Questioni have replaced the fans, the relay and the pig tail plug going the relay. and the relay over heated again with in a few hours. when i put the new relay in the fans just ran constant but i didn't think any thong of it till now. am i missing something?
AnswerHi Alex,
The fans are turned on to either slow or high speed by the powertrain control module (engine computer) based upon the coolant temperature of the engine as measured by the sensor near the thermostat housing and as well as by the internal pressure of AC refrigerant system which is related to whether the compressor is 'on' or not.
The fans should not run at all while you are cranking the engine over to start it. Is that the case?
If you ask for AC after the engine starts you might see the fans come on as soon as that is done, depending upon how full the refrigerant system is, but it should come on pretty soon after the ac compressor is activated.
Finally when the temp of the engine coolant reaches 214F the fan will come on slow, and when it reaches 230F it will come on high.
The wire from the pcm to the relay that determines what the relay will do (off-low speed-high speed) is light green/dark blue on pin 2 of the relay and the other end is at pin 73 of the pcm.
The first thing I would do is unplug the plugs at both ends of that wire (the relay and the pcm) and measure whether that wire even though disconnected is shorted to ground (i.e. show 0 ohm resistance between it and the - post if the battery). If it shows infinite resistance that is good, otherwise the wire is shorted somewhere along its length which you need to trace and correct.
Another possibility is that the engine coolant temp sensor is shorted to ground or otherwise inaccurate in its resistance as a function of its temperature. It should read 7k-14k across its terminals when cold, and drop to about 1k ohms when the engine is warmed up. I would also measure the resistance to ground of its signal wire on pin 2 of its plug and with the other end of the plug disconnected from the pcm (pin 26 tan/black color) just the same way you checked the relay wire. If it is shorted to ground that would need to be corrected.
Once you get the relay working as it should, the other thing to do is make sure it is mounted firmly on the frame rail as it depends upon that connection to dissipate heat from the relay which will otherwise cause it to fail as you have already observed.
With all this in mind, go back and run through these observations and tests. Good luck and let me know what you learn. Also if you write back tell me which size engine (L) you have.
Roland
