Initial Inspection and Diagnostic Overview

IMPORTANT
Many of the voltage tests described in this section depend on the fuel pump relay being energized during testing. The ECU energizes the fuel pump relay for approx. 5 seconds when the ignition is turned ON, by grounding terminal 47/T of the relay (terminal 47/7 on models with CIS-MOTRONIC). After this time, the relay is de-energized, and the ECU waits for a "cranking" signal before it will again energize the relay. Once the engine is started, the relay will stay energized as long as the engine is running. If the engine dies, the ECU will turn the relay OFF after 5 seconds and wait for a "cranking" signal before it will energize the relay again.
When performing tests where the ignition is ON and the engine is not running, it may be necessary to disconnect the fuel pump to prevent it from running, and energize the fuel pump relay by manually grounding the circuit to relay terminal 47. If the engine must be "cranked" during a test procedure, you will also need to disconnect the power output stage of the ignition coil to prevent the engine from starting on the residual fuel.
If you are performing a voltage check WITHOUT manually energizing the fuel pump relay, and do not get a voltage reading (especially when looking for battery voltage in a circuit) try turning the ignition OFF, wait a few seconds, then turn the ignition back on and check the reading within 5 seconds. You may want to have a partner work the key while you perform the tests.