Replacing the Tachometer

My tachometer stopped working properly and would only move up to around the 800 RPM no matter at what speed the engine was actually running.

The tachometer signal is derived from the engine Crankshaft Position Sensor (CKP), which is bolted the engine adaptor plate adjacent to the flywheel. The ECM uses this signal to calculate the actual engine speed, which is then fed to the tachometer in the form of a square wave at 1 - 12 volts with the equivalent of two pulses per engine revolution. As the speed of the engine increases so does the frequency of this signal. The ECM uses this information in conjunction with that obtained from the manifold absolute pressure sensor in order to provide idle stabilisation and reference for injection timing.

Back to my problem: Since the engine was clearly ticking over evenly and with no other problems being experienced during driving, it seemed reasonable to assume that the CKP sensor and the ECM were both working okay, and that the problem lay either with the cabling between the ECM and the tachometer, or the tachometer itself.

If you have access to an oscilloscope, it's worth checking that the signal is reaching the tachometer. The problem could simply lie with a poor connection! I know of one other MGF owner whose tachometer was behaving erratically and then completely died, even though the car was performing okay. After undoing, cleaning the connectors at the CKP, the ECM and ECM-main loom connector using a electrical contact cleaner, and then reconnecting them all, the tachometer worked perfectly !!

The easiest place for checking the signal with an oscilloscope is at the fuse panel sited under the dash board. The ECM connector and the main harness to engine harness connector (sited under the ECM mounting bracket on the left hand side of the engine bay) are not very suitable for probing, since all the pins are environmentally sealed at the rear of the connector pins.

  • To avoid any chance of causing an electrical short, make the oscilloscope connections whilst the ignition is turned off.

  • Do not probe into the front face of a connector, since this is likely to cause damage to the pins and lead to later faults.

    Click to enlarge Click to enlarge The tachometer signal is on a WHITE + BLACK STRIPE wire that comes out of connector C0223 on pin 2 (2nd pin from LH side on lower set). Use a short length of stiff wire or paper clip and insert this into the back of the connector.

    For 'scope GND, use the instrument and warning lights Earth on a BLACK wire on pin 9 (2nd pin from LH side on lower set). Use a similar length of stiff wire in the rear of pin 9.

    The tachometer signal at this point on my car was okay, so the problem lay with the tachometer itself.

    Click to enlarge Click to enlarge Click to enlargeThe Tacho itself.



    REPLACING THE TACHO.

    NOTE: Before removing the nacelle covers on the steering column and the instruments, DISCONNECT THE BATTERY AND LEAVE FOR AT LEAST 10 MINUTES FOR THE SRS AIRBAG SYSTEM TO DISCHARGE.

    Remember too that disconnecting the battery will mean your radio will loose it's security code and will have to be re-entered.

  • Undo the three screws that are underneath the steering column and that hold the two nacelle halves together. These halves clip together and excessive force will easily damage the clips. Mine did!

  • Gently prise out the rubber grommet from around the ignition key-switch.

    Click to enlarge Click to enlarge Click to enlarge The clips seem to be designed to pull apart, but are fragile and can easily break.


    Click to enlarge Testing out a way to undo them with my old nacelles, it seems that if you can get your fingers into the square holes for the indicator and light stalks, then gently pull upwards and downwards to separate the two half at this point on either side.

  • The rest should then come apart, but make sure that the farthest ends of the nacelles halves, underneath the instrument panel, have separated before trying to pull the separate halves out. You may need to reach underneath and separate them at that end in a similar way.
  • Gently ease the lower half over the ignition key barrel, and remove.
  • Then ease out the top half.

    Click to enlarge Remove the 4 screws that secure the instrument cowl the dashboard facia.


    Click to enlarge Release the cowl and disconnect the panel light dimmer switch multi-plug from the rear of the cowl. Leave the switch in the cowl.


    Click to enlarge Remove the 4 screws that secure the instrument pack to the dashboard fascia.


    Click to enlarge Click to enlarge Note the 3 little rubber covers on the lugs. They are inclined to fall off easily. Make certain they are all in place and not on the floor when you come to re-assemble.



    It is difficult to undo anymore until you release the speedometer cable.

    Click to enlarge Open the bonnet and remove the under bonnet closing panel (the big plastic panel that covers the and windscreen wiper mechanism)

  • Release the upper cable section from the retaining clip just above the join of the two sections.

    Click to enlarge Disconnect and separate the upper half of the speedometer cable from the intermediate cable.


    Click to enlarge Prise out the rubber grommet from the bulkhead, and slide it down the cable, so as to allow free movement of the cable. Note how mine had split - a possible water ingress point?



    Then......back in the passenger compartment.........

    Be aware that the rear face of the instrument panel is a thin film PCB and can easily be damaged if mistreated

  • You should now have freer movement of the instrument pack.

    Click to enlarge Reach behind the pack and unclip the retaining clip on the speedometer cable, and slide it up the cable out of the way.


  • The speedometer cable should now come loose, though it might be a tight fit on the spigot and need to be gently persuaded.
  • Now release the 3 multi-plugs from the rear face. Note that the electrical contacts are directly onto the thin film PCB inside the connectors and similar care must be taken.
  • Remove the instrument pack.

    Now to extract the tachometer........MAKE SURE YOUR HANDS ARE CLEAN......you don't want to leave a finger smudge on the dial faces!

    Click to enlarge Click to enlarge Remove the 2 screws and support brackets. Then release the 4 clips that secure the instrument panel window to instruments pack and separate.

    Click to enlarge Remove the 5 screws that secure the tachometer to the instrument casing.


    Click to enlarge Notice the markings and how three of these screws provide the electrical connection for the Ignition (IGN), Ground (GND) and Tacho (TA) signals.


  • Remove the tachometer from the front of the pack.



    Refitting is the reverse of this procedure - don't forget to check that those 3 little rubber covers are in place!