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Discussion Starter · #1 ·
I'm hoping you might be able to help me with a long standing problem....

I have a 2003 2500HD with the 8.1L and the Allison transmission. Since the truck was new, I've found that the transmission shifts erratically when the weather is warm (about 90F), and when I'm in stop and go traffic. The engine revs quite a bit before downshifting. This becomes really annoying after a while....

I live in a rural area, and it hasn't been too big a problem, but I recently purchased a travel trailer and would like to get this resolved before setting out on the first trip.

Any help on this would be appreciated.

Thanks.

Bob
 

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A good start would be to put a scan tool on it. Althought your Check Engine Light may not be lit, the Allison has its own transmission control module, and some trouble codes may be stored in it!
 

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GM Suggests -

Shift Speed Diagnosis
This category contains the following topics:

Inaccurate or inconsistent shift points
Extended shift pattern
Erratic shift pattern


Refer to Inaccurate Shift Points .
Refer to Tow/Haul Switch/Indicator Always On or Inoperative .


Inaccurate Shift Points
Checks
Cause

AT Output Shaft Speed Sensor Assembly (22)
Pinched or broken wire
Loose connector
Incorrect air gap
Inadequate signal
Damaged coil
Damaged rotor teeth
Loose connection

Throttle Position Sensor
Pinched or damaged wire
Incorrect resistance
Loose connector
PCM malfunction

AT Fluid Pressure Manual Valve Position Switch Assembly (40)
Loose connector
Loose bolts causing leakage
A pinched wire
No signal to the PCM

Axle Ratio
Ratio is incorrect or ratio has been changed from its original value.

Tire Size
Tire size is incorrect or the size has been changed from its original value.

Check the PCM for current DTCs.
 

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Discussion Starter · #4 ·
Wow! Thanks, Matt. This advice is exactly what I was looking for. I'll print this out and bring it to the dealer.

I'm a few thousand miles, and a few months out of warranty now, I wonder if they'll cover repairs if the error codes indicate the date and/or mileage when the error occured....

Bob
 

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Allison Transmission Does Not Shift Properly
Condition
Action

Engine idle speed too fast during Neutral to range shift
Adjust to correct idle speed.

Faulty throttle sensor/circuit
Repair or replace the sensor or circuit.

Faulty or sticking bleed ball in C1 position housing
Repair the transmission.

Excessive clutch running clearance
Rebuild the transmission and adjust clearances.

Incorrect shift calibration for vehicle
Install the correct calibration.

Instrument panel tachometer incorrect
Repair or replace the tachometer.

Incorrectly calibrated electronic speedometer
Calibrate the electronic speedometer.

Faulty speed sensor/circuit
Repair the circuit or replace the speed sensor. Refer to the speed sensor DTCs.

Degraded fluid
Change the transmission fluid and filter. Refer to Automatic Transmission Fluid/Filter Replacement .

Loose speed sensor
Tighten the speed sensor retaining bracket bolt. Refer to Speed Sensor Replacement .

Incorrect fluid level
Correct the fluid level. Refer to Transmission Fluid Checking Procedure .

Shift adaptives not converged
Drive the vehicle until shift adaptives are converged.

Low main pressure
Repair the transmission.

Intermittent conditions
Inspect the wiring harnesses and connectors.

Loose or damaged speed gear
Tighten the output flange bolt or replace speed gear.

Sticking valves in control valve body
Overhaul the control valve body assembly.

Leaking trim solenoids
Repair or replace the trim solenoids.
 

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Discussion Starter · #7 ·
Thanks, Matt.

So when they do the scan, will it track down which of these errors may exist, or do the technicians need to check all of these conditions (that sounds time consuming)...
 

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Thanks, Matt.

So when they do the scan, will it track down which of these errors may exist, or do the technicians need to check all of these conditions (that sounds time consuming)...
When they connect a scanner - it may indicate a possible trouble code...

ie. DTC P0741 displayed as a stored code would point them to:

DTC P0741
Circuit Description
The transmission control module (TCM) uses data from the engine speed sensor and the input speed sensor to calculate torque converter slip value. The TCM then compares this calculated slip value to a preset value in the TCM calibration.

Diagnostic Aids
Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
A bent terminal
A backed-out terminal
A damaged terminal
Poor terminal tension
A chafed wire
A broken wire inside the insulation
When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
You may have to drive the vehicle in order to experience a condition.
 
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