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Thread: Electric Speedometer conversion

  1. #1
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    Electric Speedometer conversion

    I've got an engine swap project I'm doing on an old Corvette with a mechanical speedo. During this process, I'm also going to add a carpc.

    I'd like to convert the original speedometer from mechanical to electric since my new transmission is all electronic. It seems that OEM electric speedo's use a stepper motor. I have one from a '93 Lexus ES300 I may try to use.

    That all said, I'm wondering if anyone has tried this type of project yet. If so, is there an OEM source that is easy to integrate from a controller standpoint? Seems like I read somewhere that a particular vehicle just 'x' amount of voltage to move the needle 'y' amount. Seems like other speedo units may work on a pulses per mile signal straight from the vss (or massaged by the ecu).

    Any insight would be much appreciated. I'm thinking that once I have a direction on the motor to source, I'll use an arduino or maybe even the car pc to make the conversion from my speed sensor to what the gauge wants.

  2. #2
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    Hi! If the new transmission is all electronic them the VSS (Vehicle Speed Sensor) is a pulse train. Exactly 4000 pulses per mile on my Camaro and most GM car to my knowledge. This pulse information is feed to the ECM (Electronic Control Module) and to the speed indicator on the dash. A microcontroller can easily read this information and compute MPH , Odometer and Tripmeter.

  3. #3
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    That's the plan - It's an LS1 & 4L60E combo. I just need to figure out the best way (best hardware to use?) to convert the mechanical speedo to electric. I'd like to keep the speedo & tach OEM looking, so I'll be removing the needle and mechanical drive in the existing speedo to replace with electric drive.

    I have the stepper motor(I'm assuming) from an ES300 handy to play with. It has the factory Lexus driver board attached. Would be interesting to know what signal it's expecting --- or if that isn't common knowledge, it may be easier to control the stepper directly. I just don't yet have any experience working with an electric speedo and I no longer have the ES300 to sample the signal from.

  4. #4
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    4L60E is an auto transmission, isn't it?

    Have you tried looking at available aftermarket gauges? Quickest solution is most likely to get one you like the look of and is already electric. Otherwise, might need some hardware to create a signal for that Lexus unit and then play around calibrating it and such.

  5. #5
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    Yes, the 4L60E is a common 4 speed electronic automatic transmission used in rear wheel drive GM vehicles. Information on it's VSS output is not difficult to come by.

    I've looked at aftermarket gauges a little as a source for parts. Really though, I'd like to source an OEM part from another vehicle since really, I'll be maintaining the look & feel of the OEM gauges. Also, in the future, an OEM part should be less expensive and easier to find in case it ever needs replaced. Really, all I need is the stepper motor that drives the needle - then integrate it into the original dash.

    I figure I'm going to need a microcontroller to interface the transmission (or signal from the ECM) to the speedo & plan on that being a project to teach me about working with microcontrollers. That's looking ahead at this point though & with the documentation out there, I don't believe this should be a difficult part of the plan.

    Step one of this project is getting the electric speedometer motor assembly (from the ES300) to move in a predictable manner and without destroying it - or if there is a motor assembly out there that is well known/hacked/reverse engineered, sourcing it & getting the info pertaining to it. There seems to be a large void of information on the subject out there. This is what I was hoping some members might have experience with & helping me sort out the basics and get started.

  6. #6
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    Quote Originally Posted by 86atc250r View Post
    I've got an engine swap project I'm doing on an old Corvette with a mechanical speedo. During this process, I'm also going to add a carpc.

    I'd like to convert the original speedometer from mechanical to electric since my new transmission is all electronic. It seems that OEM electric speedo's use a stepper motor. I have one from a '93 Lexus ES300 I may try to use.

    That all said, I'm wondering if anyone has tried this type of project yet. If so, is there an OEM source that is easy to integrate from a controller standpoint? Seems like I read somewhere that a particular vehicle just 'x' amount of voltage to move the needle 'y' amount. Seems like other speedo units may work on a pulses per mile signal straight from the vss (or massaged by the ecu).

    Any insight would be much appreciated. I'm thinking that once I have a direction on the motor to source, I'll use an arduino or maybe even the car pc to make the conversion from my speed sensor to what the gauge wants.
    you do not need to make the old speedo electric that is too much work. use the manual cable and add on if these sensors
    http://www.jagsthatrun.com/Pages/Spe...eedometer.html

  7. #7
    Constant Bitrate
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    Quote Originally Posted by 86atc250r View Post
    I'd like to convert the original speedometer from mechanical to electric since my new transmission is all electronic. It seems that OEM electric speedo's use a stepper motor. I have one from a '93 Lexus ES300 I may try to use.

    That all said, I'm wondering if anyone has tried this type of project yet. If so, is there an OEM source that is easy to integrate from a controller standpoint? Seems like I read somewhere that a particular vehicle just 'x' amount of voltage to move the needle 'y' amount. Seems like other speedo units may work on a pulses per mile signal straight from the vss (or massaged by the ecu).
    Here's some that reads 0-5V on the analog input: http://www.mcnallyelectronics.com/products.htm
    But they do not use stepper motors but an air-core sin/cos motor driven by a Cherry Semiconductor CS-4172 SPI air-core driver chip, on the SPI from an Atmel AVR ATmega16 microcontroller: http://www.avrfreaks.net/index.php?n...wtopic&t=78510

    http://shop.mcnallyelectronics.com/p...4&productId=30
    Custom dial face

    We can do a custom dial face. Our gauge reads 0-5V on the analog input, and reads a Type K thermocouple on the temperature output. Need to keep white letters on black background and 240 degrees deflection, you specify number of major graduations and numbers, no more than 5 minor graduations per major grad.

    DIY Air-Core Instruments: http://www.mikesflightdeck.com/diy_a...nstruments.htm

  8. #8
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    Thanks for the info.

    Here's what I've found so far with what I have on hand (in case it helps anyone in the future).

    To recap:

    I have an instrument cluster from a 93 Lexus ES300 (no longer have the car). The speedometer driver/stepper is a self contained module that has a controller on board (so the stepper doesn't need to be controlled directly).

    I have a 94 Celica (daily driver) that has a simliar driver/stepper (not as user friendly for modifying).

    I have a 79 Corvette that is getting an engine swap to a late model LS1 - it has a mechanical cable driven speedo that I'm wanting to convert to electric.

    I put a scope on my 94 Celica the other day, to check out the signal being output to the speedometer. I verified that the Lexus unit wanted pretty much the same signal by hooking up the Celica's wiring to the Lexus driver, then putting the car in gear (while on stands).

    The signal needed is 5v square wave at approx 1 pulse per second per mile/hour (this will change with the scale of the speedo in the Corvette which is only a little different than that of the Lexus - or Celica for that matter). The high pulse width is equal to the low pulse width no matter what the frequency.

    If you feed this signal to the Lexus speedometer driver, the needle moves exactly as expected.

    So, now I have two steps left.
    1.) mount the Lexus driver/stepper into the Corvette cluster and attach needle so it looks "factory"
    2.) determine if it's possible to program the LS1 PCM to output the correct signal. If not, build/buy a microcontroller to do the conversion. This step will come much later down the line as I must have the engine running, wiring integrated, etc before I can test.

    Seems like this should be an very easy way to make the conversion. I won't have an odometer, but that's not important in this car as the original Odo is not accurate anyway. Eventually, I may see if I can integrate a mechanical/electric Odo from a mid 90's car or do a digital one that looks "right" in the dash.

  9. #9
    Constant Bitrate
    Join Date
    Dec 2008
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    Seems some modern cars use stepper motor gauges instead of air-core gauges: http://www.avrfreaks.net/index.php?n...wtopic&t=78510
    Quote Originally Posted by jayjay1974
    Quote Originally Posted by toalan
    Wow an SPI stepper motor driver, I never knew such an animal existed.
    They do exist, for gauge stepper motors (Switec X25.168) which seem to have replaced air core movements in more modern cars.

    Freescale MC33976
    http://www.pointstep.net
    The Pointstep gauges connects directly to the OBD II interface.

    Switec X25.168 Stepper Motor http://steppermotorsonline.com
    Motor specs can be found here: http://www.microcomponents.ch/produc...ec/switec.html

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