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Transmission / Drive Train Severe "Jolt" Just Before Downshifting

jspartan

NAXJA Forum User
Location
Nomad
99 XJ - 4.0L - AW4 - NP242 - 175k Miles
4" Lift, 33" Tires
Trans Fluid & Filter Changed 10k Miles ago
Owned as a DD for 18 months.

Thanks in advance for any help. Haven't had any real problems until about 2 weeks ago. The issue started very slowly and is now constant. Hope I can describe it well enough to give you an idea.

When going up a hill with the transmission in "D", moving along at highway speeds there will be an almost violent jolt felt through the entire vehicle. The jolt happens as you are increasing throttle to maintain speed going up a hill. As if it is activated by the transmission attempting to drop down into the lower gear, but it does not drop into the lower gear. If you anticipate the jolt and decrease throttle it will not happen. However you are now not maintaining speed. If you give it a strong increase in throttle it will shift down just like it should have automatically.

This started out of the blue about 2 weeks ago. It has progressed to now doing it whenever the above conditions exist (all the time). But now not just in "D". If you keep the trans in "3" it will do the same in the above conditions just not as violent a jolt.

Again if you can anticipate the jolt you can ease off the throttle and stay in the present gear or give it more throttle than necessary and it will successfully shift down. Also you can shift down manually with the gear shift and it works fine.

Replaced TPS yesterday did not help. Adjusted TV cable no difference.

Thanks again for any help.
 
You are using Dexron-III/Mercon in the AW-4, and the fluid is fresh and clean ? I strongly suggest you spend $7.95 for a Service Manual at www.pacificcoastmanuals.com and then follow the transmission diagnostics section.
 
I would suspect a faulty Transmission Control Module (TCM) before a faulty Toque Converter, however either one is a logical possibility. While I do not recommend throwing random parts at an undiagnosed problem, around here a used TCM is $15 and worth a try if the diagnostics suggest a TCM issue.

I do strongly recommend using only genuine Jeep engine sensors and NTK O2 sensors.
 
Are you sure this is not a misfire you're feeling?

At this point I'm not ruling anything out. Maybe I don't know what a misfire is but the pattern is it only happens in either 3rd or 4th when it is about to shift down. To me a misfire is when one or more cylinders stop producing power.

The jolt I'm feeling is more like all cylinders decided to make the engine turn backwards for a split second. It's harsh man.
 
Worn out ujoints?
 
I would suspect a faulty Transmission Control Module (TCM) before a faulty Toque Converter, however either one is a logical possibility. While I do not recommend throwing random parts at an undiagnosed problem, around here a used TCM is $15 and worth a try if the diagnostics suggest a TCM issue.

I do strongly recommend using only genuine Jeep engine sensors and NTK O2 sensors.

A bad TCM is fairly rare, but not totally unheard of. Does it feel like it's trying to shift without unlocking the t/c first? Possible the t/c lockup solenoid is sticking? Does the t/c reliably unlock and the rpm jump up a few hundred rpm if you tap the brake with the left foot while keeping the throttle even with the right foot?
 
Great idea on the brake trick. Will try tomorrow morning. After a lot of reading I was leaning towards attempting to test the TC solenoid. I think I saw where it is solenoid #3. No idea where that is. I will try and found out the test procedure for that solenoid.

Thanks.
 
Great idea on the brake trick. Will try tomorrow morning. After a lot of reading I was leaning towards attempting to test the TC solenoid. I think I saw where it is solenoid #3. No idea where that is. I will try and found out the test procedure for that solenoid.

Thanks.

The solenoids are accessible by dropping the pan and the filter/screen. From the trans computer connector or at the wiring harness connector at the firewall, you can measure the resistance, which should be 11-18 ohms as I recall. Typically though if they're failing electrically, they don't activate properly which would leave the t/c unlocked (or not shifting right if it's #1 or #2).
 
More information. This morning I tapped the brakes. The rpms did go up as it should. However I think the following may be helpful. Anytime the jolt was happening I could tap the brakes and give it slightly more throttle and no more jolts. Or I could anticipate the jolt and before it happened, tap the brakes add slightly more throttle and avoid the jolt altogether.

Not sure what that means but it sounds like it could be an important clue?

Thanks for the info on the solenoid. Will try and address this weekend. Question. Is there any other troubleshooting I could do before dropping the pan and checking the solenoids. Like a diagnostic on the TCM, TCU, ABC or anything else?

Also shouldn't I be seeing a check engine light? I don't. Would hooking the XJ up to a scanner maybe show any codes even though there is no CEL?

Thanks again!
 
The jolt I'm feeling is more like all cylinders decided to make the engine turn backwards for a split second.



A slight engine misfire is also a good suspect. Test the Throttle Position Sensor, and confirm it is a genuine Jeep part. The TPS has direct data input to the ECU and TCM for shifting the AW-4. Testing the CPS is also a good idea.

There may be pending OBD Trouble Codes that can help focus the diagnostics.
 
I would also check your slip-yoke and u-joints, I've seen that cause what your describing.
 
I think I found how to test the TPS. I will do that first since it seems fairly quick and easy. Even though it was replaced a few days ago it was from Advance Auto and not OEM. Then I will see if I can figure out how to test the CPS, ECU, TCM if still necessary. I will buy the manual suggested if I hit a wall. Saving the solenoids inside the trans for last.

A few months ago I inspected the drive train. I didn't notice any slop in the u joints etc. but there could be now. I don't think the pattern fits them being the root cause but if there is too much play there I could see where it could make the situation worse.

Thanks everyone for your help and suggestions. Once the issue is found and resolved I will write up what happened for the next guy.
 
Saving the solenoids inside the trans for last.

You can measure their resistance from either the TCM connector, or the wiring harness connector up against the firewall.

Not sure how you'd test the TCM, aside from maybe getting a probe on the t/c solenoid wire and verifying that it dropped power to the solenoid when it was supposed to. Or swapping it out. Most of the electronic failures would result in it not locking up.
 
I tested the TPS using the chart / procedure described here . . .

https://www.cherokeeforum.com/f2/throttle-position-sensor-219877/

311677d1501220757-throttle-position-sensor-tps-20tests-20final.jpg


I had two exceptions;
a) The TPS signal that should have been > 0.26 volts but < 0.95 volts was 0.973 volts. While this only 0.023 volts too much. It is still a significant percent of such a small amount of voltage. So does this mean the new TPS sensor is bad?

The other was a resistance test from this thread . . .
http://www.jeepforum.com/forum/f11/how-test-tps-610386/
SNIP >>>>
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.

<<<< SNIP

b) My reading was not infinite but 1.054 on the 2K scale. Which if I'm reading that correctly is a very very HIGH resistance?

All other tests described above passed. So I should go to the dealer for the OEM TPS?
 
If the TPS shows .973 it is out of specification and should be replaced with a genuine Jeep TPS.


The resistance test is for the Crankshaft Position Sensor.



TPS TESTING

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 little to 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.
 
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The Solution to my Issue

I would like to thank everyone who took the time to offer suggestions and advice. With the limited information I gave, all of your responses were valid and I appreciate your input. Special mention to @trippled and @Tim_MN. I checked many of the things suggested by everyone and followed the rabbit trail down to a few more.

The biggest problem ended up being my interpretation and description of what I thought was happening. To my defense it originally started with only the down shifts but the problem was not the transmission. The issue ended not having anything to do with the drive train. Not sure why it initially started happening only as the transmission was about to down shift. The jolt was because of the engine cutting out for a split second then running again while the Jeep was under strain going up a hill. I had no idea that it would be that violent.

To everyone who suggested u joints and other drive train wear points I'm going to recheck my entire drive line for excessive play again. Maybe the reason it was so severe when the engine cutout was because of too much play.

It progressed from being only on the down shift to also a rough idle and the engine missing under acceleration. Then to missing in almost any condition.

I checked the TPS and it was so slightly out of spec that I replaced it anyway. It made no difference. Today I was running diagnostics on the crank position sensor and it passed every one. As an aside there are thoughts that the resistance check only between two of the wires is not conclusive.

Went to look at the cam position sensor under the distributor cap and saw a few problems. Although the spark plug wires looked in good conditions from the outside on the inside the metal connections looked bad and two wires broke from the metal attachment to the plugs. The inside of the distributor and the rotor condition was, well I don't understand how it ran at all.

I replaced the plug wires, distributor cap and rotor. It now runs great.

What I learned.
I have a better understanding of testing sensors. While frustrating not finding the problem it was also nice to check off what wasn't the problem.

Check the basics first. There was a time I would have automatically checked the simple things first. A partial basic tuneup ended up being my answer without even replacing the plugs.

Thanks again everyone.
 
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