98 xj TPS P0122 and P0700

Beave

NAXJA Forum User
TPS problems on a 98 Cherokee 4.0 auto 294k miles

I’m getting a P0122 and a P0700. Engine runs great with no issues. But the shifting is delayed till 4000-4500 and will not downshift on acceleration. As soon as I clear the codes they immediately come back. I have disconnected the clock spring under the column. And I’ve tested the tps and to me it tests good by the specs ive found but WOT is a little low. I have not replaced or removed the tps so unsure of brand.



Supply 5.10 volts.

Signal at tps and pcm .84 idle 3.94 WOT (I know this is lower than what most get)

Ground has tested good.

Can not for the life of me find a good pin out that matches my tcm. The one I have found is backwards and going by the color of the wire i get nothing and also probing where the pinout location says I still get nothing.



So I’m leaning towards the signal is not making it to the tcm.



Can anyone’s help me out what pin i should be probing at the tcm or anything else I need to check?
 
If you disconnect the TCM and shift the transmission manually 1/2, 3, OD, and if shifts promptly and correctly, the transmission is OK. TCM failure is uncommon, TPS failure is common. The TPS is the most common and most logical suspect.

The Throttle Position Sensor is directly involved with transmission shifting characteristics. Your test result are out of specification. The P0700 can be ignored.


Possible Causes of the DTC P0122 Jeep
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
- Faulty throttle position sensor
- Throttle position sensor harness is open or shorted
- Throttle position sensor circuit poor electrical connection

TPS failure 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.


TPS TESTING

The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the 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. This will vary in an approximate range of from .26 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle.

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 fault 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.


Cheap crappy Chinese "Lifetime Warranty" parts are often out of specification or even failed right out of the box. Many times they have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors.
 
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Thanks for the reply Tim. I have already tested the tps by that method. It was within spec. Even went further and tested for the signal at the pcm plug and it was also within spec. What I’m having trouble with is testing for the signal voltage at the tcu. Initial testing I could not find voltage resembling that of the tps.

Further more. Although I have a cheap code scanner. Even the p0122 is showing up under the transmission section and not the engine section. Which makes me think the tcu isn’t receiving the signal.
 
If that TPS was in either of my XJ's it would be replaced. The TPS spec of .26 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle is right out of the Factory Service Manual.
 
I just replaced it with a box store unit. Still the same problem. But now signal at idle is .34 and wot is 4.2
 
Acceptable test numbers now.

Inspect the wire harness plugs for corrosion and bent or pushed back wire plug pins. Inspect the wire harness for melted or chafed wire insulation, or other damages. Typical problem areas are anywhere near the valve cover, or sharp casting edges.
 
So I’m back on the case after a few days off.

I have inspected, cleaned every connector and grounds that I could find. All good

Have tried a tcu out of a running jeep. Still the same.

If I unplug the tcu I can clear the codes.
Which leads me to believe it is tcu related.

I’ve found a pinout for the tcu connector that I know is wrong year. It shows 2 grounds. One for the tcu ground and one for “sensor ground”
One black and yellow one dark brown and tan. I also have both colored wires. One ohms out to a good ground. The other not. I also can not find another ground on the tcu connector.

Also I know it doesn’t always work this way but the ground at the dipstick is also has the same color and it had no continuity from the tcu connector to the dipstick side.

I’m thinking I may have a bad sensor ground at the tcu but have no way to know for sure.

Can someone help me with a pinout for the tcu connector for a 98. I really don’t want to gamble and splice into the tcu connector and try making a jumper ground to see if that fixes it. I also would prefer not to cut the entire harness apart to try to chase a single ground that may or may not be what I’m even after.
 
I have since found the tps ground at the tcu harness and it is good. So here’s what I’ve got total.

TPS tests good at tps, pcm and tcu .34 idle 4.2 wot tried a new tps and a used tps out of a good running jeep.

Have also tried tcu out of good running jeep.

Good grounds. At Tps and tcu.

Can clear codes if I unplug the tcu.

But come right back if I plug it up.
 
I know I’m having a conversation with my self here but I’m doing so in case someone else is searching for help with the same problem.

So my xj has had a rough life that’s obvious to me. And it seems as though it’s had a new engine put in it at some time.

So I found something new with googling that may or may not be the culprit of my problems. I have a 98 that’s for sure. Vin on the jeep matches my title and it says 98. Also searching for parts using my vin also comes up for a 98.

Tcu part numbers match on rock auto.

However my pcm part number does not.

I have a 97 pcm. Confirmed with snap on scanner. PCM vin does not match my vin plate or title. And a quick search with my pcm shows it is a 97. PN# 56041277ad And from what I can tell. It’s kind of a one off year. And a 97 is only compatible with a 97 according to fs1inc.com

Now has someone just switched the pcm and nothing else. Or other stuff also. I’m not sure.

Anyone have any input on where I should go from here.

Is there any differences?
 
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