With Winter almost upon us, well here in Oz, it's time to fix up those water leaks before the carpet starts to stink and rot. Probably one of the most common and the hardest leaks to fix in an early Falcon is the plenum box floor. This is the leak that gives you that cold foot bath at the first corner.
Just in case there is still some doubt to the location of the plenum box, it's the area directly below the grill outside the windscreen where air enters to feed the under-dash vents. Leaves and other debris that fall through the grill is blown to the ends of the plenum box where it blocks the drain holes. As more water enters via the grill the leaves compost and eventually rust-out the floor of the box. Trying to seal the leak from the underside of the dash is a waste of time as most of the area is double skinned. The only successful way to stop the leak is to open the box end and weld from the top.
Firstly examine and clear the area around the vent units under the dash. The most likely place for a leak will be in the corner behind the vent. To remove the vent loosen the two nuts which are on hooked shafts in side the vent door. Then remove the side wall plastic trim, the fire wall insulation and on the driver side pull back the wiring harness.
To gain access the plenum box the front guards have to be removed. Start by removing the chrome radiator grill, headlight surrounds etc. This will expose the half dozen or so bolts around the apron and behind the headlights. There are also two bolts inside the car and one on the door pillar as well as the row inside the bonnet.
Fig 1. Front guard removed, plenum box cut and spot-welds drilled.
With the guards off, the drain holes of the plenum box are visible. It is now necessary to remove the top of the box. Using a cut-off wheel or a metal cutting jigsaw, make a cut in line with the outer edge of the engine bay side-wall towards the windscreen pillar stopping at a point about 100mm before the windscreen. Then make a second cut from the vertical spot-welded seam to the end of the previous cut. Finally drill out the spot-welds along the seam between the two cuts.
Fig 2. Plenum box top removed, rust holes in box floor visible.
Removing this section of the box will reveal the rust holes in the box floor. Repair these with sheet metal. Before replacing and welding the top make the drain holes bigger. Also drill a 38mm hole in the top which enables this vulnerable part of the inside of the box to be covered with bitumus paint after welding. Then use an old floor bung to seal that hole.
© BigH 1994
DISCLAIMER
While this method has been tried and works, NO responsibility will be excepted by the author, the Club or any of its Members.
