Short Circuit in A/C control panel

The most common causes for blowing the air conditioning fuse is a blower motor overload or a short in the airconditioning control panel. It is best to eliminate the blower as the fault symply by unplugging it. If the fuse still blows it is best to disconnect the control panel next, this is the easiest way to determin whether the fault is in the A/C circuit board or not.
At this stage most people would now bite the bullet and order a new part. There is some good news. Most circuit boards can be saved with very little effort.

AC control panel circuit boardThis page documents how we rectified a short circuit in a 306 N3 air contitioning system. The procedure could also be suited for later models but this has not yet been tested. This repair was well worth the trouble as a new circuit board costs close to $1200.

This is how.


  1. Isolate the short circuit by disconnecting the A/C panel.
  2. If this confirms the fault then procede with removing the control panel and dismantle.
  3. Remove the large diode adjacent to the blue connector. See figure A and B.
  4. Plug the A/C harness back into the crcuit board.
  5. Does the fuse still blow? If not, then we are almost there.
  6. Check all funktions and if all is ok, reassemble the panel.

ImageImportant! It is not unusual for the earth and main power supply pins on the board or in the connector to be burnt due to the high current flow. This can be easely rectified by soldering an earth and positiv power supply wire directly in the place where the diode used to be. In figure B the locations are marked: + and respectivly.


This is why

The diode we removed is a Zener Diode and its function is to protect the sensitive logic circuits from voltage spikes by directing current to ground when the voltage exceeds the predetermined amount at the diode. A failed diode permanently lets current pass, which is our short. Although we won't have protection after the removal, at least the circuit board will have a new lease of life. There is quite a number of boards out there that are enjoying a long and happy life after the operation.


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