The 993 has a known issue with defective low-speed resistors that power the oil cooler and AC condenser fans. They fail from moisture exposure and heat cycling.
Replacing them as originally installed involves disturbing AC and Oil cooler components and their hoses on a 25+ -year-old-car that maybe shouldn't be messed with and will take a few hours to accomplish.
This DIY is a minimally invasive how-to to make resistor assemblies from resistors available from eBay or really any electronic supply store and relocate the resistors to a more accessible location behind the headlamp pods. If the existing resistors are abandoned in place and pigtail wires are obtained from another source this DIY involves no body or component disassembly at all!
Please read this document completely from beginning to end before attempting this project. The author of this page is not a professional automotive technician or engineer therefore, please feel free to consider this project at your own risk.
The items needed:
Quantity 4, 100W 0.8 Ohm aluminum housed resistors, two used in pairs to replace each single fan resistor in each assembly;
Quantity 2, Discarded OE fan resistor wiring pigtails with plugs on the end. These can be the ones clipped from your original resistors abandoned in place or possibly request a couple failed & discarded resistors from someone on a 993 technical forum or secure some through a dismantler. If you clip the pigtails from your old resistors the one on the left side is easy to get to through the headlight pod opening. The one on the right side will require removing the wheel well liner to obtain.
The below wire, sheath, and heat shrink tube insulation items are only needed if the salvaged pigtail wires recovered from the old resistors are too short for the pigtail plugs to reach the wiring harness sockets. Doing some trial layouts will help in determining if extending the pigtails is necessary.
Quantity 4, 1-foot sections of GXL grade heat-resistant stranded primary wire in 12 or 14 gauge and
Quantity 2, 1-1/2 foot sections of heat-resistant insulating sheath to cover them.
Quantity 2, 2-inch sections heat shrink tube insulation suitable for insulating 12 to 14 gauge primary wire.
The above three items are easily purchased in small quantities on eBay.
An alternative wire and sheath source to lengthen the plug wires is to secure quantity 2 discarded Bosch oxygen sensors. Most any shop that service European cars have a couple of these in their trash bin. Also if you were considering refreshing your car's oxygen sensors save the old ones for this project. The oxygen sensor pigtail includes two white heater wires and a wire sheath that are the proper gauge and are made of heat and abrasion-resistant material and are ideal doner wires and sheathing to lengthen the resistors' pigtail wires.
General fabrication supplies needed include:
- Solder & a 75 Watt soldering gun;
- Aluminum roof flashing sheet, something purchased at a home improvement store;
- An inexpensive pop-rivet gun and;
- Small diameter aluminum pop rivets;
- A role of 3M type 33 or 88 Electrical tape;
- A roll of aluminum (heat resistant) duct tape.
Making the resistor assemblies:
- Cut off and use the wire pigtails and wire sheath from the OE resistor assemblies.
- Extend the pigtails if needed. Abandon the original pigtail sheath and splice the added wire using staggered solder joints along the original pigtail wires so the spices don't overlap each other eliminating the possibility of them shorting together. Insulate the individual wire splices with some heat shrink tube and snake the wires through the replacement outer sheath before soldering them to the resistors. This method avoids using electrical tape not designed for potentially high temperatures.
- Once the pigtail is soldered to the two resistors in parallel paint the resistor's bare solder connections with nail polish to minimize corrosion over time as shown below.
- Trim the aluminum sheet and fold it in an "L" so it forms an angle bracket with a length about an inch longer than the length of the resistors and their wire attachment points.
- One side of the L will form the base to attach the resistors and should be about 2" wide
- The other side forming a vertical wall about 2-1/2" inches tall.
- The vertical wall is then cut in a curve matching the profile of the outer body curve above its planned placement.
- This curved edge will also act as a stop when sliding the bracket forward to its final installed position. By acting as a physical stop from sliding forward it will protect the forward-facing electrical connection on the resistors from touching the body.
- This vertical wall will tuck behind the rolled edge of the headlight opening in the body above it.
- Trial tuck the trimmed and folded sheet above the existing headlight bracket on the frunk side of the headlight opening as shown below.
- Attach the resistors to the aluminum sheet using 4 pop-rivets, 2 per resistor, so the sheet metal bracket extends past the exposed resistor end electrical connections.
- You may need to slightly ream out the existing mounting holes on the resistors with a drill bit so the pop-rivets can pass through them.
- Apply the pop rivets from the sheet metal side (bottom) of the resistor mounting attachment points.
- The resistors should be attached to the bracket in such a position so it precludes their end connections from physically being able to touch any part of the body when the complete assembly, resistors & sheet metal is tucked as far forward as it can into its final position. Cover the sharp edges of the sheet metal with electrical tape where it will touch the body of the car to protect the body paint from being scratched.
- Shown below is the assembly partially placed in position. When fully placed the vertical wall formed by one side of the sheet metal folded "L" should be tucked behind the body opening edge lip above it. By doing this it will completely clear the headlight pod when the pod is slid back in. Once the assembly is in position plug the pigtail into the wiring harness and tie the wires neatly as needed using tie-wraps.
- Use a small patch of heat-resistant aluminum duct tape, as shown below, to anchor the assembly in its final position so it does not move over time.
- Reinstalling the headlight pods can be greatly facilitated by spraying their mounting tracks with a little Teflon or Silicone spray lube and rubbing a little of the lube with your finger on the pod's electrical connection socket seal
- Once installed the resistors can be tested by pulling the two relays that control each fan in the fuse box and jumping each relays' two low-speed spade sockets.
The spade sockets to jumper are labeled on both relays, they are spades 30C & 87C and are located directly across from each other in each relay socket.
Relay Test Jumper In Position - The jumper can be made from a piece of 14 to 12 Gauge solid core copper primary wire, easily pulled from a roll of solid core Romex primary wire. Strip the ends of a small section of wire and hammer the ends flat to form small spades that will fit into the relay's sockets. There is no need to have the ignition on in the car to do the test. Also if while you have the relay out you would like to test the fan motor through the high-speed setting, jumper spade socket 30 to spade socket 87.
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| Relay Test Jumper wire with the ends hammered flat |
- The resistors do heat up a bit and if one would like to test how much just run the fans with the jumper for three to five minutes and they will reach their terminal temperature somewhere a bit north of 130 degrees although I did not take a measurement. It turns out the pigtail wire connections to the resistors and the pigtail wires themselves tend to stay cool when I ran this test, go figure?
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