Thanks for posting this hack! *NM*

This is a revival of a post I made on PPBB in 2009. This subject comes up every spring.

---

Just before I left for BRBS I discovered that my passenger-side radiator fan wasn't working on the low speed. Knowing that I was going to be doing some hard driving in warm weather, I devised a hack to remediate the problem at least until I could fix it properly. The hack worked well, keeping the temps in check, even in sustained high-revving runs in hot weather.

At BRBS I did the same hack on two other cars with similar success. It seems to be a common problem that many of the older cars have at least one non-operational low-speed radiator fan.

Description
The radiator fans in the Boxster run at two speeds. The low speed runs when the car engine temp is higher than 206°F or the A/C is on. The high speed runs when the coolant temperature is higher than 215°F or the A/C freon pressure is higher than 16 bar. The low speed is controlled by a ballast resistor which is prone to eventual failure.

Diagnosis
With engine coolant temperature under 206°F, turn on your car and the A/C. Put your hand under the front bumper in front of each of the front wheels and feel for moving air from the fan. If there is no moving air then likely one of the ballast resistors is no longer working.

To determine that the problem is the resistor and not some other part of the system, get in the car, start the engine, and put your finger on the appropriate low-speed fan relay (see below for diagram). Press the A/C (snowflake) button on then off. You should be able to feel the relay click. If not the problem is the fuse, the relay, or the wiring.

You also need to verify that the problem isn't the fan itself. Leave the car running with the A/C on and wait until the temperature reaches 206°F. The fans will kick into high speed and at this point, chances are both fans will be running. If not, there is a problem with the actual fan, the fuse, the relay, or the wiring.

Hack
There are four relays that control the radiator cooling fans: left low speed, left high speed, right low speed, and right high speed. This is also the order that they are found on the relay panel. They are relays 19, 20, 21, and 22 in this diagram:



The relay panel is under the left side of the driver's side dash. The bottom of the relay panel is about the same level as the OBD2 plug so you really have to get your head under there to see it. In this picture you can see the clutch pedal and dead pedal for reference. Note that it is not necessary to remove the fuse holder cover as shown on the left in this picture (this pic is actually from a different hack).



Remove both the low-speed and high-speed relays for the fan that isn't working. Cut and strip a piece of wire about 1.5" long.



Insert one bare end in the output socket (pin 87, center bottom horizontal slot) for the low speed relay and the other end in the output socket for the high speed relay.



Put the relays back and test the hack. Start the car (coolant temp less than 206°F), turn on the A/C and verify that both fans are now operating.

Fix
The proper fix is to replace the ballast resistor. I will cover that procedure in a different post.



Edited 1 time(s). Last edit at 06/04/2018 03:27PM by Boxsterra.(view changes)

Stefan, that is one of the best written and illustrated step-by-step repair guides I've ever read. Well done!

Very clear and detailed guide to checking and keeping the cooling fans running!

I checked both low and high speed fans with my Durametric and found no problems. I also tried your low speed "hand" test...success!

Thanks for taking the time to write and post this DIY guide.

Bruce
2001 Boxster
1984 Carrera Targa
1977 911S (Euro)