fuseblower

There we were out in the French countryside on our way to collect a chest freezer with our 5.5 meter trailer when.... the engine died. Terminally dead... it would crank but I knew from the dashboard that with no fuel showing and no battery voltage something serious had happened. To cut a long story short... a kind couple turned up in a white van... who happened to live just around the corner and we coupled up the trailer to the van, put the jeep on the trailer and towed it to theirs. I collected it next day and now it sits in the yard waiting for a learned diagnosis.
My research here suggests the crankcase position sensor is the most likely candidate. I have a donor vehicle of the same year (2002) and could swap them over. I have had this WJ for many years and it has never missed a beat! Making up for lost time I reckon!
So my learned friends, is this the case and how difficult is it to do? Starter motor off and you can then reach it? Are there any resistance tests I can do before removing the CPS?
Grateful to you for being interested and more so for replies.
Philip Jones

dave1123

The CPS is a Hall-Effect switch that senses changes in it's magnetic field caused by objects passing close by it. IDK if there is any way to test it except for checking for a pulsed output while cranking the engine. I DO know that Mopar sensors are made with a rare earth magnet while aftermarket ones tend not to be. When I changed mine, a small screwdriver would actually stick to it. If you can't find one locally, which I doubt, rockauto.com has them and you can get express shipping at a price! If you can get an aftermarket one, use it until the Mopar part arrives.

A CPS failure won't show up as a "key code", but will with a scanner. The way the system works is the PCM looks for a signal from the CPS showing the crankshaft is actually turning. It then turns on the ASD relay and fuel pump relay. The ASD relay energizes the ignition and fuel injector circuits.

Without the gauges activated, I'm inclined to think you've got a loss of voltage in the primary power supply at the fuse/relay box (PDC) under the hood. You should be showing the warning light test sequence and volts and fuel level at least. All the gauge information is sent from the PCM, not the individual sensors. I think there is a PCM fuse, but I'm not sure.

Just checked. There is a fuse #19 at 10 amps;

fuseblower

Thanks Dave1123,
You were not to know that I am an electrical engineer! I am lacking system information about the WJ control.
The tip about the rare earth magnet in the Mopar unit might explain its cost and perhaps it might generate a more reliable signal. As a Hall effect sensor the inductor should not be open circuit - and should probably show a few ohms..
Clearly all engines need a reference pulse to initiate the timing cycle and the only place this can be obtained is from the crankshaft. So if it is absent it isn't going to run. I have read elsewhere that the PCM , in this situation, turns off the fuel pump, spark generation and kills the guages. Do you know if this is true?
I forgot to mention that the cooling fan is running all the time as well! When I reconnect the battery the guages flick up and then return to zero (not the end stop but zero on the dial).
I have tried the key code function and there is no code. Also my scanner (which seems to be communicating with the WJ) is also not reporting any faults.
Does any of this further intel assist you with a diagnosis?
Fuseblower

dave1123

It's been a while, but I should have remembered I ribbed you about your username when I found out about your profession! Please excuse the wandering mind of an old man! Heck, I remember screwing around with vacuum tube radios! My introduction to solid state electronics was digital flip-flops and logic gates!

The way I understand it, with the loss of the CPS signal, the PCM shuts down both the ASD relay and fuel pump relay, thus killing the engine dead. When my 4.0s CPS started to get funky, it would randomly shut off the engine whenever it got it into it's mind, but would instantly restart. The original one lasted 179K miles before it screwed up.

Trying to diagnose with a keyboard is an exercise in futility without being hands-on. Even then, I've seen mechanics stare at an engine, then make a proclamation about what was wrong, without doing any testing, like the answer came to them in a dream! Most of the time, that's how I feel about it, just going thru the system in my mind and making suggestions. Anytime the gauge cluster gets involved, It don't condemn the PCM, but do suspect the wiring and power supply to it. I've had my whole cluster go nuts with wild gauge swings just because of a loose battery connection!

Sorry I can't be more specific and I hope you get it figured out.

EDIT: I'm sure you've looked at this, but I understand on the 4.7, the harness to the CPS can get fried by the exhaust pipe.