Transmission Shifting In and Out of 4th and 5th Gear (Hunting) When Pulling/Carrying a Load, Unable to Manually Select 4th Gear (Install 5th Gear Inhibit Switch)

This bulletin is being revised to modify the service procedure. Please discard Corporate Bulletin Number 02-07-30-051 (Section 07 -- Transmission/Transaxle).

Some customers may comment that when pulling a load, the transmission shifts in and out of fifth gear too frequently, or they would like to be able to select 4th gear manually.

A customer who requests the fifth gear inhibit switch must understand fueling systems of diesels and the resulting noise that may be heard. This noise might be objectionable to some drivers. The following is provided in an attempt to enlighten the driver/operator of the vehicle.

Diesel engine fuel injection systems that use a common rail system, as the Duramax™ diesel does, may or may not have what is referred to as pilot fuel injection at the lower engine speeds. Pilot fuel injection is defined as a pre-injection of a small quantity of fuel 5 to 20 crankshaft degrees before the main fuel injection. This pre-injection softens the main injection pressure rise and reduces combustion noise. At engine speeds above 2600 RPM, and a fuel quantity supply greater than 35 cu mm, it is necessary to eliminate pilot injection to meet emission requirements. When driving at or above these speeds, main injection combustion noise may be noticeable. It is normal and not harmful to the engine.

When traveling in 5th gear, the pilot injection to main injection transition is typically in the 136 km/h (85 mph) range, however it may also be heard under heavy acceleration and when pulling loads up a hill during shifts at or above 2600 RPM. Customers that install the fifth gear inhibit switch may encounter the fuel knocking of main fuel injection at speeds around 107 km/h (67 mph) in fourth gear between 2600 and 2700 RPM. This sound is considered normal and is to be expected.

Install the 5th gear inhibit switch using the following procedure.

1. Open the hood and disconnect the negative battery cable from all batteries.
2. Remove the heat shield over the TCM.
3. Disconnect the TCM wiring connectors.
4. On vehicles with the L18 gasoline engine, remove the three retainers that retain the Throttle Actuator Control module to the engine side of the cowl, just under and behind the fender to cowl brace, and position the module to the side.
5. Remove the sound insulator retainer from the left upper most end of the dash insulator and carefully move the insulator to the side.
6. Approximately 76 mm (3  in) to the right of the stud for the sound insulator retainer, there is a small inward dimple in the dash. This is the pilot for the drill. Drill a 14 or 15 mm (37/64 or 19/32 in) hole through the dash panel.
7. Place a thin bead of sealer around the retainer, P/N 22573692 and insert the retainer into the dash panel.
8. Insert a pocket screwdriver into the center of the retainer to open the retainer.
9. Remove the instrument panel trim panel from around the IP cluster.

•	For vehicles with an air bag selector switch, remove the accessory housing panel with the air bag selector switch and disconnect the wires from the air bag selector switch. Go to Step 10.

•	For vehicles without the air bag selector switch, remove the IP bin from the center IP dash area. Insert one P/N 15040483 housing. Go to Step 10.

10. Remove the blank panel from the housing (if applicable) and install the switch and wire assembly, P/N 15174508.
11. To the short white wire from the switch, properly crimp and solder a terminal, P/N 12110844.
12. To the yellow, green and blue wires from the switch, properly crimp and solder a terminal, P/N 12084912.
13. Position a piece of mechanics wire into the back side of the dash in order to pull the wiring through the opening in the dash, routing the wires under the HVAC control and steering column.
14. Insert the three long wires through the previously installed dash pass through.
15. Pull the three wires completely through the dash pass through.
16. Seal the wires to the dash pass through with sealer.
17. Cut off a 1524 mm (5 ft) section of the wiring conduit.
18. Route the three wires into/through the conduit.
19. Position the conduit so it drops down from the dash pass through. Reposition the insulator blanket and reinstall its retainer.
20. Reposition the Throttle Actuator Module and install the retainers (If applicable).
21. Route the newly created wiring harness along the vehicle's existing wiring to the TCM area using 3 to 4 plastic tie straps to secure it in place.
22. Open the gray/clear TCM connector body cover.

•	Insert the green wire into cavity 20.

•	Insert the blue wire into cavity 12.

•	Insert the yellow wire into cavity 28.

23. Properly close the TCM connector, re-connect the connectors to the TCM, reposition the TCM and install the TCM cover.
24. Locate the Relay Block-Body to the right and up from the parking brake mechanism. Inspect for a cream colored connector plugged into the Relay Block-Body, usually in the top right hand corner of the Relay Block-Body .

•	If there is no cream colored connector in the Relay Block-Body, obtain P/N 15306105 and perform the following steps.

-	Remove all terminals and wires from P/N 15036105 connector.

-	Insert the white wire from the new switch assembly into cavity "E" of the connector.

-	Place the connector into the top right hand corner of the Relay Block-Body.

•	If there is a cream colored connector in the Relay Block-Body, perform the following steps.

-	Disconnect the connector from the Relay Block-Body.

-	Insert the white wire from the new switch assembly into cavity "E" of the connector.

-	Reinstall the connector into the Relay Block-Body.

25. Use a couple of tie straps and tie the newly installed wiring to the underdash wiring.
26. Reinstall the IP trim bezel.
27. Reconnect both batteries.
28. Use normal SPS reprogramming procedures to reprogram the TCM.
29. Follow the procedure outlined in Corporate Bulletin Number 02-07-30-032 and complete the FastLearn procedure.
30. Reset the customer clock.
31. Road test the vehicle in the normal driving mode at 72 km/h (45 mph), 5th gear. When the speed is stable and road conditions are favorable, depress the newly installed 5th gear lockout switch. The LED should illuminate and the transmission should downshift to 4th gear. The transmission then should only have 1st, 2nd, 3rd, and 4th gear operation as long as the switch LED is illuminated.
32. Depressing the switch will allow 5th gear operation to resume. Fifth gear operation will also be reestablished any time that the key is cycled to the "OFF" position. This is normal.

Parts with the (*) are required for Crew Cab type vehicles that do not have an air bag "On" or "Off" switch.

Parts are currently available from GMSPO.

Addition of the 5th gear lockout switch is not a warrantable repair. The following information is a suggested repair guide only.