Cranks with ECU disconnected. Why doesn't it crank with ECU connected?

P0340 is bad connection/signals from the camshaft sensor.
Check the connection is good to the ECU.

limited information generic answer ---is motor compatible with vehicle and ecu ---was ecu flashed to vehicle

Hi Sue, The links below are YouTube videos showing how to fix a car that doesn't start/cranks but doesn't start, Also there's videos down the right side of the links that may help as well....................CHEERS..
Fixing Car That Does Nothing When You Turn The Key

Fixing Car That Cranks But Doesn Start Up

Car Cranks But Will Not Start

Hi Adriel, The links below are YouTube videos showing how to fix a car that doesn't start/cranks but doesn't start, Also there's videos down the right side of the links that may help as well....................CHEERS..
Fixing Car That Does Nothing When You Turn The Key

Fixing Car That Cranks But Doesn Start Up

Car Cranks But Will Not Start

Hi Daw, As you've already checked the crankshaft sensor, sounds to me as if the problem may be from the camshaft pickup sensor. If that one also proves fine you'll need to have the anti theft system re-coded. The anti theft unit cuts the injection. Regards John

ecu can be reset by disconnecting battery. this is likely not your problem..

Could be the crank sensor is bad but If the check engine light does NOT come on when cranking at the times when it won't start, then your ECU is in need of repair. The engine will only start after the check engine light comes on and the ecu is what controls that. It's likely going bad, causing the delayed start problem typical of early 90's jeeps. Repair or replacement services for your jeep ECU are available on ebay.

If the fuel pump is'nt running then it's a bad relay or blown fuse. If the pump is running then it could be a few things, a bad timing belt, bad cam sensor, bad crankshaft sensor, could be the timing is off which you can adjust with the distributor, or it could be a bad o2 sensor or MAP sensor.

Sounds like a blown ECU ( engine computer)

connect a test light to the plug on the ignition coil with it disconnected and crank the engine. if the light flashes the crank sensor is ok and you should replace the ignition coil. if it doesnt i would replace the crank sensor.