I have a '91 Volvo 940 Turbo. I noticed today that my ABS light came on for the first time ever. I got home, and noticed a humming coming from under the hood even after I shut the engine off. I'm guessing this is the ABS module? It's a black box, says Bosch and "made in germany" and seems to have brake lines coming in and out of it. Well that box kept "running & humming" for 30 mins with the car off. I unhooked the battery cable and it stopped. The brake fluid in the master cylinder is above the min level and seems full. Any idea what I can do here? Is there a reason for this to keep on running/pumping/whatever it does after the engine is shut off? Thank you very much!

 

I can't help you with ABS diagnostics on a 91 940 as they have a different system to my car, but I believe that car SHOULD have on board ABS diagnostics. Wait for someone more knowledgeable to confirm.
As a temporary fix, you can remove the main harness going into teh pump to get teh car driveable again.

Regards, Andrew.