Gauge Cluster lights up but gauges unresponsive + no bus

[Skruffy]

I know this is a common problem but all the usual fixes are not working for me, no surprise since my jeep is very unique.

What I have:
2001 XJ with an LS3 engine in it. I kept the original XJ computer and plugged the XJ sensors right into the LS engine. The XJ computer is supposed to act as a gauge mule and I expected all of the gauges except the Tach to work. None of them respond, other than the oil pressure gauge moves 1/4".
I checked all the fuses, no problem there. I checked all the grounds, even the dreaded G108. All good. I even re- grounded the transmission control unit thinking there was a hidden ground in that harness somewhere that was causing me problems.
If i hold the Odmeter button down while turning the key to on, my guage cluster goes through its little dance no problem.
I cleaned and recleaned all the connectors to the cluster. I also checked for continuity on the grounds (good) and the BUS + and - (good) it is getting the signal from the computer.

I measured the voltage at the DLC pins 3 to 4 and 11 to 4 I get 2.49V and 2.51V. I measured the impedance and got 60 Ohms.
I unplugged the TCM, ACM, RKEM. Still no gauges!
I even filled the gas tank.


Another problem that I discovered on this odyssey is that my scanner will not connect with the jeep computer. The scanner works on all my other vehicles. I get good voltage at the DLC. The ground on pin 4 was bad so I made a new one. I think that this has something to do with my problem. Any thoughts from you gurus out there?
Anyone in the san diego area that knows about these things?

 

[Anak]

How many of those sensors depend on a path to ground at the sensor? I know that on my '96 the oil pressure and engine temp are single wire, depending on their threaded connection to the block for their ground. Is it possible that you have done too thorough a job of sealing the sensors (i.e. too much teflon tape) and they lack a ground path?

Probably easy enough to test with a ground jumper to the body of the sensor. See if that wakes up one particular gauge.

Just a thought. And seeing that your fuel gauge is affected it is probably not the answer, though I don't know what all you may have needed to do at the tank.

 

[Skruffy]

The speed sensor and oil pressure sensor have three pins, so are grounded. I didn’t touch the fuel tank sender circuit at all so it should be fine. The coolant temp is ungrounded.

Jumper across those sensors to ground did not help. I think the problem is elsewhere.
Thank you for the suggestion!

 

[Skruffy]

Any body know of a can bus guru in the San Diego area?

 

[sidewaysstarion]

It sounds to me like you have something in the harness pulling the 5V rail on the PCM to ground. Do you have a CEL on the cluster when you key on? Is there a no bus error on the cluster?

 

[Skruffy]

SOLVED!!


On one hand I am embarrassed to report my success but on the other hand super happy to have identified the problem.


When I deleted much of the XJ harness I removed the 12V supply to the computer (A22). Foolish I know.


Reconnected that and the gauges jumped to life.