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VSS Circuit Repair

Posted by BoostedBen, Dec 21, 2011

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  1. BoostedBen

    BoostedBen Proven Member

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    So my buddy has a 2g GST that has had ongoing issues with the VSS. I am down to trying to trace the circuit all the way back to the ecu.

    I have already looked up what pin it should go to, now what I need to know is how the ecu send the signal to the cluster. I believe it should one of the connectors underneath/behind the radio.


    He has a heavy foot and has gotten 2 tickets in the eclipse in the past month so I am trying to fix this and get him out of a couple tickets. They were minor speeding infractions and the judge has suspended them for a month and said he will drop them if he brings proof that the speedo has been fixed.

    Any help would be appreciated. :hellyeah:
     
  2. luv2rallye

    luv2rallye DSM Wiseman

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    Bumping is not allowed.

    The ECU is not involved with the speedo. The signal from the VSS yellow wire is 4 pulses per shaft revolution that goes to the speedo on a yellow-white wire (speedo connector C-05 pin 36). It also goes to Data Link Connector pin 14. The VSS actually grounds the signal (electronically with a transistor) to create each pulse.
     

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    1999 Mitsubishi Eclipse GST
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  3. steve

    steve DSM Wiseman

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    Not quite true. The ECU, cruise control, speedometer and TCU (on automatics) all connect to the VSS and provide a pullup voltage for signaling. The VSS then pulls that voltage to ground via a transistor to signal.

    It's usually all or nothing with the electrical side of the VSS. It either switches or doesn't. If the reading is off it's either the gauge or the gear on the VSS slipping. I guess it would be possible for a bad VSS to drop pulses.
     

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    1991 Mitsubishi Eclipse GSX
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  4. BoostedBen

    BoostedBen Proven Member

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    We've replaced the VSS with a new one, and swapped the gauge cluster with a known working one. So it leads me to believe that there is a broken wire somewhere. He has a tucked harness so I am working on unsheating it and tracing all the entire circuit.

    It was under my impression that the VSS put out a waveform, and the ecu translated that to a signal, and sent the signal to the speedo.

    Sorry I have more experience with cable driven speedos.

    Suppose I should just create an additional thread when this one gets buried?
     
  5. LiquidX

    LiquidX DSM Wiseman

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    Sure, if your are looking to get it deleted and receive a warning.




    So what exactly is the issue his car is having? I know that my gauge reads 5mph faster than what the actual speed is and that's just needle placement.
     

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  6. steve

    steve DSM Wiseman

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    Sure it creates a waveform but the ECU doesn't pass it to the speedo, the speedo reads it itself. It looks roughly like a 5v squarewave each time the VSS pulses.

    Disconnect all the connectors tied into the VSS, check each one for opens and shorts to ground. Leaving the VSS disconnected, while the ignition is on measure the voltage on the VSS signal at the DLC as you plug each other one in starting with the ECU. Also check the power to the VSS to make sure it gets turned on.

    It would help if you described how it's failing.
     

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    1991 Mitsubishi Eclipse GSX
    manual · 1G DSM
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  7. BoostedBen

    BoostedBen Proven Member

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    The speedo is not getting any signal as far as I can tell.
     
  8. steve

    steve DSM Wiseman

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    So the speedometer doesn't do anything?
    What have you checked so far?

    When I lost the speedometer on my daughters 3000GT I tested the gauge by feeding it a squarewave from a 555 timer I threw together. That proved that the gauge worked. I checked the power and ground for the power to the VSS and hooked up a LED with a current limiting resistor to make sure the VSS pulsed. Then I verified that I had continuity from each of the connectors back to the VSS, in the process I found one of them had gotten water in it and shorted the VSS to ground.

    You have to get creative too to debug it.
     

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    1991 Mitsubishi Eclipse GSX
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  9. gofer

    gofer Moderator

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    What year is your friends DSM and have you actually pulled the VSS and inspected the gear?

    :dsm:
     

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    1997 Mitsubishi Eclipse GST
    11.634 @ 126.050 · 2G DSM

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  10. BoostedBen

    BoostedBen Proven Member

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    Its a 95 or 96. Yes, the VSS has been replaced with a brand new unit. The gauge is not getting signal. When its spun with a drill it does output a waveform. And as far as I can tell with a basic multimeter there is signal going to the ecu, but nothing at the connector behind the gauge cluster.

    I need to know how its wired to the gauge. Which I think I posted in the first topic.
     
  11. LiquidX

    LiquidX DSM Wiseman

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    Was the needle ever removed from the speedo?
     

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  12. gofer

    gofer Moderator

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    The problem, as far as I'm concerned, sounds mechanical and believe it or not there are a few different VSS gears out there depending on what transmission is in the car.
    I thought a speedo gear was a speedo gear up until a few months ago after I had Shep rebuild my '99 transmission. I put the same VSS gear back into the transmission after the rebuild and every 3000 or so miles my speedo would stop working. After going through 3 VSS gears I called Shep and asked him WTH? He told me that the VSS drive gear inside the transmission is different from '95-'96 and '97-'99 M/T and they might have installed the '95-'96 drive gear in my '99 M/T. I looked up the part number and sure enough, it was different...

    Before you start ordering parts and wasting money though, pull your VSS gear and check the condition of it. You can tell if you've got the wrong one in there because it will be worn flat on one side of it. See picture below...

    The top VSS gear is for a '97-'99 M/T, notice its a bit smaller in diameter? Also notice the right side of it is chewed up and flattened a bit, thats because I was using the incorrect gear in my transmission. The '95-'96 gear is the new one below, which is whats been installed in my transmission and working ever since I found the issue.

    VSS gears.jpg

    AWD M/T 95-96: MD746492

    AWD A/T 95-96: MD757055

    AWD M/T 97-99: MD749150

    AWD A/T 97-99: MD760150

    FWD M/T 95-99: MD746492

    FWD A/T 95-99: MD757055​

    Above are the P/N's for the VSS gear you'll need specific to the transmission. Its also important that you use the correct bolt that holds the VSS in the transmission, I've been down that road before a few years ago. If you need the bolt too, the P/N is MF240029. Hope this helps...

    :dsm:
     
    Last edited by a moderator: Jul 10, 2014

    Street Build 15K  11  61

    1997 Mitsubishi Eclipse GST
    11.634 @ 126.050 · 2G DSM

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  13. BoostedBen

    BoostedBen Proven Member

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    No clue, but there is no voltage of any sort at the cluster connector.

    I consulted this diagram, and there is no voltage what so ever getting to pin 36 on c-05. Is that still to hard to understand? The sensor is outputting something, it just isn't getting to the speedo. The circuit is broken somewhere between the sensor and the speedometer. I am just not good with wiring diagrams.
     

    Attached Files:

  14. steve

    steve DSM Wiseman

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    The sensor doesn't output anything. It pulls a provided voltage to ground. The ECU, Cruise Control, TCU provide the voltage.

    As you show above, the FSM doesn't show the speedometer circuit in detail so I can't say if it also pulls the VSS signal up.

    When you checked pin 36 on c-05, were the other connectors plugged in and the ignition turned on?
    Have you checked pin 14 on the DLC? Is it also not showing any voltage?
    How about pin 86 on the ECU?
    Pin 19 on the Cruise Control computer?

    The connection doesn't have to be broken, if anything else pulls the signal to ground the speedometer won't work.

    The diagram says that the yellow wire from pin 3 on the VSS goes to harness junction point 21 where it turnd into yellow wires with a white stripe, one runs to connector C-22 pin 5 and then to harness junction point 22 where more wires split off and one of the yellow wires with a white stripe goes to C-05 pin 3. Another goes to C-21 pin 18 before ending at B-38 pin 14 (the OBDII connector)

    If you look at the diagram for the Cruise Control, at harness junction point 22 the yellow wires with a white stripe connect to a white wire with a red stripe that goes to C-21 pin 19 before ending at B-29 pin 19 for the Cruse Control ECU. This is also shown on the MFI diagram with the addition of showing that the ECU is connected to one of the yellow wires with a white stripe that branched off harness junction point 21 and runs to B-56 pin 86.

    I hope that helps.
     

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  15. BoostedBen

    BoostedBen Proven Member

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    Thanks steve, That will give me several places to look when I get back to the house tomorrow.

    There is signal getting to pin 86 for the ecu, and the car does not have cruise. So that narrows it down a bit, but I will keep looking.
     
  16. gofer

    gofer Moderator

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    Found this in my VSS notes that I gathered back when I was troubleshooting my VSS circuit, thought I'd post it just in case it might help you too.

    VEHICLE SPEED SENSOR
    The rotating shaft of the vehicle speed sensor is mounted directly on the transaxle speedometer driven gear. The rotation of the driven gear is transmitted to a magnet via the rotating shaft. The IC built into the Magneto Resistive Element (MRE) in the top of the magnet detects the rotation of the magnet and generates a 4-pulse electric signal for each, rotation of the speedometer driven gear.

    MAGNETO RESISTIVE ELEMENT (MRE)
    ---Characteristics---
    The magneto resistive element (MRE) has the characteristic that its resistance value varies according to the direction of the magnetic field. When the direction of the magnetic field is parallel to the Magneto Resistive Element (MRE), the resistance value is maximum; when the direction of the magnetic field is vertical to the Magneto Resistive Element (MRE), the resistance value is minimum.

    Speedo MRE characteristics.jpg

    ---Operation---
    Since the resistance value of the MRE varies with the variations in the magnetic field when the magnet rotates, the output voltage also varies. The shape of the wave of this output voltage is shaped by the comparator and then, after it is divided by the frequency divider circuit into a 1-rotation, 4-pulse signal, the transistor turns it ON-OFF when it is output to the transistor base to produce a 4-pulse electric signal per one rotation of the speedometer driven gear.

    Speedo MRE operation.jpg


    ELECTRIC TYPE SPEEDOMETER
    The electric type speedometer uses an electronic circuit to control the electric signal from the vehicle speed sensor and a cross coil type speedometer that has a magnet fixed to the pointer shaft and two coils that pass directly through it. Furthermore, the odometer and trip meter are directly driven by the stepping motor with built-in gear.

    Speedo guage.jpg

    ---Operation---
    It shapes the wave of the electric signal from the vehicle speed sensor in the shaping circuit and, after calculating vehicle speed at the calculation circuit, it controls current flow to the cross coil and stepping motor.
    It determines the direction of the current flowing in each cross coil by using the quadrant indicating circuit and, at the same time, it determines the current value using the duty data circuit, and sends current from the meter drive circuit to drive the pointer.
    It determines the current value flowing to the stepping motor by using the pulse motor data circuit, and sends current from the pulse motor drive circuit to drive the odometer and trip meter.

    Speedo gauge operation.jpg


    CROSS COIL
    When current flows in the cross coil, a magnetic field is generated in coil 1 and coil 2. Since the two magnetic fields pass directly through, the actual magnetic field is a combination of the two magnetic fields.
    Since the magnet is fixed to the pointer shaft, the magnet rotates to try to align the magnetic axis of the magnet in the direction of the combined magnetic field and thus rotates the speedometer pointer. NOTE: The speedometer pointer is attached to the pointer shaft 20* counterclockwise to the magnetic axis of the magnet.

    Speedo gauge current coil.jpg

    RELATION BETWEEN POINTER ANGLE AND CURRENT VALUE
    The angle of the pointer is determined by variation of the current value flowing in coil 1 and coil 2.
    Example: When the pointer is at 90*; Maximum current flows in coil 1 and no current flows in coil 2.

    NOTE: When the pointer is at an angle of 0*, it indicates a speed of 0mph.

    Speed gauge current value.jpg

    :dsm:
     
    Last edited by a moderator: Jul 10, 2014

    Street Build 15K  11  61

    1997 Mitsubishi Eclipse GST
    11.634 @ 126.050 · 2G DSM

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    manual · 2G DSM
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  17. MadSciHighPSI

    MadSciHighPSI Probationary Member

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    I know I am resurrecting and old thread.
    Was it a 0...5Vdc signal square wave? Also, what frequency were you using?
     

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  18. steve

    steve DSM Wiseman

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    Sorry after 8 years I don't remember any specific details and I'd have to find the 3000GT manuals to figure it out again starting with who provides the voltage for the sensor to ground. I forget if it was the ECU or the cluster that sources it. From that you'd know what the voltage if the VSS signal was 0-5v or 0-12v. I believe the manual also provided the freq expected from a normal VSS. Since I was just trying to make sure the cluster worked once I saw the gauge moving it was time to start looking elsewhere for why I wasn't getting any signal.
     

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