No Codes at OBDII with known failure

Hi All, I can't get any codes thrown from my ECM - it always reads "No Codes, Pass" with things like the CPS discon.

1996 Jeep XJ SE Cherokee, 4.0l, Auto Trans, 4x4, 247K miles.

How this was discovered - yesterday I was troubleshooting a "cranks but no start" condition after the Jeep engine suddenly died while driving down the hiway. Turned out to be the ignition coil.
First troubleshooting step was to hookup the Actron CP9135 scanner & found no codes. There was (unknown psi) pressure on the fuel rail. I was alone so couldn't check for spark while cranking.

I tried unhooking these sensors then cranking the motor: CPS, Distributor, expecting to throw a code & maybe get a hint where the prob was. No Code thrown. I unhooked MAP, then TPS, then Idle Solenoid (now just looking to get a code) & after cranking still no code!

Someone came along & cranked for me & I found no spark from the coil, but signal in, replaced the ignition coil & engine is running good again.

But, just to confirm what I saw yesterday, I unhooked each of the above sensors one by one & cranked the motor, & got the expected engine symptoms (either no start or runs horrible & dies) but no codes!

Everything hooked back up & engine runs great again.

The scanner does the "linking" and all that and appears to be actually communicating with the ECM.

Has anyone seen this?, scanner prob?, ECM prob?

OBD-II does not throw codes immediately. It also MAY ignore start up issues, not sure.

Does your scanner look for pending codes? Not all do.

I had one spark plug wire disconnected on a 96 ford for hours of testing that never threw a code, but it was still in the relearn cycle since the battery disconnect/rebuild. Seems it does not throw codes until all the emission monitors are locked in from a proper set of relearn procedures.

Old school repair know how is still a must know science.

KRinAZ,

'96 is supposed to be OBDII but if its an early build it may not have the SCI receive and transmit wiring to the Data Link connector (see pic for later OBDII XJ SCI wiring.

The 'Key Trick' may reveal set DTC [codes] for your '96.


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96 was the first year for most OBDII vehicles and was a work in progress.

The 96 cherokee is electrically different in some ways than 97-01. There are differences with some of the wiring and sensors such as for the fuel pump, fuel gauge, instrument cluster, CCD bus, transmission, etc. all which connect to the 97-01 PCM.

Possibly the problem could be your with your Actron CP9135 scanner.

Scanners normally include generic OBD and proprietary OBD for specific vehicles. Possibly when Actron was creating the OBDII firmware they didn't include certain functions for some vehicles in 96.

I would email Actron and ask them if you should be seeing DTC's when connected to your 96 cherokee.