Porsche 964/993 A/C and oil cooler fans

A/C and oil cooler ballast resistor change

Ballast resistors

The A/C and oil cooler radiators are placed in the left and right front fenders in both the 964 and the 993. Both have powerful fans, and they are both supposed to run in two speeds depending on the cooling needs.
High fan speed is obtained by connecting 12V directly to the fan motor, and low speed is set by switching a high power resistor in series with the motor. Such resistors are often named "ballasts", and they sadly have a tendency to be damaged by heat or corrosion in the 964 and 993. This is probably due to them getting very hot under normal operating conditions.
The spare part (993 616 521 01) is of higher quality, and is accompanied by a extra cooling mounting flange. Once in there, you should check both the A/C and oil cooler fan resistors, all cars I've inspected have had both inoperative. Both the fan shroud and the radiator itself are often clogged with dirt and leaves, so this is probably a good chance to clean this up as well. The resistor value is 0,45Ohm og 52,5W, and normal off-the-shelf resistors can be used as spare parts instead of the Porsche part. However, this requires a different mounting method.

Oil cooler

The Climate Control Unit (CCU) controls the oil cooler fan speed. The oil temperature is measured by a sensor mounted near the oil cooler radiator in the right front fender. The fan is normally off at cool engine, and first starts at low speed, then switches to high speed when the oil temperature increases. Relay R04 control the fan speed, and really consist of two relays, one for low and one for high speed. A damaged ballast resistor will make the fan to operate at high speed in intervals. This is not good at all, the oil temperature will have to rise unnessecary high before the fan starts at all. You definitely don't want excessive oil temperatures in your engine!

Air conditioning

The condenser is where the air conditioning system gets rid of the heat from cooling the evaporator. The condenser radiator is placed in the left front fender. It's cooling fan shall start at low speed as soon as the A/C function is turned on by pressing either of the A/C buttons on the CCU. A pressure switch is mounted on one of the A/C pipes, and will set the fan speed to high as soon as the pressure increases in the system. This could be called a emergency switch, the radiator normally has more than adequately cooling effect when the fan is running on low speed. If the low speed does not work due to a damaged ballast resistor, the pressure swich will run the fan on high in intervals. You definetly don't want this situation, excessive pressure over time in the systyem can cause leaks. The 993 evaporator is prone to develop leaks, maybe the failing resistor are the cause of this? Relay R14 control the fan speed, identical with the R04 oil cooler fan relay.

Testing the fan motors

The simplest way to test the fan sppeds is to jump the connectors in the relay sockets with a piece of wire. R04 and R14 are identical, and you can jump terminal 30 and 87 to start the fan at high speed, or 30C and 87C for low speed. See the underside of the relay for terminal identification. You can also pop off the relay housing with a sharp knife, reinsert the relay, and press twe two relays inside with your finger to run the fans.

A/C condenser resistor change

Start with removing the front part of the ABS inner wheel arch cover. Dismount the lower (bottom part) front fender. Consider removing the complete polyurethane front fender if you need to change the oil cooler ballast too.
The resistor is mounted on top of the radiator, and it's very difficult to dismount it without loosening the radiator. However, this is really more simple than you expect, only a few bolts holds the radiator with fan shroud in place, as well as a rubber stud on top. Let the radiator hang in its hoses while you unscrew the allen bolt holding the resistor in place. I use a allen bit on a ratchet tool.
Be sure to get good thermal contact between the radiator and the resistor when mounting it. I always use some white electronics thermal mounting paste to ensure good cooling of the resistor. You may have to change the allen bolt when mounting the new resistor, some are a bit short to fit the increased thickness due to the cooling flange.

Oil cooler resistor

The oil cooler resistor is mounted a bit inaccessible, and the complete polyurethane front fender must be removed to get to it. You also have to remove the right wheel and the front part of the ABS wheel arch inner cover to get to the wires.
Be prepared for corroded screws on the front fender parts. The resistor is easily reached trough the front aluminium fender once you're in there.
You could consider mounting the resistor elsewhere on the oil cooler assembly to avoid removing the front fender, but be sure you mount it on a metal surface to ensure good cooling.
An alternative repair method is described here, it is possible to avoid removing the front fender.

Other recources

Bruce's DIY resistor change thread on Rennlist