Lucas Air Mass Meter

Stripdown sequence and technical information

This is it: the magical bits you find once the cover has been removed (by carefully slicing through the sealant that secures it).

Use a fine pair of pliers to disconnect the air temperature sensor from the back of the terminal block.

The connector assembly is secured by two screws. Undo them…

… and carefully extract the connector assembly. Take care not to bend any of the copper contacts.

Slacken the wiper clamp bolt (connector assembly shown still in situ here)…

… and then slide the wiper off the shaft. Note that the shaft is ‘D’-shaped so the wiper will only fit one way.

Undo and remove the screws that secure the resistor pack. The mounting plate is steel but the resistor pack is made of ceramic and is VERY fragile. The green areas are the resistors. Black lines in the green areas are laser cut marks, whereby each resistor is adjusted to a precise value.

Right, the point of no return! Undo the detent spring clamp bolt, whilst holding the black spring seat. With the detent spring removed, the spring will try to unwind, rotating the seat. Make a note of how far the seat rotates until all the spring tension is released, this will assist reassembly.

With the spring tension released, the spring and seat assembly can be slid off the shaft. A screwdriver under the centre boss will help.

Now you need to turn the AFM over and identify the series of ‘pins’ that keep the baseplate in place. Use a small drill bit to drill the pins out, then slice through the sealant around the edges and prise the baseplate off. In the photo, the positions of the pins are marked by holes, drilled to allow self-tapping screws to be used to refit the baseplate. The screwdriver in the photo is indicating the flap that moves in response to airflow. At this stage the flap should be free to swing quite freely, without binding.

Extract the circlip from around the shaft…

… and recover the spring and washer from below the circlip. The spring serves to preload the flap assembly ‘upwards’ in the AFM housing…

… and once the circlip is removed, the flap assembly can be extracted from the housing.

As the preload spring lifts the flap asssembly in the housing, so these shims serve to stop the flap contacting it and binding. You can clearly see the shaft bearings here; they should be free to turn, with no noise and no play.

Once disassembled, the AFM components can be cleaned as appropriate (avoid cleaning the carbon track of the resistor pack, it’ll probably do more harm than good). The individual resistor values can be measured.

The shaft bearings should be drifted out and replaced if rough or worn. With new bearings fitted it is possible that the top clearance of the flap will be incorrect, in which case replacement shims should be obtained or fabricated from shim steel, as here:

As you can see, the shims are of the order of a few thousandths of an inch thick! It is essential to get the shim thickness correct as otherwise the flap will bind at top or bottom as it swings in the housing. You will soon spot it if it does…

Asuming the shaft bearings have been replaced and the shim setting is correct, drill into the AFM housing at the locations of the original securing pins. You need to go deep enough to allow self-tapping screws to bite, or else you could drill deeper still and tap for machine screws….!

Replace the baseplate using sealant around the edges.

Reinstall the tension spring and seat. Rotate the seat to preload the spring to about where it was originally. Hold the seat and refit the detent spring and its screw.

Refit the resistor pack.

Refit the wiper and gently tighten its clamp bolt.

Install the connector assembly, using sealant. Ensure the springy contact on the wiper is making contact with the large round pad on the connector assembly and that that the ’kill switch’ contacts are opened by the lever on the wiper.

Reconnect the air temperature sensor.

The AFM can be reinstalled to the car for fine-tuning of the wiper tension spring, before finally refitting the top cover with sealant.

©IRS 2004