1988 jeep 4.0 battery installed backwards now cranks but wont start

To start with, code 12 is loss of battery voltage within last 100 engine starts, which is normal if you had the battery disconnected, code 55 just means the computer has completed testing. I'm sure your problem is fuel related. Why did you replace all that stuff?
Listen close when you turn the key on, you should hear the fuel pump run for a couple of seconds to prime the rail. If not, start at the fuel pump connector and check for power when someone turns on the key. If there is no power, work backward toward the battery. You should check with a dealer as Chrysler/Jeep had a recall on the relay /fuse box under the hood(power control module), which would fail causing problems such as you are describing. They are really expensive, and the company was sued due to the many failures. If yours is bad, and you get no factory help, try to find a salvage part. I have installed used ones before that worked OK, but there is no guarantee how long.

Bill, No known inertia switch for your vehicle unless installed aftermarket. Could be Cam or crank sensor,security issue,etc., have a good tech. scan for trouble codes, diagnose and estimate repair. Check the attached links,instruction and guides, Good luck
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So your engine cranks but wont start

Did you reset computer
You can do this by removing battery positive and leaving off for about 5 mins make sure you have radio code though

Since you do have A fully charged battery and also you do receive fuel smell after crank start and the car wont start,either you do have an Ignition fault means not giving the spark,advice remove the spark plugs and check whether are wet with fuel,see whether are black,if so change them and before start check by leaving an HT lead off a spark and use one of the old ones connected to the HT lead and earthed to the engine,ask somebody to start the engine checking for any spark comes out. If yes then put the HT lead back to the new spark on the engine and try. If ther is not any spark,then you have to go backwards from the HT leads to distributor,Rotor Arm,Electronic igniter,Coil if is fitted,Fuse and relay. Good Luck.

Jiggle the wire connectors going in to the computer under the hood -- first check battery cables/terminal connections at battery -- beleive it or not, connection could SEEM good because you get crank & lights, but if it's faulty, loose, engine won't start - computer requires good solid voltage flow.. next check a fusible link very close to the battery off of the positive cable. check fusible link carefully. this could cause your problem for sure.

If it doesn't turn over it may be your starter going. or a weak battery. start with that first, insufficient fuel pressure won't prevent you car from cranking, but weak battery or bad starter will. If you can jump it and it starts its probably the battery if it doesn't crank then its probably the starter. Good luck

The number one problem with these cars is with clogged fuel filters. They must be changed regularly. When you say it won't start, do you mean it cranks but won't fire? That could be a fuel pressure problem due to a clogged filter.

Its the key fob battery!

Mine had enough to open the doors etc but not enough to deactivate the immobiliser!!!

Jeep Northampton fixed it. Brilliant Garage, would recommend them to any Jeep owner, lovely people.

Did you have the same cranking with no start problem with original engine or did the problem begin when you changed it? I would check codes if you have not already, and would also clean business end of idle air valve (next to tps) DO NOT turn the end plunger while cleaning or it may not seat properly. Do not loose the rubber "o" ring while working either!

timing gear