Puzzling check engine light.

Hello. First off, I've searched and read all the 157 threads that poped up under my search. I still couldn't get a concrete answer. My problem is on a 1998 4.0 AW4, 231, 129k. When I went to start the truck today, the check engine light never went out. I went in the house, picked up my OBD II reader and tried to read codes. It connected and communicated with the PCM, but 0 codes were given. Tried it 2x, no help.

Tomorrow I'll try disconnecting the battery for a few minutes and see if that helps. However, I don't think it's really the PCM putting the light on since I'm not getting any codes, and the reader isn't giving me an comminications error message.

Any ideas what the problem might be? I'm starting to think the problem is inside the instrument cluster.

what kind of scanner is it? Maybe its still stored in the PCM but your scanner cant read it?

Innova 3100

http://www.canadiandriver.com/articles/pw/obdreader.htm

Today I still couldn't get the light to go out. I couldn't disconnect the battery (terminals frozen onto battery, will replace the battery and cables from Jeepers Creepers anyway ). What I did was unplug all 3 connectors on the PCM for 15 minutes which should be the same as cutting B+ to it to let it reset ).

I then went into the truck, put the key in and turned it to run ( didn't crank the engine ). All the lights came on and went out as usual ( except the check engine light ). The check engine light came on immediately, before all the others, and never went out. Being that I never cranked the engine, I still don't think it's put on by the pcm.

More, info. I disconnected the PCM and tried putting the key to the run position and the check engine light didn't come on at all ( not even for the dash lighting initial test ).

check and see if the scanner shows any code after you disconnected the PCM. You wont have a check engine light with the PCM unplugged because the PCM controls it.

buffalomatt350 said:

check and see if the scanner shows any code after you disconnected the PCM. You wont have a check engine light with the PCM unplugged because the PCM controls it.

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I'll check again. For kicks, I unpluged the coolant temp sensor while the engine was running. That should of set a code. Will see if I can pull it up.

As for the PCM controlling the light, I am aware or that. I just wanted to confirm that the dash was/wasn't causing the light to come on instead of the PCM.

Dumb question perhaps, but are the pins in the test ports arranged 100% in the same matter in all OBD II systems ( regardless of make or model )? Thinking maybe the pins might be arranged differently?

While I have a stealership 1 block away from my house, I'm trying to avoid a $70 bill and try to figure out why I can't read the codes ( if there are really an set ).

Calata said:

I'll check again. For kicks, I unpluged the coolant temp sensor while the engine was running. That should of set a code. Will see if I can pull it up.

As for the PCM controlling the light, I am aware or that. I just wanted to confirm that the dash was/wasn't causing the light to come on instead of the PCM.

Dumb question perhaps, but are the pins in the test ports arranged 100% in the same matter in all OBD II systems ( regardless of make or model )? Thinking maybe the pins might be arranged differently?

While I have a stealership 1 block away from my house, I'm trying to avoid a $70 bill and try to figure out why I can't read the codes ( if there are really an set ).

Click to expand...

The pins in any OBDII connector should be compatible. There's something wrong there.

Even though it didn't pick up a code, have you tried using the OBDII tester to clear all codes, to see if that will put the light out?

Matthew Currie said:

Have you tried using the OBDII tester to clear all codes, to see if that will put the light out?

Click to expand...

Yes, and the light will not go out, which is why I though the dash might be shorted or something.

If your reader will let you(depending on how complex it is) cycle through the numbers and find what readings it is giving for the O2 sensors. My check engine light was on because of the rear O2 sensor(one of the wires snapped) and it never gave a distinct code, but it was showing a straght 1.0 reading for the O2 sensor which obviously looked wrong.

Just something to check. Again, I guess it depends on your reader.

Calata said:

I'll check again. For kicks, I unpluged the coolant temp sensor while the engine was running. That should of set a code. Will see if I can pull it up.

As for the PCM controlling the light, I am aware or that. I just wanted to confirm that the dash was/wasn't causing the light to come on instead of the PCM.

Dumb question perhaps, but are the pins in the test ports arranged 100% in the same matter in all OBD II systems ( regardless of make or model )? Thinking maybe the pins might be arranged differently?

While I have a stealership 1 block away from my house, I'm trying to avoid a $70 bill and try to figure out why I can't read the codes ( if there are really an set ).

Click to expand...

if you have an auto zone near you they'll read the codes for free. maybe there's something wrong with your reader.

I've figured out my problem. I finally got the computer to spit out code P1495. I'll deal with that problem later, but I'll post what the problem was.

Based on past experience, I've read the ECM with the engine running ( always worked on my other cars ). When I tried it on the Jeep, it wouldn't register a code ( not 100% sure why thou ).

As for the light not clearing, it's because the engine was always running. I came to that conclusion because today I put the key to RUN and tried reading it. Got the code. Managed to extinguish the light. Cranked and started engine, light came back on in 2 seconds. The reader was still plugged in. Tried reading it again, no codes. Tried extinguishing the light, wouldn't extinguish.

So, for what ever reason, the PCM doesn't like to be linked to when the truck's running. I figured the above little knowledge might be helpful to others in the future.