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Timing Connector Location/High Idle Prob

Discussion in 'Chevy Trucks' started by Stan, Nov 11, 2010.

  1. Stan

    Stan Senior Member
    Messages: 579

    1994 3500 7.4 TBI:
    Going nuts trying to find this wire. The pcm is located next to the master cyl unlike my K1500 which is near the glove box. The problem I'm having is high idle. Searched for vacuum leaks, no codes. It is running @ open loop upon start up and will go into closed loop. Noticed the timing @ idle according to the scanner is 16 degrees. Idle is around 800-850 rpm once warmed up. Once this wire is located can I set the base timing using the scanner -vs- timing light?
  2. brad96z28

    brad96z28 PlowSite.com Addict
    Messages: 1,526

    the connector should be near the firewall behind a plastic cover if I remember correct.
  3. Stan

    Stan Senior Member
    Messages: 579

    inside or firewall side? left or right?
  4. brad96z28

    brad96z28 PlowSite.com Addict
    Messages: 1,526

    Outside. left I think
  5. Pro Lawn Care

    Pro Lawn Care Member
    Messages: 36

    I have All-Data I'll check and get back to you. If mem serves it is on the pass side just below the glove box by the blower motor. 1 wire with a black connector.
  6. ss502gmc

    ss502gmc Senior Member
    Messages: 473

    Yes what pro lawn said,,, under the glove box and its probably taped up in the main wire harness but it does have a black connector on it so its easy to spot.
  7. tac48

    tac48 Member
    Messages: 41

    Here is the info found on the connector location. Hope it helps.

    Bulletin No.: 416514
    Date: August, 1994


    The location of the set timing connector is incorrectly called out in "SECTION 6A" of the "Driveability and Emissions" Service Manual.

    The correct location of the set timing connector is inside the cab where the heater box and the carpet meet on the passenger side floor. Pull carpet back to expose the single wire and connectors on the outside of the control module harness. The wire colors are tan with a black stripe.

    Group Ref.: Engine
    Bulletin No.: 366002
    Date: January, 1994



    The timing connector, which can be disconnected to return engine timing to base for adjustment purposes, has been moved from the engine compartment to inside the cab. This new location is not currently reflected in the Service or Owner's Manual.

    The new location is under the dash directly below the glove compartment. The tan/black wire is routed for a short distance outside of a large wiring harness. This large harness is routed between the heater blower motor and evaporator case. The timing connector is taped to the wire bundle.
  8. Stan

    Stan Senior Member
    Messages: 579

    Thanks for all the replies, I searched that side with a fine tooth comb. Looked at my '94 k1500 and it was there just as all-data states. The computer on the k1500 was there as well...however the computer on this is mounted inside the engine compartment next to the master cyl. Checked the harness going into that but does not have a single lead to disconnect.
  9. cubicinches

    cubicinches PlowSite.com Addict
    Messages: 1,412

    The wire breaks out of the main harness located under the black plastic cover on the passenger side firewall. Tan wire with a black tracer. Once the wire is disconnected, you still use a timing light to set base timing.

    If you're having high idle problems, check the intake manifold gaskets for vacuum leaks... common problem on TBI 7.4's. Also check the gasket at the base of the throttle body for leaks. Check IAC counts on the scanner. If they're close to zero at idle, you've definitely got a vacuum leak. It's not likely an ignition timing issue.
  10. Stan

    Stan Senior Member
    Messages: 579

    "Joe's kid" must have been under the dash and taped the wire to the main harness.....found with the help of a razor blade cutting through layers of tape.

    Checked the intake with carb cleaner and all areas. Replaced the IAC, the TBI base gasket, switched out the MAP, replaced the EGR, thermostat, wires plugs you name it. The lowest I have seen idle after closed loop is around 800 rpm.

    I did something very barbaric, I elongated the holes on the on the TPS and had helped a little. Don't laugh, I was desperate.
  11. cubicinches

    cubicinches PlowSite.com Addict
    Messages: 1,412

    Does it stay in closed loop after prolonged idle, or does it go back to open loop?
  12. Stan

    Stan Senior Member
    Messages: 579

    Stays in closed loop once warmed up.
    Is timing @ 16 degrees normal @ 850 rpm? What should actual curb idle be?
    Last edited: Nov 13, 2010
  13. cubicinches

    cubicinches PlowSite.com Addict
    Messages: 1,412

    Don't go by what the scanner says. Now that you've found the connector, put a timing light on the thing. There should be a sticker under the hood or on the air cleaner which gives the timing spec and idle speed spec. Base timing is probably 4 degrees. Confirm that the base timing is correct with engine at full operating temp and the timing connector disconnected.

    The TBI 454 trucks are plauged by idle problems, I've owned and worked on many of them. Here's a bunch of things that you should check after confirming the base timing with a light. All of these can affect the idle.

    Check it over again for vacuum leaks... big or small.

    Use the scanner to confirm that the coolant temp sensor is within range when the engine is fully warmed up. Scanner should show 192+ degrees at full operating temp. If not, confirm that the engine is actually coming up to full temp (check condition/operation of thermostat). Or confirm that CTS is not faulty.

    Make sure your O2 sensor is active and responding quickly at full operating temp. This will affect closed loop operation.

    Scan the TPS voltage and make sure it is close to .4 volts at closed throttle. Setting it at anything else will only cause you issues elswhere in the throttle range.

    Scan the IAC counts. On a fully warmed up engine, they should be around 20ish ideally. Drastic deviations from that number are indications of other issues.

    Remove the throttle body and clean the throttle bores, the throttle plates, and the IAC passage thouroghly. Make sure they are spotless.

    Depending on what scan tool you're using, you can perform a minimum airflow check. Minimum airflow is corrected by actually changing the position of the throttle plates at idle. You want to make sure all else is correct before doing this. Re adjust TPS afterwards.

    I beleve some trucks in some years had a PROM update for idle. Scanner will give a PROM ID, which you could take to a dealer to see if there was an updated one issued.
  14. Stan

    Stan Senior Member
    Messages: 579

    Using snap on mt 2500.ck'd the timing but tab is beyond rusty, it was the first pointer past TDC so I'm assuming its 4btdc. After trk was driven this morning here's the following info:
    Closed loop rpm @ 800-825. O2 8-counts @ 0. Iac pos @ 8. Coil enabled A+B on.
    After some driving:
    Open loop @ 775 rpm. O2 cross counts @ 0. Iac pos @ 77. Iac coil enabled iac A off, iac B on.