Choke issues

Discussion in 'Tech Discussion' started by Michael Anderson, Nov 5, 2018.

  1. Michael Anderson

    Michael Anderson Junior Member

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    Hi, so the other day. My choke light came on. And then that my truck was idling high. I realized that my choke wasnt working. I have oil pressure, i keep blowing the choke fuse, tried jumping the termials on the oil pressure switch to see if the dash light would turn off, but fuse blowes again. So yeah. Any help would do.
     
  2. Charlie

    Charlie Full Access Member

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    :welcome:
     
  3. chengny

    chengny Full Access Member

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    The choke fuse feeds the line side of the oil pressure switch (on PNK/WHT 350) - nothing else is directly connected to that fuse. However the load side of the pressure switch supplies two branch circuits; DK BLU 931 (goes to the negative leg of the CHOKE light) and LT BLU 78 (supplies the coil in the choke heater). When the pressure switch contacts are closed, in addition to 350, circuits 931 & 78 are effectively connected to the choke fuse - and a short in either branch could be causing it to blow.

    press_switch__CLOSED.jpg

    If the fuse blows when the switch is open (i.e. engine not running so no oil pressure to close the switch), the short can be safely assumed to be somewhere in the PNK/WHT 350 circuit.

    If it does not blow until the engine is started (and oil pressure causes the switch to close), the short is in either DK BLU 931 or LT BLU 78. In the case of LT BLU 78, in addition to the lead - that connects the switch to the heater - the heater itself could possibly have an internal short. Disconnecting the lead from the coil should tell you whether or not that is the case. If the fuse still blows with the heater lead pulled off, the problem is not with the heater coil.

    If those two diagnostics fail to pinpoint the location of the short, you are pretty much left with only the actual wires - the two that make up the LT BLU 78 & DK BLU 931 circuits.

    LT BLU 78 is easy to inspect. After the switch, it enters the loom that crosses behind the distributor, takes a turn forward in the same loom that contains the alternator wires and exits the loom right at the choke heater.

    DK BLU 931 is another story. After it leaves the switch, it dives into the loom that carries the engine wiring harness up to the firewall connector block. It transits the FW and emerges behind the fuse block. It bypasses the fuse block and heads up to the CHOKE light along with the other wires in the instrument cluster harness. If you are intent on fixing the CHOKE light - and the short is in 931 - plan on doing some digging. On the other hand you might consider cutting the 931 lead near the switch. You will regain choke heater operation but the CHOKE light will no longer work - it will always be out.

    choke_light__ON.jpg



    choke_light__OFF.jpg
     
    Last edited: Nov 6, 2018
    Michael Anderson and K201979 like this.
  4. K201979

    K201979 Full Access Member

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    Good info!
     
  5. 73c20jim

    73c20jim Supporting Member Supporting Member

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    This answers a question I was going to ask. Thanks @chengny Right now I have the choke wire plugged into a IGN spade in the fuse box.

    Is the choke fuse protected here, or do I need to run an inline fuse? And what size?
     
  6. chengny

    chengny Full Access Member

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    You are alright as you are now. The IGN sockets are actually protected by the B/U LPS fuse. See the wiring dwg below:

    Pages from X_7305_1973_GMC_Truck_Wiring_Manual_CK_10_30_Pgs_Only.JPG
     
  7. MikeB

    MikeB Full Access Member

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    Great info above. It reminds me that I need to get one of those GM oil pressure switches to use in series with the choke. I have a gauge sender in the port below the distributor, so maybe I can use the port above the oil filter boss.
     

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