Erratic or Premature Upshifts, Harsh Dowshifts, Engine Lugging, or Low Power
| Subject: | Erratic or Premature Upshifts, Harsh Downshifts, Engine Lugging, or Low Power and Performance (Install Transmission Modulator Relay) |
| Models: | 1998 Chevrolet and GMC C6-7 Conventional MD, B7 School Bus Models with 3126 Caterpillar® Diesel Engines (RPO LG5) |
| MT643, MT653, AT545 Allison® Automatic Transmissions (RPOs ME3, ME4, ME5, ME6, MF1) |
Condition
Some owners may comment of any of the following conditions for the above vehicles:
| • | Erratic or premature upshifts |
| • | Harsh downshifting |
| • | Poor engine performance |
Cause
These conditions may be caused by a missing transmission modulator relay.
Vehicles equipped with AT 545 or MT transmissions have an electronically controlled transmission modulator. The engine ECU triggers a relay in order to make the modulator operate. On AT 545 equipped vehicles, the relay is located in the relay center. On MT equipped vehicles, the relay is located on an underhood (UH) support bracket.
Correction
First verify that a transmission modulator relay was installed for either of the transmission types. Install a transmission modulator relay if the relay is not present. Use the following Service Procedure. Use the appropriate part number listed below.
Service Procedure -- Relay Installation
Notice: Use care when installing the relay. There is a slotted cardboard template over the modulator panel. Do not pierce this cardboard. The relay has 5 prongs. The side of the relay with 3 prongs should go inboard. The side with 2 prongs should go outboard. Install the relay in the proper direction in order to prevent misindexing and damage to the cardboard template.
On C6-7 Conventional models with 3126 Cat® Diesel Engines and MT643 and MT653 transmission, use the following procedure
- Locate the relay support bracket. This bracket is under the hood on the passenger side, near the inner fender. The relay support bracket can hold up to four relays.
- Remove the dummy relay (no schematic on the relay.
- Install the transmission modulator relay in its place.
On B7 School Buses with AT 545 Allison® Transmissions and 3126 Cat® Diesel Engines, use the following procedure:
- Locate the relay center position marked "Trans Mod." The relay center label is located just below the relay center and is attached to the front of the dash sound deadener.
- Install the relay into the position marked "Trans Mod."
Parts Information
P/N | Description | Quantity |
|---|---|---|
12177235 | Relay, Trans Modulator (MT Transmissions) | 1 |
12193602 | Relay, Trans Modulator (AT 545 Transmissions) | 1 |
Parts are currently available from GMSPO.
Warranty Information
For vehicles repaired under warranty, use the following information:
Labor Operation | Description | Labor Time |
|---|---|---|
N2847 | Relay - Replace (Transmission Modulator) | Use published labor operation time |
