Tech Tip No. 3
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!