Quote:
Originally Posted by cjohnson6772
Ok I've been doing some thinking since it would be a royal pain to run another ign wire to my pcm based on location:
First off I think I'm going to change to pin 33 on C2 to turn on my second fan since my first fan already stays on most of the time anyways. Also, could I just wire the diode and resistor in parallel instead of adding another ign wire? If my thinking is correct, this should be the same since the pcm would always see the 12V from pin 86 on the relay passing through the 1kOhm resistor.
If I did leave the diode and resistor out all together, would it actually damage the pcm or just throw the code?
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The PCM grounds the fan relay control line(s) to turn on the fan(s) in response to engine coolant temperature. In addition, when the fan(s) are supposed to be off, the PCM monitors the state of the fan control line(s) to detect a fault in the circuit.
Under normal circumstances, the control line is held at a logic high because the relay coil acts as a pull-up resistor on the control line. If you add a trinary switch to the PCM's control line, and the switch closes, the trinary switch will ground the control line. The PCM doesn't know anything about the trinary switch and sees this as a problem. Then it throws a code (P0480, I think).
To answer your first question, the resistor and diode in parallel would not work. The diode, when oriented correctly, is there to stop the trinary switch from sending ground back to the PCM. A resistor in parallel would defeat that because it would provide a current path around the diode. In addition, the resistor would drop the voltage low enough that that the PCM probably couldn't even turn on the fan anymore.
For your second question, if you leave them both out, you'll get a code but it shouldn't damage the PCM.
The real problem in all this is that when the PCM throws the code, it defaults to the fans always being on until the fault is cleared. So, your fans will always run with the ignition on after you get the code.