View Full Version : Electrical issue???

04-07-2006, 07:11 AM
I have some sort of electrical issue that has taken up residence beneeth my hood, and I want it gone! Can anyone offer suggestions as to a possible cause?

Here is the background... It started gradually running richer and richer at startup (cold), and got to the point where I parked it because it was so pig rich It would barely stay running. I thought it was the MAP sensor (because it through on the "check engine light" once and that was what it pointed the finger at. So I replaced it. It started and idled great after that but ran like $hit everywhere else in the RPM band? It is still running poorly throughout the rpm band but will idle ok if you don't touch the throttle. I have since put the old MAP sensor back on, as it seems to make no difference now which one is on there. It runs the same, and the voltage signal output back to the PCM is identical, with either one. (which tells me it was never bad to start with) :banghead:

Things I have done: (besides pull out my hair) :nod:
1. I checked the 5V source coming from the PCM to the MAP and it was right on 5V. The output signal that the MAP sensor sends back to the PCM is messed up though. It is soposed to vary from .5V (idle) to 4.49V (WOT). It will drop no lower than about 2V? I don't know why, but I do know that explains the rich condition! I even went so far as to test the MAP when I had it off. By attaching a small length of vacuum line to it and sucking on it (creating a vacuum) while supplying a 5V source to it's input, I could get it to drop to .5V on it's output voltage. So it would seem the only reason for it not to drop to .5V on the vehicle is insufficeint manifold vacuum. (ie...a vacuum leak) But I cannot locate one, and my vacuum gage, at idle, reads 19?
2. I checked the TP sensor and it is also getting it's 5V supply from the PCM. It is functioning like it should on it's voltage signal back to the PCM varies from .7V (idle) to around 4.5V (WOT).
3. I have checked for vacuum leaks and came up empty handed, as I thought a vacuum leak could cause the high MAP voltage signal.

The idle air control circut I sopose could be a culprit, but I have no way to check it (need a DRB scan tool) and I don't see how that would effect the MAP voltage. I don't know if, when they fail, it produces the symptoms above? I am, what I would consider to be, a fairly smart guy when it comes to working on vehicles, but I am just about out of ideas. Other than spending money (like I did on the MAP sensor) replacing sensors that probably aren't bad, one at a time! This however seems to be more of a simple problem causing the MAP and perhaps other sensors not to function right, rather than a sensor failure problem. :confused: :confused: :confused:


(I am continuing to search for a vacuum leak)

04-07-2006, 08:00 AM
just a thought..
dissconnect the map and the pcm, apply 12v to the sig wire at the harness
end (map)and measure voltage drop at the pcm connector end(map)
then remove the map sig wire from the pcm connector, plug pcm back in
and measure voltage at the pcm pin (koeo& koer) trying to isolate wire
to wire short, or an internal pcm feedback issue,
sorry I type slow, and get beat up for it constantly....now if I could write
a program to "interpolate" a guitar fretboard to hot keys....

apologies ...an addendum...also as most sensor strings are spliced into a common ground string , a poor splice at the map grnd to the string will cause
a constant high voltage, on the map sig wire

04-08-2006, 09:03 PM
Is it throwing any codes? What's the O2 sensor reading when it's running rich? I've seen on the 4.0Ls where the wiring harness comes loose at the firewall and rubs through on the back of the fuel rail, causing all sorts of problems. I had one that ran rich at idle and no matter what I did, ended up being the upstream O2 signal wire rubbed through on the fuel rail. I don't recall ever running into this on a 2.5L, but that doesn't mean it's not possible!

Worse case, run an overlay wire(s) and see if that helps.

-Chuck James

04-10-2006, 10:37 AM
Is it throwing any codes?
Nope, no codes. I had thought about the fact that somewhere there could be bare wires from rubbing on things. I will check that out.
I didn't do anything on it this past weekend. Instead I chose to work on something I knew would have a positive outcome. :nod: I took the carb of my Polaris ATV, gave it a good cleaning, and put it back together. Runs like new now. :thumb: I've got a 4 day weekend coming up this weekend. I plan on spending some quality time on the Jeep then.

Thanks for the suggestions guys.

\/\/\/ my Trail Boss