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Thread: Heater resistors A/C vs no A/C

  1. #1
    Supporting Member Turbo Mopar Contributor
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    Heater resistors A/C vs no A/C

    Interesting
    My FSM (87,88,91) all show a diode in the resistor modules in the wiring diagrams for A/C but no diode on heater only. OK but the microfiche for 87,88,89,90 all mention the same part number for A/C vs no A/C, note that each use may use different part numbers. 91 uses different part numbers for A/C vs no A/C.

    What happens if a non-diode resistor module is used in a A/C vehicle? I can always solder a diode into the wire harness, if need be.

    I do notice at times my A/C starts with the A/C button off. Perhaps I have a flaky AC switch. I thought I would ask before I dug in too deep.

    What do you think?

    Car is my 87 Ti Peabody
    Regards,
    Miles

    DD '87 Sundance T1, 12PSI, SLH with rear disks
    Project '87 CSX #432
    Donor car '88 Daytona Shelby Z, TII with S60 Turbo/smec, 3 bar.

  2. #2
    Supporting Member Turbo Mopar Contributor
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    Re: Heater resistors A/C vs no A/C

    Is the diode there to clamp kickback currents from the blower motor? If there is something else connected to the blower circuit that could be sensitive to a reverse polarity spike the diode would be necessary. I don't have wiring diagrams for your car so that's just an educated guess as to the purpose the diode serves. When a coil(motor windings in this case) is energized it creates a magnetic field. In a DC motor there is usually iron which is magnetized by the coil. If the power is disconnected the manetic field collapses and the residual magnetic field held by the iron is absorbed by the coil. This causes the coil to generate a pulse of electricity. This pulse will be of reverse polarity to the original magnetic field generated and will cause a spike in the wiring. The diode may be in place to short this reverse current in order to avoid damage to something else in the circuit upon turning the motor off. Hope that helps

  3. #3
    Supporting Member Turbo Mopar Contributor
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    Re: Heater resistors A/C vs no A/C

    Lightbulb010 a very nice explanation of a diode in a flyback configuration. I have been using flybacks to prevent reverse emf on relays and solenoid for a while now. and the diode connection is across(parallel) the motor terminals in a reverse bias configuration.
    In my case the diode is forward biased.after the resistor (in series). It ultimately will go to the ECU (if all the switches are closed) which the ECU uses to determine that the fan is on. Or so I assume. In this type of measurement I would use a pull down resistor on the micro digital I/O line but who knows what is in our ECU. I do have a series of schematics for the LM mostly for sensors. injectors etc but not for AC.
    Regards,
    Miles

    DD '87 Sundance T1, 12PSI, SLH with rear disks
    Project '87 CSX #432
    Donor car '88 Daytona Shelby Z, TII with S60 Turbo/smec, 3 bar.

  4. #4
    Supporting Member Turbo Mopar Contributor
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    Re: Heater resistors A/C vs no A/C

    Maybe they just put it there to drop the voltage then. Seems kind of strange to me but they may have used it to save money. The input to the ECU might be at 5v logic level so they just stuck a diode in there instead of using some kind of buffer in the ECU. The cars with heat only don't have a connection from the HVAC to the ECU do they? I can't imagine why they would. Interesting indeed.

    I noticed on my 87' van the resistor module has disc shaped resistors held between springy contacts where as the resistor module I pulled from the 94' Shadow parts car has wirewound resistors. Some of the part number variances could be due to that.

  5. #5
    Hybrid booster
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    Re: Heater resistors A/C vs no A/C

    a c system with reverse surge protection ??
    and no connection to the ecu ?

    if you're diode is in the fan wiring (? ..if I read this right))

    maybe today we should open our manuals to the automatic temperature control system ...

    when I did the heat to ac / heat conversion I had to change the fan power lead and everything attached to it at the support strap
    when I did the auto temp control I don't think I had to change that little separate wiring section again
    I had to add the dash wiring but that section to the support strap and fan stayed the same

    the wire coil resistors went away with the auto temp set up as it does it electronically - to add there are some heavy heat sinks on the box mounted module so there's probably a lot of current and heat running around loose in the auto temp system
    controlling that might be what you're seeing lol
    Last edited by Dr. Johny Dodge; 11-14-2017 at 03:13 PM.

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