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Author Topic: Importance of Grounds for EFI and Transmission Controller  (Read 15735 times)
El Hombre
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Location: Puerto Rico
Miracles?.... by appointment only!

« on: September 29, 2014, 05:02:04 pm »

The following is a case I worked at the shop that more than confirms the importance of grounds for EFI and Transmission Controllers or TCM. Even though this is not a case invloving a Ford Smilie 2 it does prove the fact grounds ARE important and SHOULD NEVER be overlooked. So here it goes..........


  • 1994 Isuzu Trooper.
  • 3.2L DOHC V6 Engine MPFI.
  • Automatic TCM controlled 4L30-E transmission and 4x4 Drivetrain.

Case or Problem:

  • Intermittent engine stalls.
  • Transmission would not shift to second gear.
  • "Check Transmission" indicator  would start flashing and TCM will fall under "Fail Safe" state forcing 3rd gear to be permanent in DRIVE.
  • "Fail Safe" would be cleared after a IGN OFF - ENG ON cycle but the condition would repeat as soon as 2nd gear was attempted to be applied by the TCM.

Diagnostics and Repairs:

  • Since this is an OBD-I system, a KOEO test (per manufacturer's procedure) showed the following TCM codes:
    • TCM-DTC 37 = Torque Management Serial Line Faulty. This is a signal the TCM sends to the ECM for it to control ignition advance when shifting and prevent shift engagement shock in the drivetrain. .
    • TCM-DTC 23 = Engine Coolant Temperature Sensor Circuit Signal High (aka too cold)
    • No ENG-DTC present
  • TCM to ECM serial line wiring short between TCM "DLC Output" and "DIAG Output" wires found and repaired to correct code 37. This short was "created" in another shop.

  • Replaced TCM and noticed problem became intermittent and that code 23 was still present.
  • Confirmed that code 23 was due to the absence of the thermostat causing a "engine too cold" condition and also affecting the application of the OD. This alone literally busts the cooling system myth of too much engine coolant flow in the system causing an "engine hot" condition or possibility.
  • Installed new thermostat in the engine and code 23 was corrected.

  • Intermittent code 37 was still present with the replacement TCM and fail-safe triggering was also intermittent. Intermittent engine stalling symptom was also and still present.
  • Reverified serial line circuit between TCM and ECM in addition to a possibility of a short to ground or power and everything check OK.
  • Per the manufacturer's pinpoint procedure, the replacement TCM was also at fault even though the expected voltage of 4-5 vdc was present and 12 vdc was also present between ground and IGN ON pin.
  • Had to figure a procedure to verify voltage at diagnostic connectors pins using a separate/isolated ground. Noticed that the ground terminal showed a voltage of -6 vdc....WTF
  • Verified that the diagnostic connector ground was shared with a the ignition module, HEGOs, ECM and TCM shielding. 
  • Traced the location of that ground, which was supposed to be on top of the upper intake behind the Ignition Module/coils assy........... and noticed it was NOT hooked or PRESENT. Did notice somebody modified or worked in the section of the wiring harness the ground (G125 per the nomenclature)..... was supposed to be. Note the blue tape on the harness sections.

  • I then proceeded to slice the harness section and found that the IGN GND wire in one of the ignition module connectors, did read -6 vdc with key in IGN ON.
  • Identified the ground wires (4) and reinstalled the ground where it was supposed to go.

Final Results and Conclusions:

  • Transmission shifting restored, no more engine stalls and OD working as required.
  • All TCM-DTCs eliminated and engine coolant temperature restored to specifications.
  • Original TCM was damaged due to the elimination of the G125 ground and the created short between the "DLC Output" and "DIAG Output" wires
  • The absence of the G125 ground also caused the intermittent engine stalls.
  • The damage to the TCM could have been prevented, if (and I have to say it) if flaky shops and technicians do not perform "repairs" assuming grounds are not that important or simply, incompetent service.
  • Does their existence and work favor my business?..... Umh...DUH! .....however, I have to end up picking up the pieces left and finish trying to change the customer's bad perception of the service industry.  Phew
  • GROUNDS are important to not only ensure the optimum or correct functionality, but also to prevent expensive components failures.
  • This case CONFIRMS the myth regarding coolant flow and the "dwell time" in the radiator to ensure cooling of the engine.
  • Final service and components tab for the customer?........ above $500 and below $1,000.



'86 Mustang GT, X-303 cam, Holley DP 700, RG Tran., 289 Heads   Ford Smilie 2

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