94 Explorer: Is the PCM the issue?

I have a little over 150K miles with my 94 Explorer (bought new). Over time I have been developing issues, and now need to ask for some help.

Currently, the following things are not working or working erratically:


Temp Gauge: Only one sensor on the 4.0 V6, feeds to PCM. Just recently failed. Changed sensor, no difference.

Pinging/Knocking: Has always had a little, recently got a lot worse. Did not try to change sensor.

Speedometer: Within last 6 months started to show erratic operation. Usually it shows higher speed then what we are actually driving (i.e. driving 65 on freeway is showing 80).

Cruise Control: Has not worked in last two years.

I have checked all fuses and they test good. I have replaced the PCM power relay about a year ago to fix an issue with the fuel pump, so I am reasonably confident that is ok.

Question for the forum: Does this all add up to a bad or failing PCM. If so, is it an easily replaced part? How much should a new one cost?

Thanks in advance for your help.

John O

The pinging/knocking could be an EGR issue possibly. Since it sounds like you have a service manual for your Explorer, see if you have a PFE sensor. If so, you want want to consider checking it as they were common items to fail. The pinging could also be caused by a faulty coolant temperature sensor. Did the sensor you replaced have Light Green/Red and Gray/Red wires going to it? AutoZone.com shows that there is a sender/switch for your Explorer (single wire) as well as the sensor for the PCM (2 wires). Which did you replace? The single wire sender/switch would be for the gauge, the sensor is for the PCM as you indicated.

The speedometer and cruise control are probably both related to the vehicle speed sensor (VSS). Does your '94 use a speedometer cable, or is it electronic? If electronic, then VSS sounds like a likely culprit for both. If it's cable, then the VSS probably is the issue for your cruise control.

Bottom line, your symptoms don't sound to me like an indication of a failing PCM.

-Rod

Rod,

Thank you for the reply.

I am working out of a Haynes manual. I did replace the Coolant Temp Sensor with the Gray Housing and two wires. That is indeed the one which feeds the PCM. The Haynes Manual states that on some models there is a second sensor next to the first to connect to the gauge. I did not see any other sensor in proximity to the first, so I am assuming that the PCM drives the gauge in this model (4WD, XLT). After removing the original sensor, the cold resistive value was correct, so it may have been OK. The Haynes manual does not show the temp gauge in its drawings so I am a little in the dark on that. Anyway, the PCM Coolent Temp sensor did not help as the gauge is reading the same. I will make another attempt to find the second sensor. Would have been nice if the manual had more complete info on that.

I have also purchased the VSS and will install sometime this week. If that does not correct the speedometer issue, I would probably go back to the PCM issue. If I understand my speedometer, I think it is driven electrically from the PCM. Again, the manual is a little vague on that.

The reason I am thinking PCM is that all the issues I am having seem to have the PCM in the loop. The engine knocking should be detected by a knock sensor, that feeds to the PCM, and the PCM is supposed to retard the timing. That is definitly not happening. The cruise control is controlled through there, and I cannot find any other item in that system that has failed (i.e. fuses, brake dump switch, etc.). I found rebuilt PCMs for my model at about $100.00 on-line, so it wouldn't be a budget buster to try changing it out.

Thank you for taking the time to answer questions. have a Great Christmas Holiday! http://www.automotiveforums.com/vbulletin/images/icons/icon7.gif

John O.