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6.11 - Instrument Panel Electronics ( John Ping,  October 20, 2001 )

On XJ40s, whether Model Year 1988 / 1989 or Model Year 1990 through 1994, the "Instrument Panel" is microprocessor controlled. Although it sounds technically sophisticated, it quite conventional by modern design / production standards. All signals, regardless of their original source, pass through the Instrument Panel's "electronic pack" prior to be routed to the appropriate gauge or display. All instruments within the Instrument Panel are replaceable on an as needed basis. Thankfully, the Instrument Panel has a solid reputation for being quite trouble-free (especially, the 90-94 years). All Instrument Panels were manufactured by Lucas and are considered precision devices.

The early models (pre-90 Model Year) utilize a combination of analog gauges (speedometer and tachometer) along with quasi-digital segmented display for ancillary parameter monitoring. As true to the early years, this Instrument Panel is subject to problematic quality control. The design is known to suffer from "cold solder" joints and other electrical connection concerns. Typically, the problems can be resolved with some judicious troubleshooting and re-soldering.

The Instrument Panel was revised to a more "driver friendly" conventional analog gauge layout for the 90 through 94 Model Years. The later models use 58 position stepper motors as the "four" ancillary gauges (voltage, fuel level, coolant temperature and oil pressure). As previously mentioned, the signals for these instruments are feed into the electronic pack microprocessor and then independently supplied to each Instrument Panel gauge. The speedometer and tachometer are the only true analog instruments, but again, being supplied via the electronic pack.

Note: Instrument Panels between the early model years and the later model years are NOT interchangeable. Panels can be interchanged between the associated Model Years, but the original "electronic pack" should be retained if possible to ensure the odometer reading for the vehicle is correct.


6.11.1 - Instrument Panel Removal ( October 20, 2001 )

Removing the Instrument Panel is a simple task. Do not be intimidated by the electronics or the fact that you are "pulling something out of the dash" as it is truly easy. The following is a step-by-step method for removing and replacing the Instrument Panel.

Step #1 - Disconnect the negative battery cable. Always ensure that this action is performed as there are continuously hot circuits in the area of the driver's footwell and Instrument Panel.

Step #2 - Remove the driver's footwell upholstery panel. There are typically four or five No.2 Phillips head sheet metal screws and two fir-tree type push-pin type connectors holding the upholstery panel to the frame.

Step #3 - Remove the driver's footwell sheet metal panel. Again, there are typically four No. 2 Phillips head sheet metal screws securing this panel to the frame.

Step #4 - Remove the "passive restraint module" (or other electrical components) from the left side of the steering column. This action is to provide additional working clearance when removing the Instrument Panel from the dash. The passive restraint module (or other components) is typically mounted with one or two No.2 Phillips head machine screws. There is no need to electrically disconnect these devices, just drop them from their mounts and position out of the way.

Step #5 - Release the Instrument Panel two multi-pin electrical connectors on the backside. Lying on your back within the driver's footwell, locate the rear of the Instrument Panel (crème colored plastic device approximately 15" in width). Connector "A" is the smaller of the two and is located on the right side (based on your current positioning). Squeeze the small release tabs in the center of the connector while pulling outward. The connector will release without significant effort. Connector "B" is the larger of the two and is located on the left side (based on your current positioning). Squeeze the small release tabs in the center of the connector while pulling outward. Again, the connector should release with minimal effort.

Step #6 - Remove the Instrument Panel mounting screws. The Instrument Panel is secured using four No. 2 Phillips head machine screws. Again, while lying on your back within the driver's footwell, locate the two screws on each side of the lower Instrument Panel. At the bottom front corners of the Panel, locate a single machine screw on the black frame. The second machine screw for each corner is located within the Speed Control / Trip Computer or Lighting Control Pods. An access hole to allow removal of these fasteners is provided in each Pod. Disconnecting or removing the Pod is not required. With the four fasteners removed, the Instrument Panel is ready for removal.

Step #7 - Dislodge the Instrument Panel from the dash. While lying on your back within the driver's footwell, use your hands to apply force on the rear side of the Instrument Panel attempting to push it towards the steering wheel. It will dislodge without tremendous effort as its just a simple press-fit.

Step #8 - Re-position the Steering Wheel and Remove the Instrument Panel. Release the steering wheel positioning level and pull the steering wheel outward as far as possible. It is not necessary to remove the steering wheel to facilitate Instrument Panel removal. With the Instrument Panel dislodged from the previous step, guide it outward and sideways for complete removal. The Instrument Panel should now be within your arms.

In this case, re-installation is honestly the reverse of removal. Just ensure the Instrument Panel is properly re-fit in the dash prior to installing fasteners. The Instrument Panel dash pad has locator tabs at the front corners, which mate with slots on the lower dash frame. Significant effort is not required to re-install the Instrument Panel into the dash.


6.11.2 - Instrument Panel Electronic Pack ( John Ping,  October 20, 2001 )

The Instrument Panel "Electronic Pack" is mounted to the rear of the panel. As mentioned before, it is a crème colored rectangular shaped plastic module with dimensions of approximately 15" length x 5" width x 2" height. Inspect the two multi-pin female type connectors located on the rear of the pack to ensure / rectify any visible defects such as deformed pins or corrosion. Always use a quality electronics parts cleaner to removed films or dust/dirt from these connectors. No petroleum-based products should be utilized! With the Instrument Panel inverted to allow access to the "microprocessor electronic pack", remove the ten (10) No. 1 Phillips self-tapping screws which are located around the perimeter and connector portion of the module.

With the cover plate removed from the module, the microprocessor board is visible. There are no "owner" repairable items on this board. The microprocessor provides output to the gauges / displays via the use of three "printed circuit board type" ribbon strips. Release each ribbon strip by pulling up on the strip locks on each end of the ribbon. With the locks released, the strip can simply be pulled upward to release. This step should be performed to the other two strips.

The electronics pack can now be disconnected from the Instrument Panel by releasing the two plastic clips at the bottom and pivoting upward on the module to approximately a 90-degree position such the hinges on top will release. Note that the hinges on the module are split to allow separation from the Instrument Panel.

Note: The electronic pack has a specific "speedometer calibration". If replacing this item, ensure the replacement electronic pack is marked with the exact "speedometer calibration" as the original electronic pack or the speedometer indication will be incorrect. The odometer reading is stored within the electronic pack. When installing a replacement pack, the odometer will not readout with the original vehicle mileage.

As with the Instrument Panel, installation is truly the reverse of removal. Ensure the three printed circuit ribbon strips are securely installed on their connectors and the locking devices are in proper position prior to re-installing the rear cover plate.this case, re-installation is honestly the reverse of removal. Just ensure the Instrument Panel is properly re-fit in the dash prior to installing fasteners. The Instrument Panel dash pad has locator tabs at the front corners, which mate with slots on the lower dash frame. Significant effort is not required to re-install the Instrument Panel into the dash.


6.11.3 - Gauge / Warning Light Pack ( John Ping,  October 20, 2001 )

A word of caution, the gauges are precision devices and are easily damaged if handled in a careless manner. Ensure that your work area is clean … including your hands. To access the Instrument Panel gauges, it is necessary to remove the Instrument Panel clear bezel. This step is accomplished by releasing the ten (five top and five bottom) plastic clips located on the gauge pack. With the ten clips removed, carefully separate the two housings.

Support the gauge pack in a manner such that the instruments are not touching any surfaces. With the gauge pack vertically supported, it is now necessary to remove the wood veneer panel. It is attached to the gauge pack with seven No. 1 Phillips head self-tapping screws. Fastener access is from the rear (printed circuit side) with two screws along the top, three screws in the middle section and two screws along the bottom. The Vehicle Condition Monitor (odometer) will release with the veneer panel. Ensure the four-pin black connector is released (pull upward) and the two odometer light bulbs are removed. Carefully separate the veneer panel from the gauge pack. Keep the gauge pack positioned vertically to ensure no instrument damage.

Remove any instrument as desired by first locating the mounting tabs for the affected instrument on the rear of the gauge pack. Press the small tabs to the right and left side of each pin to release the pin. This is precision work … be careful! Each instrument has three pins. The ancillary gauges can be replaced independently, but the speedometer / tachometer is replaced as a single module.

Install the new gauge as desired by pressing the three pins into their circuit board female receivers. Carefully position and re-install the veneer panel using the seven small self-tapping screws. Carefully position and join the clear bezel housing to the gauge pack. It is wise to clean the inside and outside of the clear bezel using a glass cleaning solution prior to re-attachment with the gauge pack.

A quick check for electrical circuit continuity can be performed using a conventional digital multi-meter. Trace out the circuit path from each instrument pin to the printed circuit ribbon strip end. Each circuit should have minimal resistance. Mount the electronic pack to the rear of the gauge pack (hinge and clip mechanism). Carefully mount the three printed circuit ribbon strips to the connectors on the electronic pack. Ensure the ribbon strips are fully seated and the connector locks are properly positioned. Mount the electronic pack rear cover plate and you are finished.

Follow the directions for re-installing the instrument pack into the dash.

Note: If the tachometer fails for no apparent reason, it its possible the inline buffer resistor (approx. 6.8k ohms) between the negative side of the ignition coil and the Instrument Panel has failed (open circuit). All models may not incorporate this design, but absolutely check the wiring schematics to determine if it applies to your vehicle. If this problem occurs on your vehicle, just bypass the original failed resistor and install a new one in the circuit to restore tachometer operation. It's a very simple matter to splice the new resistor circuit into the existing wiring harness near the ignition coil.

Note: The warning lights (tan holders) used on the Instrument Panel are the "long-life type". They will rarely need replacement as they are seldom illuminated. The illumination lights (white holders) on the Instrument Panel (center of the electronic pack) should be inspected (slight twist to the left and pull out) for deterioration (smoking / graying of the inside of the bulb). Replace any suspect bulb prior to re-assembly. The two odometer (VCM) lights at the bottom of the electronic pack (tan holders) are always illuminated and it is prudent to replace these bulbs (or swap with other low priority warning bulbs if replacements are unavailable) prior to re-assembly.


 

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