722.6 RPM sensor testing? - Mercedes-Benz Forum

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post #1 of 2 (permalink) Old 08-16-2010, 03:26 AM Thread Starter
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722.6 RPM sensor testing?

Hi, I have removed the conductor plate from my '98 C250 turbodiesel which was in 'electric limp mode'. Apart from not changing gear the car drives perfectly, the only other fault is that it will not crank on the key, however it cranks and fires up easily if I manually close the starter relay. I don't know the history of the car, it was another Fleabay 'bargain' and came with these faults. The trans fluid was medium to dark in colour but there were no particles and it didn't smell burnt.

I had hoped that the starting issue was due to a faulty inhibitor switch on the conductor plate, but it checks out OK (contacts normally closed). All the solenoids click when a voltage is applied (tested after removal).

My question is this: can I test the Hall Effect sensors? I know they work by apply a voltage between the connections marked + and - and I was thinking of rotating a lump of metal near them, then looking for an output. Please, does anyone know what voltage they work on? Battery voltage, or a lower voltage from the ETC? I don't want to fry these chips!

I used a DVM on resistance, as mentioned in another post, but could not find any continuity between any of the 4 connections.

Finally, does anyone know a source for a new conductor plate in the UK without going to MB?

I didn't have the codes read, because from researching the forums it seemed that the conductor plate was the most likely problem and I figured I might see some mechanical defect. I might consider breaking the car if it can't be fixed, anyone want a good engine in Norfolk?

Many thanks, Alan
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post #2 of 2 (permalink) Old 08-16-2010, 03:27 PM Thread Starter
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OK, I connected the conductor plate to the plug under the car and my DVM neasured 6 volts at the + connection to the Hall sensors. Each output gave 6 volts, which dropped to zero when steel was placed next to the sensor, so they appear to be OK. Unless I am advised otherwise, I will reassemble with a new filter and refill with the required liquid gold.

I found a local garage man who says his Snap-on 'Ethos' handheld diagnostic machine will read and clear the trans codes for £30 - does anyone know if he is right? He has the 38 pin adapter and the menu on his device includes transmission.
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