BACKGROUND:
In the TJ as in most modern cars, there are multiple computers - they
pass information to each other on a digital buss. This is the DaimlerChrysler
PCI (Programmable Communications Interface), which interconnects the various
microprocessors via 10.4 Kbps signals sent on a single wire using a scheme
called VPWM (Variable Pulse Width Modulation) (an SAE standard).
The primary CPUs in the vehicle are the PCM (Powertrain Control Module),
the SKIM (Sentry Key Immobilizer Module, if present), the ABS module (if
present which it is not on any Rubicon), the Cruise Control, the Instrument
Cluster, and the TCM (Transmission Control Module). That last is the source
of that annoying upshift arrow on the NV3550, but also monitors stuff
like clutch fluid volumes, so you can't just start carving out circuits
- in fact the DRBIII is needed after changing clutch parts such as cover
or disk, even.
It does however mean that if lightning strikes your Jeep, it's going to
be too dead to even go into "limp home" or Open Loop mode.
THE TEST:
"Instrument
Cluster Actuator Self test" while standing by with a pen and pad
to write down any error codes:
1) Start with the ignition key in the OFF position.
2) Depress the odometer/trip odometer button.
3) While holding down the button, turn the ignition to ON (not START).
4) Release the button and watch the displays and gauges. You will see/hear:
a) A single chime to test the chime generator
b) The number "8" will scroll across the odo display - note
any bad segments
c) Decimal point test for the display
d) The software version (example "SOF 8.9")
e) Last six digits of the VIN
f) A fault number you need to write down (or if none, it will say "no
FAULtS")
g) Indicator test (one at a time the indicators come on in sequence)
h) Gauge test - the needles move to calibration points - confirm full
travel on each gauge face.
Once the test is over, the gauges return to normal. If the gauge(s) or
indicators are defective, you must return to the dealer for service -
only a DRBIII tool can be used for further diagnosis or to replace an
instrument cluster (it has to be programmed with the VIN and the option
list for your vehicle)
Thanks to KaiserJeep on JU for originally bringing this up.
