Brake and ABS lights are on, also getting low oil pressure( recently changed oil pump). Wasn't started for a while and now the AC compressor wont come on and power windows work occasionally.
Thoughts? 2001 Grand Cherokee.

 

ABS light probably is just a front wheel sensor. The back sensors are housed and protected. Or, could be the the tone ring which is the serrated portion on wheel area off the axle which provides the sensors a way for reading things to make the ABS work. The sensors go bad because they are being exposed always to road grime and/or generally wearing out. Or, their wire(s) break and/or develop a bad contact connection somewhere. The tone ring can develop rust and stuff, and can be cleaned up with a wire wheel easy enough to fix the issue.. which is most likely one of the two problems causing your ABS light to be on. There are other less common reasons, but these are the ones to check first.

Low oil pressure most likely (if you are not hearing bad noises.. or now have a broken engine bc of actually having none or really low oil psi) is likely the oil pressure sender unit. A $10.00 part located near the oil filter that goes bad. Replacing it will get the gauge to read oil psi once again. You could hook up an actual oil psi gauge to see what the psi is... I just replaced the sender unit and all was well afterwards getting good oil psi readings once again, coming from having intermittent low to no oil psi with the old failing sender unit.

The compressor may be one of three things.. most likely. First thing to check with one of those recharge cans having a psi gauge is to hook that up on the low side to see if there is really low or no psi. If so, that shuts off the compressor, and recharging to fill it will get it working again (if bad leak it'll not work for long until fixing the leaks). Next thing to check would be some sort of switch, relay, fuse, or the wires... One of those things could be bad, and fixing it will get the compressor working again.. Lastly, the compressor could be worn and out of spec. There is a shim on its clutch area to remove that will draw it closer again to being back in spec. Doing that; removing the shim will work to get it operating again for a long time many years most likely, and it is easy to do watching youtube videos will help with removing this shim. However, if highside psi and low side psi is very really abnormal, doing this to the compressor removing the shim could potentially cause catastrophic a/c system failure when getting the compressor to run full time with high side psi issues... So, I would consider it imperative to have the low side psi checked and verified at minimum prior to removing that shim, and idealy the high side psi too if can swing that with a shop that'll hook you up and test it out for you, but still not totally necessary.. And plus, instead of removing the shim you also could probably find and install a brand new or junkyard compressor instead too. Not too common I don't think though with that one thing, because I haven't heard about that happening ever having catastrophic failure r/t fixing a compressor not running by removing a shim...

Not sure on the windows, or the regular brake light. The brake light I would think is mostly for letting oneself know that the parking brake is still engaged.

 

"Occasional windows" are caused by broken wires the the door bundle, that rubber boot between the door and body. You can unlatch the boot on the body and push it back into the door to inspect the wires. The best way to fix them is to solder the 2 ends together and use shrink tubing over them. Don't use a butt connector because the reason it broke where it did is because it's got to flex there. Actually the best way is to replace that wire and butt splice it inside the door and body.

There are 3 functions for the brake light. The first is to signal low brake fluid in the master cylinder. The second is you have the parking brake on, and the third is a backup for a burnt out ABS light.

Did you spin up the oil pump when you installed it? You should have filled it with oil before you installed it and spun it up with an electric drill by removing the cam sensor after you installed it and before you fired it up. The initial startup of a dry pump puts tremendous wear on the gears in it and causes the clearances to get too large. It's a gear pump and by design, they don't like to pull a vacuum when they are dry. There is another way I've seen it done, but sort of "out of the box." Apply pressurized oil from a tank thru the oil pressure sensor port. This is called "pre-oiling" and is done on radial aircraft engines if they've been sitting too long.