I'm the person who repairs the ABS/TRACS modules for the 70 series Volvos.
Your problem points to a major grounding somewhere in the power circuit.
Try to measure the amps the motor is using when you bypass the module. This is not going to be the total amperage used since the unit also activates solenoids when it is in use.

If you suspect the grounding to be outside of the module or the motor.
Try using a short cicuit tracer, it is not common anymore but you may be able to find or make one. It is basically a flasher or a low amp circuit braker in line with a buzzer or a light, or both and a small compass.

If you suspect the module or the motor be the problem, you may want to open it to look for signs of damage.
Any way you look at this, you don't want to spend the money on a module or a motor/pump assy to have it damaged by another problem somewhere else.

Anyway, I have not repaired a module for the 850 series, I had some inquires but I have not received a unit yet.
My 95 850T gets a lot of use of the ABS and TRACS but I have never had any problems. My 98 V70 burned the module, that's how I got into repairing them.