Tech Tip No. 2
ENGINE CUTOUT AT 4000 RPM
The engine in my early style Audi Coupe quattro
cuts out at 4000 RPM, but it doesn't do it all the time. Why?
Before I answer this question, let's cover just what should happen
as the engine runs. Information about engine speed, throttle position, crankshaft position, rotor position, coolant temperature, intake air temperature, and intake air pressure are processed by the fuel/timing
control unit. This Hartig Electronic Ignition System or "computer" processes all the information and the correct ignition timing for these conditions is selected from the computer's memory. When the
crankshaft is at the correct position the computer sends a signal to the ignition control unit. The ignition control unit will then switch off the ignition coil primary voltage and cause the coil to fire.
The receipt of correct information from all these sensors is
critical. If incorrect data is obtained the timing will not be correct and severe damage may occur to the engine. In an attempt to prevent a gross error in ignition timing the computer has a built in self check. The
self check system limits the engine speed to 4000 RPM if the input signals are out of range or obviously defective.
What do you check?
-- Check the air temperature sensor and its wires. An ohmmeter connected between terminals 18 and 19 on the 25-pin plug at the computer should read approximately 13 to 33 ohms at room temperature. If this sensor fails or a wire to it breaks the computer will shut the engine down at 4000 RPM. This is the most common cause.
-- Check the idle switch. It must open when the throttle is moved more than 3 degrees. Remove the 25-pin connector from the computer. Connect an ohmmeter to terminal 15 of the 25 pin connector and ground. With the throttle closed it should read 0 ohms. With the throttle opened it should read infinite ohms. If the switch is saying we're idling and the speed sensor says we're going 4000 RPM the ignition system determines a conflict of data and shuts down the engine.
-- Check the terminal on the brown wire on the top rear of the manifold. It should be clean and well grounded. There must be a good ground on the intake manifold.
-- Check the 25-pin connector on the computer. It must be plugged in all the way with the metal clip holding it there. If it's not or if there are corroded or oxidized connections the computer will receive corrupt data and do its safety shut down.
-- Check the connections on the fuse and relay panel. Poor connections caused by corrosion and/or overheating will cause the same problem.
-- Check the fuel distributor frequency valve and the wires to it. If the frequency valve has become disconnected or has broken wires this will cause engine shut down at 4000 RPM.