Posted: 24/November/2007 at 10:16pm | IP Logged

Thanks to Mowog for this info. As per his warning about fire risk, if you are at all unsure about what you are doing then do not try this at home, Its here for informational purposes only.

The Problem

I recently purchased a 1996 Primera 1.6 (P10), and one of the only faults was that the heater fan only worked on speed 4. After asking some of the helpful people on this forum, it transpired that the 'heater fan resistor card' was not functioning. This is a metal card a bit larger than a credit card that is attached to a plastic base, and is held into the bottom of the fan unit by two screws. Stick your head in the passenger footwell and look behind the bottom of the glovebox, and you should see a large plug with 4 wires going into the resistor card.

The Boring Bit

Now, all the advice I could find was that Nissan would want about £30 for a new card, so a secondhand replacement was my best bet. However, I began to think about how the card worked...

Fan speed 4 is direct - the current from your battery straight into the fan motor. If you look at the resistor card you can see that it consists simply of metal pathways of differing thicknesses through with the current must pass to operate the fan. The length and width of the path determines the resistance, and therefore how fast the fan motor will move when current is applied.

A bit of GCSE science should tell you that this resistance must generate heat, and that is why the card is mounted in the body of the fan - presumably it sits right in the path of the cold air coming into the heater. Obviously dry heat means a potential fire risk, so there has to be a fail safe of some sort.

If you look at a failed card you should see that it has some crusty stuff on it. I am fairly sure that this is burnt solder. Follow the pathways along the card, and you will see that any fan connection passes through a point at the top-middle of the card, where the path is broken, each end finishing with a small square shape. My guess is that the connection is completed across these two points with a little solder, so that if the card becomes too hot the solder will melt and the fan will switch off.

Fixing it

Clean up the two small square shapes with very rough emery paper, until very shiny. Get a fine soldering iron very hot, and apply the solder generously onto the tip. I tried applying straight to the resistor card, but the flux in the solder made the surface dirty again and the solder would not stick. Now hold the loaded soldering iron into one of the squares until a blob of solder sticks. Repeat for the other square, and then carefully link the two by dragging a blob of solder across. When the two are joined, your card should be functioning again.

Disclaimer

This is entirely guesswork on my part, so there is no guarantee that your soldered repair will melt at the same temperature as the original. I can see no reason why it should be hugely different (the temperature at which the solder I was using melts seems to me to be well below the temperature at which the internal components of the fan might spontaneously catch fire), and my card has been functioning well for a while now. But obviously using none manufacturer approved parts could pose a fire risk, even if only a small one.

If you do effect a repair in this manner, I suggest you use fan speeds 1-3 sparingly, just in case.