DME Relay Technical Information and Testing

Introduction

The DME relay used on 1985.5 and newer 944s is essentially the same as the fuel pump relay on early 944s (which sits on the fuse/relay panel under the dash, driver's side, left-hand drive). The DME relay contains two relays/contacts:

• Primary relay/contact: Powers the DME control unit (and, on turbocharged cars, the KLR control unit) and actuates the secondary relay.
• Secondary relay/contact: Powers the fuel pump and the O₂ sensor heater.

If either relay/contact fails, the engine will not start.

DME relays are prone to failure over time, primarily due to broken solder joints inside the relay. Often the relay can be repaired by resoldering the broken joint. Another cause of failure is a diode in the primary relay circuit path; this diode can be replaced with a part from most electronics supply stores.

Failure Symptoms

• Engine fails to start, or is difficult to restart when the engine (and relay) is still warm after a short shutdown. The car often restarts after 30 minutes to 2 hours of cooling.
• Engine suddenly dies while driving, then restarts after cooling.

Quick Field Test

Tap on the DME relay and then attempt to restart. If tapping temporarily restores function, a broken solder joint is the likely cause.

Emergency Operation (No Spare Relay)

If the DME relay has failed and a replacement is not available, the car can be moved by jumpering the relay panel terminals — but only while the engine is running. Remove the jumpers immediately when the engine is stopped. Leaving jumpers installed with the engine off will continuously energize the DME and run the fuel pump, draining the battery.

Warning: Do NOT leave the emergency jumper installed when the car is not running.

Tools

• 2 wire leads with female spade connectors (for testing)
• 3-lead emergency jumper with male spade connectors on each lead (for emergencies and testing)
• Multimeter
• 12 VDC power supply (for bench-testing the relay)
• Magnifying glass (for relay inspection)
• Soldering iron and solder (for relay repair)

Testing

1. If a multimeter is unavailable, use the Emergency Repairs section below to test the relay indirectly.

2. On the relay/fuse panel under the hood, remove the DME relay (position G5).

3. Set the multimeter to resistance (ohms). Measure resistance across the following terminal pairs (labeled on the underside of the relay):

   • Terminals 30 and 87b
   • Terminals 30 and 87

   Both readings should be infinite (indicated as "1." or "OL" on a digital meter, meaning the contacts are open).

4. Connect a 12 VDC power supply across terminals 85b and 87. Measure resistance across terminals 30 and 87b. The reading should be very close to zero (0.1–0.3 ohms), indicating the primary contact has closed.

5. Connect a 12 VDC power supply across terminals 85 and 86. Connect the negative lead to terminal 85 and the positive lead to terminal 86 — the diode in the circuit will prevent the relay from picking up if polarity is reversed. Measure resistance across terminals 30 and 87. Reading should again be very close to zero (0.1–0.3 ohms).

6. If any reading is out of spec, the relay should be replaced or repaired.

Emergency Jumper

The emergency jumper uses terminals 30, 87, and 87b (not 86, 87, and 87b).

Note: Some sources suggest jumpering terminal 86 instead of 30 to avoid continuous energization when not running. However, the wire on terminal 86 is a 1.0 mm wire intended only to carry coil current. The wire on terminal 30 is a 4.0 mm wire designed to carry the full current demand of the fuel pump, DME, and injectors. Always use terminal 30 to supply jumper power.

The jumper consists of three leads with male spade connectors:

To start the car with the emergency jumper:

1. Insert one connector into terminal 87.
2. Insert the second connector into terminal 87b.
3. Insert the last connector into terminal 30. You should hear the fuel pump energize. The car can then be started with the ignition switch.

Alternative field fix (from a reader): A paperclip bent around the correct posts and the relay inserted back into the panel has been reported to work for getting home in an emergency.

Installing the emergency jumper and attempting to start can also serve as an indirect relay test — if the car starts normally with the jumper, the DME relay is almost certainly faulty.

Warning: Remove the jumpers immediately when the engine is stopped.

Inspecting and Repairing the DME Relay

1. Remove the relay cover by looking at the bottom of the relay for two plastic tabs. Pull outward on the bottom lip and upward on the terminals on each side until the cover releases.

2. Carefully inspect all solder joints inside the relay for cracks.

3. Resolder any cracked joints. Because it can be difficult to identify a bad joint visually, resoldering all joints is a reasonable precaution.

4. As a precaution, replace the diode for the primary relay as well.

5. After repair, reinstall the cover and verify the relay using the Testing procedure above.