Troubleshooting Fuel Supply Problems

Introduction

This procedure is used to troubleshoot fuel supply problems, including failure of the fuel pump to start during engine cranking.

Fuel Pump Not Starting During Engine Cranking

Note: Labelling on early cars (early) terminal boards may be inconsistent, missing, or difficult to read. It may be necessary to determine the terminals to use for testing by looking at the labels by the stabs on the bottom of the relay.

Work through the following checks in order.

Fuel pump fuse

Model	Fuse
Early cars	Fuse #2 on the Auxiliary Fuse Panel
1985.5 and newer 944 and 968	Fuse #34 on the Central Electric Panel

Bypass power to the fuel pump

Remove the Fuel Pump / DME relay and install a jumper to bypass the relay and the DME computer, powering the fuel pump directly. On all 944s and 968s, jumper terminal 30 to terminal 87b.

If the fuel pump starts, the pump, its power supply, and the wiring from the relay panel to the pump are good. The problem then lies in the Fuel Pump / DME relay, the DME computer, or loss of the signals that tell the DME to start the pump and keep it running. Continue with the next check.

If the fuel pump does not start, use an external 12 VDC source to apply power directly to the pump. If it now starts, check the wiring from the relay panel to the pump and perform the primary-supply-voltage check below. If the pump does not start even with power applied directly at the pump, the pump is bad and should be replaced.

Fuel Pump / DME relay — primary supply voltage

Remove the Fuel Pump / DME relay and check for supply voltage (approximately 12 VDC) from the battery. Inserting a male spade connector into the relay-board terminal makes the voltage easier to check. On all 944s and 968s, check terminal 30.

If no voltage is present, the battery is dead or the wire from the battery to the relay panel is broken or disconnected.

Fuel Pump / DME relay — primary coil voltage

Remove the Fuel Pump / DME relay, turn the ignition switch to the "run" position, and check for approximately 12 VDC at the relay primary-coil terminal. On all 944s and 968s, check terminal 86.

If no voltage is present, the ignition switch is bad, there is a problem with the alarm system (if equipped), or there is a broken wire between the battery and ignition switch, between the ignition switch and alarm module (if equipped), or between the alarm module and the Fuel Pump / DME relay. If the car has an alarm system, bypass it using ELECT-23 and repeat the test.

If primary-coil voltage is present, turn the ignition off, reinstall the relay, and proceed.

Fuel Pump / DME relay — primary coil activation

Disconnect the plug from the DME computer. Turn the ignition switch to the "ON" position and check for supply voltage (approximately 12 VDC) from the Fuel Pump / DME relay to the DME computer electrical connector. On all 944s and 968s, check terminal 18 to ground and terminal 35 to ground.

Engine Cranking / Running Signals

For the fuel pump to start and keep running, for the ignition coil to develop secondary voltage, and for the injectors to fire, the DME computer must see an engine-start signal or an engine-running signal (greater than 200 RPM). When it does, it completes the circuit for the Fuel Pump / DME relay secondary coil and the pump starts. It also provides the current path for the ignition coil primary and secondary, so current through the primary induces the secondary voltage seen at the distributor. The same cranking/running signal goes to the injector drivers (2), which provide the current path for the injectors to open. The injectors are batch-fired — all four fire together. On 944s, injectors 1 and 2 are wired together and injectors 3 and 4 are wired together, but all are powered from one injector driver inside the DME computer.

The engine-start signal is normally generated by a combination of voltage from the ignition switch to the starter solenoid and a signal from the reference sensor. If the starter-solenoid signal is not present, a cranking signal can instead be generated by the combination of the speed sensor and reference sensor signals.

If all of the previous tests passed, check the cranking-signal inputs below.

Speed sensor

Using an oscilloscope, check the speed sensor output at terminals 8 and 27 on the DME computer plug. You should see a signal greater than 2.5 V. The speed sensor generates an engine-speed signal by counting the teeth on the flywheel ring gear, producing two output pulses for each ring-gear tooth.

Without an oscilloscope, check the sensor by resistance: look for 600–1600 ohms between terminals 8 and 27 on the sensor connector, and infinite resistance between terminals 27 and 23.

Reference sensor

Using an oscilloscope, check the reference sensor output at terminals 25 and 26 on the DME computer plug. You should see a signal greater than 2.0 V. The reference sensor generates a signal relative to engine position at TDC, producing a single pulse triggered by a bolt cemented in the flywheel.

Without an oscilloscope, check by resistance: look for 600–1600 ohms between terminals 25 and 26 on the sensor connector, and infinite resistance between terminals 26 and 78.

Starter solenoid signal

Turn the ignition switch off and disconnect the DME computer plug. Connect a DC voltmeter from terminal 4 on the DME plug to ground. Crank the engine and check for approximately 12 VDC. If no voltage is present, the ignition switch is bad or the wire from the ignition switch to the DME computer is broken. To check the wire, measure the resistance from terminal 50 on the ignition-switch plug to terminal 4 on the DME plug.

DME Computer Checks

The best way to test for a faulty DME computer is to swap it with a known-good unit. Most owners don't keep a spare on the shelf, but borrowing one from a friend for testing is the best DME test available.

Warning: You cannot test an early cars (early) with a DME computer from a later car, or vice versa — they use different injectors. A later DME will not fire early injectors at all. An early DME may fire the injectors in a later car, but the current draw will be excessive and can damage the DME's injector drivers. Even if the drivers survive, they will likely shut down after a few cycles from the excessive current.

Beyond the swap test, there is very little that can be done to check the DME's operation directly. A 944 DME computer normally only fails from water intrusion (due to a leaking battery tray) or from bad solder joints, which are easily repairable. If the car is hard to start and you suspect the DME, you can sometimes get it to start by tapping on the outside of the case with a rubber mallet.

To inspect for broken solder joints, remove the DME computer from the car and open it. Inside you'll find two circuit boards mounted face-to-face, bridged at one end by a ribbon cable and plugged into the main terminal at the other. Separate the boards, slide one out of the connector, and lay them open like a book to inspect the joints. For the detailed procedure, refer to DME-02. Pay particular attention to the solder joints where the ribbon cable joins the two boards — those are a common source of problems.

Other Fuel-Related Problems

Symptom	Possible cause	Checks
Good fuel pressure but no fuel to the engine	Injectors not firing	Crank the engine while listening for clicking from the injectors. Injectors should fire twice per engine revolution.
Good fuel pressure at the fuel rail but injectors not firing	No voltage to the injectors	Disconnect one fuel injector plug, turn the ignition to ON, and check for approximately 12 VDC on the injector plug terminals (12 VDC on one terminal, 0 VDC on the other).
Good fuel pressure but injectors not firing, voltage present	Injector firing signal not present	Perform the Speed Sensor, Reference Sensor, and Starter Solenoid Signal checks above.
Fuel pressure too high	Bad fuel pressure regulator	When fuel pressure runs too high, the injectors draw too much current and the DME's injector drivers shut down. Disconnect one fuel injector connector and crank the engine. If it starts where it would not before, the fuel pressure regulator is bad and should be replaced — disconnecting one injector reduces driver current enough to let the drivers fire.
Fuel pressure low	Clogged fuel filter	If you can't verify when the fuel filter was last changed, replace it.
Fuel pressure low	Fuel pump check valve stuck	Not a normal low-pressure cause — usually associated with the system depressurizing after shutdown, as the valve typically sticks open. If the car has sat for a long period, the check valve can stick closed. New fuel pumps normally come with a new check valve. Replace the check valve.
Fuel pressure low	Bad fuel pump	Perform the fuel pump delivery-rate check (FUEL-04), assuming the fuel filter and check valve are good.