2000 XJ with 4.0 and automatic. Got in after work and it started fine and idled fine while warming up. Then I left the parking lot and less than 100 yards down the road I felt it hiccup and the check engine light came on. Then it wouldn't go over 30 miles per hour. It would rev up fine but it wouldn't shift up. Went to O'Reilly's and ran code reader.
Found 4 codes:
P0121 (this code came up twice)
P1687
P1281
P0320
A few months ago I changed the crankshaft position sensor after the Jeep decided not to start one day. That fixed that problem and been running great until this.
Anybody with experience like this?

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The shops computer suggested:
​​​​Clockspring assembly
TPS (throttle position sensor)
PCM (power control module)
Engine wiring harness
MAP sensor
TCM (transmission control module)
BCM (body control module)?
Passenger door module
Connector C200
Oil pressure sending unit
CMP (camshaft position sensor)
Distributor
CAS (crankshaft angle sensor)
Ignition coils
Thermostat
ECT sensor (engine coolant temp.)
Fuse #10

Needless to say, that didn't narrow it down much and I just limped her home to consult you all and the rest of the web.

 

What can happen is that one problem triggers a myriad of codes.

I always start with the lowest number code, work on resolving that, then clearing codes and see how things go and what codes return. Oftentimes when all of this comes up at once, resolving the one root cause will take care of all. Please, please....whatever you do, don't throw parts at this based on the printout of what the parts store told you here! You will drain your bank account. Use a testing based methodology to resolve this.

So for you if I was working on this XJ, I'd absolutely start with the P0121 code which is for the throttle position sensor. I think it's especially important to start with this code as the TPS directly effects shifting characteristics of the AW4 and you are having problems shifting up through the gears.

Here is more on the throttle position sensor. Keep in mind that this code is for the TPS circuit which includes much more than the sensor itself. Wiring, connectors, input signal from the computer, etc.

Good luck and keep us posted!
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is 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. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

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: -

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

• 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 fault TPS can report values outside the denied 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 excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

 

Nice writeup, tj!

 

Thanks for all the detailed information!
I took your advice and started with the TPS and it worked! I wasn't sure how to test the output of my old sensor while it was plugged in. (I guess just poke the multimeter's lead through the wire insulation?) I wasn't sure if that's what I should do, so I just tried replacing that first. Picked one up from Orielly's for about $53 with a lifetime warranty. It's SMP brand which is on Rock Auto for about half price but less warranty plus shipping costs and waiting.
So I'm super grateful for the help.
I still have to go back to the auto parts store to clear the CEL/MIL and verify that there are no more codes but I think that fixed it.
Also do you think I should use threadlocker on the TPS? The two torx head bolts seemed to have some yellow on the tips.
On another note,
This morning I noticed it revved higher than usual (about 2k rpm) on start up and sustained it for about 10-15 seconds before returning to normal but it was super cold out. Also whenever it is at idle if the blower motor is on (a/c or heat) it bumps up the rpms every ten or fifteen seconds for just a second or two. Is this normal? I have yet to determine if it is doing this with the new TPS, but I think so.

 

I did my TPS last week and had the exact same experience as you. It went away permanently after a battery reset and a couple of drives

 

my son has a Jeep renegade 2020 would this be his problem? His check engine light is on and it won’t go over 25 mph

 

You are in the wrong sub forum, this is for XJ _1988?-2001