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Air Conditioner/ Radiator fan wiring using ECM signal and trinary switch

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  • Air Conditioner/ Radiator fan wiring using ECM signal and trinary switch

    OK guys, not an engineer so help me out. I'm planning on running a single electric, single speed radiator fan. My goal is to trigger the fan (via a relay) to come on with engine temp and A/C requests. If the A/C is OFF, the fan needs to come on by engine temp only. If I have the A/C ON, I want the fan to run to aid condenser cooling. I will use a TRINARY switch to let the A/C trigger the fan as well as incorporate the high/low pressure cut-off features for the compressor clutch. I want to use the ECM "high speed" signal (yellow wire) to trigger the fan relay for engine cooling.
    FINALLY the questions: I'm concerned about power feedback TO the ECM from the trinary switch on the A/C system because they are both tied to pin "86" on the relay. Basically, if the A/C calls for "fan", the circuit will also back-feed the ECM yellow signal wire. What is the voltage of the Cummins ECM fan high signal wire (12 vdc?, 5 VDC? Does it matter?). I think a diode on the ECM signal wire would work(?) but I wouldn't know what size? Also some relays are spec'd with a diode, but its function is to prevent voltage spikes when the relay coil field collapses during opening the circuit. I know enough to be dangerous, and really don't want to fry the engine computer...any help out there?
    Last edited by treetopflyer; 09-15-2021, 08:50 PM.

  • #2
    P.S. I searched the forum for "A/C" and "Air Conditioning" and found nothing...there is a reference to such content in one of the threads I read, but can't find the source posting? Any A/C interface/ installation information would be greatly appreciated.


    • #3
      I plan to run two relays. The fan high and return on the Cummins wiring completes a ground loop which would close the relay and kick on the fan.

      Here is a wiring schematic I found online for those who plan to use a trinary switch. What I plan to do with mine is just run a second relay in place of the manual switch/optional fan temp switch shown in this diagram which would be grounded by the Cummins wiring to close the ground loop when the ECM commands fan at 194F.

      If the ECM is not commanding fan for engine temps, that relay would be open and if you turn on the AC with a cool engine and the AC switch turns on the fan, it can't feed anything back through that open relay.

      Someone smarter than me please intervene if I'm wrong!

      Click image for larger version

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      • #4
        Also - if anyone knows of a good electric clutch that would be a good fit for the R2.8, let me know. I'm wanting to try it in my J truck build since I have the space.