Crank Position Sensor... running, or just starting?

[TC535i]

Does the DME look at feedback from the CPS when the car's running at any point, or does it just monitor it when you're starting the engine?

[Jeff N.]

The CPS feeds RPM data to DME based on the rotational speed of the dampner. The notches in the dampner send a pulse to the DME so it can calculate the precise rotational aspect of the crank. There is a "skip notch" if you will that tells the DME where TDC is. This allows the DME to adjust the timing based on engine load (AFM signal) and fuel (function of RPM via CPS, AFM load, TPS indication).

Soooo...the DME absolutely needs the CPS to function at all times. Just try pulling the plug on the top of the engine if you need further convincing.

PS - all specific to Motronic 1.3. Any other version - your mileage may vary.

[TC535i]

The CPS feeds RPM data to DME based on the rotational speed of the dampner. The notches in the dampner send a pulse to the DME so it can calculate the precise rotational aspect of the crank. There is a "skip notch" if you will that tells the DME where TDC is. This allows the DME to adjust the timing based on engine load (AFM signal) and fuel (function of RPM via CPS, AFM load, TPS indication).

Soooo...the DME absolutely needs the CPS to function at all times. Just try pulling the plug on the top of the engine if you need further convincing.

PS - all specific to Motronic 1.3. Any other version - your mileage may vary.

Believe our 89 735i uses 1.1, but you're probably right. Lots of problems lately with the car just CUTTING out (abrupt jerk) and coming back while accelerating, and now it won't even start. Screwed around for a few hours testing fuel pump, relay, etc etc...

Finally decided to look at the CPS, and the bracket is loose, there's about 1/2" of play in it, and I think it's been contacting the teeth of the wheel. Not good.

I'll get that replaced, hopefully that gets the car back on her feet.

[shogun]

In case of the M70 V12 engine it is like this. Copied just for your info in a similar case on a 750 E32:
quote:

If you start it and it idles for a short period 800-900 rpm and after that it drops to 500-600 in LHM, can indicate that the faulty bank is firing during startup. When starting, all injectors will work (despite all sensors/MAF) to make the mixture rich and enable it to fire up. When the DME's notice the engine runs, this startup mechanism switches off and the engine control management takes over.

If this is the case, most likely its due to one of the sensors. Have you tried swapping the cilinder identification sensor yet?

Have you checked the 4 electrical connections in front of the engine (2 crankshaft sensors and 2 cilinder identification sensor connections)?

Keep us posted.

Sean750

[Bill R.]

you hit the nail on the head.

Believe our 89 735i uses 1.1, but you're probably right. Lots of problems lately with the car just CUTTING out (abrupt jerk) and coming back while accelerating, and now it won't even start. Screwed around for a few hours testing fuel pump, relay, etc etc...

Finally decided to look at the CPS, and the bracket is loose, there's about 1/2" of play in it, and I think it's been contacting the teeth of the wheel. Not good.

I'll get that replaced, hopefully that gets the car back on her feet.

[Jeff N.]

Locktite that buzzard!

you hit the nail on the head.