Help with pic chip tye and code for car Aircon condenser Fan control.

[540itouring]

Hi , i work with electronics and electronic repairs and know how to create a hex code from a .asm file and program a pic but do not know much about pic code designs . I would like to build a small pic chip project (i have only programed microchip devices) that cat enable or disable a relay coil to switch the AIRCON Condenser fan off at higher road speeds. The fans are wired to switch on and off every time the compressor is on but i would like to turn them off at road speeds 35Mph and above and enable the fans at below 35mph. The onle things i can tell you is this...

1 , Requires a input from speed signal from dash or gear box (i have no problem where to find the signal)
2 , Requires a output to drive a relay coil (i can add a output transisor to increase switching load if required)
3 , When speed signal is below 35mph relay must be off
4 , Road speed 35 mph or above relay switches on.
5 , some kind of a delay to prevent rapid switching of relay * 35mph


I would like any help or advice on what chip to use , and info on program design and how simple or complex this would be. The car for info only is my 1994 jaguar XJS V12 which i have converted to twin speed cooling fans which are controlled by a electronic temp control module and run at half speed when the aircon compressor is on. The car does not require cooling fans above 35mph due to the ram air cooling the engine and switching the ac fans off will save a load on the car battery and alternator.

Thanks for any help in advance.

[Archangel]

Hello 540itouring,
Welcome to the forum.
I would disagree with the blanket statement " car does not need cooling fans over 35 mph . . . ", to the extent of under NORMAL conditions. It will need them when you are following too close and probably will need them when climbing a long hill .
I have had cars with electric fans which experienced failures and they would heat under those conditions, with a/c on. It would be a good project I suppose if you allowed a high temp override to enable the fans. First thing you should do is make a flow chart on paper of what your program needs to do, and in what order, decide how many I/Os it needs to control. To measure temp, do you want a sensor or simply a switch, that will determine if you need analog functions. You want to count pulses from the speed sensor or tach. or abs sensor so you will probably need a timer, likely a 16 bit timer. Those are my immediate thoughts.