Yeah this thing is really hard to see. I’ve never seen mine because it’s so deep down there. Here are some images clipped from eBay auctions for 1985 engines. It is on the engine not the trans. But it is really close to the boundary that’s why we say on the bell housing. Good luck.

Maybe you can use the 6000 imp gauge and a green tach amp board if you don’t have this sensor. Meaning you can grab the signal off the front sensor and use a gauge off an 82/83. Not optimal but it may be easier than figuring out a sensor for an old om617 block. You may actually need Frank Reiner’s 6000 imp gauge.

ETA - yeah this makes sense. I think BillGrissom told me the ring gear has 140 teeth and provides the signal to the pickup coil. That means it has 140 pulses/revolution X 6000 revolutions/minute / 60 seconds/minute = 14000 which is the full scale impulses/sec (Hz). That’s what is stamped on the back of the gauge. So I think that little sensor just reads the teeth.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Herewith a shameless highjack of this thread!!

The wheelspeed pickups for the ABS systems are variable reluctors (VR) that have a sine wave output of rather less than 1 volt. The speedometer of the W126 apparently requires a signal of 1 volt or greater. Would a simple, low cost (< $15) amplifier intended for audio applications be usable to boost the ABS VR output above the 1 volt threshold needed to drive the speedometer?

Maybe?

The VR sensors have a very peculiar drive circuitry that has different input characteristics than a microphone or audio preamp output. If had to guess the VR input is very similar to a phono moving magnet cartridge input stage. That’s kind of a different animal from a simple audio amp

The frequency range is right but I don’t think the input impedance is anywhere what is needed to read a VR sensor.

Schematics like the first one in this paper would likely work.

https://leachlegacy.ece.gatech.edu/headamp/

Usually all this circuitry is built into the abs controller. Are the amps separate on the old benzes?

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

In the attached diagram the peak to peak voltage as shown in the upper left is about .5-.7 V for the VR sensor which is the subject of this inquiry. My question is, would it be possible to use an amplifier such as the KEMO M031M (the link will not copy), a 3.5W audio amplifier which is powered by 12 V, to increase the amplitude of the VR signal to a level above 1V?

Attached Thumbnails

 

Frank,

I don’t really know. First off that sensor setup is oddball to me because it needs a magnet to add extra field. This is likely due to our abs system not using an excitation signal for the magnetic field.

Oh ok. Just educated myself. This is a “self powered” sensor. You likely can read back the signal with your audio amp. Almost any amp should be able to read this given the magnet in there.

https://www.aitekinstruments.com/application/files/8817/3756/5035/INTRO__PRINCIPLES_OF_OPERATION.pdf

I think this is why the CPS in M104, M103 etc get heat soaked after a couple of decades. The heat must demagnetize the backup magnet.

Given this, why go through the fuss of amplification when you can just swap out wheel sensors?

If you want to try the amps I’d just buy one and try it. Measure the pulses with a scope and see if anything useful comes back.

ETA. Not sure what your goal is in all this but I thought I’d mention these amps are a lot cheaper than the kemo.

https://youtu.be/AMYkNyTwPl0?si=XUWZyFgP5Gs-W7OS

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles