Hey Guys, Need a bit of advice for by 2004 TJ Wrangler Auto which no longer works in reverse. This is a time line of what happened;
Dropped wrangler off at mechanic for general check over as planning a road trip in near future. [Wrangler is working fine - no problems what-so-ever with the transmission - no slipping / sticking / grinding / noises etc ] Ask mechanic to degrease engine bay while he has it.
Mechanic does check over and finds no issues, mechanic reverses car out and takes it to local car wash to degrease engine bay, drives car back and parks it at his workshop.
Mechanic calls me to advise car ready to be picked up
20 mins / half hour later I turn up to pay him and pick up car.
Go to reverse out and reverse won’t work. Will work very, very slowly if rev it flat out. take it for a short drive thinking maybe it has some water in it to dry it out. Reverse now won’t work at all. Take it straight back to Mechanic who says that my transmission has gone. All the other gears work perfectly fine - no issues with shifting in and out of gear, no sticking, no grinding, no clunking. Transmission fluid is not low and does not smell burnt. Mechanic ran diagnostic which says check reverse gear ratio [unsurprisingly enough]. Do you have any idea what could be causing the sudden problem with the reverse gear only? or will I need to rebuild / replace transmission?
Cheers
Kylie
A sudden reverse gear failure on a Jeep can happen. Usually requires a rebuild/replace.
We can’t see inside the trans from over the internet nor, sadly can your mechanic standing right next to it 
… but @insightful sure seems on the right track. The trans needs to come out and apart and rebuilt.
What happens if you slip the trans into 4L and try reverse?
If an electric solenoid is used to initiate reverse on your transmission, besides the ideas above, suspect that part too. Spraying the engine compartment with a high pressure wand can force water into places it shouldn’t go, and that might have been the reverse solenoid in your case. Or the water pressure could have knocked an electrical connector off. Might pay to just park it in the sun with the hood open and let it dry out for a few days, see if that helps. Meanwhile do a look-see for juggled electrical connectors.
I used to wash the engine compartment of my truck like that using a pressure wand, but I only did that when I was going to drive the truck for a long drive afterward, to allow the engine compartment heat to dry everything out. If the shop tech washed the engine compartment, then only a short drive, and parked your wrangler, water contamination of the electrical system should definitely be a suspect here.
Suggest going forward to not use a high pressure wand to wash the engine compartment. I stopped doing it b/c of weird problems it would cause, like this one.