Jaguar XJ-S. Manual - part 75

 
 

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of my work comes from wholesalers, so I get boxes of them without any reference to what the customer's problem was. 
 In general, they have any combination of the following due to solder joints: 

A few % lean due to bad solder joints in the power supply 

Full lean intermittance on Map sensor 

One or both O² circuits not tracking properly 

Dead or intermittent (chattering) fuel pump circuit 

“A rough guess is that about 10% have only a bad fuel pump circuit.  Out of these I'd say that on more than 80% you 
can induce one or more of the other failures by just flexing the circuit board.” 

“You shouldn't pay more than $100 for a used 6CU unless it's been rebuilt by someone who knows what they're doing.  
The best replacement would be a 16CU.  They are a much safer buy "used" than the 6CU's.”  Not only do the 16CU’s 
appear to be far more reliable, they also have significantly better processing and programming built in. 

 

HOT ECU’S:  No, not stolen.  Walter Petermann reports that some 16CU’s have microprocessors inside that get very 
hot when running.  Apparently several different type microprocessors were used, and one type in particular pulls half an 
amp and therefore gets quite toasty.  The good news: apparently it doesn’t hurt anything; while the 16CU has the 
occasional microprocessor malfunction just like any comparable electronic equipment, there’s no significant history of 
heat-related failure.  Just the same, Petermann likes to take one of two actions when he finds one of these hot ECU’s.  
His preferred fix is to replace the microprocessor with one of the type that doesn’t get so hot.  “Unfortunately the thing 
has 64 legs and they are soldered on both sides (and through the hole) of the board. Replacing them takes almost as 
long as rebuilding a 6CU.  It's something you never seem to get comfortable doing.  There's traces running between the 
pads on both sides of the board.  If someone wants to try it, the replacement chip is an NEC D78C10AGQ.” 

His other fix is to install a 12V cooling fan (commonly available for cooling computer CPU’s) inside the ECU.  The fan 
is installed entirely within, there are no openings made in the case; it merely circulates the air around inside the box, 
thereby moving the heat from the chip to the aluminum case more efficiently.  “Yes, it's a PC fan.  It comes with a 
finned aluminum heat sink and claws to attach to a chip.  Unfortunately the uP in the ECU is not the correct size, so the 
heat sink is not used and must be removed.  You can glue the fan in place on the side of the ecu about 1/4" above the 
microprocessor.  There's a convenient small ledge running along the center line of the ecu case that it can also be glued 
on for extra strength.  Power can be taken from the filtered 12V source of ECU pin #22.  This is on the yellow 'tubular' 
cap C304 on the end that's close to the center-line of the ECU.  Ground goes to pin #35.” 

 

THROTTLE POTENTIOMETER:  If your XJ-S has ragged throttle response as though it has a bad accelerator pump, 
here’s some news for you: it doesn’t have an accelerator pump.  However, the throttle potentiometer, located 
underneath the throttle pulley on top of the engine, can cause similar symptoms when bad.  When operating properly, 
the resistance across this pot varies smoothly as the pulley is rotated.  If it is breaking up, it confuses the EFI computer 
on accels.  The EFI computer, using inputs from the oxygen sensors, can usually keep the engine running reasonably 
well at constant throttle, but it stumbles during throttle changes.   

The original 1980-88 pot, 73200 or EAC2670, is both unreliable and expensive.  Sounds like a Lucas part, but it’s 
actually made by Bourns.  Clearly, simply stamping “Lucas” on the top has an effect on reliability! 

 

THROTTLE POTENTIOMETER ADJUSTMENT:  When you replace the throttle pot, the Jaguar manual says you 
must adjust it using their special electronic tester.  Below is the alternate method. 

The throttle pulley assembly must be unbolted from its tower to adjust the pot, meaning the linkages to the butterflies 
are disconnected; but the idle stop is part of the throttle pulley assembly and therefore is unaffected.  You can even start 
the engine in this condition, as long as you don’t go above idle. 

 
 

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Have the ignition on, and the throttle pot assembly rotated to the idle stop.  The engine may be running or not.  Connect 
a digital voltmeter (the old needle type just won’t do -- go to Radio Shack and fork over the $40) to the red and yellow 
leads from the pot without disconnecting the pot from the harness.  This can be done by pushing the probes under the 
insulation on the connector. 

Adjust the pot by loosening the three small mounting screws and rotating the pot until the voltage reading is within 0.32 
- 0.36V.  Tighten the mounting screws and reassemble.  Make sure the reading remains within limits when the linkage 
is all together and the engine is running at idle. 

 

THROTTLE POTENTIOMETER REPAIR:  If you wish to attempt to repair the throttle pot, Matthias Fouquet-Lapar 
sends a procedure:  “Carefully undrill 3 plastic melt points from the bottom.  After about 2 mm you’ll see a screw.  
Using a small screwdriver undo these screws. 

“Open poti.  It’s actually a very good quality poti, (double contacts), but clearly showed excessive wear on light part 
throttle position.  Using my VOM I could find several places where the carbon really had gone away. 

“Mmmmh.  I decided to cut off the 4 contacts, shorten them by about 1 mm and resolder them, so they would end up on 
the unworn surface.  This is a pretty sensitive operation, be sure that you feel confident that you can handle it. You need 
some good small tools, a fine solder iron, some PCB cleaner and a very quiet hand. 

“Resolder everything, adjust contact height as before to have the correct pressure when putting it back together.  Also 
check if the contacts are really off the worn track. 

“Put it back together, check again with a VOM.  Smile, since you just saved a bunch of money.” 

“Total repair time was about 2 hrs.  However, I think one can do this kind of repair only once, or maybe a single contact 
could be used instead of two, giving a potential of a third repair.” 

This author has had two throttle pot failures so far.  In one of them, the conclusion was that the problem with this pot is 
that the wipers have little points on them so they contact the resistance track in a very narrow groove.  Although the 
track itself is quite wide, the wipers cut little narrow grooves through it until they’re riding on the ceramic below.  I 
fixed it by replacing the wiper in entirety!  I made a whole new wiper out of brass stock (available at any hobby shop) 
and fixed it to the plastic rotor with two tiny flush-head screws.  The wiper configuration is completely different, with 
wiper arms cantilevered from one end rather than the double-ended wipers of the original, but most of the difference 
was simply to make it easier for me to make with hand tools.  The significant difference was that each contact touches 
the track with an area of metal about 1/8” wide rather than the point used before.  Basically, the individual wipers were 
shaped by bending the end to form a folded edge.  The pot was temporarily assembled with a piece of fine sandpaper 
laid over the track, and the rotor turned back and forth a few times to polish a smooth contact surface on the wiper.  

Paul Bachman says, “this thread reminds me of a repair that I once did to a mechanical television tuner with a similiar 
failure mode.  I went down to the local jeweler and bought some small scraps of 24k gold (about $2) and soldered them 
to the wipers.  This provided a low resistance contact with very low wiper friction.  Never had another problem.” 

Of course, wiper modification doesn’t always work.  Peyton Gill:  “I had hoped to be able to repair the pot by 
relocating the contacts on unworn surface.  When the pot was opened I found that the contacts had worn through the 
conductive surface.  One set of contacts had worn so long on the ceramic substrate as not to be usable... As worn as the 
old pot was I’m surprised that it didn’t cause more problems than it did.  The nature and extent of the wear would lead 
me to recommend that everyone check their pot with a VOM.” 

In the author’s other pot failure, the resistive coating that the wiper slides on was properly positioned to overlap the 
metal conductor at one end of the travel, but at the other end it barely touched the metal -- and was making intermittent 
contact as a result.  Solution: go to an auto parts store and buy a kit for repairing a rear window defroster.  This kit 
includes a remarkably tiny bottle of copper-colored paint, a tiny brush, and a template for repairing the conductor lines 
on a rear windshield.  Using the tiny brush, apply some of this paint on the metal conductor and overlap it just slightly 
onto the resistive surface.  The result is a very reliable connection. 

 

 
 

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THROTTLE POTENTIOMETER REPLACEMENT:  If rebuilding the pot simply isn’t going to work, Jon Heflin 
suggests you replace it with a better pot -- from Jaguar!  From 1988 on, the XJ-S was fitted with a red pot, EAC9634, 
that is electrically similar but has a much better reputation for reliability.  While the earlier black pot was made by 
Bourns and had Lucas printed on it, the later pot is apparently actually made by Lucas and has the name in raised 
letters.  It is not a direct replacement because it requires replacing a couple of other parts to make it fit, so if you go to 
the Jag dealer and ask for a new pot for your model year they will still sell you the old style pot.  However, the new red 
pot costs so much less than the Bourns pot that you can buy the parts needed to make it fit and you’ll still be out less 
cash total.  Heflin says the red unit is “far superior in design and quality, it's even spring-loaded so as to assure it returns 
to exact same starting position.  I discovered that all that is necessary in order to use the updated unit is simply obtain 
the new style throttle shaft, shaft adapter, press pin, and an aluminum adapter to which the pot mounts.  The old style 
pot mounts with 3 screws and the new with 2.  The total for all the upgraded parts plus a new pot was right about 
$100.” 

Kelly Spongberg says, “After many false starts, I did acquire all the necessary parts to put the new style "red" TPS on 
the V12.  The parts are as follows: 

Part #  

Description 

EAC-9634 Potentiometer 
EAC-9567 Adaptor 
PA-103161/J 

Pin-Spring (came only as bag of two?) 

EBC-3568 Coupling 
EBC-3566 Throttle 

shaft 

“One strange note, EAC-9567 fits, but with only two of the three bolt holes lining up.  It works fine with the two bolts, 
just seems odd they would make a new adaptor for this upgrade and it not be perfect.  Perhaps it fits the 6.0L as well, or 
is from that car's parts?? 

“Both the local Jaguar parts departments were caught unaware that the original black TPS has been discontinued, and 
also that more than one part was required to install the red unit.” 

The “throttle shaft” is the steel part visible on top of the turntable and held to it with two screws; you can remove the 
two screws and slide it right out.  The only difference in the one for later cars is the configuration of the bottom tip.  
The “adapter” is a sleeve that fits the bottom of the throttle shaft and the top of the shaft on the red pot.  The “pin-
spring” is a standard roll pin.  You can see photos and illustrations of these parts at 

 

http://www.jag-lovers.org/xj-s/book/ThrottlePots.html

 

Patrick MacNamara says:  “The Bourns type throttle potentiometer is no longer available from Jaguar.  The dealer did 
not know if the two pots were interchangeable electrically and neither did British/Auto USA.  The dealer actually had 
no suggestion other than to try an aftermarket supplier for the old Bourns pot.  Thanks very much!  I actually had to 
provide him with the part numbers for the upgrade. 

“The 87-89 parts catalogue lists coupler and drive shaft for the new style throttle pot as EAC 9568 and EAC 9566 
respectively, but these have been superseded by EBC 3568 and EBC 3566. The strange thing is that if you pull up EAC 
9568 on the Jag parts computer it will list it as still available, but if you pull up EAC 9566 it says it has been superseded 
by two parts EBC3568 and EBC3566 which are clearly the coupler and drive shaft. 

“My 87-89 parts catalogue does not show the VIN changeover numbers for the new throttle pot, but does state engine 
number changeover as 8S61794.” 

 

THROTTLE POTENTIOMETER SUBSTITUTION:  It’s a potentiometer, and potentiometers are available at any 
electronic supply house for pennies.  Right?  Well, not right.  The resistance range of this pot is no problem; the rotation 
range is.  This pot must run through its entire range in 90° of rotation, and most generic pots from electronic stores have 
something near 270° of rotation.  Because this pot is used as a “voltage splitter” with 5V applied to the full range and 
the wiper picking off nearly zero volts at idle and nearly 5 volts at full throttle, you cannot merely use 90° of a 270° pot 
-- no matter what the pot’s rating is. 

 
 

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AJ6 Engineering offers a throttle pot kit that involves a little gearbox.  As the throttle turns 90°, the pot turns 270°, 
allowing use of the commonly-available and cheap generic pots.  Roger Bywater says their kit “just fastens in place of 
the original and actually costs less than the factory potentiometer on its own.  Then if a replacement potentiometer is 
needed a few years down the line - not that ours is any less reliable than the others but none of them last for ever - it 
will only cost £35 (at today's prices).” 

Another solution for a misbehaving throttle pot is replacing it with a pot from another type car.  Brian Sherwood relates 
his experience:  “From my spare parts pile I found a TPS from a Ford, an ’83 3.0L V-6, I think; part number E7DF-
9B989-AA.  It turned in the proper direction, same degree of travel (90 degrees), and had the same resistance as the 
original at both ends of its travel (from .05 to 3.5K ohms).  But no, it doesn’t bolt right up.  I cut a slot in a steel bushing 
to fit the drive lugs inside the Ford TPS.  The other end of the bushing I crimped down to fit the D shape of the shaft on 
the bottom of the throttle pedestal, that the old TPS used to fit over.  A piece of aluminum strap was used to clamp the 
Ford TPS to the bottom of the pedestal.  The TPS fits snugly in the recess underneath, and loosening the strap allows 
adjustment.  I cut the wire and plug from the old one, crimped on some female spade lugs to fit the terminals of the 
Ford unit  (would have been neater to use the proper Ford plug, too, but I couldn’t find that.)  Red wire=wiper, 
green=high side of pot, yellow=low side of pot.  With new TPS plugged in and ignition on, I adjusted the TPS to give 
.36 volts measured between red and yellow wires, at idle position.  Bolted everything down, and went for a test drive -- 
worked great.  Only difference from the original is that now it doesn’t surge at cruise anymore!” 

Gill, having failed to repair the pot, then went looking for a replacement pot:  “What type Ford is it for?  That’s the 
same question the guy at Pep Boys asked!  I told him it was for a V12 Jag (always enjoy the expressions I get from 
these counter guys). 

“The pot I got at Pep Boys is a Borg-Warner part #EC1046.  This was matching cross reference from Brian Sherwood’s 
Ford part #.  The only challenge was machining a coupling to go between the throttle assembly which has a shaft with a 
flat and the pot which has a hollow shaft with two splines within.  I took an old worn out 1/4 inch drive socket (5/16) 
and drilled out the drive end so I would at least have a little flat on one side for the throttle assembly shaft.  The other 
end I took a file and worked a slot on each side to go up into the splined hollow shaft of the pot.  Crude but effective. 

“I reused the wiring and connector off the old pot.  The replacement pot has three small male spade connectors which I 
had the luck of having the correct size female ends.  About 1/2 size of standard spade connectors.” 

Despite all these success stories, this author was unable to find a workable Ford TPS at a local parts store.  There were 
many, many Ford TPS’s to choose from, and basically all of them had the requisite total resistance, but all had too 
much resistance at the idle end of their travel -- the wiper was physically prevented from getting anywhere near the 0Ω 
end, and in fact won’t get near enough to attain the 0.32-0.36V adjustment at idle.  Of course, you can just forget about 
that adjustment, which will result in the EFI system operating in “run” mode at all times, even at idle, and never 
actually going into “idle” mode.  This may be acceptable, although it may result in an unstable idle, especially in earlier 
cars with unheated oxygen sensors. 

A better solution appears to be to select a GM throttle pot instead.  Some GM throttle pots have a simple metal lever on 
the shaft, so all you have to do is devise a lever on the Jag turntable shaft to engage it and a plate to mount it on.  The 
GM pots all seem to have plenty of range for the idle adjustment.  Be sure to select one that turns the correct direction; 
some of them may rotate the other way! 

Also try to choose a pot that’s relatively small and compact.  If you select one with something hanging out the side too 
far, then trying to adjust the pot’s position at idle may have it running into one of the four posts supporting the turntable 
assembly.  Interference can be avoided by carefully aligning the shafts when making the coupling so that the pot is 
swinging in between the support posts, but it’s a pain trying to keep such things straight in your head; better to choose a 
pot that could spin all the way around without hitting anything so you don’t have to worry about it. 

Regardless of which pot you select, David Johnson came up with an excellent method of mounting it.  Basically, unbolt 
the turntable support from the four studs it stands on and take off the sleeves that establish the turntable’s height.  Cut 
all four sleeves into two pieces each.  Slide one piece onto each stud, and then slide on a rectangular mounting plate you 
have fabbed up.  Slide on the remaining portions of the sleeves, and bolt the turntable support back on.  “The kerf was 
exactly the thickness of the aluminum sheet (3/16), so everything reassembles as if the plate was meant to be there.”  
You can establish the vertical location of this mounting plate by choosing where you’re going to cut the sleeves in two.