# Need to find a compatible magnetic speed sensor for speedometer

#### Alec_t

Joined Sep 17, 2013
10,690
would I need to be concerned about frying things if voltage went too high.
Yes, if the ECU is expecting signals from a Hall effect sensor. Probably not if it's expecting signals from an inductive sensor.

#### MB107

Joined Jul 24, 2016
103
It appears that what you have is a fairly standard magnetic impulse sensor, generating an AC signal. As already mentioned I wouldn't be terribly worried about using a 1KΩ sensor, you can simply add an 820Ω resistor in series if absolutely necessary. The important consideration is the number of pulses per mile, and the signal level. You will need to determine what is the lowest signal voltage that the speedometer will work at and the peak voltage at max road speed.
A magnetic impulse crankshaft sensor or abs sensor will do the job, or if you are fussy about the voltage level, a 5V Hall type sensor (often used for camshaft sensors) should work with a voltage level spot in the middle of your desired range. It will have the added complication of requiring a 5V supply , which can be easily obtained, and it will have a square wave output. I would be surprised if the speedometer is fussy whether it is a square wave or AC pulse.
Before you start, you will need to accurately determine the number of pulses per mile. By far the best option is to use a signal generator to drive the speedometer and graph frequency V's speedometer reading. If you don't have access to one you can easily make one with an arduino.
. Your Tyres are 205/65-15 which turn 792 times per mile. Your top speed is 140MPH = so your tyre turns at max speed are 30.8 times per second.
Going by the max figure you quoted in your original post of 400HZ, you will need to generate 13 pulses per wheel rotation.(30.8x 13 = 400.4) which is accurate to 0.1%
This is very easily achieved by making a new split collar in two halves,the same diameter and approx weight as the existing one on the OE driveshaft, with 13 evenly spaced teeth attached to it. It is then a simple matter of mounting the sensor on a bracket so that it is pulsed as each tooth passes.

Most of that work is already figured by the manufacture. Mercedes already uses a 4 tooth wheel on the transmission output and so will I. Consequently this produces ~ 13 pulses per wheel rev depending on the differential ratio. My toothed wheel design is shown in some of the first few posts. Speeds of operation have been calculated to be between 30 and 1000 in/sec at the diameter wheel proposed (5MPH to 180MPH). Also running between 10Hz to 500Hz. Challenge seems to be the low surface speeds, the point where the speedometer just starts to sense motion. At the other end of the spectrum is at high speeds not producing too much voltage that I fry something.

In this car the signal does not go through an ECE it goes directly to the speedometer which I assume must have some kind of frequency to dc voltage converter built into it.

Since these sensors are very linear I believe that matching the threshold voltage at a low driving speed is the key to getting this right. For example if the stock vehicle produces 5V pk-pk at 10MPH then I need to find a sensor and gap combination that will also produce 5V pk-pk at 10MPH.

#### MB107

Joined Jul 24, 2016
103
Well I believe I am there. Monday I will be placing an order for a model 0010M-52954 sensor from Spectec. Unfortunately because it is custom made I had to by 3 for a total of $550.00. Not to bad considering retail over the shelf sensors are about$150 in the 1.8K Ohm range. Unfortunately finding one off the shelf with 1.8K Ohm and under 1/2" diameter was not happening. It will also take about 1 month to deliver.

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#### MB107

Joined Jul 24, 2016
103
OK so it all works pretty good except the speedometer needle has a slight bounce to it. Not bad but not like Mercedes Benz would make it. I believe I have found the problem in that the tone wheel is difficult to get concentric to the shaft. There is nothing to pilot on so it can be off by the tolerance of the bolt holes.

So was wondering if there might be a way to clip the signal if it exceeds 10V. That would make all the peak voltages the same for the usable part of the operating range.

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#### Alec_t

Joined Sep 17, 2013
10,690
A 10V Zener diode would clip the sensor signal. The diode power rating would depend on the value of any series resistor between sensor and diode, and the value of the input impedance of whatever reads the signal.

#### MB107

Joined Jul 24, 2016
103

#### Alec_t

Joined Sep 17, 2013
10,690

#### MB107

Joined Jul 24, 2016
103
OK I just received the diodes tried a 12V 1W diode in the lathe with the tone wheel offset to simulate the problem and the results seem positive. The sensor does not appear to even get warm, however I really cant get the lathe to simulate over about 50 MPH. The only way to really tell will be to get it in the car. Stay tuned.

Thank You Alex

#### Alec_t

Joined Sep 17, 2013
10,690
The sensor does not appear to even get warm
That's encouraging.
I really cant get the lathe to simulate over about 50 MPH
Would a drill press give a higher speed?

#### shortbus

Joined Sep 30, 2009
7,326
however I really cant get the lathe to simulate over about 50 MPH.
Your lathe must be really slow. 100MPH should be around 1400RPM

#### Back to school

Joined May 22, 2019
46
Just curious but what transmission did you put into the car?