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Stumped - Engine not starting

Tiny
HARRY P
  • MECHANIC
  • 2002 JEEP GRAND CHEROKEE
  • 4WD
Ok so i'm still tinkering with my 2002 Jeep GC 4.0 4X4. It cranks, but doesn't start. No sparks and no response from PCM to 2 different scanners. According to AllData, the 5v sensor supply is somehow tied to the issue. At the TPS I get 0.45v from the reference, which should be 5v give or take. So there's a shorter sensor somewhere on that circuit. But I can't figure out which sensors that includes. I've unhooked every sensor I can find and still only getting 0.45v and no response from pcm.

I think what I need to know is what else is on that circuit? So far I've checked all 02 sensors, TPS, CKS, MAP, coolant temp, and maybe a couple of others.

Side note: the security/key light does stay on, but Jeep people tell me that if that were the issue it would start and then die after 2 seconds. So idk what to make of that.

HELP!
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Saturday, June 20th, 2020 AT 7:43 PM

26 Replies

Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
The PCM could be limiting voltage with the security activated. Why arent you dealing with the security issue first? Just asking.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,574 POSTS
On that vintage the security shouldn't touch the 5 volt supply. It shuts off the fuel pump.
There are 2 5 volt circuits as I recall, one feeds cam, crank, TPS, MAP. The other deals with the transmission, Oil pressure, A/C. Unplug connector 2 from the PCM and see if the 5 volts changes. Next would be to use a signal tracer to feed a signal into the 5 volt reference line and trace it, it could be shorted somewhere in the harness. You could cut the Orange 5 volt feed wire near the PCM and see if you get 5 volts back on the PCM side then connect a meter to the harness side and start moving the harness around to see if you can find the problem.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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  • 2,292 POSTS
94 TA: I don't think it's a security issue because it doesn't start at all. If it was a security issue it would run for 2 seconds then die. Plus the PCM isn't responding to scanner. Oh and I forgot to mention that the ABS and BCM also report "lost communication with PCM" codes

Steve: I thought about cutting that wire but generally don't like cutting wires right at the PCM. But if it's what has to be done, then so be it.

Thanks guys. I'll dig in tomorrow and let you know what I find
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
Im not saying this is the case but it has some similarities. I posted about my buddies GMC truck a while back and he had the same exact thing going on. 0.45 volts on all the reference lines. That ended up being a section of harness melted down and shorted out and when it did it smoked the 5v B+ circuit in the PCM. So had to fix the harness and get a new PCM.

And as for the security. I wasn't sure how their system worked but it seems to me if it shuts the fuel pump down its not going to run anyway. Just a thought.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,574 POSTS
Most of the domestic security systems work by killing the pumps power be turning off the relay. They also will kill the ignition pulse on some by shutting off the transistors in the PCM.
One way I have "saved" vehicles that fried the 5 volt is to make a stand alone regulator using a 7805 or similar regulator to generate the 5 volt bias and make the sensors think they are getting power from the PCM. I've also tested systems that way, inject the 5 volts into the circuit and see if it wakes up or if that gets pulled to ground, that can be helpful in the case of a grounded reference circuit and you don't want to blow the replacement parts quick.

Harry, If you want to take the time you can de-pin the contact from the connector, same results just makes it more work that a simple cut and splice.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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I thought about the pump relay being bad or shut off and bypassed it by jumping the #87 terminal straight to the positive terminal. No change. Good spray at the rail. Can hear the pump running and sounds normal. I'll try cutting the 5 volt wire and check it again. If there's a change there then I'll supply the 5 volts and see if it runs. If so then I'll pretty up my work and sell that POS before it has a chance to do something else.

Is there a way to get a list of what runs off of that 5v line? Maybe some spot I missed in AllData?
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,574 POSTS
There are 2 5 volt circuit feeds as I recall, one feeds cam, crank, TPS, MAP.
The other deals with the transmission, Oil pressure, and A/C controls.
I'm not sure if they share the internal PCM 5 volt feed though. That's why I suggested pulling the C connector off the PCM as it carries the second 5 volt feed entirely but doesn't carry PCM power like the connector with the engine control 5 volt feed.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
CARADIODOC
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I'd like to add a note of great value. On Chrysler Engine Computers, the computer turns the 5.0-volt sensor supply off when it sees excessive current, (short), to protect the computer. In all my experimenting with cars donated to my college's Automotive program by Chrysler, I never damaged a computer by purposely installing shorts for my kids to find. Be aware that once the short is removed, the supply stays shut down until you cycle the ignition switch off, then back on, to reset it.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
  • MECHANIC
  • 2,292 POSTS
Ok. If i'm not mistaken, the cam sensor is built into the ignition coil pack. Since its the only thing I haven't disconnected and tested yet, I'll do that first thing tomorrow. If not I'll snip that supply wire and check for a short to ground. If I end up needing to create my own backyard 5v supply, what do I need to get? We're getting a bit above my level of expertise here so thanks a million guys
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
You dont need to cut it to check it for a short. Just unplug the connector from the PCM and put 1 lead on that pin and the other to block, frame and body to check for a short. You do need to cut it to see if your voltage drop is a short or the PCM. By cutting it you remove the load and if the PCM is ok you will have 5 volts if not you wont. If you do then your 5 volts is going to ground somewhere.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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Ok. Gotta be the PCM. I snipped that wire and checked the voltage coming out. Just 0.45v with the key on. Ordering one now. Hoping that will put this thing to bed
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
94 TRANSAM
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Make sure you check the other end to ground for a short to make sure thats not what fried the PCM.

Motor, body, Battery and frame.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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Good point TA. I'll doublecheck it friday when I can tinker again
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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So I dropped a known good PCM in it and still absolutely no change in it. No start. No response from PCM to either of my obd2 scanners, even though other modules are responding and giving codes for "lost communication with PCM." I'm thinking that there's a wiring issue that I'm missing. Anyone know of any common wiring issues to look towards?
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,574 POSTS
Have you checked all of the power and grounds to be sure there isn't some power missing?
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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Yeah, I've got good ground and 12v in the correct spots. But only 0.45v coming out of the 5v reference line. Gotta be a short somewhere in there. I'm guessing it's in the wiring itself. I'll dig into it again soon but for now it's on the back burner. I've got a 2004 Blazer needing an o2 sensor and to be driven for inspection, a 2005 trailblazer needing to he driven for inspection and a few dents pulled (I'm learning to play fun with dents too), and a 2006 Liberty needing both front wheel hubs done. I've got enough on my hands, lol.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
If you cut that wire and still had.45v with 2 PCMs then you have to have a 12v PCM feed wire that is low or dead. Look up the pinout on the PCM and check all the feeds with it plugged in, key on. IE under load.

Remember the #1 rule in power. There is very little difference between an open and a short.

An open circuit under load will test the same as a shorted one.
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
HARRY P
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  • 2,292 POSTS
Ok. Can someone grab that pinout/wiring diagram for me please? I can't get into alldata or prodemand at the moment. It's a 2002 Jeep grand Cherokee 4.0L with part time 4x4
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
94 TRANSAM
  • MECHANIC
  • 680 POSTS
Here ya go.
Images (Click to make bigger)
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Sunday, August 30th, 2020 AT 10:36 AM
Tiny
4DRTOM
  • MECHANIC
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Hi,
I think your looking at a wiring issue a short or cracked and open wire in a harness somewhere. I'll be following this Im super curious what the issue ends up being. Ill run it by my brother he is a jeep freak
Tom
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Sunday, August 30th, 2020 AT 10:36 AM

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