awg

We have discovered the wiring is different in some respects for the '96

If the fuel pump whirs, that is strongly suggestive your PCM is not completely dead, as both the ASD and fuel pump relay must be working for that to take place

There is a good Youtube on tracking down "No 5V on Jeep Cherokee XJ"

sorry dont have the link, but its from a pro technician doing a "lesson"

I would search hard for that one, it involves uplugging components till the short goes away

Could be the wiring though...one my last XJ, nearly every wire in the rear harness had cracked plastic and you could see copper

I let out a girly scream when I saw that

It had a periodic problem of losing all voltage (including tailights) for a few milliseconds, not long enough to stall, but it would be like a big miss

Edit; 99% sure this is the Youtube..extra special bonus, its a 96!

Dave51

Try this to retrieve codes. Should work on a '96::

Put the key into the ignition, push down the odometer reset button, then turn the key to RUN with your finger on the button - and then release the button. The odometer will go through 1111111 through 999999, then display the car's serial number. It will then show the error codes (it might show them starting with P).

Don Oswald

Thanks. I've watched the Scanner Dan video several times previously. He gives me some hope.

Don Oswald

Dave51 - unfortunately the odometer is that old analog type, not electronic.

Dave51

Well that's a drag.

Do you have anti-lock brakes?

awg

I would go back to the section in the '96 Manual that outlines the diagnostic tree for the ASD

( or check out a previous link I put up that is a diagnostic manual for the '95, even though its OBD1)

If you have no spark it is possible you have a short in the FETs that drive the spark ground, so even though the PCM still works the fuel pump, it wont give spark

A diagnostic tree is very helpful, otherwise you are blind

It certainly could be a shorted wire, as another poster suggested, who seems like a very experienced pro tech, cutting the PCM wires one by one is the diagnostic for that, but he is a pro and you are not, so its not so easy to proceed

One thing I can tell you is for a '96, only another '96 PCM can be substitued

Dave51

Seems like it would have to be.

Instead of cutting wires, why not just disconnect modules? I'd say the short is more likely to be distal vs proximal.

Or just check everything near the exhaust system (I mean, ScannerDanner could have saved a lot of time by just looking under the hood and seeing what was smoking).

Start with CPS, then oxygen sensor, then ABS (if so equipped).

Is inability to link OBD reader part of this problem or something different?

Probably part of the same issue, but I'd still check the voltage at the DLC:


Anxious to hear what the solution is!!

Dave51

Those odd voltages of ~2.5 V are because that's how messages are being sent:

awg

I agree your suggestions are all preferable to cutting the wires, that is a last resort imo

The poster who mentioned it implied he has done it a few times, (and it may save you time if the short is in a wire close to the PCM, and you cut the wires in the best order)

I think you would have to know what you are doing even to intepret results of cutting wires one by one

Dave51

Yeah, that would be the downside of simply unplugging connectors.

I also wonder if the failure to receive a tachometer signal is also a separate issue, or part of the overall problem. It is not clear in your '96 manual where the tach signal comes from (they just say PCM) but TTBOMK it comes out of the CPS. I'm thinking to focus on CPS wire.

Dave51

OTOH, if CPS isn't getting 5V to begin with, I guess PCM wouldn't be getting a great signal to forward to tach...

awg

here is the "Diagnostic manual" this is a Chrysler OEM publication for their technicians

unfortunately it is the OBD1 '95 version, but it has the full diagnostic trees in it !

The OBD2 version is not available for free download,(that I know of) but I have seen it on USA ebay as a book

" Powertrain Diagnostic Procedures Manual" - Jeep Cherokee Forum

agreen

Yeah, it's very simple. Unplug all the sensors (one by one) that use 5v. There's only a handful of them. 5v ref returns? It's a sensor (the last one unplugged). If it doesn't return, make the cut. If 5v returns, it's shorted in the wiring.

Don Oswald

Spent most of today trying to trouble shoot this with my son who is a a Boeing 787 tech. After trying all of Scanner Dan's recommendations, and several other found on the interweb, we unplugged all the 5v supplied sensors without identifying a problem, connected them back up and jumped the ASD (no luck), then I crawled under and put my stethoscope up to the fuel tank and didn't hear the fuel pump running (that my son said he previously heard running). Pulled the fuel pump relay and jumped 12v directly to the pump and the pump ran and we got a good spray out of the Schraeder valve when we depressed it. Prior to this we only got a dribble of fuel out of it.
The fuel and volt gauges work when we turn on the key, but still no check engine light and "No Link" when I connect up my OBD II scanner. The tach doesn't move when cranking the engine either. Cleaned and checked the engine to firewall ground, the negative battery cable to fender well, and the cable to block connection. All checked okay initially but we cleaned them anyway.
So as a last resort we've ordered another PCM. I don't like replacing expensive electrical/electronic stuff without knowing for certain that it is faulty but we've run out of ideas.
Should have the replacement PCM in 7-10 days so will update this thread once we install it. Crossing our fingers.

awg

You need a spare PCM if you want to keep your '96 running long term anyway