Hello,

1994 xjs 4.0 L - Trip computer is acting up(erratic readings) and I am also not passing California Smog. Does the trip computer affect performance or only display information from the rest of the car?

I have tried disconnecting the battery, shorting pos and neg leads for 30 sec, hoping to do a total reset of the all the electronics.

Was surprised to see the clock on the trip computer still showing correct time and the radio presets still set after reconnecting the battery.

Trip computer is still erratic.

thanks,

Bob

 

No, the TC is only calculating upon numbers and co provided by the ECU. It takes data. It doesn't give

 

The trip computer is a useful diagnostic tool, if it is reporting numbers that are out of the ordinary, it means the engine is doing something out of the ordinary - as evidenced by your smog failure.

In what way are you failing smog?

 

Are you getting FF codes when you first turn on the key ignition? This will assist you in determining where the problems are within the car. Turn on the ignition key, don't start the car. watch the computer screen. there should be a FF code pop up. I have a spare computer module from a 94 XJS, if you need to replace yours. But I doubt that your computer is faulty.

 

Thanks for the replies.

The trip computer does not show FF codes ( I am familiar with their use - helpful in the past). On starting, TC shows "check engine" then clears to normal display.
No FF codes are posted. I think this is unchanged from before the erratic behavior. I have always assumed that the "check engine" display was like the check engine light. First displayed to verify operation and then turns off unless there is a fault.

Numbers are erratic not just out of normal . For example, speed was once stuck at a reported at 22.4mph when standing still. Other times all numbers look normal.

The reset button does clear all the numbers except the clock.

Any idea why the clock did not reset when I disconnected the battery?

Smog test shows high HC etc. I wondered if the trip computer issue has put the car in a "safe mode" - in some cars this is a retarded spark, rich mixture operation to get you home...

Bob

 

When mine went wrong it did affect the performance.
I did 100 miles in 1/2 hour. Never moved so fast.lol

 

"Speed" displays the average speed recorded since reset. So a static display while parked is correct.

Was it by any chance Noon or midnight when you reconnected the battery?

 

The clock has a 2 minute memory, the battery needs to be disconnected longer than that to get the clock to reset.

The communication between the ECU and trip computer is one way, it's just an information display, it cannot do anything to the ECU.

Usual suspects for high CO and HC's is spark plugs/wires etc - misfire, O2 sensors getting old and lazy, thermostat stuck open, coolant temp sender not reporting correct resistances to the ECU or a bad mass air meter.

Lacking any further data, I would chase the O2 sensors first, followed by checking thermostat and coolant temp sender and then plugs/coils.

 

I believe on the 94 XJS, the is a connection for the on-board computer module. It is under the dash on the LH (driver side). You can go to O'Reilly's or Auto Zone, Advanced Auto, NAPA and they can connect their diagnostic test module. The readings will identify the issues you are having. I have a 93, it does not have the computer connection. That is why I get FF codes on my trip computer screen.

 

My 94 V12 doesn't have the OBDII diagnostic socket. It wasn't required until 96.