What TDC means in words ? E30 325i early1986 Motronic 1.0 ECU two sensors CPS and TDC

[Pekka Finland]

I still have cold start problems, engine stopping at the traffic lights, above normal fuel consumption etc. after changing the Coolant Temperature Sensor, Cold Start Fuel Injection Sensor, Fuel Pump Relay + Fuse and Main Relay. The above was pointing to a corrpted CPS. Then, of course, I was looking for CPS sensor in the front end of my 2.5 L 171 hp motor without result and it also seemed that there is in the engine block two wire plugs both looking like CPS sensor plug. Luckily I found an answer at this forum written by "richardodn" Nov. 2008 stating that in the early model E30 325i's with Motronic 1.0 ECU there are two sensor plugs in the motor block and two sensors in the bell-housing of the gearbox. These sensors are called CPS (crankshaft position sensor) and TDC which I assume relates to timing. The same original Bosch sensor part is used for both CPS and TDC purpose. I have not yet started changing those as I have to first lift my car for checking what the existing sensors look like and find out the right Bosch part number in those - just to be sure.
I very kindly ask from which exact words the abbreviation TDC comes from ?

Best Regards,
Pekka

[bluptgm3]

TDC - (Typically, cylinder piston 1 is at) ‘top dead center’ of the crank throw up-stroke. With both valves closed the cylinder is ready to combust. The piston is as close to the cylinder head as it will ever be.

My understanding of the ‘eta’ engines is that they have two sensors looking at the flywheel, looking thru ports in the bell housing. One checking engine speed and one looking at crank position, by looking for the rectangular chunk of metal on the flywheel.

Use the last seven characters of the VIN in www.realoem.com to determine vehicle specific part requirements.




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[Pekka Finland]

Dear bluptgm3

Thank you very much for explaining from which words "TDC" comes from.
I did not find out that despite reading quite many expert articles with pictures about Motronic 1.0 ECU and the two sensors CPS & TDC. Surprisingly there was nothing in the YouTube.
Otherwise I have the necessary data about the issue - exept having not yet checked at the existing sensors in the bell-house of the gearbox what the matching Bosch part number is before buying new ones.
Yes, you are right - the both sensors are looking at flywheel at the back end of the motor.

Best Regards,
Pekka

[richardodn]

A caution on the flywheel sensors. They are the same part. Function is determined by the harness connector they're plugged in to and must be inserted to the correct bellhousing hole. Easiest way to keep them straight is to only do one at a time. Otherwise be sure to mark both the connection and installation position.

Engine speed sensor (counts flywheel teeth, position D) is the black connector. Reference point sensor (senses flywheel pin, position B+) is the gray connector.

[superj]

Top dead center can be in the compression stroke or the exhaust stroke, also. The crank turns twice compared to the cam

[82eye]

for clarity :










the second sensor is referred to as a reference sensor. tdc as a term does not refer to the sensor itself. it may confuse you to keep calling it that as you move forward.

[bluptgm3]

for clarity :

Position/speed sensors

the second sensor is referred to as a reference sensor.
tdc as a term does not refer to the sensor itself. it may confuse you to keep calling it that as you move forward.

Or iMAGE




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