Jaguar XJ-S. Manual - part 149

 
 

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From VIN 134286 on -- the beginning of the 1987 model year -- the 1-wire oxygen sensors were upgraded to 3-wire 
oxygen sensors.  This change eliminated the legal requirement for a service interval counter, so it was omitted.  See 
page 290. 

In the late 80’s, the pulse generator on the transmission was eliminated in favor of a sensor on the final drive unit.  
While the transmission-mounted pulse generator was a shaft-driven device, this later unit has no moving parts; it is a 
simple pickup mounted adjacent to a toothed flange on the ring gear carrier inside the differential.  The pickup does not 
produce a usable signal by itself, however; it’s connected to a “speed interface unit” (so named to distinguish it from the 
“interface unit” used to process fuel injector pulses for input into the trip computer), and the speed interface unit in turn 
provides a signal to the speedometer, cruise control and trip computer. 

 

SPEEDOMETER SENDING UNIT 90° ADAPTER:  John Shuck sends this report:  “I’ve actually repaired these little 
expensive jobbies.  Take apart the crimps and inside is a small square piece of metal that actually does the drive and is 
probably sheared.  Now go to a speedo shop and have them square you a piece of speedo cable about 2 inches long.  
They put the round cable in a die.. hit it with a big hammer..bingo..square.  Cut this to length with a die grinder and 
reassemble.” 

Another possibility might be to find the right size square key stock.  Industrial supply houses usually have a good 
selection. 

 

SPEED WHILE STANDING STILL:  Technical Service Bulletin J85/28 addresses “Speedometer Needle Movement - 
Vehicle Standing”.  This TSB was issued in April of 1985 (shortly after the introduction of the trip computer) and the 
specified repair involves the trip computer, so apparently there was a spurious signal problem with the new speed 
circuitry for the trip computer.  The cure is to pull the trip computer out of the dash and splice in a capacitor between 
the yellow wire on one harness connector and the black wire on the other harness connector.  The capacitor is described 
only as “Lucas 60600903”. 

The yellow wire is the pulse signal to speedometer, trip computer, service interval counter and cruise control, and the 
black wire is ground.  A capacitor installed in this way is simply a filter for spurious signals that may be affecting all 
four of those systems. 

 

TACHOMETER ZEROING:  Bob Gallivan reports that “my tach was out of whack, reading low ~ 750 rpm.  Found 
that it can be zeroed in by a white plactic lever that sits ~ at the 6-10 O'clock position on the back of the tach face.  
While in the car it can be adjusted using some lock wire to pull the lever (up or down).”  Note that Gallivan’s car is an 
’84, and this author’s ’83 has no such plastic lever. 

 

OIL PRESSURE SENDING UNIT:  Many people confuse the two separate items on the XJ-S, both located at the top 
rear center of the engine, just below and behind the turntable.  The smaller item is the warning light sender, and is a 
relatively cheap item.  The larger part is the sender for the gauge, and it is more expensive and less likely to be available 
at a generic auto parts store. 

The sending unit is a simple variable resistor.  Jim Isbell says “I have opened up one from a series 3 XJ6 and found a 
mechanical diaphragm to wirewound pot contraption.  It essentially acts as a variable resistor that shows high resistance 
at low pressure and low resistance at high pressure.” 

Mike Cogswell reports that earlier Jaguar senders were different than the later -- and they shouldn’t be mixed.  “Turns 
out that the S2 E-Types (and probably XJ-6s of the same vintage) used 80 psi gauges while the V12s used 100 psi 
gauges.  The gauges are identical except for the markings, but the senders are different since they are the same 
resistance at different pressures.” 

 

 
 

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FUEL LEVEL SENDER:  Gregory Wells of Coventry West, Inc., says, “Regarding the fuel level transmitters, I trust 
everyone is aware that these are notorious for the floats becoming partially filled with fuel and thus giving a very false 
reading.  In the last three years, I don't think we have changed a tank or transmitter without finding that the hollow float 
was partially full of fuel.  I have seen people throw away the hollow plastic float and replace it with an appropriately-
sized cork to eliminate the problem.” 

This author found some fuel in his float when lining the tank, but not much; perhaps 1/10 full.  Still, it seemed the thing 
to do to do something about it, so I drilled a very tiny hole in one end, squeezed the fuel out, then plugged the hole with 
a #2 (tiny!) stainless steel screw.  The screw was 1/4” long to begin with, but I cut it shorter so it was barely any longer 
than the wall of the float was thick.  Then I coated the entire float with the same stuff the tank was being lined with; the 
tiny screw was backed out far enough to get some of the liner on the threads, then seated again. 

I didn’t expect this to make any difference -- but I was wrong.  It made a big difference in how the fuel level gauge 
read.  Previously, filling the tank resulted in the gauge reading on the F, perhaps a hair under.  The level would drop 
fairly quickly to 1/2 full, then drop more slowly for the bottom half of the gauge.  After draining and sealing the float, 
filling the tank resulted in the gauge reading quite a ways above the F, probably pegged.  And it seems to move linearly 
from full to empty.  Similar results have since been reported by Steve Holst and others. 

Len Olsen provided an easier fix: go down to your local Ford dealer’s parts department and ask for part number C0AZ-
9202-B.  This is a brass float that will fit the XJ-S sender with only minor spreading of the wire loop at the end of the 
arm.  It also costs less than five bucks.  It reportedly comes in packages of two for some reason, so how much you’ll 
pay may depend on whether you can convince the Ford dealer to break open a package. 

Peter Cohen adds, “For what it's worth, the second character is a zero, not the letter O. In Ford part number parlance, 
that is the first year that the part was used.  In this case, probably 1990.” 

This is perhaps one of the easiest fixes on the XJ-S.  Drive the car until the tank is nearly empty, then peel back the 
carpet in the trunk and remove the sender.  Replace the float and reinstall. 

Earl Huff says, “I replaced the fuel sender float on my 1985 XJ-S a few days ago with the recommended Ford float.  
The good news is that it was easy, all went well, and the gauge for the first time reads as it properly should.  The 
puzzling part is that the original float appeared to be perfectly intact with absolutely no gas inside it.  Does anyone have 
any idea why the new float made a difference?  I guess there is really no reason to seek an answer to this being that the 
problem is solved.  However, I would certainly advise anyone to go ahead and replace the float even if the old one 
looks fine.” 

Apparently Jaguar became aware of this problem and did something about it.  Around 1987-88, the hollow cylindrical 
plastic float on the fuel level sender was replaced with a solid float that looks like a bicycle pedal.  The Ford brass float 
won’t fit -- but there shouldn’t be any need to fit it. 

Also note that the fuel level senders used in the H&E convertible are totally different, and the Ford float can’t be used 
there either. 

Another plausible method of improving fuel level sender performance would be to fit a weak spring to the arm.  The 
spring should be weak enough that when you hold the sender in the same position it’ll be in the car, the weight of the 
float will still overcome the spring and move the arm to the bottom of the stroke.  But the spring should take some of 
the weight of the float, so when it sits on the surface of the fuel in the tank it sorta bobs on top rather than being nearly 
submerged. 

 

VOLTMETER ACCURACY:  If you’re not sure to trust your voltmeter, Michael Minglin suggests “Pick up a cheap 
cigar lighter adapter, clip the leads and connect to a voltmeter.  This will allow you to monitor the voltage, with 
reasonable accuracy,  under different driving conditions.” 

If the voltmeter proves inaccurate, read the section on FLAKY INSTRUMENTS on page 588. 

 

 

 
 

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Fuel Guage:  Hess & Eisenhardt Convertible 

 

Mike Cogswell describes the system in the H & E convertible:  “Each tank has its own fuel level sending unit.  The 
upper unit is apparently the standard XJ-S tank unit.  The lower one is similar, except the mounting plate is horizontal 
instead of vertical.  The two gauges are wired in series.  There is a small circuit board in the H&E harness that 
theoretically turns on the low fuel level light. 

“My gauge is wildly inaccurate.  Because they are in series I’m guessing that my top one basically hits bottom well 
before the bottom one starts to drop.  As a result, my gauge is very non-linear.”  Another owner says, “My H&E gas 
gauge is like the stock market: sometimes up, sometimes down, always moving and seldom based on reality.” 

The low fuel warning light is no help.  Paul Hackbart says, “It is as unreliable as the gauge, as the signal is derived from 
the gauge.” 

One suggestion that’s been made is to disconnect the sender in the upper tank altogether and connect only the sender in 
the lower tank to the gauge.  In theory, this would result in a full reading for the first 14 gallons after a fillup, and then 
linearly drop from F to E as the 11 gallons in the lower tank are consumed -- odd behavior, but perfectly usable.  
Unfortunately, this presumes that the upper tank is used first, followed by the lower tank -- but the various 
idiosyncracies noted with this fuel system (see page 248) indicate this may not be a valid presumption.  Indeed, part of 
the reason the gauge fluctuates so much may be due to the fuel draining into the lower tank and then being pumped 
back into the upper tank, so that both levels are fluctuating constantly and randomly. 

 

Warning Lights 

 

The sensors and circuits that operate the indicator lights are discussed here.  The bulbs themselves are discussed on 
page 635. 

 

IGNITION (ALTERNATOR) LIGHT:  See page 569. 

 

OVER VOLTAGE LIGHT:  Yes, this is a different item than the alternator light.  On the author’s ’83, the alternator 
light is the fifth from the right and has a lightning bolt on it.  The overvoltage warning light is the fifth from the left and 
has a little picture of a battery on it.  It comes on when the voltage exceeds some set value, around 15.6V according to 
Joe Bialy.  In other words, it tells you when the regulator in the alternator has failed causing the alternator to 
overcharge.  The voltmeter should confirm the problem. 

Bialy provides some tests:  “The OV unit looks like a big old time thermal blinker unit.  It's mounted right by the main 
fuse panel on the driver’s side, plugs into a three terminal socket.  Take its R/B wire to ground and see the light come 
on bright.  If you have a variable DC supply, break the white wire and feed it varying voltages to see it in action.” 

 

BRAKE WARNING LIGHTS:  On the ’83 XJ-S, the parking brake light and the warning light for low fluid in the 
reservoir are separate lights.  Mike Morrin says, “On the earlier cars (XJ6 and XJ-S) they are different lights, but they 
both come on with the handbrake as the bulb test for the brake warning bulb.  My recollection is that the circuit that 
drives them is shown incorrectly in the XJ-S manual.” 

 

HANDBRAKE SWITCH CONNECTIONS:  Alan Baker provides some guidance:  “To check the wiring, (mine's a 
RHD if that makes any difference), remove the black plastic cover at the base of the handbrake lever.  The 3 wires on 
the micro-switch, starting from the front, should be black/white, then black/yellow, then black followed up at the rear 

 
 

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by a ring connector earth.  If the wires are switched, the warning light will remain on.”  You can look at an illustration 
of these connections in Fig. 9.22 in the Haynes manual. 

 

BRAKE FLUID LEVEL SWITCH:  The switch in the cover for the brake fluid reservoir is supposed to light an 
indicator on the dashboard when the level is low.  The rubber cover over the connectors has a bump in the center.  
Pressing the bump forces the float downwards and closes the contacts, providing a circuit and bulb test. 

Unfortunately, the switch is garbage and the indicator may never come on, or may stay on all the time.  The float for the 
switch is a piece of cork, which rots, soaks up fluid and sinks, etc.  The protective metal cover over the cork float gets 
full of junk and jams the float.  The contacts within the switch are exposed to the air and fluid within the reservoir, and, 
despite evidently being silver plated, get corroded and fail to make a connection. 

The cork is easily replaced with one from a wine bottle, and the metal cover’s problems are solved by removing it and 
throwing it away.  The contacts themselves can be serviced by using a tiny screwdriver to pry the switch assembly out 
of the top of the reservoir cover; don’t lose the little metal sleeves that keep the contact screws from tightening down 
onto the plastic.  But all such efforts won’t solve the basic problem: you should not trust your life to that switch. 

The Jaguar repair manuals indicate that either this switch or the pressure differential sensor will turn on the warning 
light, but after VIN 107981 the differential pressure sensor was deleted and replaced with a couple of simple fittings on 
the brake lines.  So, the fluid level switch provides the only warning you are likely to get prior to actual brake failure.  
This switch therefore needs to be ultra-reliable, since it is rarely tested and failure to work when needed can be fatal.  
While it’s easy enough to get it working with the procedures above, there’s no good way to get it to keep working.  The 
switch is crap, pure and simple. 

The only truly safe solution is to replace it, lock, stock, and barrel, with something reliable.  Something reliable was 
suggested by Walter Acker IV: the brake fluid reservoir from a Mitsubishi, which comes with the typical Japanese 
ultra-reliable floating-magnet switch built in.  A suitable remote reservoir is used on several Mitsubishis, including the 
93-96 Mirage 1.8L 2-dr, the Gallant 89-2/93, some later Gallants, and all Eclipses 90-99 and probably later.  Basically, 
find a Mitsubishi in the junkyard and check! 

If you happen to find a Mitsubishi with ABS, the reservoir is similar except that it has an extra part in it.  Doesn’t hurt 
anything; it will work too if that’s what you find in the junkyard. 

If you want a new reservoir, the part number is MB534534, and costs less than $40 from any Mitsubishi dealer -- but 
you’ll have to order it and come back to get it when it comes in, they don’t keep it in stock because it never fails.  That 
part number includes the reservoir itself, cap, diaphragm, float, and switch, but not the mounting bracket nor the hoses. 

The Mitsubishi reservoir actually looks very similar to the OEM Jaguar reservoir.  The wires for the fluid level switch 
go to the bottom instead of the top, since the switch is magnetic and is actually outside the reservoir.  Only the float -- 
with magnet -- is inside. 

The mounting bracket used to hold the reservoir in a Mitsubishi is not readily usable in the XJ-S; it will be easier to 
fabricate your own.  Joe Bialy says, “I mounted it in exactly the same place the old one was at.  A simple "T" shaped 
piece of sheet metal bent 90 degrees to mount in the 2 existing holes.  Then wrap a 3" hose clamp around the whole 
thing.”  This is an excellent plan; this author used it in my ’83, but rather than a simple “T” I designed the bracket 
shown in Figure 29 to hold the reservoir a bit more securely.