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<H1>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) #02-07-30-051A - (02/14/2003)</H1></TABLE>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)

2001-2002 Chevrolet Silverado HD2500/HD3500 Series

2001-2002 GMC Sierra HD2500/HD3500 Series

with 6.6L Diesel or 8.1L Gas Engine (VINs 1, G -- RPOs LB7, L18)

and Allison® Automatic Transmission (RPO M74)

This bulletin is being revised to modify the service procedure. Please discard Corporate Bulletin Number 02-07-30-051 (Section 07 -- Transmission/Transaxle).<A name=ss1-1243294><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1243294&pubid=271&evc=sm#ss1-1243294" target="_blank">

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 name=ss2-1243294><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1243294&pubid=271&evc=sm#ss2-1243294" target="_blank">
<H5>Advise the customer:</A></H5>

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.<A name=ss3-1243294><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1243294&pubid=271&evc=sm#ss3-1243294" target="_blank">

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

  1. <LI =1>Open the hood and disconnect the negative battery cable from all batteries.
    <LI =1>Remove the heat shield over the TCM.
    <LI =1>Disconnect the TCM wiring connectors.
    <LI =1>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.
    <LI =1>Remove the sound insulator retainer from the left upper most end of the dash insulator and carefully move the insulator to the side.
    <LI =1>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.
    <LI =1>Place a thin bead of sealer around the retainer, P/N 22573692 and insert the retainer into the dash panel.
    <LI =1>Insert a pocket screwdriver into the center of t

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Thanks for the info.

However, the tsb does not say who will pay for the installation.

If the dealer is to do it free, I'm on my way.

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