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Replacing Faulty Immoboliser & ECU Mx5 Mk2/2.5 NB

BaZa
Just Run In
Posts: 21
Joined: Thu Jan 31, 2013 9:29 pm

Replacing Faulty Immoboliser & ECU Mx5 Mk2/2.5 NB

Postby BaZa » Wed Oct 30, 2013 3:44 am

Many thanks to BaZa for taking the time to write up this How To.

ECU, Immobiliser and Key Compatibility
In order to ensure you source the correct replacement parts that will work with your Roadster you will need your ECU part number.

Locating and Removing the ECU
The ECU sits behind the carpet at the passenger side footwell.

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Firstly remove the sill cover. This is held in by plastic clips and should just pop up & out by prising with a flat head screw driver.

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Now unclip the carpet from the door sill lip and pull it back. This will reveal the cover plate which protects the ECU.

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ECU Cover Plate
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Remove the ECU Cover
There are two round machine headed bolts A&B which you will need to either totally grind off or grind a slot in and unscrew with a large flat headed screw driver. (I just grinded them off completely with a Dremel power tool for ease). Now undo bolt 1 and nuts 2&3 and the cover should now remove easily. You will now see the ECU unit. See Below:

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The large characters are the important part number you need when sourcing replacements. B6MV in this instance. A replacement ECU, Immobiliser, Key and Ignition Coil Antenna will need to be sought from a working Roadster with the same number as the one you are replacing. To remove/replace the ECU unbolt the metal bracket which holds the main cables located on the RHS of the ECU. Then unplug the various cable plugs from the bottom of the ECU. Replace with your new ECU and re-fix the bracket and cable plugs.

Locate and Replace the Immobiliser

The immobiliser sits behind the under-facia to the right of the steering column. Firstly remove the facia closest to the pedals by unscrewing the self tapping screws 1&2 with a star
headed screw driver. Also remove the cover to the underside of the ignition barrel by removing the three screws with the same screw driver (No.3=normal screw and No’s 4&5=self tappers).

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Unplug the cable plug from the bottom of the old immobiliser. The old immobiliser should pull down from the steel bracket that slides into the back of it. (I needed pliers to pull it down as it was quite stiff). Plug the cable plug into your new immobiliser. Now fit the new immobiliser to the steel bracket by sliding the bracket into the slot on the back of the immobiliser and push up.
In the picture below you can see the old immobiliser still in place and the new one plugged in and hanging down.

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Attaching the New Transponder Key to the Coil Antenna/Transceiver

The coil antenna is incorporated within the plastic ring that fits around the key hole on the ignition barrel. The coil transfers and receives all data signals to and from the immobiliser via the thin wires you can see in the pictures.Remove the coil antenna by unclipping it from the ignition barrel.

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A transponder chip is incorporated within the key handle. This needs to be taped to the coil antenna/transceiver. Ensure it fits centrally over the coil.

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Now tuck the taped key and coil antenna/transceiver well out of the way from the ignition barrel to ensure no signal interference.
Note: At this stage if you have sourced an ignition barrel that fits with your new key you have the option of replacing the ignition barrel, and a full lock set to be changed also, Then this would negate the need for taping the key and coil together.

( Please Note, That putting the key in the coil alone will render the immobiliser all immobilised so we do recommend fitting the new ignition barrel and lock set from the donor car so it will not affect your insurance, the key in the coil is a quick fix to get you back up and running.

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Ensure your battery is well charged as it may well have been sitting idle for some time and lost its charge. Check with a volt meter and ensure it reads at least 12–12.5V.
Now insert your own key in the ignition and fingers crossed your Roadster should start. All being well you can now replace all covers, fascias and the carpet etc. For aesthetics, replace the coil antenna/transceiver that’s been used in the repair with the one included with your new parts. Just clip it on and tuck the cable out of the way before replacing the ignition barrel cover.
Mk 1 1992 Black 1.6 with Silver Hard Top. Garaged needing alot of work (welding etc.)
Mk 2 2001 Red 1.8 with matching Hard Top
Mk 2.5 2002 Black 1.6 with Silver Hard Top

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