Tech Tip No. 3

Ned Ritchie

Copyright 1994, 1997

MORE ENGINE CUTOUT AT 4000 RPM

The Tech Tip on the engine cutout at 4000 RPM for the early style Audi Coupe Quattro was great. I was able to fix the problem in my Quattro, however, my wife has a Turbo Automatic that cuts out at 4000 RPM? I checked all the items mentioned in the Tech Tip and was not able to correct the problem. Is there something else to check in an automatic?

Yes, in addition to the throttle switches and sensors described in Tech Tip No. 2 the Turbo Automatic needed an additional safety feature.

Since engine power is delivered to two wheels instead of four as in your quattro the stresses on the two wheel drive are about doubled. This power is delivered through a torque converter, a weak link in the drive train. In order to protect the torque converter and the two wheel drive train if the driver uses "left foot braking" the engine is shut down at 4000 RPM. Left foot braking is when the driver maintains full throttle and simultaneously applies the brake with the left foot to control the speed of the car. This technique is used when going into a corner to keep engine power up so that you can accelerate out of the corner with maximum power.

How does the engine control unit know that this is happening? Simple - It monitors the brake light circuit. It has a circuit that is grounded through the brake light bulbs. When you step on the brake, power is supplied to the brake lights. What was the ground for that monitoring circuit is now 12 volts. The circuit in effect hollers out, "Hey, he's stepping on the brake again." If the engine is below 4000 RPM nothing happens, but if it above 4000 RPM the "little man that runs around inside the control box" turns off the engine.

If your foot is NOT on the brake it means that the brake light monitoring circuit is sensing the equivalent of you stepping on the brake. What could cause that?

     A brake light switch that is stuck on

     Both brake light bulbs are burned out

     A corroded connection in the brake light circuit

     A broken wire in the brake light circuit

This is a problem that has plagued many mechanics. Who would ever think that replacing the burned out brake light bulbs would fix the problem!

INTENDED ACCELERATION ^®

© Copyright 1997-2000 by Ned Ritchie