Jaguar XJ-S. Service manual — part 74

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An oxygen sensor failure causes the engine to run rich, which results in high emissions. The EPA requires that
emissions systems have to work for 50,000 miles, but the 1-wire sensors needed to be replaced every 30,000 miles. To
meet the EPA requirements, up to VIN 134285 Jaguar provided a warning light on the dash that lights when the EFI
system detects a bad sensor -- right? Wrong. The light is there, but it’s only a counter that tells you that you’ve driven
30,000 miles since the last time the counter was reset -- which, hopefully, was the last time the oxygen sensors were
replaced. The “service interval counter” that turns on the light is connected to the speedometer sender, not the EFI
system. Your sensors may still be just fine when the light comes on -- or they may have given up the ghost 10,000
miles back! There are reports of the oxygen sensors failing before the counter turns the indicator on. It is
recommended that the sensors be replaced when the light tells you to, and if the car doesn’t appear to be running
correctly don’t overlook the possibility that they have gone bad even though the light hasn’t come on yet. The test of
the closed-loop operation described above will tell you if they aren’t working. Another clue: the fuel economy will
take a precipitous drop if they aren’t working, like 2 mpg or more.

The service interval counter is in the trunk, hidden under the carpeted panel on the left side near the filler cap; you need
to remove a carpeted panel in the forward left corner of the trunk to get to it. The rearward-facing end of the black
plastic box has an electrical multiconnector, a metal device that looks like a solenoid, and a single-wire connector. If
you lean your head in a little farther, you’ll see that towards the right (and hidden around the corner of the panel the
counter is mounted on) there is also a small protrusion pointing rearward with a tiny white plastic tit on top. To reset
the timer to zero when you replace the sensors, use a pointy object to push the center of this white plastic button in (a
long ways, like a half inch) until you hear the thing reset.

If all that wasn’t clear enough, you can look at pictures of the service interval counter at:

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

Another option, obviously, is to simply unplug the service interval counter. Or remove the light bulb.

Note that the service interval counter described only appears on cars with oxygen sensors up to VIN 134285 -- the end
of the 1986 model year. 3-wire sensors were introduced with the beginning of the 1987 model year, and they last
60,000 miles between replacement. Since the EPA only requires that the emissions systems work for 50,000 miles, the
service interval counter was omitted.

There was a different type of “service interval counter” behind the dashboard in the early pre-H.E. cars that obviously
has nothing to do with oxygen sensors since those cars didn’t have any.

The mid-90’s cars come with the OBD or OBD II features required by California law, and therefore have fault codes to
indicate when sensors aren’t working.

3-WIRE OXYGEN SENSORS: Starting with VIN 134286 -- the beginning of the 1987 model year -- the XJ-S came
with “three-wire” oxygen sensors with built-in heaters to keep them at operating temperature even when the exhaust is
relatively cool. Such sensors were expensive at one time, but Tomco part number 11014 is available at Discount Auto
Parts for only about $40 each. Rusty Beard found another generic: “The Part # is 341-90-06.”

Regarding wiring up the generic 3-wire sensor, Peter Cohen says, “I bought a pair of Tomco (boxed as Lucas) 3-wire
oxygen sensors. These come with 2 white wires and one black wire. The instructions say to hook black to black and
white to either of the white wires. Only problem is, there are no white wires on the Jaguar. It has one black, one G and
one NS.” Rusty Beard explains how to connect it: “Black wire to G (to ECU). The white wires to Black and NS (This
is just a heater circuit).”

RETROFITTING 3-WIRE OXYGEN SENSORS TO EARLY CARS: It’s a very good idea to retrofit the 3-wire
sensors into the earlier cars that came with 1-wire sensors. Even discounting the lower emissions, better fuel economy,
smoother idle, and other possible benefits of upgrading, installing 3-wire sensors can actually save you money; the 1-
wire sensors have a useful life of 30,000 miles, but the 3-wire sensors have a useful life of 60,000 miles -- apparently
because the heating helps prevent the buildup of deposits.

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Randy Wilson reports on replacing one-wire sensors with three-wire sensors in his 6-cyl sedans: “I did it to get the O

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sensor up to temp, period. I run tri-Y headers on my cars, which puts the tap point for the O

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sensor (the collector) a

fair distance from the engine. It’s enough distance that, combined with headers not retaining the heat in the exhaust
stream, the sensors would cool off too much at around-town cruise speeds.

“This will not get you into closed loop any quicker. The ECU will not go closed loop until the engine reaches a critical
temp. But it will help in making sure the sensor signal during low-load closed-loop cruising is valid.”

You will need to provide a 12V power source to each oxygen sensor heater. In the later cars that came with 3-wire
sensors, the power comes from the fuel pump relay -- which means it shuts off after two seconds if the engine is stalled
for any reason. Whether or not this is of any importance, it’s not difficult to use this same wiring scheme on earlier
cars. Even though the fuel pump relay is at the wrong end of the car, there is an NS wire from this relay to the
supplemental air valve at the front end of the right side air filter housing. The two heated sensors will draw less than 5
amps total, and this existing circuit can easily handle that much additional load. Simply splice into that NS wire
anywhere convenient and run leads to both oxygen sensors.

Once you have converted to 3-wire sensors, you can leave the open-loop circuitry at idle in Park or Neutral alone, or
you can pull the shorting plug in the trunk to allow the car to run in closed-loop at all times like the later cars. You also
can decide what to do with the service interval counter in the trunk; you can leave it there and merely reset it without
replacing oxygen sensors every other time it goes off, or you can simply remove it and throw it away and just remind
yourself to replace the sensors every 60,000 miles.

OXYGEN SENSOR TESTING: MOTOR/AGE Magazine gives the following instructions for testing an oxygen
sensor itself:

“An oxygen sensor can be tested safely with a digital voltmeter, but an analog (needle type) voltmeter will destroy the
sensor in a heartbeat. A simple bench test has been suggested by Tomco Carburetors to check O

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sensor function. All

you need is a vise, digital VOM and a propane torch.

“Lightly clamp the oxygen sensor in the vise with the sensor flutes facing upward. Attach the digital VOM leads to the
sensor. If the sensor only has one lead, attach the VOM ground to the sensor body.” Ed. note: connecting the VOM
ground to the sensor body is the way to connect all sensors used on Jaguar XJ-S’s. If there are three wires, two are for
a heater; only the signal wire should be connected to the VOM.

“Set the VOM on the 2V scale, then light the propane torch (using regular heating tip) and apply direct flame to the tip
of the sensor flutes. The voltage should rise to about 0.8-0.9V within 60 seconds. Move the flame away quickly from
the sensor tip and the voltage should drop drastically; move the flame back and forth and the voltage should respond
quickly. If the changes aren’t almost instantaneous, it’s time for a new O

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sensor.”

David Littlefield says, “AutoZone claims they will check your oxygen sensors on their equipment for free as part of
their customer-friendly tool lending program. I think other parts places might do this as well, but I haven't tried it.”

FEEDBACK MONITOR SOCKET: Near the ECU within the trunk is a 4-connector socket on the end of a harness
with a dummy plug in it. Two connectors are a black ground wire and a KB wire with battery voltage on it. The other
two connectors provide a voltage between 0 and +5V (relative to the black ground wire) that corresponds to the amount
of correction the ECU is providing in its attempts to maintain the proper Lambda feedback. The GW wire (pin 4 on the
ECU) is for the A bank and the GK wire (pin 10) is for the B bank.

While there is a special Lucas tool that plugs into this socket, Roger Bywater points out that an ordinary voltmeter can
be used: “On a 6CU the monitors are voltage outputs. On a 16CU the monitor outputs are square waves which can be
converted to a voltage reading by placing a 47 μF capacitor across the voltmeter leads.”

Now, trying to make sense of what voltages you find. First off, you need to figure out whether the EFI system is
operating in open-loop or closed-loop mode. If the engine is cold, it will be in open-loop mode. If the car has a jumper
in a red 2-connector socket at the end of a wire near the ECU and the shifter is in P or N, it will be in open-loop mode at

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idle. You will need to pull the jumper to get the system into closed loop at idle with the shifter in P or N. If the car is a
later model with no jumper near the ECU and is warmed up, it will be in closed-loop mode.

If the ECU is in open-loop mode, both connectors should be reading a rock-solid voltage; this voltage will be 2.5V on
the 6CU as well as the 26CU and later ECU’s, but it’s lower on 16CU ECU’s. Walter Petermann explains: “The μP on
the 16CU does not put out a 0-5V signal. I just checked it with the scope, and it's about 3.8v max.” Since the open-
loop voltage is at the halfway point on the range, it will be about 1.9V on a 16CU. Whatever the open-loop voltage is,
it is the voltage that indicates the ECU is providing no correction at all to the fuelling; whatever the baseline fuelling
map calls for is what the engine is getting.

If the ECU is in closed-loop mode and the system is operating correctly, the voltages should be fluctuating up and
down cyclically. Roger Bywater says, “print this out and pin it on the wall:

Low Lambda = weak exhaust = ECU corrects rich = monitor rises

“Lambda voltage is low when fueling is weak so the ECU reacts by correcting rich. The monitor then moves high
(6CU), or increases the high part of the duty cycle until continuous high (16CU), with full rich correction. The monitor
outputs go low when the ECU is correcting by weakening (i.e. Lambda high = rich exhaust).”

If the ECU should be in closed-loop mode and the voltages are held solid against one extreme or another or are stable at
the open-loop voltage, something’s wrong with the car.

OPEN-CIRCUIT AT THE OXYGEN SENSORS: If you disconnect the oxygen sensors, the ECU will interpret the
lack of voltage as a sign of lean operation and trim richer, right? No. Roger Bywater explains: “There is some
circuitry which applies a mid-range bias to the inputs through some quite high value resistors.” What this means is that,
when the oxygen sensors are working properly, they overpower this mid-range bias circuit to send feedback signals to
the ECU. But when the oxygen sensors are disconnected, the mid-range bias circuit provides a backup signal for the
ECU to work with. Since it is mid-range, the feedback response is neutral, and the feedback monitor voltages will hold
steady at the open-loop voltages. In short, disconnecting an oxygen sensor effectively puts the ECU into open-loop
mode.

Interestingly, if you have a 16CU ECU, the same cannot be said for disconnecting the ground shield on the oxygen
sensor wires. Bywater: “The bias network is grounded externally via pin 23 through the Lambda screen so if that
ground connection is lost the bias goes high and causes the 16CU to trim rich if the Lambda sensor is still connected. If
the sensor and ground return are both lost it all sits at mid-range. The 6CU does not seem to react at all to
disconnection of ground to pin 23, it still drifts to mid-range as do the monitor outputs.”

IDLE MIXTURE ADJUSTMENT: The Electronic Control Unit for the EFI system is in the right side of the trunk, and
there is an opening in the ECU casing -- usually facing downward near the rear end of the box as it is mounted. Roger
Bywater: “There is a rubber cap and possibly a metal tamperproof plug inside which can be persuaded out with a fine
screwdriver, a turning motion and a little patience. The ECU needs to be removed for this really. Then the spindle is
accessible.”

The real challenge, of course, is getting a tool in the hole to be able to turn the knob. Bywater: “It is exactly like a
radio volume control shaft - 1/4" diameter with a flat on it. If the correct tool is not available it just needs a short piece
of tube to slip over the spindle then just carefully indent one side to engage the flat. Travel is about 3/4 turn. You
cannot do any damage by turning the idle fuel adjustment (it moves in ratchet clicks either way) other than by excessive
force.”

This adjuster affects the baseline fuelling map (before Lambda correction) at idle. If you’ve read the previous few
paragraphs, perhaps you have already figured out how to adjust it. First, you need to make sure your engine is actually
at idle; check the voltage at the throttle pot as described on page 295, and adjust it if it is not correct. Adjusting the idle
mixture will be frustrating if the ECU thinks the engine is actually off idle. You also should ensure that the idle RPM is
correct.

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Once idle is established, you should look at the voltages at the feedback monitor socket while in open-loop mode to
determine what the open-loop voltage is -- it should be 1.9V on a 16CU (with the 47 μF capacitor across the
connections mentioned above) or 2.5V on all other ECU’s. Then, if the car has the red 2-conductor socket on the end
of a wire with a jumper in it near the ECU, you will need to pull the jumper to put the EFI system into closed-loop
mode at idle in P or N (with the engine fully warmed up, of course). This should start the feedback voltages cycling.
Monitor the voltage on one of the two feedback voltage connectors and adjust the idle mixture knob until the readings
are cycling around the open-loop voltage. Then reconnect your VOM to monitor the other feedback connector; if its
voltage is also cycling around the open-loop voltage, you’re done. If its voltage is cycling above or below the open-
loop voltage, adjust the mixture until both sides are about the same amount off in opposite directions. Now you’re
done. If you had to pull a jumper, reinstall it.

What do you do if the 1-wire oxygen sensors cool off and quit working during this operation? Good question. The
best advice -- short of retrofitting 3-wire sensors -- may be to rev the engine up for a few seconds to heat them back up,
then drop to idle and check the setting before they cool off again.

Bywater elaborates: “The need for adjustment in Lambda mode is because if the ECU has to apply a lot of correction
the Lambda swings make the idle unstable and it hunts up and down in time with feedback.”

Note that the described procedure will result in nearly stochiometric mixture at idle without closed-loop correction,
which should pass emissions tests -- but might not provide the smoothest idle. For a smoother idle in P or N, you might
consider adjusting it a little rich, although in closed-loop mode (as soon as you shift into D) the ECU will just pull it
back to stochiometric anyway.

The idle mixture needs to be adjusted periodically as the engine wears. As the engine gets worn, it develops
progressively less and less manifold vacuum at idle. This reduced vacuum is picked up by the MAP sensor in the ECU
and interpreted as meaning the engine is drawing more air, so it responds with more fuel -- and the engine starts to run
rich at idle. Adjusting the idle mixture will bring it back into line.

ECU INTERNAL CONNECTIONS: The harness connects to the ECU with a 35-pin connector. However, within the
ECU itself, many of the wires from the harness are simply connected together. When trying to figure out how things
work, it may be helpful to know what wires are connected internally. To save you the trouble of opening it up to see, I
will provide the info here based on the ECU in my ’83 (part number DAC 2597). The following groups of connectors
are all wired together inside the ECU:

1-2-19

(ground)

8-9-27-28

(“B” bank injectors - open)

11-29

(“B” bank injectors - hold)

12-30

(“A” bank injectors - hold)

13-14-31-32

(“A” bank injectors - open)

16-17-34-35

(ground)

On the schematic in my repair manual, I have drawn boxes around each of these groups to remind me of these internal
connections.

ECU INTERCHANGEABILITY: The Lucas 6CU and 16CU ECU’s are “pin-compatible”, meaning you can replace
one with the other without making any other changes -- provided that everything else in the car is the same, such as
engine compression. Don’t try putting an ECU from a 12.5:1 spec car into an 11.5:1 car. The 26CU is not
interchangeable with the 6CU/16CU.

The 6CU has a reputation for poor reliability, especially in the fuel pump control circuit -- see page 253. Walter
Petermann (Corsaro, page 704) thinks the fuel pump control circuit is just the most obvious symptom of trouble; “Most