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Thread: 7730 Pin E8 Question for Fan 1 Relay

  1. #1
    Fuel Injected! 84Elky's Avatar
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    7730 Pin E8 Question for Fan 1 Relay

    Need some help with the Fan1 circuitry of the ‘7730 ECM, and in particular Pin E8. The pin-out document (here: http://chevythunder.com/fuel%20injec...%20pinouts.htm)
    shows that Pin E8 provides 12v with Key on and 13.9v with engine running. Is this correct, or does E8 actually provide a ground to the relay?

    Here’s why the question. My Fan 1 relay is wired as follows (which appears to be in accordance with the GM manual):
    Pin 87 – 12v to Fan 1
    Pin 30 – Battery 12v
    This provides 12v to the grounded Fan 1 when the relay coil is energized via Pins 85 and 86.

    But here is the wiring for Pins 85 and 86:
    Pin 85 – 12v from Ignition Switch
    Pin 86 – To Pin E8 of ECM
    With this wiring, how is the circuit from Pin 85 to 86 completed if E8 is Positive 12v? Doesn’t it take a ground from Pin E8 to cause the relay coil to be activated?

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    most ECM outputs are actually switched grounds.... this is no exception. a 7730 pin E8 can be in 2 states: open circuit or connected to ground through a quad driver.

    sounds like you have everything wired correctly. some relays have internal diodes that require specific polarity on pins 85/86, but they're not that common.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    Super Moderator Six_Shooter's Avatar
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    The reason that you will have 12V at the pin at the ECM, with the key on, is that the 12v ignition signal is passed through the coil of the relay, and will be found on any connected wire for any length, as long as a ground signal is not applied to that opposite side of the coil. Once ground is applied, that entire length of wire will then have a ground potential, and will no longer read at 12V, but be equal to (or very close to being equal) to ground.

    Same thing happens with conventional light bulbs and many, many other electrical items.
    The man who says something is impossible, is usually interrupted by the man doing it.

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    Fuel Injected! 84Elky's Avatar
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    Thanks for the prompt replies. I'm an electrical idiot and can't get too complicated; but in my simple thinking if Pin 85 is +12V from the ignition switch (and it is), seems the other side of the relay coil circuit (E8 from the ECM) must be Ground (negative) in order to activate the coil circuit and turn on the fan? Then anytime the fan is not requested, E8 is something other than Ground. Am I understanding this correctly and is that the "switched ground" being referenced?

    Sorry to beat this to death, but am trying to understand so I can measure when the ECM has activitated Pin E8 to request the fan on and don't want to fry the ECM. So how does one detect ground and/or otherwise at Pin E8?

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    Super Moderator Six_Shooter's Avatar
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    Yes, that is correct. When not being turned on, or "switched to ground" you will read around 12V on the ECM side of the relay coil.

    Use a DMM set to 20 volt scale or DC voltage (if auto ranging), depending on how your meter is set-up.

    Probe the pin, when not triggered you will see around 12V like I have said, and when triggered you will 0 or nearly 0 volts on that wire. Sometimes there will be a small voltage differential between the pin of the ECM and the ground reference you are using. If there is, it should be in the mV range.

    Alternatively, you can use continuity setting and when the fan is triggered the meter will show a connection to ground, or beep, depening on how the meter is set-up.

    Yet an 3rd way would be to use a light, connect one side to ignition, and the other side to the pin, and when the fan is triggered the light will turn on.
    The man who says something is impossible, is usually interrupted by the man doing it.

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    LED would work nicely with little current draw. connect the cathode(negative) side of a LED to the E8 circuit and connect the anode (positive) side to +12V. if the LED has an integral resistor, you're done. if not, you'll need to calculate and impliment a resistor on either side of the LED and then you'll be done.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    Fuel Injected! 84Elky's Avatar
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    Thanks again for the prompt and to-the-point replies. Also appreciate the detailed steps and explanations for testing!!!

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    Fuel Injected! 84Elky's Avatar
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    Quote Originally Posted by Six_Shooter View Post
    Yes, that is correct. When not being turned on, or "switched to ground" you will read around 12V on the ECM side of the relay coil.

    Use a DMM set to 20 volt scale or DC voltage (if auto ranging), depending on how your meter is set-up.

    Probe the pin, when not triggered you will see around 12V like I have said, and when triggered you will 0 or nearly 0 volts on that wire. Sometimes there will be a small voltage differential between the pin of the ECM and the ground reference you are using. If there is, it should be in the mV range.

    Alternatively, you can use continuity setting and when the fan is triggered the meter will show a connection to ground, or beep, depening on how the meter is set-up.

    Yet an 3rd way would be to use a light, connect one side to ignition, and the other side to the pin, and when the fan is triggered the light will turn on.
    Tests work exactly as indicated. Thanks again. But more questons ---

    UPDATE --------
    All now OK for Fan 1 and have learned something. Fan 1 now on with AC on. It would apparently not come on when the original post below was made because Pin C9 was wired at the end of the clutch loop (per GM specs) so that a voltage was only sent to the ECM via C9 when the the AC clutch was truly engaged. Because I'm low on freon and need to charge, the clutch was engaging and disenagaging every 1-2 seconds (very rapidly) via the low pressure switch. This rapid clcling apparently did not allow enough time for the ECM to ensure that the clutch was on and thus the Fan 1 was not turned on. So on a lark, wired C9 directly to the AC hot wire at the other side of the low pressure switch and bingo -- Fan 1 on with AC on. So that must mean that continuous C9 voltage for a certain duration is an indication for the ECM to ground E8 and turn on Fan 1.

    But now something with Fan 2. Fan 2 is wired identically to Fan 1 at the relay, but uses F8 to turn on the fan if the calibration-set Fan 2 enable temperature is reached. That was working great before all the work with Fan 1. Now, when Fan 2 enable temperature is reached, the voltage when probing Pin F8 before the relay is all over the map from 13+v to near zero, pulsing 3-4 times a second. If the wire from F8 is plugged into the relay, can feel the relay clicking 3-4 times a second. The fan runs but not at full speed because of the varying voltage it's being supplied. Have I fried my ECM? Any other ideas?

    DISREGARD and see above under UPDATE -------
    Fan 1 does not come on with A/C request, and I seem to remember that it did. Should it come on with A/C request ('7730 $8d)? The fan circuitry is OK because when Fan 1 on Temp is set to 15*C (59*F), fan is on immediately as expected. And Pin E8 is reading about 13.5v engine running, AC on. Ditto voltage at Pin C9 to the ECM, so the ECM knows that the AC is on.

    If the fan is not supposed to be on with A/C request, then no issue; but if it is supposed to be on, what's happening under the covers in the ECM to tell it to ground Pin E8 to activate the Fan 1 Relay? Is it due to 12v on Pin C9 (A/C request)? Is it D12 (not sure what this does -- this pin is not wired on my car)? Something else? Asking this because if fan is supposed to be on with AC, need to chase what the issues are. Thanks!
    Last edited by 84Elky; 09-20-2013 at 03:57 PM. Reason: Further Information

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    EFI GearHead ! EagleMark's Avatar
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    I'm just going through 7727/30 $8D $A1 and a 8708 working on this harness and seems both have 2 fans. Fan 2 seems to always come on with AC, but not fan 1...

    The part showing that these signals are 12 volt is very confusing when trying to wire relays! When in fact it is ground that completes the circuit to turn on...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    Fuel Injected! 84Elky's Avatar
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    Quote Originally Posted by EagleMark View Post
    I'm just going through 7727/30 $8D $A1 and a 8708 working on this harness and seems both have 2 fans. Fan 2 seems to always come on with AC, but not fan 1...

    The part showing that these signals are 12 volt is very confusing when trying to wire relays! When in fact it is ground that completes the circuit to turn on...
    Yep, ground it is. But strangely enough, Fan 1 comes on with A/C request and also in Diag mode with A & B shorted, and can't find that Fan 1 is set on in that area of the code like Fan 2. Just something else to look at when Ihave time.

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    EFI GearHead ! EagleMark's Avatar
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    In the 8708 I found a Sense/Fan request wire as well! Is there any sense wire in 7730 $8D ? Shows same pin D21.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  12. #12
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    Quote Originally Posted by EagleMark View Post
    In the 8708 I found a Sense/Fan request wire as well! Is there any sense wire in 7730 $8D ? Shows same pin D21.
    Not sure what you mean by "Sense/Fan request wire", but it appears Pin D21 on the '8708 serves the same purpose as Pin D12 (note "12") on the '7730. D21 appears to go to a A/C Pressure transducer on the 8708. On the '7730, D12 is designed to be connected to a grounded (normally closed) High Pressure Switch. Most of these switches open at about 430# pressure and that is a signal to turn on Fan 2 (I believe by supplying ground to Pin F8).

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