1996 Jeep Cherokee Sport 4dr with a 1997 4.0l (not HO) engine. Original motor threw a rod.

I've had this problem for a little while now.

Very high voltage, tach bouncing wildly, GEN light coming on, stalling when coming to a stop, no start, etc. After reading a bunch on here (best Jeep advice ever!), I sprayed half a can of throttle body cleaner into it and pulled off and cleaned up the Idle Air Control Valve, and it seemed to handle much of the problem, but it started happening again, eventually dying by the side of the road, and not starting. Cranking fine, no gas smell, but won't start.

To make a long story short (too late), I plugged in my little bluetooth OBD reader, and fired up my laptop. Using ScanMaster ELM but saw no errors. I turned it over and it fired up awesome. Voltage was at 14V and rock solid. No bouncing tach, no sputtering under load. It ran better than it had in a months.

I figured the software did something so I unplugged the scan tool. Immediately the voltage went wild.

Plugged the scan tool back in and it runs great, not a blip.

WTF? Could this be a bad PCM?

 

Or maybe the PCM's plug is dirty?

 

PCM Plugs all look great. No dirt, or corrosion even on any of the three plugs either on the plug or the PCM pins. Damn near looks new.

 

This is an odd one. I've only seen one other post where the Code reader/scanner actually make the Heep run. There was no follow-up on these posts to ID the fix.


Me thinks it's a ground issue, fixed by the DLC ground, where the scanner (hooked up) completes the ground path from the battery to the chassis. Meaning, the ground cable from the battery negative post to the right inner fender is compromised (corroded) and/or the bonding strap from the firewall to the #14 cylinder head bolt is compromised (corroded).


Disassemble these grounds and scrub the ring terminals clean using 400 grit sandpaper, and wire wheel the fender and firewall sheet metal nice and clean. Reassemble and give another try (without scanner hooked up).