Heater Matrix

So after a fair bit of research on the Cobra Club forum, I delved into the modification of the Mk2 Polo Heater box.

I managed to source one from eBay (searching for Polo Mk2 breaking, then PM the sellers) for £20 complete with the matrix, blower, controls and demister duct. Although you don’t need the controls or demister duct for the cobra build.

I used a dremel with a cutting disc on for most of the modifications to the pipework, it’s tough stuff, dont think you’d get far with a stanley knife!

My fan speed control had broken and instead of paying the outrageous £50 for  replacement, I decided that my three speeds could be Off, 50%, On. So I ordered a high power resistor (100W 1Ohm) from RS Components for a couple of quid and will put this inline for the half speed mode. I’m yet to test it, but it should do the job.

What else you need:

  1. Sikaflex black EBT+ Adhesive
  2. 1no. Black plastic 110mm gutter reducer
  3. 4no. Black push-fit 40mm end plugs
  4. 1no. 110mm Black plastic access cap
  5. AK heater plenum (comes with kit)
  6. Adhesive foam strips from car builder solutions
  7. 75mm black silicon duct hose (1m)
  8. 40mm black pvc duct hose (1m)
  9. Chrome air inlet

Here’s what I did:

  1. First up, remove the matrix, give it a good clean with degreaser and then flush it with water (or you can use matrix/radiator cleaner, but my matrix had been recently replaced and was only 6 months old, so no need for the cleaner).
  2. Then check/replace the o-ring that sits between the control valve and the matrix, you can pick these up from B&Q in their assorted O-ring packs. It is approximately 21mm internal diameter and 2.5mm thick.
  3. Modify the guttering offset to remove the outer section of pipe. This will leave an inner section that provides a 70mm connection for the ducting and a flat plate to seal it onto the heater at the open end (opposite to the motor end).
  4. Modify the access cap piece to trim the chamfered end of the pipe off, this will give a wider area to bond onto the heater. Remove the inner sealed cap as this will foul the motor. Drill a 10mm hole though the side of the access cap pipe for the supply cables to push through and I also filled a small section from the bottom so that it could straddle the raised plastic section on the heater that the electrical connections are attached too. Basically you want to make sure you have a good fit between the access hatch and the heater at the motor end.
  5. Slightly roughen the flat section of the gutter reducer and the area that you will bond it to the heater. Apply a generous amount of Sikaflex to the fan housing and press on the offset piece. Make sure you align the offset so that the ducting coming out of the inner pipe section does not clash with the heater pipe connections.
  6. Connect the spade connectors onto the motor terminals and push through the hole in the access cap pipe section. Apply Sikaflex to the edge of the pipe section that you trimmed down earlier, align any notches required in the pipe and press onto the fan housing. Apply Sikaflex to the cable penetration to seal around the cables.
  7. Take the AK plenum box, mark out the locations of the 4no outlet duct connections and mark out the centre points on the top section of the plenum box. Take note that the connections pieces (end plugs) have a small flange to them, so make sure you dont space them too closely together or too closely to the edges of the plenum box. Pilot drill the centre points with a 4mm drill bit, then take a 40mm hole saw and cut the 4no holes in the plenum box.
  8. Modify the end caps to remove the blanking section so that they can be used as bulkhead connectors.
  9. Apply Sikaflex around the edges of the holes on the inner face of the plenum box. Push the end caps through the holes so their flanges abut the inner face and seal with the Sikaflex.

So, thats as far as i’ve got on this one. Further updates to come to cover installation in the tub…

Finished up the heater today by the following:

  1. Wired up the resistor using the old spade terminals (trimmed off with a dremel) and then you can re-use the existing mounting screws. I have retained the three connections (-ve, full speed and half speed).
  2. Blanked off the gap above the matrix to ensure the air gets passed through the matrix rather than around it. To do this I simply used a shaped piece of wood screwed at either end and then sealed with a bit of Sikaflex.


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