124 Speed Sensor Problems?

[chilsta]

Hello

I've just fitted aftermarket cruise (will post more info on this when time allows).
All is working well, but it's not picking up a signal from the speed sensor.

When I bought the car it had a factory speed sensor on the back of the speedo but nothing was connected to it. I fitted a factory junction box from a donor car to the sensor & an OTG to the junction box.

I've now connected the cruise's speed signal wire to the junction box as well, but when I put the cruise system into diagnostic mode, all works well apart from the beeping/flashing LED to indicate the presence of the speed pulse. Have checked continuity between back of speedo and connector into new cruise ECU. Also tried directly connecting the cruise speed wire directly to the speed sensor.

So my questions - is there a way to test that the hall effect sensor's outputting anything? I found here that it should have around 2V when loaded with a standard cruise ECU, but doubt that this system will apply the same load. Unfortunately I don't have access to a meter with Hz reading.

I'm going to remove the sensor now and see if there's anything obviously wrong with it, but doubt this is going to get me anywhere.
Any idea if the sensor's a relatively cheap part to get from MB & could anyone help with a part number? I'm going to be in northern Germany next week and not going to have a chance to order one from the UK, so part number will save me from my lack of being able to speak the lingo.

Many thanks

 

[chilsta]

I forgot to mention that I've got an imminent 2,000 mile round trip, so would really appreciate any help with getting this working!

Have taken the sensor out, glad it was an easy job & it's a small relatively inexpensive looking device. Is this likely to be stocked by MB?

As well as the single pin connection that I've seen used before for the OTG & cruise, there's a 2 pin connector. Is this likely to have the same output? I'm wondering whether it would be worth trying that in case the larger single pin output has a bad connection internally. Trouble is I don't have a connector. Will try to figure out what the 2 pins are. Think I'll try measuring across the various pins with a meter and see if anything happens when I pass a magnet near the end of the sensor...

 

[chilsta]

No continuity between either of the pins on the 2nd connector and the large connector, or to ground. I've read somewhere about this being for a 12V feed. Could that be the problem, that the sensor needs 12V to operate but because the car originally had no need for the speed sensor, no power input was connected to it? If that's the case, why would there be 2 pins for a feed?

Or does the sensor generate enough power for a pulse with the hall effect on its own without the need of external power, which is what I originally assumed. Hmm...

 

[chilsta]

I found the problem and the cruise is now working fantastically- the lower connector is required to power the sensor (I had assumed it was some sort of spinning magnet/coil system that was creating its own power).

The two pins are marked with a sideways T (-) and +:

The upper, larger connector gives the pulse output.

I connected these up and everything worked as it should.