A couple weeks ago I left the key on while I was at work and when I went to jump it off it'll fire right up and die within a second or two. I've checked fuel pump, fuel pressure, firing, throttle body sensor, crankshaft positioning sensor, map sensor, fuel resistor ballast and replaced the ignition switch ( which I thought was the problem because it had 2 burned/melted connectors) but nothing has worked. I pulled the ECM and pulled the cover off and didn't see anything burned or melted or any obvious problems. The damn thing will fire up with no problem as many times as I try but just wont stay running. Also it doesn't have any throttle response unless I hit gas while the key is still in the start position ( I know I shouldn't do that but I was trying to get as much info as I could). please help!! Thanks in advance

 

Pick-up coil (camshaft position sensor).

Got any engine codes?

 

I haven't pulled any codes. Did mean crankshaft position sensor or camshaft position sensor? Would the camshaft position sensor cause it to fire right up perfectly then die right off?

 

The codes will be important.

I believe leaving KO would either fry the coil or the pick-up coil (camshaft position sensor). Since it does fire the coil should be OK.

I am thinking that it is the PCM that is shutting the engine down after certain parameters are met (or not met). If the code doesn't reveal anything then I would test the CKS and CPS (you can do this a voltmeter)(an oscilloscope would be better)(but we can get into that later).

 

The 1989 Jeep you have is what is known as the RENIX (87-90). The Renix does not use a check engine light or have codes.

In 1991, when the XJ went to the OBD1 protocol, it started using a check engine light and you could retrieve a simple two character code with a "key-on, key-off" sequencing. 96-2001 XJ is OBDII which is a considerably more advanced diagnostic system with more detailed codes, but you need a code reader or a scan tool to retrieve codes.

With the RENIX, in order to do diagnostics, you need a decent multi-meter and the ability to understand and use it properly. Here is a link to a great site for all things RENIX from my buddy, Cruiser54. Lots of great tips and information. Be sure to bookmark it!

http://cruiser54.com/

As for your issue, you mention you left the key on and may have a wounded battery. The 4.0 engine is very sensitive to low voltage. And yes, you can have a battery that is strong enough to start the engine, but marginal enough to give the engine management system fits and create a whole bunch of different symptoms, with one of the main symptoms of a weak battery being engine stalling. Basics first; always. Have your battery "load tested"; any parts store will do that for free. Also freshen all engine grounds, this vintage of XJ is especially sensitive to poor grounds.

Good luck and keep us updated!

 

Ooops!

 

Nonetheless, checking the CPS (sync signal generator) would seem to be warranted. See p. 23:

http://jeep.avtograd.ru/cherokee/inj.../89jeepefi.pdf

 

10 4, I'll give it a check. I'll double check grounds. I appreciate it guys and I'll keep you updated

 

Any way to test the sync signal generator besides continuity. If so which pins?

 

Put your voltmeter on the fuel pump ballast resistor and watch it. One side should be at 12-volts briefly when the gn turns on, then again as soon as you crank and the ECM sees the crank turning and stay there once running. The other side will be +12volts while the resistor is bypassed during cranking, then drop some (to 7-volts I think) once you let off the key. You can also just jumper across the resistor to see if it's dropping too much voltage for a weak pump.

Also if you haven't already, check Cruiser's site at crusier54.com and refresh all the grounds.

 

Did you refer to the manual from the link I posted? Besides continuity, it explains how to check for power and signal generation.

 

See sequence of events in Start Mode and why SSG could be the culprit:

 

You can run a Renix with the sync generator unplugged...

I like the ballast resistor suggestion from lawsoncl!!

 

Jumping the wires going to ballast was the first thing I tried

 

No I haven't checked that link yet. I just got back to messing with it this evening but I'm definitely going to check it out. I'm ready to get this jeep back to running. Turkey season is right around the corner