Hello everyone.
Back again with a very unique problem with the jeep. I really need help on this one as it is way beyond my comprehension.
This is a 2003 Jeep Grand Cherokee 4.0L inline 6. Over 125K on it and still going strong,,well sort of.
Anyway here is the problem: Wife went to the store and was coming home when Jeep just died in the middle of the road. Some guys pushed it into parking lot and mentioned that she was out of gas. I knew it had a half tank in it and was not empty. Turned out that when you turn on key NONE of the gauges would work and engine would not start.
The key will come on, the engine will turn over strong but not start, just spin, as it has new everyready battery.
Sometimes after it sits for awhile it will start right up and die as soon as you try to move it.
I jiggle the ecm connections and nothing helps it. I sprayed all the terminals and replaced them, nothing helps.
Now here is the tricky one: I accidently discovered that if I turn on the headlights and then turn on the key the gauges work and the jeep starts right up.
Once it starts I can turn the lights off and it will stay running. Don't know how long it will run after that , I need to do more testing.
So if anyone has any ideas on this I would really appreciate the help.

 

Take a good close look at the positive (+) battery clamp. The smaller of the 2 wires goes to the PDC under the hood and supplies battery power for everything except the starter. If it's the least bit corroded inside the lead terminal, it could cause an intermittent voltage drop enough for the PCM to shut off the engine. When you pull the headlights on, it burns a good connection and it'll start. This happened with my 2000 WJ. A cheap pair of bolt-on battery cable clamps solved the problem. Just like you, I only discovered the headlight trick by accident trying to check for battery power. I thought it was dead.

 

OK, I been working on this thing forever and made little progress so far. It looks like it boils down to maybe 3 things that can go wrong. One is the ECM which I have coming to me from a vendor, I also had him remove the SKIM program.
Second it could be the SKIM system, so I have removed that SKIM thing and stowed it far from vehicle.
Third , it COULD be the CRANKSHAFT POSITION SENSOR, which I have also bought and it's in the mail.
So, the ECM and the SKIM are friendly and easy to R&R.
HOWEVER, that CPS thing is a night mare and difficult to get to.
So, I need help here,,, does anyone know where the cps plugs into? I found the CPS hiding at the top of the bell housing on the drivers side, impossible to get to.
Then I see that the connecter wire from it, goes up and over the bell housing to the RIGHT side of the engine and basically disappears! Can anyone tell me exactly where that thing goes to and where does come out on the other side and what does it connect to? I have tried every angle and can not even find the wires as they come over the bell housing.
ANY help here would be greatly appreciated. AM I looking for a junction box, a single connector to connector or?
BTW, I did check that battery wire idea and that was not it. Turns out that the light trick was just a fluke and only worked one time.

 

The connector is just underneath the plug for the coil pack. When I changed mine, I tied a string to the plug and pulled it out, then tied the string to the new one and pulled it in. The reason it's over there is that's the side the one for the 4.7 is and they used the same harness.

The easiest way to get to the CPS is what I did. remove the crossmember bolts and lower the trans down as far as you can without tearing any hoses up front or cracking the exhaust. Then you'll have a bunch of room for a socket and ratchet without extensions. Otherwise you'll need about 3 feet of extensions and a swivel socket.

Then there's this way; go thru the floor like this;

 

My vote is no to the cps, ckps etc because it starts when he turns the headlights on. Back to inspecting the battery/fuse block, ign switch

 

+10!!!

Don't mess with the CPS! That's not causing the lights out situation! Fix that and you'll probably fix your no-start.

Troubleshooting 101: FKPF!! FIx Known Problems First!

By the way, the easiest way to get to it is with a 30" extension, a u-joint, and a 2-3" extension. Piece of cake.

 

If that's aftermarket, send it back without opening the package. 1, that's not your problem, and 2, you ONLY want OEM (dealer) parts when it comes to Cherokee sensors. These beasties are just picky about that!

 

Well everyone, the problem has been solved. After tinkering with the idea of changing the CPS I decided that I agreed with those of you who said it was NOT the CPS so I left it alone. Next the Jeep would not start even turning the lights on or kicking it or slamming the hood or anything else strange. So, I bought another EMC and had the owner of it REMOVE the security part of the programming, before shipping it to me.
I then installed it and it started right up. Seems when the EMC goes bad it can cause strange things to happen. In this case the security programming apparently went haywire and shut the system down for "safety reasons". I don't know what is safe about a car running down the road and just shutting off midstream , no power , no brakes, no steering etc. The idiot that designed the shutdown feature should be fired.
Anyway, I want to thank you all that have contributed to the thread and I really appreciate your efforts.