The TPS can be tested. Check the sensor wire harnesses for chafed or melted insulation. Stalling and not starting is more commonly the CPS, which can also be tested.

P0123 JEEP - Throttle Position Sensor/Accelerator Position Pedal Circuit High Input

Possible causes
- Throttle position sensor wire harness is open or shorted
- Throttle position sensor circuit poor electrical connection
- Faulty throttle position sensor
- Faulty Powertrian Control Module (PCM)

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds and the ECM/ECU may behave oddly until you remedy this.

TPS

You may have one or more of these Physical Symptoms:

1) The engine loses power and is stalling.
2) The engine will idle, but may die as soon as you press the gas pedal. When driving, it seems as if all power is gone.
3) Sometimes it feels as if the transmission is failed or isn't shifting properly, if at all. If you quickly jump on the gas you might be able to get the transmission to shift, but it won't shift properly by itself. Shifting manually, the transmission goes through all the gears.

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics. The TPS function should be verified early in the troubleshooting process, when a transmission issue is suspected.

TPS TEST

You should have 5 volts going into the Throttle Position Sensor (TPS). It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. At idle, TPS output voltage should be greater than .26 volts but less than .95 volts. Move the throttle and look for smooth meter response up to the 4.49 at WOT. The other wire will be the ground and should show no voltage.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture, and vibration, leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following:

-Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".

- High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.

-Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.

-Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted accordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.

- Engine Misfire: A faulty TPS can report values outside the acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessive misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

CPS Testing

Crankshaft Position Sensor Connector (CPS/CKP)

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TESTING PROCEDURE 1991 - 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

 

thanks for the info, I'll see what I can find

 

I had something similiar happen on one of my jeeps. Although, it was not hard to start it would only die while driving.
At the time I knew the wires to my o2 sensor were pulled loose. CEL was on but no other problems...Until the Intake Air temp sensor went out. That is when all the problems started. It would just quit. I would have to let it set a few minutes and it would start back. I never had an issue with it not cranking after it sat a while.

It only happened a couple times before I figured it out. My point is that if you have more than one sensor that is going bad it might cause the problems you are having.

 

****UPDATE****
I tested the TPS which has reading a little low so I put a new one in. I drove to work with no problems. On the way home, I had the defroster on and when the compressor kicked on, it seemed to put a substantial drian on the electric system (down to 11-12 volts) but the jeep never died or "chugged." I turned off the defroster and made it home without any problems. The next day I was driving to work and it died about 2 blocks from my house and wouldn't start back up. After trying to start it a few times, I got a P0320 code. After about an hour, I called a tow. As soon as the tow showed up, the jeep fired up. As I drove home, the belt was squealling really bad any my volts were reading 11-12, which is obviously low. I pulled into my garage and it died and won't start. I feel like I'm back to square one. Sounds like a Crank Sensor or some short somewhere.................................?

 

It looks like you have two separate issues.

First of all, resolve the voltage issue. If your belt is squealing, it might be loose or in need of replacement. Inspect it closely for wear/glazing, etc. I recommend the Goodyear Gatorback if you go new. A loose belt might be responsible for the alternator not working properly. The belt on the 4.0 needs to be pretty darn tight to keep quiet.

The P0320 is NO CRANK REFERENCE SIGNAL AT PCM. This is most likely either the crank sensor itself or a dirty connector for the crank sensor, wiring, etc.

 

A friend told me my alternator could be causing all my problems. Any thoughts?

 

As was previously posted, begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds and the ECM/ECU may behave oddly until you remedy this.

Have the alternator and the battery Load Tested at the auto parts store. Buy a new Gator Back fan belt while you are there.

 

Well I'm going to spend tomorrow on it. I've got to clean everything, I went offloading a few weeks ago and everything is still caked with mud. I'll let you know what I came up with.

 

Alternators are not particularly fond of water/mud.

Get that charging system working PERFECTLY first thing before moving onto more complex possibilities.

With a healthy battery and charging system, you should have a verified 13.5 to 14.0 volts measured at the battery with a simple multimeter. Do you have a meter? If not, buy one. They are essential to anyone doing any work on their XJ. A basic one can be had for around $10 - $20

 

The alternator is caked with clay right now

 

I may have solved it-

I went outside to fiddle with it and it started up just fine, I turned it off and checked the CKS with the multimeter, looked ok. Turned it back on, left it running and started checking connections. After about 5-7 minutes it died. I tried to start it but it wouldn't fire. I got under the jeep and lightly tapped the CKS. I then heard what sounded like a relay. I tapped it again and heard the same thing. I tapped it for a third time but didn't hear anything. I tried to start the jeep and if fired up.
I picked up a new CKS from jeep for $75 (same price as autozone), swapped them out and everything seems ok so far. I drive it around for about 10 minutes and then let it idle in the driveway for another 15 and it didn't die. I hope this is it.