On your jeep, the tachometer is controlled by the PCM. To me, a freaking out tach indicates a bad sensor/bad connection to the computer.
This is how I am thinking.
1. The computer runs the tach and spark plugs.
2. The tach signal in the computer is generated based on the coil output.
3. The tach and coil are working intermittently, and linked.

Based on those three things, the PCM is trying to generate a signal intermittently.

This in combination of the p0340- loss of cam position sensor signal, and p1391 intermittent loss of cam or crank.

I decided to look up what would set those codes really quick and this is what I found...


P0340- Set condition- at least 5 seconds have elapsed with Crank position sensor signals present, but no Cam position sensor signal

AND

P1319 û set condition û crank or cam position sensor was intermittently lost 20 times for 2 ôtripsö

I think you really need to look into the Cam Position Sensor because...

1. Both of those 2 codes talk about the Cam Position Sensor. (one code is saying that that it is being lost, and the other saying its been lost for a while)
2. The Tach and ignition system is not functioning right.

A relay would cause the CoilÆs to not fire, but wouldnÆt really effect the tach. I donÆt know the intimate details on how that particular computer is setup, but I bet that an intermittent flow of juice through a mosfett would not cause an internal sensor to register crazy RPM. I do bet that an intermittent signal from a cam sensor could be breaking and getting signal super quick causing a really weird sensor reading which would cause a really weird output (freaking out tach).

I forget, Have you check out the connector on the Cam Position Sensor?

The connector coming from the computer should have 3 wires... I donÆt remember the order, but...
1 wire should have 5V reference voltage.
1 wire should have a good ground.
1 wire is feeding the computer with the signal from the cam position sensor.
To test for 5 volt reference from the computer...
1. Disconnect the connector
2. Turn the key to run
3. put my volt meter on DC volts
4. put my ground probe on the bat ground
5. test all 3 pins for DC
6. a pin should have 4.5-5V DC
To test for ground....
1. Disconnect the connector
2. Turn the key to run
3. put my volt meter on Ohms
4. put my ground probe on the bat ground
5. test all 3 pins for continuity
6. a pin should have practically no ohms << 1 ohm

If you have a good ground and good clean 5 volts reference, I think it is time to check the cam position sensor.
Unfortunately... You need a digital storage oscilloscope to check it right.

BUT as Marty pointed out they sometimes like to die at the oil pump. I know on some of the cars I worked on the if you remove the bolts holding the sensor you could actually see the chunk of metal that spun around and generated the signal. IÆd try pulling the bolts that hold the sensor to the little stand off and see what is underneath. If you can see something that might rotate, crank the engine see if it spins, put your hand on it (not cranking) see if there is play...

Good Luck
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