Trans seems to downshift often and early - JeepForum.com
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post #1 of 26 Old 06-04-2021, 05:15 PM Thread Starter
Kahoona360
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Trans seems to downshift often and early

HI
I have a 2001 Cherokee Sport, 6 cyl with 4 speed auto with 130,000 miles.

When driving at 55 or over it downshifts when there is the smallest hill or if I press the pedal even slightly. Given the torque this engine has I would not expect it to need to downshift very often at all. I have to concentrate on careful throttle application to keep it under control. It shifts back up correctly. It is not the torque converter coming out of lock.

The only things that have been changed are Tires that are about 2" taller and I did add one pint of Lucas Trans additive which I have always done with any older vehicle I buy. I did use the recommended Dextron III fluid. I changed out the dash cluster to a full instrument one. Otherwise the engine and trans are flawless. There are no other symptoms.

I did read this
https://www.jeepforum.com/forum/f11/...speed-1421686/

Before I start checking these things I am wondering if any of the things I changed could have caused it. If so can I compensate somewhere and if not which of the possible things are most likely?

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post #2 of 26 Old 06-04-2021, 05:34 PM
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Taller tires will cause this. The axles are geared for stock diameter tires. When you change tire diameter, you need to re-gear the axles if you want to keep the engine/transmission operating like stock.

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post #3 of 26 Old 06-05-2021, 02:48 AM
jtec
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I'm thinking maybe try adjusting your transmission TV cable.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #4 of 26 Old 06-05-2021, 06:56 AM
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Quote:
Originally Posted by jtec View Post
I'm thinking maybe try adjusting your transmission TV cable.
Could you please explain to the audience why this should be done?

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post #5 of 26 Old 06-05-2021, 08:23 AM
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The cable can stretch, and the XJ Cherokee throttle valve cable adjustment is a simple 1 minute job.




The root cause of the transmission behavior likely is larger tires and stock gearing. This is assuming the Throttle Position Sensor is genuine Jeep part, and is functioning within the correct tolerances.


TPS TEST

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 Transmission Control Unit also uses TPS data to shift the transmission.

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 .25 volts at minimum throttle opening (idle), to 4.8 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.

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post #6 of 26 Old 06-06-2021, 08:52 AM Thread Starter
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Thanks. I'll check that out today and report back.
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post #7 of 26 Old 06-06-2021, 08:56 AM
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ravenworks to answer your question as CJ7-TIM mentions a 2minute easy check.


I don't make this S..t up, kinda matches OP issue - from FSM-
"If cable setting is too loose, early shifts and slippage between shifts may occur. If the setting is too tight, shifts may be delayed and part throttle downshifts may be very sensitive."

Thinking was a quick easy no cost check would be a good mention.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #8 of 26 Old 06-07-2021, 09:23 AM Thread Starter
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Quote:
Originally Posted by jtec View Post
ravenworks to answer your question as CJ7-TIM mentions a 2minute easy check.


I don't make this S..t up, kinda matches OP issue - from FSM-
"If cable setting is too loose, early shifts and slippage between shifts may occur. If the setting is too tight, shifts may be delayed and part throttle downshifts may be very sensitive."

Thinking was a quick easy no cost check would be a good mention.

LOL Thanks. I did hear something about a cable having adjustments for that now that you mention it. I will check that out too.



We are holed up in Tombstone Arizona to get some mechanical stuff done on the camper and the Jeep. Hanging out with the cowboys. This doesn't effect dirt performance at all so we hope to get some good trail time here.
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post #9 of 26 Old 06-07-2021, 06:29 PM Thread Starter
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Hi
Found a good article on the TV cable here


http://www.allpar.com/SUVs/jeep/cher...nsmission.html



and a Youtube that showed how here



http://www.allpar.com/SUVs/jeep/cher...nsmission.html


just in case the next guy needs to see it. It was easy to do, I wondered what that strange thing was on top of the engine. Took it for a test drive and found a new dirt road I didn't know about too. I can't tell. On small curvy roads with ups and downs it is perfect but then it was before. It is on the wider roads where I am going places and have to hold a steady speed over 60 or so that it shifts up and down. I will have to report back after I need to make another trip. Meanwhile I will check the TPS as soon as I try the new road and take the other half shopping. I will report back. Thanks for all of the help.
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post #10 of 26 Old 06-19-2021, 11:25 AM Thread Starter
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No difference with the TV cable adjustment done. I am sorry to keep you waiting but now there are other problems to fix and the daily temps here are running around 106 so I am slow to get stuff done. Who knew retirement would be so much work. I will try to get the TPS check today. It is only on rolling hills at 55 and over but that is what there is often. On the other hand se did get it off road on some steep twisty switchbacks and it was a lot of fun. That 6 pulls great.
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post #11 of 26 Old 06-19-2021, 01:17 PM
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At that speed, with oversized tires, I would fully expect it to be hunting for the right gear.
Overdrive in that year was intended and engineered to cruise at 55-65 with the highest fuel economy (aka, lowest power) it could get...on stock size tires.
By running 31s it simply doesn't have the power to stay at that speed in overdrive, so it downshifts. If you want to fix the issue, it's going to take different gears in the axles.

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post #12 of 26 Old 06-19-2021, 03:59 PM Thread Starter
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I tested the TPS and it reads .9v at closed throttle and 3.6v at WOT. I understand this is within normal range. The voltage rises and falls very smoothly and is not affected by tapping. Seems I am OK there.



Should a 235-76-15 tire really make much difference? Hardly any difference in driving except that annoying tendency to shift down.



What about moving the TV cable just a little in either direction?



If not I'll just have to get used to treading lightly and keeping it in 3rd sometimes.
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post #13 of 26 Old 06-19-2021, 04:36 PM
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Well, you indicated in your original post that the tires were 2" taller than stock (that would be about 31"s), which would make a big difference.
But 235/75/15s are not even close to 2" taller than stock. That's very near stock size, only about 10mm wider and 7.5mm taller depending on the trim package.
So no, those should not be an issue here.

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post #14 of 26 Old 06-19-2021, 05:10 PM Thread Starter
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Quote:
Originally Posted by Kruzin View Post
Well, you indicated in your original post that the tires were 2" taller than stock (that would be about 31"s), which would make a big difference.
But 235/75/15s are not even close to 2" taller than stock. That's very near stock size, only about 10mm wider and 7.5mm taller depending on the trim package.
So no, those should not be an issue here.

Nothing is ever simple. Makes me nuts. I went to our regular independent tire dealer and showed her the extremely skinny and small tires that were on it when we bought it. I asked her to give me a quote for some tires that were about 2" taller (to raise the car about an inch) since the clearance was not great at the diffs. I chose one and they put them on for us. They were about 2" taller and much bigger and heavier.

The sport came with 215/75R15 that year but who knows what was put on since. With what you are telling me then I don't have a problem from bigger tires since it was geared for a 215/75R15. Thank you. Now all I need to figure out is why it drops back into third easily. Or live with it. I wonder if that hurts the trans.
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post #15 of 26 Old 06-19-2021, 05:39 PM
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Quote:
Originally Posted by Kahoona360 View Post
I tested the TPS and it reads .9v at closed throttle and 3.6v at WOT.


That TPS is out of specification. You should be measuring closer to .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle. The throttle position sensor is DIRECTLY involved with transmission shifting characteristics.

I have 31x10.5x15 on my 2000 with stock 3.55 gears and the transmission only downshifts oddly on steep mountain roads. The quick and dirty solution is to shift into 3rd.

Cheap crappy Chinese "Lifetime Warranty" parts such as the TPS 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.

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 .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

“We the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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