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Alarm - Interior Scan Sensors Retrofit

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The Ford Mondeo hit our roads in 1993, and has consistently been a sales success. Now on its 4th incarnation, it remains a drivers favourite.
For more information, visit the Ford Mondeo forum on, the definitive resource site covering all Fords from the present day to the 1970's.

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Overview Guide
Ford Model: Mondeo
Petrol/Diesel: ????
Estimated Cost: ????
Difficulty? Fairly Easy
How long does this take? 40-60 mins ish

Parts Required

Facelift model GEM unit ( one that has the interior scan function in it as there are early facelift models without interior scan functions )

Interior light unit with scan sensors

Interior scan sensor connector plug ( with a decent length on cable attached )

GEM unit connector plug ( with a decent amount of cable attached )

Around 3-4m of cable ( i used a pair of wires from an old computer network cable )

4x butt crimps

Tools Required

Crimping tool

Small flat screwdriver

T25 Torx Bit Screwdriver

Philips Head Screwdriver

Fitting Procedure

Remove 2 connection pins from the GEM unit connection plug.

Measure up enough wire to reach from the interior light unit in the roof along the front of the roof panel & then down the A-pillar trim & then down to the GEM unit. ( you will need 2 of these lengths )

Connect these 2 lengths to the relevant wires of the sensor unit connection plug at one end ( see piccies further down this how-to for the wires ) & then the other end to the 2 GEM connection pins removed earlier & mark them accordingly for what connector hole they will go into.

Using a small flat blade screwdriver unclip the current interior light unit from the roof console ( there is a small gap at the windscreen side of the light unit for this very task )

Unplug the light unit from the roof loom.

unscrew the sun visor & the sun visor clasp from the roof at the drivers side ( UK - other countries its whichever side your GEM unit is on ) - the sun visor clasp needs a small cover prising off before you can access the screw below it .

Remove the A-Pillar trim on the drivers side of the car.

Remove the Lower dash panel on the GEM side of the car ( UK drivers side & is 5 philips head screws holding it inplace )

Now feed the GEM side wiring you made earlier down the hole in the dash where the A-Pillar trim used to be & feed it down to the GEM unit area making sure its not in the way of anything essential & its secured as required ( make sure to leave a little spare to allow you to work with the GEM unit later etc )

Now feed the top half up the A-Pillar & into roof above the headlining.

Finally Feed the last bit of wiring along the front of the roof lining & to the interior light opening in the roof console ( the reason for removing the sun visor as it much easier to do with that removed )

Once the wire is routed you can put the A-pillar trim & lower dash surround back in place if you wish

Plug the new wiring loom into place ( making sure to put the earth onto the required connector pin on the interior light unit & then plug in the interior light unit to the roof loom ( trapping the earth wire for the alarm sensor - if you wish you can connect this wire up in a different way.

Fit the new interior light unit back into the roof cosole.

Replace your GEM unit with the new unit as follows :-

  • Unclip old GEM module from in the drivers footwell.
  • Put the new GEM unit in the same alignment/direction as the old GEM unit.
  • Remove the plugs from the old GEM unit one at once & once removed fit the plug into the same slot in the new GEM unit ( apart from the one plug that is shown later in the piccy ).
  • Take the top cover off the pictures GEM plug & insert the 2 new pins into the slots they go into ( should have already numbered the wires ).
  • Put the plug back together & fit it into the new GEM unit.
  • Put the new GEM unit back into place & make sure it clips back into place properly.

Follow the RF procedure from this guide & reprogram all your remote key fobs to the new GEM unit

Pictures of the fittings & Wiring Info

Wiring at the GEM

Alarm sensor GEM wiring.JPG

Wiring at the Sensors on the light unit

Alarm sensors wiring.JPG


To test the Sensors i did the following.

Sit in the rear drivers side footwell behind the drivers seat ( probably not required but i wanted to be out of the way of the sensors vision etc just to be sure )

Now double lock the car using the remote & wait around 30 seconds or so.

After the 30 seconds or so have passed move your arm out from behind the seat & the alarm should trigger ( it did for me )

You could also test it from outside by leaving a window down & then double locking the car before waiting & then sticking your arm into the car through the open window etc ( this way there is a risk the alarm may go off if its a touch windy etc - i know i have had a couple of false triggers when the wind has picked up when i have had the windows down for a prolonged period when its been hot out etc. )