[Dr K]

NOTE: Thanks to Matt De Maria for explaining how to do this.

The "upper" sensor is the "reference" sensor, which tells the DME when top dead center (TDC) passes during cranking (there is a set screw in the flywheel). The "lower" sensor is the speed sensor responding to a toothed wheel on the flywheel. Two voltage pulses are generated for each tooth on the flywheel (Bentley manual, page 200-5 and 280-11)

Tools needed:
Jack
Jack Stand
Lug Wrench (or similar, to remove wheel lugs)
5 mm. long ball-headed Allen wrench (or short socket-type with extension and ratchet handle)
8 mm socket (if you need to remove plastic tube from heater fan to get to connectors)
10 and 13 mm sockets (and ratchet handle)
Silicone spray
Torque wrench with low range (8 ft-lbs)
Flat-headed screw driver
String
(silver Sharpie)

1. Open the engine compartment (you'll need to get in from the top to unplug and plug in the sensor wires)
2. Remove plastic tube between engine-compartment heater fan and rubber sleeve by engine tin (to heat exchangers)
3. If still in place, remove the cable clamp through which the wires go - it's at the base of the intake manifold.
4. Loosen left rear lug nuts
5. Jack up right side or right rear of the car and rest it on a jack stand
6. Remove the left rear wheel
7. Identify the flywheel sensors, on the front of the engine, upper portion, right by the flywheel/transmission cover
8. Mark the flywheel sensors "U" and "L" for "upper" and "lower", and also pull on the wires for the bottom sensor and see which moves in the engine compartment. Mark that plug "L" also and note which position it is in on the sensor connector plate near the left front of the engine compartment just above the left fuel rail (I'd tell but don't remember as I write this, and hope to edit it later). You should mark the new sensors similarly. If the old wires are marked with tape collars, DG is the speed sensor (L) and BG is the reference sensor (U). The sensors are identical electrically.
9. Using your Allen wrench, remove the single screw in each sensor. It should loosen easily, but if not, use a regular Allen wrench to break it loose--this will be less likely to strip the screw head.*
10. Spray the edge of the large irregular grommet through which these wires enter the engine compartment (through the fire wall)
11. Using a screwdriver, carefully work the grommet out into the wheel well
12. The wires are inserted into the grommet from the side (cut to the edge). Bend it a little and pull the speed sensor and reference sensor wires away from the grommet (see photo)
13. Starting with the speed sensor (lower), unplug the other end of the wire from the bracket on the engine. There is a second metal place inserted into the first, from the side of the bracket (remove it by pulling out toward the rear of the car)
14. Tie a string to the plug end of the sensor, and tie the other end (needs to be about 3 feet long) to something in the engine compartment (see photo)
15. Feed the plug end of the sensor wire through the firewall (pull from the wheel well). Alternatively (if you know the sensors are really shot) you can cut the wire. Save the plug and give it to me or your regular mechanic, who may need it in the future.
16. Using the string, pull the plug of the new sensor through the fire wall into the engine compartment (this is a lot easier with an assistant feeding the plug and wire through for you)
17. Clean the bracket into which the sensors will be attached.
18. Replace the sensor and tighten the screw to 8 Nm (6 ft-lbs).
19. Unplug the upper flywheel sensor plug from the engine bracket, tie a string to the plug, and pull the plug out through the fire wall into the wheel well.
20. Using the string, pull the new sensor wire plug end through the fire wall into the engine compartment.
21. Repeat this with the other sensor wire plug
22. Plug the connectors into the bracket in the engine compartment, re-insert the locking plat, and plug the wiring harnes into these connectors
23. Bend the grommet a little and fit the new sensor wires into the openings in the grommet
24. Spray the grommet that goes through the firewall with silicone, and carefully position it so it is again through the fire wall.
25. Replace the cable clamps
26. Gap for these sensors should be set to 0.8mm (+/- 0.3 mm) but I haven't figured out how to do this with a feeler gauge while the engine is in the car and the transmission in place. Replacing the sensors as described should not change this. If you need to adjust, this is done by loosening the top screw on the mounting bracket (which has an elongated hole) and slightly loosening the bottom screw, then move the bracket to adjust. Don't forget to tighten them down again. [NOTE: A discarded sensor can be used as a sensor gap. Glue an 0.8 mm washer to the tip of a used sensor and install temporarily to adjust gap.]
27. Try to start the engine. Then swap the plug locations of the two sensors since it seems like greater than 50% chance you did it wrong, and the engine should start.

*If the sensor screw is seized in the mounting bracket, you might need to remove the mounting bracket and get the sensors off away from the car. I'm not sure this can be done with the engine in the car, but I believe it can. When I did this job, my screws came out very easily so no need to touch the bracket. If you remove the bracket, you'll need to re-gap the sensors when you replace the bracket, but if you don't remove or loosen the bracket, the new sensors should have the correct gap when installed.

Peter

[Dr K]

Above instructions updated (we did this again today). Also, here are some photos:


Here are the old connectors in place (step 7)


Ball-headed Allen used to remove sensors (step 9)


Gasket bent so wire can be removed (or inserted) (step 12, step 23)


Separating 2 pieces of connector bracket (to unlock connectors) (step 13)


Sensor plug ends in engine compartments (note string) (step 14)


Sensors will be inserted here, but bracket is dirty (step 17)


Bracket after cleaning (step 17)


Sensors in place step 21

Note gasket back in fire wall


Connectors attached (don't forget the locking plate) (step 22)


Cable clamp with wires in place (this one is in wheel well, there is another in the engine compartment) (step 24)

[Jascha]

Much appreciated picture taking of the fine install details --