40 is a great lube, but not for rubber, it makes it 
swell.  Jag makes a very expensive lube called Glietmo, but I prefer silicone.” 

Paul Bachman says, “In my experience, lubricating rubber window guides is only a short term solution and seems to 
aggravate the sticking in the long run (gets gummy as it enevitably dries out).  I have had very good success lubricating 
the guide rails on electric windows (including my 85 XJ-S) with graphite.  It is difficult and a little messy to get it 
rubbed into the right places, but once in place it works for a long, long time.” 

On the author’s car, one reason for slow windows was that the little roller that slides in the track at the bottom of the 
glass had gotten all screwed up -- see below. 

John Himes talks “...about possible fixes to the drivers window not going all the way up without using their hand.  On 
my 88 XJ-S, the problems was that one of the screws was removed by a PO, or had fallen out over time that mounts the 
window motor to the door & the others had become loose.  The motor assy. would move when you raised or lowered 
the window.  After tightening the screws & new lock washers (with the window all the way up so it would fit 
correctly), the window now goes all the way up & I no longer have the fingerprints on the window (inside anyway).” 

John Setters reports:  “Two problems caused my drivers side window not to close fully without assistance: 

 

1. Window motor mounting had come loose. 

 

2. The lift assist coil spring was binding on itself. 

“Firstly remove the door trim panel - the hardest part!  I found that I needed to close the window fully before tightening 
the motor mounting bolts.  This is the way to assure correct positioning of the closed window.  Do this by applying 
upward lift with your hand under the slide rail at the lower edge of the glass.  Then tighten the bolts. 

 
 

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“Complete lift was hampered by lack of spring tension. Although well lubricated by grease too much friction existed.  I 
applied spray CRC to the spring then operated the up and down movement to work the CRC into the spring coils.  Heh 
presto it all works fine now.” 

John Napoli suggests adjusting the track at the rear of the window.  There are two screws that hold this section of 
channel in place; one is under the door panel near the bottom rear of the door, and the other is clearly visible on the end 
of the door above the latch.  The track is removable to facilitate replacing the glass.  The bottom screw has a very large 
washer on it and fits through a large hole in the door sheet metal, so the track can be moved around quite a bit before 
tightening it down.  The upper screw doesn’t have as much room to move, but can still move a little.  It is helpful to 
loosen both screws at the same time to fully investigate the range of possibilities. 

Napoli also suggests “If you can’t find an adjustment that solves the problem, replace the lining of the rear channel.  Jag 
sells a replacement channel.  I suspect that good old aftermarket channel felt can be installed in the old channel 
assembly.”  The problem with old channel felt is that the fur wears off, allowing the glass to rub directly on the rubber 
underneath.  The friction is hence much higher than it should be. 

If you have the early Delco window motors, once you have addressed all the possible problems listed above, your 
windows will still move slowly.  The final solution is to install a relay control system; see below. 

 

WINDOW/TRACK SEPARATION:  John Napoli says:  “I once had a weird window failure in my car.  The PO had 
replaced the rear window channel on the drivers side.  One day I lowered the window and thunk -- the window drops 
down out of sight.  Opened the door up and found that the metal channel that the glass rides in had been pulled away 
from the glass.  It was as if the glass had a positive stop on the way down.  The motor kept on pulling the glass down 
after it hit the stop and pulled the arm off.  Put it back together and it soon happened again.  I solved the problem by 
taking the glass out, supergluing the arm to the glass (in the correct location!) and installing a sophisticated support that 
the steel arm would hit when the window was lowered.  It was a carefully shaped chunk of 2x4.  You need to glue the 
channel to the glass in addition to adding the stop because if the channel is loose on the glass it will eventually slide 
sideways and prevent the window from opening or closing properly.  The glue locks the channel in position and the 
stop prevents the window from dropping too far and allowing the motor to pull the channel away from the glass. 

 

ELECTRIC WINDOW REGULATOR DRIVE ROLLER PROBLEMS:  The little roller on the end of the motor arm 
on one window in the author’s ’83 XJ-S was FUBAR.  Basically, the shaft the roller turns on is attached to the arm like 
a rivet, and this attachment worked loose, allowing the roller to cock sideways and jam in the track on the window.  As 
the motor forced it back and forth in the track, the edges of the sheet metal track chewed the plastic roller all to bits.  
No, you can’t buy a new roller from Jaguar -- they want you to buy the entire window regulator mechanism.  The ‘83 
has the early Delco regulator, but all of the regulators have rollers of some type and may occasionally have similar 
problems. 

For anyone with a lathe and some plastic stock, it would be a simple matter to make a new roller.  Unfortunately, I 
don’t have either.  I went to a building supply store and looked through their collection of rollers intended for sliding 
closet doors, drawers, shower doors, etc.  I immediately noted two concerns:  1) the roller in the Jag window was 
smaller than any of these; and 2) the roller in the Jag window has a sort of offset -- the bearing surface isn’t in the center 
of the roller, but over to one side.  In the Jag, the pin is shaped like the head of a flush head bolt, so that no part of the 
pin extends beyond the roller and jams into the track.  Only a select few of the rollers in the building supply store had a 
similar feature. 

I selected a package of “Closet Door Hardware” described as “No. 8544, 7/16” Offset Hanger Pr.” and made by Door 
Products Inc.; I’m sure there are generic equivalents in just about any such store.  This package contains two hangers 
intended for the top of a hanging, sliding closet door, each with a little roller to fit into a track.  The 7/16” refers to how 
much the offset is, no importance here.  In fact, the entire hanger is forfeit except for the roller itself.  There are 
probably several other types of hangers, with various offsets, all using the same or similar roller.  It is necessary to grind 
off a rivet flare to remove the roller from the hanger. 

 
 

602

The roller acquired is the smallest I found, but still too large; it is about 20mm in diameter, while the Jaguar window 
roller is about 17mm.  However, it was a simple matter to mount the new roller onto a 1/4” bolt, chuck it up in a drill, 
and grind off the outside edge until it was the correct diameter.  This roller, along with the Jaguar original, is illustrated 
in Figure 30. 

This roller also had the offset bearing, although it was not as long as the Jag original.  When assembling, it was 
necessary to add a few washers behind it to space it out the same distance from the arm as the original had been.  I 
made washers out of plastic so they wouldn’t jingle.  I also made sure their OD was small enough to not interfere with 
the window track. 

JAGUAR (DELCO)
WINDOW ROLLER

CLOSET DOOR

ROLLER

REMOVE MAT'L

 

Figure 30 - Original Window Roller and Cheap Substitute 

I also was able to find a shoulder bolt, or “axle” bolt, that I was able to modify for use.  It was an Allen socket head bolt 
with a 1/4” diameter shoulder 3/8” long, and a 10-24 threaded end beyond that.  If I had intended to use this bolt to hold 
the original Jaguar roller, I would have had to grind a taper on the bottom side of the head to match the tapered surface 
within the roller and allow the head to recess far enough into the roller so as not to protrude at all. 

For this new roller, however, the problem was different.  The new roller has a recessed opening for the head of the 
original pin that held it, but it’s flat-bottomed, not tapered.  There is therefore no reason to grind on the bottom side of 
the head, but the top side protrudes entirely too far -- this head is much taller than the recess is deep.  So I had to grind 
about half of the height of the head off.  This removed the entire Allen socket, leaving me no way to tighten it.  So I cut 
a straight slot across the top for use with a screwdriver. 

When assembling, I put the roller and the homemade plastic washers on this shoulder bolt, screwed on a nut, inserted it 
through the hole in the arm, and put another nut on the back side to hold it.  When tightened up, this positioned the 
roller about the same distance from the surface of the arm that the original had been. 

I chose not to install this roller in the original hole in the regulator arm.  Instead, I drilled a new hole 1” closer to the 
motor itself, making the arm effectively 1” shorter, in hopes of improving the leverage the motor has moving the 
window.  This worked perfectly, but since the arm is 15” long to begin with, this provides only a small percentage 
improvement in leverage.  Still, it’s easy to do, and every little bit helps. 

Of course, the window motor and regulator assembly is an AC Delco; perhaps you can figure out what other cars it was 
used in and find a cheap replacement in the local junkyard. 

 

ELECTRIC WINDOW SWITCH PROBLEMS:  Reports of switch problems are more common with earlier Delco 
units.  That may be partially because the cars are older or that there are simply more of them, but the early Delco system 
definitely suffers from overloaded switches.  The switches are inadequate for the inductive load involved and the 
contacts get burned.  Even though the later Delco and Bosch motors are physically smaller, they still move the windows 
at a good clip and probably draw a comparable amount of current; we’ll probably hear more about switch problems in 
later cars as the years go by.