Need to find a compatible magnetic speed sensor for speedometer

Discussion in 'Automotive Electronics' started by MB107, Jan 27, 2019.

  1. MB107

    Thread Starter New Member

    Jul 24, 2016
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    I'm doing a modification to a 1986 Mercedes Benz 560SL. It involves swapping an old transmission with a magnetic pickup used to drive the speedometer to a newer transmission which has no provision for a speed sensor on the transmission. While I can use the old magnetic pickup, I would prefer to adapt an off the shelf part as the old ones are no longer available. So therefore I'm trying to find a magnetic pickup that will be compatible with the Mercedes computers. And of course Mercedes Benz does not publish spec for these parts. But I have made a few measurements and run an output test of the current sensor. The stock sensor is shown in the first picture. Where it will be mounted is shown in the second picture.

    What I have done so far:

    Measured resistance across the two terminals at 1.825 KOhm using digital meter.
    Measured AC voltage output with with the sensor running at 2.85V @ 535RPM and 7V @ 1500RPM. This was done on a lathe with the sensor ~ 0.060" away from the chuck jaws as shown in the third picture.
    I made a few calculations of the required physical installation

    Wheel Diameter 560SL in 2.5
    Wheel Teeth 4
    Max RPM 6000
    Max pulse speed Hz. 400
    Max surface speed in/sec 314.1593

    Not sure which parameters are important but I cannot find one at last in the Newark catalog with a coil resistance above 1 kOhm and it has a peak to peak voltage of 190VAC, which may not be too much as I notice they tested it with a 0.005" gap. I will be at about 0.040"

    Any help on this would be appreciated.

    Thank You
    John

    IMG_20190126_223856279.jpg IMG_20190126_223933135.jpg IMG_20190126_233953601.jpg
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A big unknown is what the ECU does at power-up to check for the pickup's presence and condition. If the new coil is 1k but the ECU is looking for a coil resistance > 1k you would have to find a way of fooling it. The obvious answer is a series resistor, but that would reduce the input voltage to the ECU so some amplification of the coil's output might be needed.
    I know nothing of Mercs, but my understanding is that ECUs generally aren't too fussy over pickup input voltage provided it exceeds some minimum specified amplitude at tickover rpm.
    Another thing to consider is pulse frequency. It will need multiplying/dividing depending on the numbers of teeth on the old and new 'tone wheels'.
     
    Last edited: Jan 27, 2019
  3. MB107

    Thread Starter New Member

    Jul 24, 2016
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    Thanks for the replay. I don't believe, in this car any ECU comes into play as it certainly would in the newer vehicles. This is an early old school electronic speedometer only sensor. No warning lights are displayed if it is not functioning. The sensor is wired directly to the speedometer, but the speedometer does have some electronics in it so who knows what is available to get fried if I do this incorrectly. Attached is a circuit diagram with the speedometer and sensor circled in red.

    Also not sure if the voltage I measured with a DVM is accurate since this was the pulse of three jaws of a lathe chuck that have very large spaces in between each jaw. An oscilloscope would certainly be more appropriate.

    As far as pulse frequency I plan to handle that with a pulse frequency modifier which are readily available from the aftermarket for people changing gear ratios.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I wouldn't expect that device to be too fussy about pickup coil resistance or output voltage, and it should provide its own output compatible with a range of speedos (check its spec).
     
  5. MB107

    Thread Starter New Member

    Jul 24, 2016
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    Sounds promising. I think I will take the risk with my speedometer. I'm now leaning toward this one.

    Thank You
     
  6. bwilliams60

    Senior Member

    Nov 18, 2012
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    Bendix makes a 1.5-2.5 Kohm sensor for its ABS systems. They can be found on most Class 8 trucks. It is a 2 wire magnetic pulse generator. You will need to figure out how to get your pulses per mile and that should do the trick.
    Maybe I missed it but why aren't you using the original sensor?
     
    Last edited: Feb 1, 2019
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  7. MB107

    Thread Starter New Member

    Jul 24, 2016
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    Thanks for the reply.

    Two reasons I/m not using the OEM. 1) it is no longer available. 2) it is too big. The yellow thing in the four view picture is a 12mm wide sensor. It is positioned ~3mm from hitting the already shortened drive shaft bolts. The stock 18mm diameter sensor will hit. Re positioning the setup is not favorable to installation of the tone wheel or protection from the elements.

    Interested in the Bendix sensor do you know what diameter it is. Getting the pulses per mile is easy. The stock MB had a 4 tooth wheel and I have a 4 tooth wheel. I can also adjust PPM using a electronic device readily available to the automotive industry just for that propose.

    After talking to the supplier of the sensor proposed I have concluded MB uses the 1.825KOhm sensor because there stock sensor when embedded into the transmission needs to have ~ 0.1" gap as the sensor is insulated from the hot transmission fluid by placing a plastic thimble type piece sealing off the sensor from the transmission internals. Mine will be external so I believe I can get away with a smaller gap and consequently a lower resistance sensor.
     
  8. bwilliams60

    Senior Member

    Nov 18, 2012
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    The outer diameter of this sensor is 0.627" and spacing to the tone ring is 0.015".
    The sensor comes in a straight or 90 degree install. I think it is close to what you are looking for.
     
  9. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Warner Group used to do reluctance sensors with integral magnet that sense plain teeth on the starter ring for a tacho. For speed; you could mount one through a hole for the planet ring in the diff, resolution wouldn't be great though.
     
  10. MB107

    Thread Starter New Member

    Jul 24, 2016
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    After fine tuning the design I have found that 12mm (0.472in) diameter is the absolute limit. Its so close I'm considering using a 3/8" version which is also available. I hate putting SAE hardware on an all metric car.
     
  11. MB107

    Thread Starter New Member

    Jul 24, 2016
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    There is actually already on in there for the ABS system using a 36 tooth wheel. I have considered splitting the signal and than trying to reduce the frequency by a factor of 9 of the signal going to the speedo. Not sure if that can be done.
     
  12. Alec_t

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    Don't know if there's an off-the-shelf module to do that, but a bit of DIY electronics could do it.
     
  13. ian field

    AAC Fanatic!

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    Messing with the ABS sensors is best avoided till you've eliminated any other option.
     
  14. cork_ie

    Active Member

    Oct 8, 2011
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    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.
     
  15. cork_ie

    Active Member

    Oct 8, 2011
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    A factor of 3 would be my calculation!
     
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