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Subject: Reference Sensor Maintenance. These will keep the car from starting!! Author: kff (moderator) : member since February, 2004 : 4035 posts Posted on: 2005-05-23 21:31:55

Hi, maybe you have seen my ...a long day write-up about changing the fuel injectors, hoses, belts and such where Tammer and I had trouble with a bad reference sensor. Well here is a discussion of the sensors.

Many M30 and M20 engines have a pair of reference sensors to tell the Motronic brain where and how fast the flywheel is spinning. These are critical to both start and run features of the car, two of the best features to have working!!!

The two sensors are identical, but they read at different places on the flywheel (they are about 2 inches apart and therefore read the flywheel about well two inches away from each other). One is the speed sensor and the other is the position sensor. They are given the role of speed or position by where they are installed in in the bell housing of the transmission. Here is a picture of the installed location with the sensors behind that rusty cover plate.


The bolts holding the cover hold the sensors in place. You can see that these are on the left side of the bell housing where it meets the engine. (Car is too low to allow me to get under there to see the sensors, but this picture is what you will see if you look under the car.

The wires for the sensors are held together with a spiral of plastic and go to the back bracket between the intake manifold and valve cover in the M30 engine. Here is that view:


To see where in the engine these are, they are above the strut brace and to the left of the radiator overflow bottle cap and to the left of the fuel line pressure dampener in this photo:


Note the difference in the color of the connector. The gray and black connectors are part of the engine harness and plug into the sensors which push up thorugh the support bracket. They are held in place like the other connectors with a wire clip holding the parts tight. Use a pick to remove this wire clip and then the plugs will come apart. It is CRITICAL to have the same sensor postition connected to the same plug as before. The sensor positions are marked with 1/2" letters molded into the bell housing. These letters are D and B and you must feel the difference since they are on the side of the mounting location that can not be seen unless you are under the car. Changing these can be done by just reaching under a non-lowered car or by jacking it a bit and suporting it with a jack stand on the left side. The Bentley manual says the speed sensor is the top one (D) and goes to the black connector and the lower sensor location (B) is the position sensor and it goes to the gray connector.

These sensors can leave you stranded when they go bad. To check them Bentley says to check the resistance between pins one and two of the three in a row and it should read 960 +/- 60 ohms. The other connections should have over 100,000 ohms resistance. Our cars are old and these can go bad due to vibrations after all these years so maybe this spring/summer is a good time to replace them. Maybe carry a spare if you don't replace them. The can be had for about 70 each from the usual on line retailers.

The M20 engine is a bit different in location in the engine compartment but the idea is the same.

And I see I have a cleaning chore this summer, the sensor cover needs some work to look like it is from the same car as the engine shots. (it is not, but the other engine looks the same so the sensor cover will get some attention). Hope this helps someone.

• Reference Sensor Maintenance. These will keep the car from starting!! (pic) (url) - kff 2005-05-23 21:31:55 (4137 views)