Here is a list of the things i have to fix on my XJ40 by the order of relevance. Any suggestion and help is welcome:

ABS :
light is on when starting then goes off after engine start. But when i brake sometimes the pad light goes on and also the abs light not always but often.
I tried a panic stop and the abs is not working.

HEATING and CONDITIONING:
the heating fan works only at max speed. the ac compressor does not start. i found a small sensor that is unter the front bumper with 2 wires and its broken maybe it's a component of the heating syste like a temperature sensor?

CRUISE CONTROL:
the cruise control engages, but when i push set, the speed does not remai stable, but slowly increase.

It's my first experience with a JAG so i need some help

 

I didn't write this, but it is very informative about the XJ40 heater fans. I was written by Martin Sellars. I used it a few years ago to repair my XJ40 fans.Air con blowers: these devices contain an electronic circuit which gives speed control for all settings but "high". This comprises a Darlington transistor 2N6284, a 68 Ohm 2.5Watt resistor, a 1N5401 diode and a small glass diode, probably 1N4148 or 1N914. All these parts are easily obtained cheaply from electronic component outlets. Blowers that won't run at low speeds almost always have a failed transistor but replacing that alone won't always fix the fault. The resistor is there to protect the transistor from voltage spikes produced by the motor, so check the value and replace if it isn't right (most hobby multimeters have an Ohms measurement feature). The small diode feeds the blower motor voltage back to the AC computer, and has steel leads which corrode away, breaking the connection and preventing speed control from working. I replaced mine with 1N4004 types, which are much more rugged, more easily handled, and cost just a few pennies more. These diodes are fitted with their cathodes (marked by a bar on the body) towards the transistor collector (the steel case). The 1N5401 diodes are very rugged, and unlikely to need replacing. When fitting new diodes or resistors, don't try to fit them inside the blower interior, like the originals, where they are prone to corrosion. Fit them on the solder side of the PCB (having snipped out the old parts) and then cover them with the original plastic flap using some tape.

Cheers

 

If the compressor doesn't fire up it generally means you are low on refrigerant. You can test it (momentarily only) by bypassing the low pressure switch, jumping the A/C compressor relay with a fused jumper. You should hear the clutch click and see the thing start to spin if the system is mechanically ok.
Next step would be to get the system recharged.