My car started overheating badly a few days back - actually it has been doing it for a while but the first hot day really caused the needle to go over half way up the dial. A quick check revealed the fan wasn't coming on, and further checks revealed the fuses and relays were all fine. The problem was the on board computer wasn't pulling a relay driver line low when it should be.
So, take it to the repairer and pay a king's ransom for a new onboard computer, or build a new controller using a picaxe? I chose the latter of course!
Hack into 3 wires (0V, 12V switched via the ignition, and the low side of the relay coil). A quick picaxe board with onboard 5V reg and relevant filtering. BC337 driver transistor. LM35 strapped to the water pipe near the radiator. Then some experimentation and this is the bit that picaxes are really good at. After a few iterations moving the chip from the car to the programmer board and it has been calibrated so 70C measured on the LM35 on the outside of the pipe corresponds with about 80C on the temp guage. readadc10 works better than readadc but it probably isn't that critical.
Turn on for 30 secs if over temp. The temperature really falls quickly when the fan comes on and a single burst of 30 secs is enough for another 3-4 mins of idling. Turn on for 10 secs when startup for diagnostic purposes.
Temp can be tweaked further if needed and this is the bit that is so much better than a pot which might shake and lose calibration. Plus a pot doesn't give control over the hysteresis or on time.
This isn't a "mission critical" application as the worst case scenario is it all fails and it is back to how it was before when the on board computer wasn't turning on the fan and the car had to be turned off at every traffic light. But I must say that there seem to be no problems with interference or resets or anything. It just works.
Now I have my own on-board car computer with a picaxe!
So, take it to the repairer and pay a king's ransom for a new onboard computer, or build a new controller using a picaxe? I chose the latter of course!
Hack into 3 wires (0V, 12V switched via the ignition, and the low side of the relay coil). A quick picaxe board with onboard 5V reg and relevant filtering. BC337 driver transistor. LM35 strapped to the water pipe near the radiator. Then some experimentation and this is the bit that picaxes are really good at. After a few iterations moving the chip from the car to the programmer board and it has been calibrated so 70C measured on the LM35 on the outside of the pipe corresponds with about 80C on the temp guage. readadc10 works better than readadc but it probably isn't that critical.
Turn on for 30 secs if over temp. The temperature really falls quickly when the fan comes on and a single burst of 30 secs is enough for another 3-4 mins of idling. Turn on for 10 secs when startup for diagnostic purposes.
Temp can be tweaked further if needed and this is the bit that is so much better than a pot which might shake and lose calibration. Plus a pot doesn't give control over the hysteresis or on time.
This isn't a "mission critical" application as the worst case scenario is it all fails and it is back to how it was before when the on board computer wasn't turning on the fan and the car had to be turned off at every traffic light. But I must say that there seem to be no problems with interference or resets or anything. It just works.
Now I have my own on-board car computer with a picaxe!