1990 Oldsmobile Ninety-Eight Regency compressor runs when car is off.

The compressor you refer to is the air suspension compressor. It provides air to adjust the ride height of the vehicle. If you will notice by looking underneath the rear of the vehicle you will notice air adjustable rear shocks, or struts. They have plastic tubing plumbed to the compressor under the hood. There is a ride height sensor for the rear of the vehicle attached from the chasis to the rear differential. Whenever the height of the rear of the car is below the threshold setting of the sensor, the system activates a relay and turns on the compressor to raise the rear of the vehicle above the minimum height specified by the manufacturer. As the vehicle ages the air envelopes of the struts loses structural integrity and seepage, or leakage develops. Some depletion of air is normal, but the compressor power supply is independent of the key position, so the compressor can run, and will run anytime the height falls below the minimum. If the car sets unattended for several days, (or weeks) the compressor can run the battery down while maintaining the minimum height. If the system leaks too much air the compressor can burn-out trying to replenish the system. If you notice the compressor runing for an extended period of time, or running too frequently you should have the system checked for the source of the leakage. (Or you can do it yourself with soapy water and a spray bottle by soaking the air bellows on the shocks or struts, and all the lines and fittings from the shocks to the compressor) If you intend to work underneath a vehicle, service stands are mandatory! DO NOT UNDER ANY CIRCUMSTANCES ATTEMPT UNDER VEHICLE SERVICE WITHOUT PROPER SERVICE STANDS IN PLACE AND STABILITY VERIFIED WITH APPROPRIATE FORCE TO PROVE THE VEHICLE CANNOT BE KNOCKED-OFF THE SERVICE STANDS!