QuestionQUESTION: Roland, A few weeks ago my Service Engine Soon light came on. I took it into a shop and they determined the cooling system relay was defective. The light no longer came on after replacement but random overheating began to occur. I took it back to the shop and they replaced the relay again. Now my Service Engine Soon light is on again and the engine still overheats. The mechanic seems to think my PCM needs replacing. If I replace the PCM, will I need it to be reprogrammed? I am looking at a part from a scrap-yard but may go new if the price is not too high.
ANSWER: Hi Chris,
If the relay in question is the fan relay, that one has been a problem that began in the mid-90's. It is solid-state and needs to be mounted externally on the body frame rail (left of the fan module) and bolted to the frame tightly so the heat is dissipated instead of overheating the relay. So my suggestion would be to notice whether or not the fans are coming on when the temperature is rising toward H, which it should be. If it isn't then you need a new relay that is properly attached to the frame.
There was a recall so if that was never availed of you could check at a Chrysler dealer to check the computer recall records to verify that you qualify and that it never was replaced by Chrysler so you might get it done an no cost. In any case, I would not replace the module into you get a good quality module properly attached to the frame. If this is not the relay that is your problem tell me which one it is.
Roland
---------- FOLLOW-UP ----------
QUESTION: Hi Roland,
Thanks for the quick response. Here's the info I forgot to include in the original question. The mechanic could switch the fans on manually using the computer diagnostic but they don't come on automatically when the engine gets too hot. The diagnostic code I get when I turn the key on and off 3 times is 35. Which indicates cooling relay or circuit problems, I believe.
AnswerHi Chris,
The 35 is indeed the low or high speed fan control circuit. It is very simple:fuse 24 in the power distribution center provides the current for the fan motors when the relay closes the contacts. That same fuse provides the current to close the relay when the pcm grounds or puts a voltage somewhere between probably 2 and 10 volts (depending upon whether it wants high or low speed) at the other end of the relay actuation coil. The wire from the relay to the pcm is light green/dark blue and it is connected to pin 73 of the pcm2 plug. Now you can observe the voltage on that wire which I believe will be 12V (or it could just be "floating") when you turn the ignition on, and then when the engine heats up the voltage should be firmly some voltage less the 12 and then drops to 0 (low speed or high speed requested). The other end of the coil having 12V on it from fuse 24 then closes the relay and allows the current to flow to the fan motors. Either the relay is bad, or the wire is "open" or shorted to ground. You can check the wire by unplugging at the relay and the pcm and testing to see that it has continuity and that it is not grounded. You could also simply unplug the pcm2 plug and then put the key in the "run" position and ground pin 73 of the detached pcm plug as a test of the entire circuit. If the fan runs, then the relay circuit is ok, and there is something wrong with the pcm or the coolant temp sensor circuit. If it doesn't run, then the felay is no good or the fans are no good. The last possibility is that the ground return wire (black) for both the fans and the relay is loose. Follow the wires from the fan to the relay and you will find the black wires are bolted to the inner frame rail nearby to the relay. That is how to resolve this. My bet is the relay. I would only replace the pcm if I found that the voltage on light green/dark blue wire did not change from the time I started the van until the time the engine got hot. If that is what you observe the pcm is not doing its job OR there is something wrong with the coolant temp sensor (but the code for that is not being displayed and the engine would be running poorly as well if that sensor was bad).
Please let me know what you learn. Remember, that relay has to be firmly attached to the frame to conduct away heat, and also that it has a history of failure.
Roland
