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  #1  
Old 10-24-2015, 09:35 PM
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Cranks but no start, no gauges.

This is a strange issue. When I turn on the key, after a second the only lights lit on the dash are the key slash through, seatbelt and fuel warning light, no gauges like normal. Doing the on of on of on of on doesn't show codes like normal.

Replaced the crank position sensor, pulled all of the PCM/ECU connectors and large bulkhead connectors and cleaned with contact cleaner (although they were very clean already), new battery and alternator, full fuel tank, checked every fuse, swapped relays around (with like kind), checked every ground, added giant ground from the engine to the chassis just for the heck of it...

It dies on my wife, I pulled the PCM fuse in the power distribution block, cleaned it and put it back in and the car worked for another couple of days.

Now however it won't fire no matter what, including leaving the negative cable off for hours at a time.

Seems like the computer has given up the ghost, but I don't want to shell out for one if that isn't it.

Help please. It is my wife's Jeep and her elderly mother is entirely dependent on her.

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  #2  
Old 10-25-2015, 12:40 AM
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Re: Cranks but no start, no gauges.

What other sensor can short out the PCM like the Crankshaft Position Sensor?
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Old 10-25-2015, 06:45 AM
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Re: Cranks but no start, no gauges.

Other sensors which can short out the 5V PCM supply are the TPS, cam sensor and MAP sensor. Just disconnect the connector from each of them, one by one , and check if the instrument panel indications come back (fuel level and volt meter will be at 0 when the PCM is shorted out, or dead)
But also a shorted PCI data bus can be the problem. Or one of the connected componentes to the bus has a failure and messes with the bus signal. You can measure the bus voltage on pin 2 of the data conector, it should be 2-2.5V. If it is 0V, than the bus is shorted. If it is around the 6-7V, than one of the componentes connected to the bus is faulty.
What year is your jeep? For 99-03 models, there is a diagnostic juntion port under the steering wheel área. This is a connector with a shorting cap on it. The shorting cap interconnects all the componentes to the bus. Removing the cap, will only have the PCM, TCM (for 4.7), SKIM and drivers door module interconnected. If the engine starts now, than you know that one of the disconnected componentes is the culprit.
If it still doesn´t start, than you can remove the connector from the TCM, the SKIM and the passenger door module and see if the instrument panel volt and fuel indications come back. If not, than there is a problem with the PCM.
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Old 10-25-2015, 09:15 AM
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Re: Cranks but no start, no gauges.

Thanks for the Rosetta Stone Frank, I will try that stuff.

How do I disconnect the cam sensors because it appears they are under the intake manifold on the 4.7? Do I just eliminate everything else possible before I get to them?

The Factory Service Manual is useless for such problems. Everything is "DRBIII Scanner" as the go to diag, but even that wouldn't do much in this situation except perhaps cloud the waters even more.

As soon as I gather my frayed wits I will dig in with your trouble tree!

Thanks again.

Mark

2002 GC
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Old 10-25-2015, 09:26 AM
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Re: Cranks but no start, no gauges.

The cam sensor sits on the forward right side of the right hand bank cylinder head. You can remove the connector without removing anything else.
See attached page for the location.
Attached Files
File Type: pdf 4.7 cam sensor.pdf (137.1 KB, 3 views)
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Old 10-25-2015, 12:04 PM
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Re: Cranks but no start, no gauges.

Turned the key on this morning and the gauges came up like normal. Very frustrating because I don't know what is causing the intermittent issue and it would be bad for my wife to be stranded!!!


Ran the codes and the only one showing is P0725 Engine Speed Input Circuit, which I would expect because I turned the key on with the CPS unplugged.


I very much appreciate your input. Now I have a much better picture of how all of this interconnects.


As an aside, I wish all of the sensors would be on the data bus and therefore not able to affect the PCM directly (like shorting out sensors that affect PCM input voltages).
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Old 10-25-2015, 12:29 PM
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Re: Cranks but no start, no gauges.

It quit again and the scanner reported loss of connection to the PCM (as expected).


Trying to get to the cam sensor connector but it is really hard to get to.


Going for the MAP instead first!
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Old 10-25-2015, 01:28 PM
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Re: Cranks but no start, no gauges.

Update 2:
Measuring less than 1 volt at no. 2 pin of the data port connector, so I guess that indicates a short in the bus.
I can't find the connector around the steering column to check the PCI stuff, but it looks like I can no longer avoid trying to get the connector off the cam sensor.
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Old 10-25-2015, 05:47 PM
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Re: Cranks but no start, no gauges.

Update 3:
Pulled the cam position sensor, pulled the connector, no love from the volt or gas gauges.
Re-cleaned all the grounds just one more time for good measure, nothing.


Should I bite the bullet and order a replacement PCU?
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Old 10-25-2015, 08:36 PM
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Re: Cranks but no start, no gauges.

I would look for the diagnostic junction port, which must sit around the steering column. My jeep doesn t have it, so i never saw it myself. A good change that the problem is the PCM though.
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Old 10-26-2015, 07:03 PM
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Re: Cranks but no start, no gauges.

The voltage at pin 2 of the diagnostic port is reading between .2 and 1 volt, wildly fluctuating. The PCM is getting a full 12 volts from the power distribution center, and I pulled the connectors of the crank position sensor (which I replaced), cam position sensor, MAP sensor, and TPS. Still reads 0 volts on the gauge cluster and empty fuel tank (which is not the case).

If it was a shorted sensor (which I think I ruled out by disconnecting all of the aformentioned sensors) it should read zero volts I would think. I can't locate the diagnostic junction port anywhere, so I guess my option is to bite the bullet and order a referb PCM.

Sound like sound logic?


Edit: Just found the section in the FSM that talked about the sensors the PCM sends 5v to, I missed the oil pressure sender, the two transmission speed sensors and the transmission pressure sensor (which is on the inside bolted to the valve body).


Makes me sick thinking about crawling back under to check those...
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Old 10-26-2015, 10:15 PM
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Re: Cranks but no start, no gauges.

Eventhough the PCM also sends a 5V signal to the AC pressure transducer and the engine oil pressure transducer, this is a separate 5V supply and as far as i know will not inhibit the engine from running. (called secondary 5V supply)
The primary 5V supply is for the TPS, MAP, cam and crank sensor and will inhibit engine operation when shorted.
Transmission speed sensors are not fed with any voltage, they produce a small voltage signal by themselves, when a target passes, and this signal is send to the TCM. The TCM also sends out a 5V signal, for the transmission line pressure sensor. This sensor does not sit on the solenoid block inside the transmission, as all the solenoids do, but is mounted on the outside of the transmission, RH aft side.
So don´t feel sick, because you don´t have to crawl under the jeep again.
Attached a drawing of where the diagnostic junction port should be, and a picture how it looks like, in case you still would like to find it.
Attached Images
File Type: png _DJP.png (53.1 KB, 7 views)
File Type: jpg Plug_DJP.jpg (168.0 KB, 9 views)
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