Issue Summary:
- In a vehicle equipped with an Allison MT545, shifts are either too soft or too harsh, and the friction plates may be burned.
- A vehicle with an Allison MT545 returns with a leak from the front of the transmission and a slight vibration at higher engine speeds.
- Before or after overhaul, a GM vehicle equipped with an Allison 1000/2000 transmission may have no upshifts, and code P0708 is stored.
- A GM vehicle equipped with an Allison 1000/2000 transmission neutralizes at highway speeds, and code P0735 is stored.
The transmission shifts are either too soft or too harsh. The transmission may have durability issues with regard to burning the friction plates either before overhaul or prematurely after overhaul.
One of the components in the electric-modulator control system has failed.
The electric modulator has only two positions: light-throttle operation and heavy-throttle operation. With light throttle the modulator is not energized; therefore, the transmission is using a set line pressure of about 155 psi.
When the throttle is depressed to 80% or greater, the modulator is then energized and pressure is raised to ensure proper clutch-holding pressure. This is also important when the vehicle is fully loaded. It takes more throttle to move the vehicle, so the modulator is energized to raise clutch pressure to ensure durability.
When the throttle is released to 60% or less, the modulator is released to avoid clunks on downshifts.
The components involved are the vehicle’s engine control module, accelerator-pedal-position module (Figure 1), modulator control relay (Figure 2) and electric modulator (Figure 3).
The accelerator-pedal-position module (APPM) sends its signal to the ECM. The ECM in turn energizes the normally open modulator-control relay, which energizes the modulator.
The modulator is provided with system voltage, either 12 or 24 volts depending on the system. Once the modulator is energized its plunger shoots out and strokes the modulator valve. This system can be used in a non-computer-controlled vehicle by the use of a throttle-controlled on/off switch.
The modulator can be mechanically faulty if it is stuck in or out, or it can be electrically faulty if it is shorted or open.
The relay could be open, preventing it from energizing the modulator, or it could be stuck closed, which would result in the modulator not withdrawing.
If the APPM signal is faulty, the relay either would not be energized or could be energized at all times.
The ECM could be faulty by not sending the necessary voltage to the relay or by sending voltage all the time. The ECM in the vehicle is provided by the engine builder (International, Cummins, Caterpillar etc.). The relay and APPM are provided by the truck maker (Ford, GMC, Freightliner, Mack etc.). The modulator is supplied by Allison.
The modulator has two wires. One is from the relay, which provides electricity to the modulator. The other is ground, which usually is attached to the chassis nearby.
Check for system voltage at the modulator on full throttle and for no voltage when the throttle is released (refer to the wire diagram in Figure 4). In most instances the faulty component is either the modulator or the relay.
- 12-volt electric modulator . . . . . . . . . . . . . . . . . . 29508036
- 24-volt electric modulator . . . . . . . . . . . . . . . . . . 29508037
The vehicle returns with a complaint of leaking out the front of the transmission and a slight vibration at higher engine speeds. When the transmission is removed, the flywheel is cracked and the front pump bushing is damaged.
The crankshaft pilot (Figure 5) is worn.
Replace the crankshaft pilot or have it sleeved to original specifications.
Before or after overhaul, a GM vehicle equipped with the Allison 1000/2000 transmission may exhibit a condition of no upshift. Scanning the truck shows that a code P0708 (transmission range sensor circuit input high) is stored. Usually when this code is stored, replacing the neutral-start backup switch cures the problem; however, when looking for the NSBU switch, the technician notices that the switch is not bolted to the driver side of the transmission case.
Beginning in 2006, GM vehicles with the Allison 1000/2000 transmission eliminated the NSBU switch on the outside of the transmission and instead used an internal mode switch. The internal mode switch is on the selector shaft inside the transmission and indicates gear-selector position to the ECM. When this internal mode switch was introduced, the internal harness in the transmission also was changed because internal-harness pin assignments were changed as well.
The switch can be tested in the same way as the previous NSBU switch, and switch parity is identical. Refer to Figure 6 for the location of the internal mode switch.
Refer to Figure 7 for identification of transmission-harness connector pins in models with the internal mode switch and in models with the NSBU and line-pressure EPC solenoid.
Verify wiring-harness integrity and replace the internal mode switch. Refer to Figure 8 for a partial wiring diagram and an internal-mode-switch parity-test chart.
Note: These parts may not be available from GM but are available from an Allison dealer.
- Detent lever (Allison part number) . . . . . . . . . . . . 29542692
- Internal wiring harness (Allison part number) . . . 29543334
Before or after overhaul, a GM vehicle equipped with the Allison 1000/2000 transmission exhibits a complaint of neutralizing while being driven at highway speeds. Code retrieval indicates code P0735 (5th-gear-ratio error) is stored.
One cause may be severe damage inside the transfer case causing it to go into neutral, which causes the computer to store code P0735. This severe damage in the transfer case is caused by a loss of fluid inside the case as a result of overheating to the point that the fluid vaporizes and escapes through the vent system.
Transmission Digest contributing editor Mike Weinberg of Rockland Standard Gear covered this problem very well in the March 2007 issue. At this time Mike and his staff recommend removing the ATF from the transfer case and replacing it with 5W-30 motor oil, and then removing the speed sensor at the rear of the case and adding an extra quart of oil through the sensor opening. In addition, Rockland Standard Gear recommends checking the fluid level every 5,000 miles and replacing the fluid every 10,000 miles. For additional information refer to the March 2007 issue of Transmission Digest, page 48.
Repair or replace the transfer case, fill with 5W-30 motor oil and add an extra quart, then recheck levels at 5,000-mile intervals and replace the fluid at 10,000-mile intervals.
August 2009 Issue
Volume 26, No. 8
- Allison MT545: Operation of electric modulator
- Allison MT545: Flywheel damage
- GM Allison 1000/2000: No upshift; DTC P0708 stored
- GM Allison 1000/2000: Setting code P0735 on highway