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S54 Fuel Injection DME Relay Testing
 
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Pelican Technical Article:

S54 Fuel Injection DME Relay Testing

Nick Czerula

Time:

1.5 hours

Tab:

$50

Talent:

*****

Tools:

Multi-meter, test light

Applicable Models:

BMW E46 M3 Coupe/Conv (2001-06)
BMW Z4 Coupe/Conv (2006-08)

Parts Required:

DME relay

Hot Tip:

Check fuses before digging too deep

Performance Gain:

Properly functioning engine control system

Complementary Modification:

Replace DME

BMW Z4 sports cars are equipped with digital engine management systems (called Digital Motor Electronics or DME). The engine control module (ECM) in these systems is programmed with software for control of fuel injection, ignition and other functions. BMW DME systems comply with second generation on-board diagnostics (OBD II) standards.

The DME main relay, the primary power source for nearly all engine management functions, supplies battery power to the engine ECM when the ignition is activated. A faulty DME relay can cause poor engine performance and hard starting problems. When it fails completely you may be left with an engine that doesn't start. If your DME is not activating, the engine is not starting and the check engine light does not illuminate when the key is ON, you will have to determine if the issue is a power supply problem or something else. This article will help you test the DME relay, the power source for the DME. Before condemning the ECM, always check power and ground.

Fault codes can be entered in any combination in the fault memory of the engine control module if the system voltage was too low due to battery discharge or due to failure or contact fault of the DME main relay.

If fault code 27C4 main relay is stored in the DME, all fault code entries listed below that were stored at the same mileage are to be ignored and deleted. The DME control module should not be replaced.

The DME main relay is located in the left side (driver side) E-box. There is a black box next to the master cylinder that houses the DME and DME relay. The vehicle used in this tech article was a 2006 Z4M with an S54 6-cylinder engine. The DME relay position varies slightly depending on year, double check your DME relay location with the latest BMW repair information.

To avoid marring the paint and trim, work with a plastic prying tool or wrap a screwdriver tip with masking tape before prying out body or interior items.

Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.

When performing electrical tests, be sure to use a digital instrument such as DVOM. Using an analog device may lead to damage to sensitive electronic components.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.

Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

The DME is located on the left side of the engine compartment mounted in the e-box.
Figure 1

The DME is located on the left side of the engine compartment mounted in the e-box. Start by loosening the T25 Torx screws (red arrows). Remove the e-bod lid. Lift up at an angle and detach the tabs.

With the lid removed, you now have access to the DME relay (red arrow).
Figure 2

With the lid removed, you now have access to the DME relay (red arrow). If you suspect your DME relay is faulty, the quickest test (for the relay itself) is to remove it and swap it with a relay of the same color. To remove the relay, pull it straight up out of the electrical connector. Swap a relay of the same color into the slot, check if you now have DME function. If you do, the relay is likely faulty. If swapping the relay does not help, continue to test the circuit.

With the relay removed, you can identify the terminals and functions.
Figure 3

With the relay removed, you can identify the terminals and functions. Terminal 30 (blue arrow) is constant power to the relay. This should have battery positive volts all the time. Terminal 85 (green arrow) is the battery negative side of the low current switch. This should be close to zero volts with the key ON. Terminals 87 (red and purple arrows) are the battery positive output to the DME and fuel injection components, Fuses F01, F02, F03 and F04.

I like to remove the electrical connector from the E-box, then reconnect the relay.
Figure 4

I like to remove the electrical connector from the E-box, then reconnect the relay. This allows me to test the wires at the bottom of the relay while it's plugged in. Press the release tab (red arrow) and pull it up to remove.

With the relay removed, let's review what you should see and where.
Figure 5

With the relay removed, let's review what you should see and where. Terminal 30 - constant battery positive. I suggest load testing using a test light and a DVOM. Connect the DVOM positive lead to terminal 30 and take a reading, it should read battery volts all the time. Then connect an incandescent bulb style test light to battery ground and touch the test light probe tip to the positive wire you are backprobing with DVOM. Your reading should hold steady, a maximum drop in voltage of 0.5 volts is OK. Anything more is a problem. In this photo, voltage held steady. Note the voltage is high due to a battery charger being connected to the subject vehicle.

Terminal 85: relay coil control.
Figure 6

Terminal 85: relay coil control. I suggest load testing using a test light and a DVOM. Connect the DVOM positive lead to terminal 85 and take a reading. It should read battery negative volts all the time. Then connect an incandescent bulb style test light to battery positive and touch the test light probe tip to the positive wire you are backprobing with DVOM. Your reading should hold steady, a maximum drop in voltage of 1.0 volts is OK. Anything more is a problem. Do not overload this circuit, be sure the test light will not draw more than 0.5 amps.

Install the relay, then using the earlier identified terminals, you can test relay function at the bottom, at the wires.
Figure 7

Install the relay, then using the earlier identified terminals, you can test relay function at the bottom, at the wires. In the next few steps, I will use my DVOM (Digital Volt-OHM Meter) and test light to check each terminal (with the relay removed). You will see expected values on a working vehicle. If you choose to test the relay while it is installed, use the specifications listed below:
Terminal 30 - constant battery positive
Terminal 85: DME relay control, close to zero volts when DME turns relay on. 0.83 as shown (red arrow).
Terminal 87 - Feed to DME loads, battery positive when relay is activated

You can also jump out your DME relay (to bypass it) for testing.
Figure 8

You can also jump out your DME relay (to bypass it) for testing. Connect a fused jumper wire between terminals 30 and 87. The blue arrow points to terminal 30 (battery positive). The red and purple arrows point to terminals 87 (DME loads). Keep in mind, it is the two larger wires you are jumping. This will also activate your DME. Battery volts should be present. Terminals may vary so double check your model against a wiring diagram.

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Page last updated: Wed 12/7/2016 03:06:25 AM