Engine Surge While Trailering (Reprogram VCM)

Some owners may comment about a perceived engine surge while trailering. The surge appears to be an engine surge, yet when diagnostics are performed, such as checking for variation in the map sensor, there is no fluctuation in the reading. All of the vehicle readings will show no indication of a surge and if there is a variation in the sensor reading, make sure that the vehicle is not getting TPS input from the driver.

There are a variety of inputs into the surge condition that may occur; trailer weight, trailer hitch, vehicle and trailer suspension, and the vehicle's powertrain. The actual condition is caused by the rolling resistance of the trailer pulling against the forward motion of the vehicle. For example, the trailer would hit a bump on the road slowing the trailer's motion resulting in a load on the vehicle's powertrain which continues to pull forward in the opposite direction. The suspensions on both vehicle and trailer take up some of this force resulting in the surge condition the customer feels. The condition can be further amplified by the driver input on the accelerator pedal. This can happen without the driver being aware that they are aggravating the condition.

The torque convertor clutch engagement speed was revised to reduce the surge. There are other variables that the customer may also want to verify; proper fifth wheel trailer hitch location, types of trailer hitches available. Also, the customer may want to vary the tongue weight, being careful not to exceed the vehicle and/or trailering guide maximum tongue weight rating.

Reprogram the VCM with the correct calibration as listed in the table below. Installation of this calibration may affect fuel economy.

The calibrations listed are electronic calibrations and are NOT available from GMSPO. Calibrations will be available from Techline starting July 1998, on the 1998 Disk 13 CD ROM.

For vehicles repaired under warranty , use: