Engine Cooling Fan Controller

The stock engine fan controller on the MR2 Turbo is poorly matched to its application. It uses an air temperature sensor mounted on the engine lid to sense when the engine bay temperature rises above 140 degrees and turn on the engine bay (intercooler) fan at that point. Usually, this means that you have to do several boosted runs before the engine bay temperature rises enough to start running the fan, by which time the intercooler is well on its way to being heatsoaked.

A common expedient is to unplug the air temperature sensor which has the effect of running the fan at all times. This is also less than ideal because it slows down the rate at which the engine warms up. Ideally, you want the engine to warm up as quickly as possible and have the fan them come on as soon as you are ready to boost.

Another solution commonly implemented is to add a fan switch to the dashboard so that the driver can turn on the fan when the engine is up to temperature. If you're like me, you often tend to forget little details like this until it's too late. For us dummies, it's better to have an automatic system control the fan.

This section describes a simple fan controller that you can build with some easily obtainable electronic parts which uses the stock EFI temperature sensor to sense the engine coolant temperature and turn on the engine cooling fan when the coolant temperature reaches whatever point you want. In fact, I was able to walk into a local Radio Shack and pick up all the parts I needed to complete this project. Here is the circuit:

The circuit can be easily assembled on a small universal circuit board such as Radio Shack part #276-150. The heart of the fan controller is an inexpensive LM339 comparator, which is a fancy name for a chip that compares two voltages and turns on a switch when one is greater than another (Radio Shack part #276-1712 that comes with 4 of these comparators in one chip even though you only use one of them). The coolant temperature sensor is a variable resistor whose resistance drops as the coolant temperature rises. The stock ECU has an internal "pullup" resistor which allows it to effectively measure this change in resistance as a change in voltage. This voltage can be sampled on the THW wire running between the ECU and the EFI temperature sensor on the coolant outlet pipe. This is a solid red wire on pin 4 of the middle ECU connector as shown in the diagram below which is the pinout diagram as you look into a 1991 3S-GTE ECU at the point where the engine harness connectors plug in. This pin location is the same on all genII and genIII engine harnesses.

The 12-14V from the car's power supply is fed to the controller and a simple LM317 adjustable voltage regulator (Radio Shack part # 276-1778) is programmed to produce a constant 10.2 volts across the variable resistor labeled SET. The circuits puts such a light load on the voltage regulator that you don't need to put a heat sink on it. This variable resistor should be the kind that has an adjustment screw on the side that lets you turn the screw about ten to fifteen times from end to end so that it is easy to set the turn on temperature precisely. Radio Shack part # 271-343 is perfect for this job. The purpose of the voltage regulator is to keep the voltage that you set and is then fed to the "+" side of the comparator steady even when the fan comes on and impacts the electrical system. The output of the comparator stays pulled to ground as long as the set voltage is higher than the voltage at THW. As the coolant temperature rises, the voltage on THW will drop until it goes below the SET point and then the 1K resistor will raise the voltage at the  base of the 2N3904 (just about any small NPN transistor can be used here) to over 10 volts and put a voltage across the coil of the S1 relay, which should be a small 1 amp SPDT relay such as a Radio Shack part #275-241.

The THW wire should be connected to the red THW wire as indicated above. This circuit replaces the engine cooling fan ECU which is the small black plastic box next to the big ECU behind the liner in the trunk. When you pull out this box, cut off the connector and connect the wire labeled ENG FAN on the circuit to the black wire with a red stripe that you find there. Attach the +12V wire on the circuit to the black wire with a yellow stripe from that same connector. Ground should be obtained from the white wire with a black stripe AND the blue wire with a white stripe (if you don't connect these two together, the engine cooling fan will not run). The air temperature sensor on the engine lid can be removed since it is no longer used by anything after these modifications are made.

In addition, the LID FAN wire can be connected to a 30-amp relay and used to control a pair of 10" engine lid fans if you so desire. Do not try to run these fans directly off the LID FAN wire as you will burn out the little 1 amp relay by drawing so much current through it.

Once the circuit is finished, put it in a suitable enclosure and wire it into the car. If you like little touches, be sure to mount the variable resistor at the edge of your circuit board with the screw pointing outward and then drill a little hole on the enclosure right where the screw faces. This will make it easy for you to adjust the temperature set point without having to open the box. Also, I always like to use connectors for little boxes like these which makes it easy to take them out of the car if anything should ever go wrong with it and you need to remove it to fix it. An inexpensive molex connector from Radio Shack will work since the trunk should never be exposed to the elements (be sure to use weather pack connectors and a water resistant enclosure if you ever plan to cut out a big hole in your trunk to fit a large trunk-mounted intercooler).

Adjusting the temperature sensor is easy. Once it is properly wired to the car, start the engine. If the fans immediately come on, adjust the variable resistor until they go off. You need to experiment to find out which way you have to turn for the fan to go off. Keep doing this if they keep coming on before your desired temperature is reached. Once the desired coolant temperature is reached (I like the point where the stock temperature gauge needle is about 1/3 of the way up), turn the resistor slowly to the point where the fan just comes on and no farther. Your temperature sensor is now dialed in and doing its job.