46 GEARSOctober2007

We usually see cars after they've been on the road about three years or so. In 2004, Pontiac

Grand Prix GTP (W body) equipped with the 4T65E and the 2004 Chevrolet Malibu Classic (Z body) equipped with the 4T40E/4T45E added a feature referred to with the following terms:• Tap Shifts — Touch Activated

Power• ERS — Electronic Range Select

Mode• DSC — Driver Controlled Shifts

…depending on the application and which literature you’re reading. Both applications give you the ability to upshift and downshift the transmission by moving a button. The button on the Malibu is located in the shift lever while the Grand Prix uses a set of paddles mounted on the spoke of the steering wheel.

OperationThe addition of the TAP shift

system led to several transmission changes, including the valve body, spacer plate, channel plate, and even the friction materials, depending on the application. Several changes were also made to the vehicle wiring and to the PCM/BCM to make the TAP shift system function.

The system consists of:• TAP switches — The tap switches

are momentary contact switches. In addition to the switch contacts, the switch assembly contains a series of dropping resistors (a volt-age divider circuit).

• A fuse controls power to the switch assembly. On the W car, the Cruise fuse located in the IP fuse block

controls the system; the Z car uses the Run/Crank fuse, located in cen-ter console fuse block.

• The shift lever position indicator is attached to the shift lever and is used to enable or disable TAP operation on the Malibu (Z body) application.

• The Internal Mode Switch (IMS), located inside the transmission, is used to enable or disable TAP

operation on Grand Prix (W body) applications.

• The PCM receives the input from the TAP switches and then com-mands the transmission to shift to the gear commanded by the driver. The voltage value received by the PCM from the switches var-ies based on whether the driver is commanding an upshift or a down-shift.

The 4T65E and 4T45E Tap Shift Feature: Putting More Control in the Driver’s Hands

Figure 1: The BCM monitors the shifter enable circuit C1.

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48 GEARSOctober2007

•A BCM interface via the serial data line is used on the Malibu to tell the PCM when the shifter has been moved into the proper position so the TAP shifts can be enabled.

Malibu OperationTo operate the TAP system on the

Malibu, you must first place the shift selector into the Low (L) position. If the shifter is in any other position, the PCM inhibits the shift. The BCM monitors the shifter enable circuit C1. (Figure 1).

When you place the shifter in L, the shift lever indicator switch pulls the voltage low, enabling the Tap Shifts to function. When the voltage is pulled low, the Driver Shift Control parameter on the scan tool will display ACTIVE as a value. When the Driver Shift Control parameter reads ACTIVE, the BCM will inform the PCM via serial data to enable the TAP shift function.

The PCM then looks at typical inputs such as VSS and engine load to determine whether it should allow the shifts, and if so, when the shifts should occur. When the shifter is in the L

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Figure 2: The shifter position on the W car is determined by the Internal Mode Switch (IMS), which is mounted inside the transmission.

The 4T65E and 4T45E Tap Shift Feature

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GEARS October 2007 49

position, the transmission range sensor (PRNDL switch) will indicate D3 range as the manual valve is actually in D3 range. The PRNDL switch values for circuits A, B, C and P will display LOW for all of the switch circuits.

When the shifter is in any other range, the voltage will be high and the Tap Shift function won’t operate. If the shifter is in any range other than L, the scan tool parameter displayed as Driver Shift Control will read INACTIVE. When the Driver Shift Control param-eter reads INACTIVE, the BCM will inform the PCM via serial data to dis-able the TAP shift function.

When you press the upshift button, battery voltage is fed through a 4.42kΩ resistor. The PCM recognizes the volt-age drop as an upshift command. To command a downshift, the driver press-es the button in the opposite direction. Battery voltage is fed through a 1.5kΩ resistor to the PCM.

The voltage level available at the PCM when the switch is moved to the downshift position is different than the upshift command value. The scan tool parameter that displays the shift switch voltage values is called the Shift Voltage Request.

Typical TAP Shift switch values with the engine running are:• No Button Pushed — 0.65 Volts• Upshift Button Pushed — 1.78

Volts• Downshift Button Pushed — 3.4

Volts

Of course, these values will vary with battery voltage. If you’re monitor-ing the voltage with the key on, engine off, the voltage values will be lower than those listed.

On Z-body applications, the BCM communicates with the Internal Pressure Control (IPC) regarding shift-er position and TAP command values. When you place the shifter in the L position, the PRNDL indicator in the dash displays L3. If you press the downshift TAP button, the indicator will display L2, and then L1 if you press the button again. If you command a TAP upshift, the PRNDL will display L2, and then L3 if the button is pressed again.

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50 GEARSOctober2007

Pontiac Grand Prix GXP Supercharged Operation

To enable the TAP Shift system on the Grand Prix, you must put the shifter in the M position. If the shifter is in any other position, the TAP shift feature won’t function.

The shifter position on the W car is determined by the Internal Mode Switch (IMS) (Figure 2), which is mounted inside the transmission. When the shifter is in the M position, the HUD (Heads Up Display) digits will become smaller. In addition, the HUD will change from showing the PRNDL position to showing the commanded

TAP gear. If the engine isn’t running, the

HUD will always read 3 in M range and the scan parameter will indicate only 3rd gear commanded, even if you push or pull on the paddles to command a shift.

The scan tool parameter for the Internal Mode Switch will display Drive 3 when the shifter is in the M position. Internal Mode Switch circuits A, B, C and P will all display a LOW value on the scan tool. In addition, the Driver Shift Control parameter will change to ACTIVE, enabling TAP shifts to occur.

The W car uses two switches mounted on each side of the steering

wheel center spoke (Figure 3). The left switch is the master, which houses a series of resistors. The right switch is a simple single-pole, double-throw switch.

If you push one of the switches, the PCM will command an upshift. If you pull one of the switches, a downshift will occur. When the PCM commands an upshift, battery voltage will be fed through a 4.42kΩ resistor to the PCM. A downshift command will cause battery voltage to be fed through a 1.5kΩ resistor to the PCM.

Typical TAP Shift switch values with the engine running are:• No Button Pushed — 0.65 Volts

The 4T65E and 4T45E Tap Shift Feature

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Figure 3: The W car uses two switches mounted on each side of the steering wheel center spoke. The left switch is the master, which houses a series of resistors. The right switch is a simple single-pole, double-throw switch.

GEARS October 2007 51

• Upshift Button Pushed — 1.70 Volts

• Downshift Button Pushed — 3.35 Volts

Once again, these values will vary with battery voltage. If you’re monitoring the voltage with the key on, engine off, the voltage values will be lower than those shown.

DiagnosticsSeveral new DTCs are available for

the TAP system; they include P0815, P0816, P0826, P1876 and P1877.

P0815 will set if:• P0826 isn’t set.• No PSM/PSA DTCs are set (Z

body).• No IMS DTCs (W body).• The engine is running.• It’s been at least 6 seconds since the

shifter range had been changed.• The PCM detects the upshift sig-

nal for longer than 2 seconds in park or 5 minutes in D4 range.

If a P0815 sets, the PCM will:• Disable the TAP shift function.• Default the shifter range to the D4

shift pattern.P0816 will set if:• P0826 isn’t set.• No PSM/PSA /Range DTCs are

set (Z body).• No IMS DTCs are set (W body).• The engine is running.• It’s been at least 6 seconds since the

shifter range had been changed.• The PCM detects the downshift

signal for longer than 2 seconds in park or 5 minutes in D4 range

If a P0816 sets, the PCM will:• Disable the TAP shift function.• Default the shifter range to the D4

shift pattern.P0826 will set if:• The engine’s running.• The PCM detects an invalid

voltage value for longer than 5 seconds.

If a P0826 sets, the PCM will:• Disable the TAP shift function.• Default the shifter range to the D4

shift pattern.P1876 (Z body only) will set if:• No TAP system DTCs are set.• No PSA/PSM/RANGE DTCs are

set.• Engine is running.

• The PCM detects the TAP enable circuit is ACTIVE and the trans-mission is in D4 range.

• The condition lasts longer than 3 seconds.

If a P1876 set, the PCM will:• Disable the TAP shift function.• Default the shifter range to the D4

shift pattern.P1877 (Z body only) will set if:• No TAP system DTCs are set.• No PSA/PSM/RANGE DTCs are

set.• Engine is running.• The PCM detects the TAP enable

circuit is INACTIVE and the trans-mission is in L range.

• The condition lasts longer than 3 seconds.

If a P1877 set, the PCM will:• Disable the TAP shift function.• Default the shifter range to the D4

shift pattern.

The TAP Shift system is a clever new way to provide the driver with more control of transmission operation. While it’s still restricted to just a few vehicles, don’t be surprised to see it appear in more models in the years to come.

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