205 GTi Tacho Modification for Aftermarket ECU
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Thread: 205 GTi Tacho Modification for Aftermarket ECU

  1. #1
    Fellow Frogger! Bluey's Avatar
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    Default 205 GTi Tacho Modification for Aftermarket ECU

    This modification is necessary when running the 205 tacho off the tacho output of an aftermarket ECU.

    It is best to do this wearing nitrile or latex gloves to prevent oil from your fingers marking the instrument face.

    Remove instrument cluster from the car and pop the 3 clips retaining the glass across the top.


    Slide a 2-3mm piece of cardboard or similar such item between the glass and the main casing at the top corners to prevent the top clips from reattaching and unclip the 2 side clips (1 each side).


    The glass will now hinge on the bottom 3 clips and pivot towards you. Make sure you physically unclip them as the glass will not come off otherwise.


    Mentally note the location of the needle's resting position. I have found that in almost every application, the bottom edge of the needle should sit in the centre of the curved line.


    Lay clean cloth across the face of the tacho on each side and rest a piece of stiff cardboard or similar across the main casing edges to support the screwdrivers as you lever off the needle. The black cover and counterweight will most likely pop off before the needle is completely removed. Keep the ends of the screwdriver tangs as close to the centre as possible and evenly apply gentle pressure to lever the needle off. I will most likely move up the shaft in short abrupt movements rather than smoothly.


    Remove the two black and the two silver face retaining screws.


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    Gently lift the face from the main casing. The top left corner will have some tacky glue holding it in place.


    Here you will see the standard tacho circuit board.


    Add a bridge wire as pictured.


    Reattach the face and fit the needle. There should be a gap of 3mm between the face and the bottom edge of the needle.


    Locate the counterweight on the needle (there is a locating hole in the underside of the weight), and clip on the black centre cover.


    Do not fit the glass yet. Take the instrument cluster to the car and connect in the blue tacho plug. No need to connect any of the others. Connect your download lead and laptop to the ECU. Start the engine and check that the engine speed on the instrument cluster matches that of the ECU. If the the tacho reading is out, take note of the amount, turn off the engine and twist the needle.


    Clean the glass inside and out with some good quality glass cleaner, a microfibre cloth and compressed air. Refit the glass in reverse order making sure to locate it over the odometer stalk first. With the bottom located, squeeze the main casing and glass at the top corners and it will snap together. This can be done in situ.


    To prevent damage to the tacho should anyone ever think of fitting it to a standard ignition setup, label the instrument cluster to notify the user.



    A trap to be aware of
    I recently opened a modified tacho that had the signal wire cut off the circuit board and resoldered on the opposite side. When repairing it I made the mistake of connecting the wire from the connector straight to the terminal on the top left of the board. The tacho read at less than 50% of the correct reading.
    For the bypass mod to work correctly, the signal wire from the connector must first attach to its original position (Red) and then bridge to the top left terminal (Blue).



    Wiring in a 12V pull-up resistor

    The modified 205 tacho requires a 12V pull-up resistor for the ECU tacho signal to operate the 205 tacho properly.
    Most ECUs will require a pull-up resistor to be wired in to the loom to lift the tacho output to 12V. Some ECUs have a pull-up resistor in built and its simply a matter of wiring the correct ECU Auxiliary Output and configuring the ECU.
    For those ECUs that don’t have an inbuilt pull-up, you need to wire in a pull-up resistor as follows



    A 1000 Ohm 0.5 Watt Resistor should be adequate but I recommend you contact your ECU manufacturer to confirm a suitable resistance. Jaycar part number for the resistor is RR0572.


    ECU tacho configuration

    Haltech Platinum Sport 1000
    Wire DP01 to tacho
    Go to main setup (F4)
    Select Outputs, Main, check the DP01 box and set the dropdown box to Tacho
    In the DP01 Pullup drop down box, select 12V



    Next select the Tacho tab. Set Duty to 50% & Pulse per cycle to 4




    Haltech Platinum Sprint 500
    Wire dedicated tacho wire to tacho
    Go to main setup (F4)
    Select Outputs, Main, check the Tacho box and set the dropdown box to 12V
    Select the Tacho tab and set Duty to 50% and Pulse per cycle to 4 as per PS1000




    MoTeC M84
    Select one of Auxiliary Outputs 1-4, (5-8 are not suitable), wire to tacho including a 12V pull-up using a 1K Ohm, 0.5W resistor.
    In ECU Manager, select Adjust (Esc), Auxiliary Output Functions, the corresponding Aux Out, Function.
    Set the Value to 4.



    Select Adjust again and select Parameters. Set the calibration value to “0.0”



    Select Utilities, Test Outputs and test the tacho output making sure to set the duty to 50%. The Tacho needle should read whichever RPM you set.




    If anyone else has info I should add in, I'll update it later.
    Last edited by Bluey; 1st January 2018 at 12:28 AM.
    Matthew likes this.

  2. #2
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    Excellent Huw, did all this but wrong wire mod from 205 drivers, Oh well time to do it again for the unlocked 6 ECU which should be just 5V or 12V square wave, not quite a coil anymore, many thanks.
    87 S1 205 GTI / GTI6 powered a project underway
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  3. #3
    1000+ Posts PeterT's Avatar
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    Matthew, I thought you said there was a potential problem connecting the output directly to pin #1 of the IC, like Bluey has done?

    '92 205 Mi16
    '90 Mi16x4

  4. #4
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    Indeed, I will look to put in a series cap to stop DC, but given the nature of this chip (a switch mode PSU driver the successor of which drives nearly all PC power supplies the TL494) it should handle this. The coil however needs signal conditioning,in this one a decoupling cap is all this needs.sufficent to not attenuate the frequency range it will present. I will put a CRO on mine and suss it out a bit more. Given the car runs on 12V and nothing much boosts this then any signal coming from an ECU should be fine, a cap is just for good measure.

    Just need to plumb in the oil cooler and fit the rad to run mine to find this out and give Huw back his $
    87 S1 205 GTI / GTI6 powered a project underway
    306 convertible (gti6 candidate)
    307 HDi wagon
    407 HDi wagon

  5. #5
    1000+ Posts schlitzaugen's Avatar
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    You can do all that, or you can just add two diodes and a resistor in line with the tach input. The whole assembly can be enveloped in heat shrink along the wire somewhere out of the way.

    No need to re-calibrate your rev counter either. That is done by trying a few resistor values.

    The main problem I see is cracking plastics which is quite likely regardless of how delicate you are. Some of these things already crumble on their own by now and I think most of us would actually break that needle. Credit due Bluey, but I think you are one of a minority if you can pull that off. I refused to try it myself. I actually contemplated cutting the gauge face along a straight line just below the needle opening from the bottom edge to the centre to allow the face be removed without disturbing the needle.
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  6. #6
    1000+ Posts PeterT's Avatar
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    You still need to bypass the filter circuit, regardless of how you create a 12V square wave signal. The filter (bypassed by the white wire) is there to shape the 400V back emf from the standard coil collapsing. It will still block any 12V signal. I recalibrate with a 12V square wave generator after I've done the bypass.

    Please elaborate on how you've used resistors to recalibrate a 205 tacho Shulz.

    '92 205 Mi16
    '90 Mi16x4

  7. #7
    Fellow Frogger! Bluey's Avatar
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    I've fixed the mess photobucket made of this thread. Pics are now viewable again.

    I've added a few more sections in the original post on wiring traps, 12V pull-up resistors & ECU configuration.

    Can someone let us know if the tacho signal from Bosch MP3.2 (Mi16/S16) and Magnetti Marelli 1AP (GTi6/VTS) ECU's requires a 12V pull-up or not?

  8. #8
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    Huw, on the 6 ECU the signal is 5V going low twice per cycle for about 1.3ms each time. The frequency is right not sure about logic if this needs to be inverted. My thinking is that the old collapsing coil puts an impulse into the speedo hence the normally high signal if inverted becomes more like the old setup. Bypass the limiting and its compatible.

    A switching FET from say a PC motherboard voltage regulator FET (600V and many amps is more than suitable) those intel CPU's use a lot of power at 3.3V.

    This is more likely to give you the 12V you are looking for, the pull up you show above will cause the ECU to sink some current as its output will be at 5V so 12-5 =7, I=V/R so (5/1000 Silly me) 7/1000 = 7 mA, not a lot but all the same. It will be OK at this level any heating will be minimal in the ECU and if it works, all the better.

    Found this in my searching, very nice
    www.dinoplex.org/tachoconversion/
    Last edited by Matthew; 9th January 2018 at 11:48 AM.
    flock likes this.
    87 S1 205 GTI / GTI6 powered a project underway
    306 convertible (gti6 candidate)
    307 HDi wagon
    407 HDi wagon

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