Stumped - Engine not starting

The PCM could be limiting voltage with the security activated. Why arent you dealing with the security issue first? Just asking.

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.

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

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.

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.

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?

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.

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.

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

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.

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

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.

Good point TA. I'll doublecheck it friday when I can tinker again

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?

Have you checked all of the power and grounds to be sure there isn't some power missing?

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.

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.

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

Here ya go.

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

Thanks Tom. That's what I'm thinking. I'm going to dig into the wiring harness probably Tuesday when I should have some time and see what I can see. Might even get a magnifying glass too.

If you have or can get a magnetic field strength meter you can use that to get to the area that a short might be. A strong field will be at the short. It will only work for a short not an open circuit.

Well a buddy of mine swears up and down it's in the electrical key switch. I'm skeptical but since it's only $20 and an easy swap I went ahead and ordered one. I hope he's right because I don't have the time tp stay signing through wire harnesses. If that doesn't do it I'll see about getting a magnetic field meter but I doubt anyone I know has one. Most mechs around here just don't play with electrical issues. Not enough money in it for them.

Hey,

Well, you could always look for a treasure hunters hobby mini metal detector. Maybe not hobby, I don't know but I'm pretty sure they make detector that is more pin point not wide area for looking at a single rock for metal content. A unit like that could be made to sense a field as well as metal the trick would be sorting out the metal reading from the field spot, this area will have a much stronger reading than the metal from a distance'
Just a thought.
Tom

Or see if you can hunt up a short finder. Basically a 12 volt circuit breaker and the meter. The breaker will snap open and reset creating a field in the wire and it will cause the meter to move until you reach the shorted area. Tool Aid 25100 Can be used on 6 volt systems as well. I've got extra breakers for mine so I can use it with lower amp circuits easier.

Hey, just wanted to maybe throw a pointer at you as I had the same problem that none of the videos or repair manuals even tell you about is on mine a 2002 dodge dakota 3.9l v6 in the pdc there's an engine connector and it was showing a no bus signal on the cluster I did find the connector was loose and barely making contact when I put my hand on it the ignition chimed and the fuel pump started running and the no bus went away so try seating in the connector just a suggestion I too am trying to figure out the 5v circuit on this and scratching my head trying to make sense of it! Lol