1996 Grand Cherokee PCM problem?


#1

Does this sound like a PCM problem? Here is what I checked.

The fuel pump doesn’t power on with the ignition key. The fuel pump will run when hot wired at the connector. The car will turn over but does not have a spark. The check engine light will not come on when the key is switched on. I checked all the fuses and relays in the main power box under the hood. My OBD scanner tool will not connect to read any codes (Linking error) I tried a friends scanner tool and had the same problem. I can’t use the ignition key to flash any error codes. WHAT DO YOU THINK… Thanks


#2

I THINK that you either have a bad PCM connection, a bad ground, or a bad PCM. Probably the last one.


#3

It’s always a possibility but there are a few other areas I would dig into first. How good are you at following schematics?
Figure 41 on the AutoZone site.
http://www.autozone.com/az/cds/en_us/0900823d/80/15/3e/8f/0900823d80153e8f/repairInfoPages.htm

I would follow the circuitry through the ASD relay first since the PCM requires an ignition pulse to operate the fuel pump relay.
Some things to consider in this equation might be the CEL bulb being burnt out or the possibility of a failed ignition switch (the electrical part).


#4

Thanks, mr josh and ok4450. I have checked the ground ciruit from the power control box to the fuel pump and it was OK. I have also checked and swaped out the relays for the ASD and fuel pump. I did not get 12 volts at the fuel pump connector with the key on. I have not yet checked the electrical portion of the ignition switch. I do have a manual that show how to do this. Thanks for the help.


#5

There will NOT be power (12 volts) at the fuel pump unless the engine is cranking, or running, or the ignition switch has just been placed to RUN. So, to check for power to the fuel pump, disconnect the wire at the fuel pump, touch the multimeter lead to the power wire and one to ground. Have someone turn the ignition key to RUN, then OFF, then RUN. Each time the key is turned to RUN, the fuel pump relay is energized, and you should get a 12 volt reading. /// Click on the web site that OK4450 got for you. Click on Fig. 42. As you can see, certain circuits supply power to the PCM (Powerplant Control Module). If there is no power in, there can be no power out, right? When the ignition is in RUN (and START), the ignition switch supplies power (12 volts) to the 15 Amp fuse #18, which powers the PCM, ASD relay, FP relay. If 12 volts isn’t there, there’s a problem somewhere back, through the ignition, to the power distribution. Usually, it’s the ignition switch. If fuse #18 has power, test to determine if that power gets to the PCM terminals numbers A2, C3, C19, C20 with the key still in RUN. Two power supply circuits are always hot (always have voltage): PCM terminals A22, and A14. Check them with the multimeter. If one doesn’t have power, you’ve got to trace the circuit back to find out why. First disconnect the battery, before disconnecting the PCM electrical connector (“plugs”). And, reconnect the PCM “plugs” before reconnecting the battery, otherwise, the PCM could be severely damaged.


#6

To ensure that you get the correct voltage readings during testing for power, remove fuses #1, #15, #16, #18, and #20. Put the multimeter red probe into each fuse socket hot side, and black probe to car’s body, or ground. Turn the ignition switch to RUN. All of the voltages should be within 1/2 volt of the battery’s voltage (12+ volts), especially, the ignition switch’s voltage. While reading the voltage on fuse #18 socket hot side (the ignition switch power out fuse), turn the ignition switch to OFF and RUN several times. If the voltage isn’t 12+ volts each time in RUN, that circuit has excessive resistance; probably the ignition switch. Do NOT put the fuses back in unless the ignition switch is in OFF; otherwise, the PCM could be damaged. // For further checks, you need the Pinout charts for the PCM (voltages and resistances) from www.alldata.com. Either buy a subscription to alldata for your car, or, go through your local public library. // You can download and print these pages, and you can download and print Fig. 42 from autozone.com. // The information is here, and there, and the rest is up to you.


#7

I suspect there is a problem near the main power buss or panel under the hood. You stated the OBD2 scanner isn’t getting a link and I suspect that it is due to a lack of power to the connector and also to the PCM module. Using a voltmeter and your manual as a guide check for main power getting to those areas. The problem could be just a bad fuse in the power distribution panel. There still may be a problem if something shorted and blew a fuse.