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.
"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.
and Design © 2002-present WanderingTrail, Ron Seegert
Common Sense and Safety should always be observed when working on your vehicle or doing modifications. Jackstands, wheel blocks, disconnecting the battery are a few of the basic safety precautions that should be used and may not be mentioned in the write ups on this site. You are responsible for your own installation, these write ups are a helpful guideline and should not be taken as an official installation instruction. My write up may be different from the kits currently out there, so alwasy double check the manufacturers installation instructions when installing anything. I try to keep the site up to date with changes that have occured as I discover them, but may not have the latest unless someone lets me know. If you feel that an install is above your capabilities after reading my write ups, I recommend getting together with a club and getting some help. Only a few times have I needed to employe some actual help from a shop to get something done. Usually welding or A/C work.
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