Preliminary Info:

1) 2000 XJ, AW4, 270ish K miles...

2) I've searched every forum I could think of... CherokeeForum, NAXJA, et. al.

3) Getting a P0122 Code that returns after clearing the CEL

4) Tasks performed to date:

a) Refreshed all Engine Bay Grounds and B(+) cables and devices (starter, alternator, etc)

b) Swapped current TPS with used spare TPS - no change in issue. Both are MOPAR. No Non-MOPAR sensors on my rig.

c) Pulled Horn relay on the off chance that the Clockspring affected this code the way it does P0123 - no change

d) Sensor and PCM readings via back probe (Green tested good - Red is what I believe is in question):

Key On Run Position: Sensor Signal (OR/BL) to Sensor Ground (BR/YL) .780V
Key On Run Position: Reference Voltage (OR) to Sensor Ground (BR/YL) 5.19V
No Voltage Drop on Sensor Ground when tested against Chassis Ground using LoadPro
No Signal Drop using Scan Tool from Closed to WOT
Key On Run Position: Sensor Signal is 3.98V at WOT - FSM says WOT should be 4.5V
Key On Run Position: PCM C1 (Black Connector) Pin 23 (Sensor Signal) to TPS Ground is 3.97V at WOT - negligible drop from Sensor to PCM

So...

Do I just have 2 subpar TPS's?

What haven't I checked?

What other sensors or modules could impact this?

What do the #s tell me (learning to diagnose electrical instead of just replacing parts)?

What uniquely quirky things on XJs might this be that isn't remotely related?

Much appreciation for any assistance in advance.

This is the rig in question...

 

One additional piece of info if needed - my rig is Cali emissions.

 

Compare your readings to the specs in this test chart. Recheck per this chart. Note the meter ground.



.

 

Here's the basic diagnostics for P0122:


P0122-THROTTLE POSITION SENSOR VOLTAGE LOW


When Monitored: With the ignition on and battery voltage above 10.4 volts.


Set Condition: Throttle Position Sensor voltage at the PCM is 0.1 volt for 1.3 seconds.


POSSIBLE CAUSES


THROTTLE POSITION SENSOR SWEEP


INTERMITTENT CONDITION


5 VOLT SUPPLY CIRCUIT SHORTED TO GROUND


5 VOLT SUPPLY CIRCUIT OPEN


THROTTLE POSITION SENSOR INTERNAL FAILURE


THROTTLE POSITION SENSOR SIGNAL CIRCUIT SHORTED TO GROUND


THROTTLE POSITION SENSOR SIGNAL CIRCUIT SHORTED TO SENSOR GROUND CIRCUIT


TCM INTERNALLY SHORTED THROTTLE POSITION SIGNAL CIRCUIT


PCM 5 VOLT SUPPLY CIRCUIT


PCM THROTTLE POSITION SENSOR SIGNAL

 

Ken,

Thanks for the info on proper diagnosis. Here's what I got following the testing procedures:

Sensor Ground; OFF; IDLE;.000V; .000V
Sensor Ground; RUN; IDLE;.015V-.020V; .0117V
TPS Signal; RUN; IDLE; .26V-.95V; .629V
TPS Signal; RUN; WOT; <4.49V; 3.91V
5V Supply; OFF, IDLE; .000V; .17V
5V Supply; RUN, IDLE; 5V +/- 0.5V; 5.20V

Appreciate any thoughts on where to go next.

 

Ken,

Thanks for the info on proper diagnosis. Here's what I got following the testing procedures:

Sensor Ground; OFF; IDLE;.000V; .000V
Sensor Ground; RUN; IDLE;.015V-.020V; .0117V
TPS Signal; RUN; IDLE; .26V-.95V; .629V
TPS Signal; RUN; WOT; <4.49V; 3.91V
5V Supply; OFF, IDLE; .000V; .17V
5V Supply; RUN, IDLE; 5V +/- 0.5V; 5.20V

Appreciate any thoughts on where to go next.

 

.

 

Thanks Ken.

I'm off today so I'll be spending it in the engine bay. By chance, do you have the test procedures with specification readings for the other sensors - CPS, CKP, IAT, ECT MAF, IAC, etc? Figure it's a good time to see if they're functioning properly while I'm at it.

 

Finally, I can help with something semi-complicated. I tested my IAC and coolant temp sensors in post 1 of my build thread.
I have links to and images of the appropriate charts in my post.)
https://www.cherokeeforum.com/f46/20...ebuild-232835/

It was awesome! I can't wait to see the resolution on your issue. Keep us posted.

 

OK, new developments. Sensors are all testing within range per FSM, however...

My hard start, rough idle, running like crap issue may be influencing the TPS code. When starting my rig it's a bear to get her up and running first key turn. This is a recent development which is why I went the route of cleaning up all of my grounds and B+ connections. Battery load tested fine. Charging is good as well...

So I moved on to test the fuel delivery system. Pump engages and pressurizes with key in run position - BUT ONLY TO 18 PSI, and then drops to below 10 PSI in less than a minute. Ran the test bringing engine up to operating temp and she held steady at 51 PSI... until she sputtered and died (not unusual recently). I haven't done the next tests to see if the issue is in the regulator or with the injectors leaking. I'm hoping the 18 PSI at Key On Engine Off would lead me ( or someone) to the culprit.

Any guidance or wisdom is greatly appreciated.

 

Perhaps you should monitor the fuel pressure until the engine stumbles and quits to see if the fuel pressure drops off before quitting (etc.).


Also suggest you use a scanner that shows live data then monitor performance parameters, especially O2S cycling and short term fuel trim (STFT). High negative (-) STFT numbers indicates it's running rich, high positive (+) STFT numbers indicates it's running lean.


BTW: P0122 is also listed as a TCM diagnostics code.

 

I'll run those parameters this week.

Replaced fuel pump this weekend. Filter was off and black as coal. Also sediment in the bottom of the tank. Would not be at all surprised if some of that got into the pressure regulator valve and kept it from operating correctly as well as affecting the pump.

Also found at least one exhaust gasket that was shot below the collector. Replacing that this week as well.

I'll post results of O2 cycles and STFT by mid week.

Thanks again Ken. Much appreciated.

 

Did you ever get this figured out? Im dealing with a similar issue.

 

The OP, Rugbysoldier has not been here since 2017 so it's likely he won't answer.

Did you follow CCKen's post above?

You should start a new thread to continue to work through this.