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    Fuel Injected! 84Elky's Avatar
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    More Fan 2 Wiring Problems and '7730 Pin F8

    Really need some help from the problem solvers and electronic brain trusts in this forum. Have just fried another ECM and have a severe brain cramp trying to solve the problem with my Fan 2 wiring. Surely I’m overlooking something or have done something stupid. Post is a bit long but necessary to include all the details.

    The attachment “My Wiring.jpg” is a diagram of how my fans are wired. If Fan 2 is enabled via the calibration turn-on temperature, the voltage on Pin F8 is all over the map, oscillating from 13+v to 0 and the relay is clicking 3-4 times a second (initially posted here: http://www.gearhead-efi.com/Fuel-Inj...in-F8-Question). As noted in the above post, this situation is apparently the result of a fried quad driver which I now know has to be caused by the existing wiring setup (more below).

    So decided to approach the problem analytically looking at each piece of the puzzle. Would really appreciate any ideas as to what the problem could be or what I’m missing, leaving out or needs to be tested.

    Resolved that the problem can only arise from the following:
    - Bad relay
    - Faulty F8 wire from relay to ECM
    - Bad fan
    - Faulty ground to fan
    - Faulty positive wiring to fan
    - Faulty positive wiring to the relay
    - Relays wired wrong
    Any other checks to be made are welcomed, please!

    Addressing each one:
    - Bad relay (No) – Have tried multiple relays and all the same. All test OK = 85-86 continuity of 60-90 ohms and 30-87 OK with 12v across 85-86
    - Faulty F8 wire from relay to ECM (No) – Unplugged wire from ECM harness and grounded it. Fan 2 on OK. Continuity of wire from ECM to connection at relay OK, so the wire is good (and because the fan runs, the relay wiring would appear to be OK)
    - Bad fan (No) – Runs properly when F8 wire is removed from ECM harness and grounded or when AC turned on (see attached diagram)
    - Faulty ground to Fan (No) – Continuity OK and Fan runs as it should when grounded and when AC turned on (see attached diagram)
    - Faulty positive wiring to Fan (No) – All tests Ok & fan runs
    - Faulty positive wiring to the relay (No) - All tests OK and fan runs
    - Relays wired wrong - Appear to be in accordance with GM wiring and Fan runs when F8 pulled from ECM harness and grounded (see Attachment “GM Fan & AC Wiring.jpg”). Also, Fan 1 relay wired in same manner as Fan 2 relay and no problem with Fan 1 at all.

    With all that, now one more detail. Thought I had fried my first ECM because +12v was passed directly to Pin F8 for 4-5 seconds due to an incorrect relay wiring during some testing. But now don’t believe that to be the case because of frying a 2nd ECM without doing anything. Iinstalled a 2nd ECM and loaded up my test Bin that turns Fan 2 on at 65*F (it’s 85*F ambient here). Fan 2 immediately on at startup and see the oscillating voltage when probing and grounding F8 just outside the ECM. Grrr!

    So pull F8 wire from ECM harness and installed a stub wire for testing. Start engine and have pulsing on the stub F8 wire with 2nd ECM. Really pissed now because 2nd ECM has to be fried – F8 stub wire is not hooked to anything.

    So install a 3rd ECM with the F8 stub wire and start engine. F8 appears to performs as it should with engine running -- only a few mV when grounded and perfect continuity.

    So now have 2 fried ECMs and am clueless as to what the issue is. Based on the above tests and with the use of the F8 stub wire, the problem has to be with the other 3 wires on the Fan 2 relay, the relay itself, or the overall wiring. But then how does the Fan run properly when F8 is grounded??? I’ve traced and probed every wire multiple times and believe everything to be as shown in the attached wiring diagram.

    Any and all help greatly appreciated!!!!! Thanks
    Attached Images Attached Images

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