Need help choosing a Variable Reluctance speed sensor

Discussion in 'Automotive Electronics' started by Senorarnolf, Aug 11, 2018.

  1. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Hello I have a question in regards to VR speed sensors. I'm looking for a replacement sensor for a vehicle application where the factory style sensor won't work for my needs. I'm trying to locate a sensor that will work for my application. My main question is how the resistance(ohms) across the two wires of the sensor plays into the signal the VR sensor produces or does it change it at all. I unfortunatley haven't been able to find much information on the original sensor other than what the resistance reading across the pins should be what other information is necessary when choosing a proper replacement. I've been sifting through information on the internet for awhile and I can't seem to find the information that I'm looking for so I'm hoping someone on here can shed some light on this subject.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    The usual VR sensor is basically just a coil of wire on a ferromagnetic core. The more turns of wire there are in the coil the greater will be the coil resistance and the greater will be the coil output voltage at any given shaft rpm.
     
  3. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Okay so what I'm understanding is that a sensor with greater resistance will produce a higher voltage at a given rpm than a sensor with a lower resistance at the same given rpm. I'm having an issue finding a VR sensor with similar resistance to original which has 1.81kohms which is much higher than what I seem to commonly see. Thank you for your speedy answer
     
  4. Alec_t

    AAC Fanatic!

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    Yes, if the wire gauge used is the same for both sensors. Thin wire will have a higher resistance than thick wire of the same length.
     
  5. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Okay awesome this is the type of information that I was having difficulty finding answers to. I have a dakota digital tachometer setup that uses a VR sensor on the ring gear however the resistance of that sensor even with the same length of wire at the same gauge has only 700 ish ohms of resistance which is a lot less the the 1.8k ohms of the original. Maybe you can clarify something else for me so I understand that the sensor produces an AC wave and the faster the object say the teeth on a ring gear passes into the magnetic field the higher the voltage what I'm unclear on is how it determines how many rpms the object is spinning.
     
  6. LesJones

    Well-Known Member

    Jan 8, 2017
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    It just counts how many pulsed occure in a fixed amount of time. So if there was 10 teeth on the gear you would get 10 pulses per revolution. So if you count how many pulses occure in 6 seconds that would be the same number as revs per minute. In practice you would want the display to update more than once every 6 seconds. So if you wanted it to update every second the electronics would multiply the number of pulses by 6 to get RPM. If you were measuring very low speeds with a small number of pulses per rev then you would measure the time between pulses. (Or a number of pulses.) and do the calculation to convert that time to RPM.

    Les.
     
  7. bwilliams60

    Well-Known Member

    Nov 18, 2012
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    Most VR sensors are between 1000 and 2500 ohms as a rule. I know that if you look at Heavy Truck, Bendix wheel speed sensors are generally around 1500 to 1750 and Meritor Wabco ABS is a little higher.
    Can you explain a little more about what you are trying to do and perhaps we can get you there. Cheers.
     
  8. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Yes of course what I have is a 1984 Jeep cj7 that I have swapped a mercedes diesel into also from 1984. The transmission and flex plate are from the jeep. In 1984 and 85 mercedes used a VR sensor to read the engine rpms and fed that information into a control module that modulates the EGR valve on the diesel engine. I have the controller wired up except for the tach signal that it requires to activate the EGR at 700rpm and deactivate at 3000 rpm. I have 2 problems with the sensor 1. The factory bosch sensor is very short and the end or the core is angled at a 45 which makes it very hard to adapt and 2. The original mercedes flywheel had 140 teeth where as the jeep flex plate that is used has 162 teeth so from my understanding due to the fact that the controller will be reading more teeth per revolution it will be reading a 14% increase in rpm. I haven't figured out if there is way way to resolve this. Here in California where I'm located in order to do a conversion of this nature it has to have an inspection done and the function of the EGR will be tested. After the initail inspection the vehicle will be registered as a diesel and exempt from future smog inspections. Sorry for the long winded response you guys have been great with answering my questions.
     
  9. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    This is what the original speed sensor looks like. Ideally I'd like the find one that is adjustable and has a longer body.
     
  10. bwilliams60

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    Nov 18, 2012
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    If it is for the tachometer only, is it adjustable. If so, could you take the pulse from the alternator and use it instead? That would be easy enough to do. You would find where the stator joins the rectifier (3 points) and jump a wire to it as an access point and then connect your tachometer to it.
    Back to your sensor. What is the optimum length and diameter that you are looking for. There are a ton of VR sensors on the market now. Perhaps we can match one up.
     
  11. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    A sensor that is between 1/2 inch and 3/4 in diameter and somewhere between 2 inches in length would be the ideal size. Although mercedes did use the signal the sensor supplied to operate the tachometer (the controller has an ac input and a digital output) it first supplied the signal to the EGR control module which uses that signal to operate the EGR valve the controller also takes input from a coolant temp sensor and a throttle position sensor. Unfortunately there is hardly any information on this system due to the fact that it was only used for 2 model years and diesels pre 1997 (in California) weren't required to be smogged so there isn't really anyone really putting time into the intrecasies of the system. What opinions do you have about the increased number of teeth on the ring gear is this going to be a major issue?
     
  12. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    On a side note I do have a speed sensor on the front of the engine that reads off of the crank pulley it's used for diagnostic purposes. It reads one pulse per revolution maybe there's a way to adapt that into something the controller can use?
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A microcontroller could be programmed to take in sensor pulses and spit out pulses at any proportional frequency you want.
     
  14. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Alec_t that's an option! This whole situation with this sensor and controller has been proving to be over my level of expertise I'm a heavy equipment mechanic by trade but I've never had to dabble into the electronics area before do plenty of electrical diagnostics and electrical repair on the job but this is a area that I have next to no practical experiance so pardon my newbie questions. Now my question is on the topic of a microcontroller is there a commercially avaible unit that would work for this?
     
  15. bwilliams60

    Well-Known Member

    Nov 18, 2012
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    I think Alec_t has a circuit made up for this if I remember correctly. Perhaps he can chime in and help you out on that front.
    As for the sensor, Google Bendix ABS sensor and I think you will find what you are looking for.
     
  16. Senorarnolf

    Thread Starter New Member

    Aug 11, 2018
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    Okay I will most certainly look into the bendix abs sensors that sounds like maybe the ticket for this quandary that I find myself in. Do you know if there is a site that sells the bendix sensors that provides the resistance of the sensor in the description that seems to be something that isn't included in most of the sensors I've seen.
     
  17. bwilliams60

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    Bendix has a literature section on their website and if you google bendix EC-60, it should come up with a complete description of the system with specs. I can tell you from a lot of experience that the sensors are generally in the 1500-1600 ohm range. If you are close to a Heavy truck dealer of any kind, they will probably have them in stock as they are very common in NA.
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Well, I designed (haven't built) a circuit using discrete components to do the job (I'm no programmer), but it's a tad complex for someone new to electronics; which is why I mentioned the microcontroller approach. I believe there are off-the-shelf units for the pulse rate conversion, but I haven't explored what's available.
     
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