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Below is a copy from a BMW performance site (
http://www.hiperformancestore.com/motronic.htm)
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Step 8, Speed Sensor and Reference Sensor!
So, you are cruising along on interstate 10 in your Motronic Basic 528e, 533i, or 535i secure in the knowledge that Robert Bosch improved the Motronic system so that a coolant temp sensor won't leave you stranded like it did with the old L-Jetronic system in your E12 528i. Now is when you find out he added two new sensors that will leave your stranded.
*Failure of either of these will stop your car dead, right NOW! Well, that's a little bit of an exaggeration, but not much. Please read on.
The Motronic Basic 528e, 533i, or 535i all have two sensors mounted on the bell housing. The first is called the speed sensor. As you may have guessed it reports engine RPM to the Motronic ECU. Failure of this sensor may or may not totally disable your car. It's supposed to limp along poorly constantly missing if this sensor fails. Reality may be different.
The reference sensor is more critical. Without it the car will not run.
These are both mounted on the driver's side of the bellhousing. The speed sensor is mounted above the reference sensor. Caution: the two sensors are actually identical, so it's possible to connect them incorrectly. If the speed sensor is plugged into the connector for the reference sensor, and vice versa, the car won't run. The speed sensor mounts above the reference sensor and connects to the BLACK plug. The reference sensor connects to the GREY plug.
Both of these plugs have the Bosch style connectors on them with the dreaded metal clip. It's probable that the connectors will have been damaged at some point in the cars life. Mechanics love to test these sensors, but they really don't like messing with the metal clips so they tend to force and break things. If the connector is damaged DO NOT try to cut off the connector and solder on a new one. You won't be able to do it. You will have to find another way to secure the connector (tape, zip ties, etc.), or replace the entire sensor.
The Speed and Reference sensors connect to the Motronic wiring harness at the crossover bracket. Watch out, these two connectors can be reversed so make sure the upper sensor connects to the black connector.
Thankfully it's very easy to test these sensors. Once again, get out your Ohm meter. Remove one of the connectors. Note that this time there are three electrical terminals present. That means three tests per sensor. The terminals are numbered 1, 2, and 3 from left to right as viewed from the front of the car. If your car's crossover bracket is missing orient the sensor so that the single tab is up, and the two tabs are down. Now the terminal on the left is number 1.
Measure resistance between terminals 1 and 2. You should have 864-1056 ohms. Between terminals 1 and 3 you should have 100,000 ohms minimum. Between 2 and 3, again, 100,000 ohms minimum. Now Bosch wants you to take this reading with the outside air temperature of 77F (25C). I find it doesn't make much difference, but if it's really hot or cold out and the sensor is a little out of spec, I probably wouldn't worry about it.
Now test the other sensor. The values we are looking for are the same because these two sensors are EXACTLY the same part number.
In the picture below I am testing the reference sensor between terminals 1 and 2. The sensor passes!
It's a really good idea to carry a spare sensor with you if you are taking a long trip. Since they are both the same you really only need to keep one with your spare fuses and relays.
Sharp observers will notice a position sensor mounted on the front of the engine of a Motronic Basic 528e, 533i, or 535i. This sensor has NOTHING to do with the running of the engine. It's not linked to the Motronic system in any way. It's used to hook up some device the dealers have.
If you have a 528e with Motronic 1.1, your car does not have the two reference sensors on the bellhousing. Instead it has a "pulse sensor" mounted at the front of the engine. Test the resistance of this sensor and look for an ohm reading from 486-594. You also need to verify that the tip of the sensor is within .7mm-1.3mm from the toothed wheel.
