A/T - Lack Of Power, Early, Late or Erratic Shifts: Overview
DATE: 1996TECHNICAL BULLETIN # 343
TRANSMISSION: 4L60E, 4L80E
SUBJECT: Lack of Power, Early, Late or Erratic
APPLICATION: GM
1993-96 4L6OE and 1994-96 4L8OE Lack of Power, Early Shifts, Late Shifts or Erratic Shifting
Some 1993-96 4L6OE or 1994-96 4L8OE equipped pickups, sport utility vehicles, motorhomes or vans may exhibit one or more of the following concerns or symptoms:
^ Lack of power especially when carrying loads or during heavy acceleration.
^ Early upshifts, no matter
what the throttle opening.
^ Late shifts in the 4WD-low range only.
^ Erratic shifts (shifts which are early during one upshift sequence and normal during the next sequence or some of the upshifts during a shift sequence are early, while others are normal).
^ Incorrect forced downshift shiftpoints or no forced downshifts at normal road speeds.
These conditions may occur on either 2WD or 4WD vehicles, but are most prevalent on 4WD applications. In addition, these conditions will most likely be intermittent and will occur without any trouble codes being set.
NOTE:
1996 applications will set a code P1875 if a short to ground or an open in the 4WD circuit occurs. These conditions may occur on 2WD applications due to the fact that the PCM harness usually contains circuit 1694, even though it is not a 4WD vehicle.
System operation and diagnosis
All 4L6OE, non passenger car applications and all 1994-96 4L80E applications use a discrete input to the PCM, TCCM or VCM to change the shift points of the transmission. This signal is provided to the PCM, TCCM and VCM using one of the following methods:
T trucks with manually shifted transfer cases (NP 231) provide input via a switch which is mounted in the transfer case shift indicator (figures 1, 1A, 1B).
T trucks with electronic shifted transfer cases (NP 233) signal the PCM (or TCCM) via a signal from the transfer case control module ((TCCM) figures 2, 2A, 2B).
K trucks with a manually shifted (NP 241) or electronically shifted transfer case (NP 243), signal the PCM, TCCM or VCM by using a switch which is mounted on the transfer case (figures 3, 3A).
NOTE:
C/K truck applications are considered full size 1500-1/2 ton, 2500-3/4 ton or 3500-1 ton. K=4WD, C=2WD. S/T applications are known as Jimmys, Sonomas, S-10/15S or Blazers. S=2WD and T=4WD.
Manually shifted T trucks (1993-95) signal the PCM of the 4WD-low position by grounding circuit 1694. This ground is provided by the shift selector quadrant switch. When the shifter is placed in the 4WD-low range, the PCM will note that the voltage on circuit 1694 has dropped to near zero (figure 1, 1A, 1B).
Electronically shifted T truck transfer cases indicate 4WD-low range position via the transfer case control module (TCCM). The customer selects 4WD-low by pressing the appropriate selector button (figure 2). This signals the TCCM of the drivers desire to engage the 4WD-low range. The TCCM engages the transfer case in 4WD-low, and also signals the PCM or VCM of its intentions by grounding pin C-1 of the TCCM. Grounding this pin drops the voltage on circuit 1694 to near zero volts at the 4WD-low input to the PCM (or VCM) (figures 1B, 2A).
Unlike the T trucks, K trucks indicate the 4WD-low range position using the same method, regardless of which type of transfer case the vehicle is equipped with. All K truck applications use a three position switch (figure 3A) which is wired to circuit 1694 or 1493 of the PCM, TCCM or VCM. When the 4WD-low range is selected, the transfer case switch will be moved to the 4WD-low position. In this position, the switch provides a ground for circuit 1694 or 1493, at the PCM or VCM (figure 3, 3A).
All K truck and T truck applications ground circuit 1694 or 1493 to provide input to the PCM or VCM when the 4WD-low range has been requested. Any range other than 4WD-low, results in circuit 1694 or 1493 remaining ungrounded and the pin voltage at the 4WD-low input to the PCM staying high at source voltage (battery voltage).
Diagnosis:
When the PCM or VCM 4WD-low input signal (circuit 1694 or 1493) is high (approximately 12 volts) the computer will assume the truck is in a range other than 4WD-low and will command the shift points based on the truck being in 4WD-high or 2WD-high. If the truck is really in four-low, the upshifts will be late for the given throttle opening. In addition, the truck will exhibit overly sensitive forced throttle downshifts.
In each case the truck will exhibit conditions which will be objectionable to the customer, but will not set any trouble codes (1993-1995 models). Most 1996 models will set code P1 875 if the circuit develops a problem. Code P1 875 will set if:
^ No TPS, VSS shift solenoid or TCC codes are set.
^ IPS angle is 17-50%.
^ Overdrive range (fourth gear) commanded.
^ Transmission temp 69-248°E
^ Engine temperature above 400°F.
^ The front axle position feedback circuit (new for '96) indicates to the VCM that the front axle is in 4WD, and the VCM calculates the speed ratio (engine speed transfer case output speed) is between .8 and 1.2 (indicating the truck is not in 4WD-low) for five seconds.
OR
The front axle position feedback circuit indicates the front axle is not in 4WD, and the VCM calculates the speed ratio (engine speed + transfer case output speed) is between 2.5 and 2.9 (indicating the truck is in four-low) for ten seconds.
If code P1875 is set the VCM will respond by commanding the transmission to:
^ Increase line pressure to maximum.
^ Inhibit TCC.
^ Shift to third gear only.
^ Turn on the check engine light (MIL).
Refer to "Diagnostic Routines" and "Figure References for further details.