LSaupe

Greetings All:

I recently purchased a 99 XJ with 6 and auto. Vehicle has 48K miles on it and runs fabulously. The transmission is smooth and predictable, except however, when it is in 4th gear and I start to encounter a load (such as an incline). It is VERY reluctant to down shift. It will drag the engine down to 1,200 RPM under 1/2 or better throttle position and finally will let go and jump 1,300 RPM and be in 2nd gear. Very abrupt. I would anticipate this would back shift much sooner and then maybe only to third.

In there an adjustment somewhere or something I could look at? Downshifting is find from 3rd on down, its just wen it gets in 4th it does not want to come out easily.

Any thoughts or experiences here appreciated.

Larry S.

tjwalker

There are a couple of possibilities but the most common one is a faulty "throttle position sensor". The inputs from this sensor are critical to shifting characteristics of the AW4.

I'd recommend you test or replace it. There is no adjustment on the 91-01 TPS. Here is more on that sensor.

Also of course be sure that fluid is RED in color, does not smell BURNT, and is at the correct LEVEL on the dipstick with a hot engine/fully exercised transmission. All very important to proper performance of an automatic transmission.
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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 acordingly 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 deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable 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.

LSaupe

Thank you for the very detailed reply.

I ran across this thread too.

https://www.cherokeeforum.com/f2/free-quick-fix-41821/

Slick761

Check the kickdown cable. Might need adjusted. Very common problem if its out of adjustment.

cruiser54

Try the TV cable adjustment but it's more likely your TPS


https://www.cherokeeforum.com/f2/free-quick-fix-41821/

LSaupe

I just tried the cable adjustment, but no change. Will look at the TPS next. Are these cheap enough to warrant just changing?

The rub is that this dang engine runs fabulously, smooth and responsive everywhere. Plus I am getting a solid 20 MPG to boot (which is better than I expected).

LSaupe

tjwalker:

Thank you for the explicit details. Much appreciated.

LSaupe

Hey all:

I checked the TPS tonight (both by direct resitance and also powered). It behaved as anticipated, very smooth and repeatable across the throttle position range.

I took it for another test drive tonight and when it was lugging the engine down I let off the throttle and it downshifted fine (previously I had done the opposite). Maybe that is a clue?

Also, I finally did a proper fuid level check. The transmission fluid was changed prior to my purchase so I assumed all was well.

The level is rather high. It is ABOVE the max line by about the same distance as the low line is from the max line (actually just about to the point of the double bend in the dipstick.

So.... obviously I need to deservice, but would being over serviced manifest its operation as I am observing? Does the good back shifting when letting off the throttle mean anything as well (clue)?

Any thoughts here appreciated.

Larry S.

Slick761

Hmm... Mine is also over by that much... Haven't gotten around to it and has been shifting great but let me know what u find out.

cruiser54

Sounds like your torque converter is staying engaged.

rklettke

I agree with this. Your problem sounds just like mine. I'm debating doing the manual switch for it. It's even worse with 31's though lol

DFlintstone

I've been learning from those guys^^, so you can blame my screw-ups on them. , . There are a bunch of wires that ground stuff like your TCU down there around your dipstick. In the earlier they are at the dipstick tube mount. Anyway pretty important grounds there. I just got mine shiny clean and put back with dielectric grease. Also the plugs to your tranny controls are there on the right side. Wiring connections turn up allot as the problem. Btw, got any codes? Does the speed sensor matter here?

I had thought if the TC was locked, it would stall/ not go into neutral when you come to a stop. Guess not...

I never quiet got that cable adjustment deal, or the "auto adjust" function on mine just doesn't work. I set it with about zero play in the cable and it seems to work fine. Hopefully someone will say something if this is really wrong.VVVVVVVVVVVVVVV....Just my .02, good luck!

LSaupe

Thanks for the additional feedback. I will definetly check those connections. Nope, no codes at all. I will get back to this on Monday. Hopefully I will find something.