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Trans seems to downshift often and early

3K views 25 replies 5 participants last post by  Kahoona360 
#1 ·
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/auto-transmission-down-shifts-cruising-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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#5 ·
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.
 
#7 ·
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.
 
#8 ·
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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#9 ·
Hi
Found a good article on the TV cable here

http://www.allpar.com/SUVs/jeep/cherokee-transmission.html

and a Youtube that showed how here

http://www.allpar.com/SUVs/jeep/cherokee-transmission.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.
 
#10 ·
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.
 
#11 ·
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.
 
#12 ·
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.
 
#15 ·
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.
 
#13 ·
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.
 
#14 ·
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.
 
#16 ·
Having reread this thread - I have not seen mention of SCANNING for codes, have you?
As mentioned the TPS is a major input for trans shifting, the MAP is also a input for shifting.

Also the vehicle (powertrain*) is STOCK?


*just tires are changed.
 
#17 ·
Having reread this thread - I have not seen mention of SCANNING for codes, have you?
As mentioned the TPS is a major input for trans shifting, the MAP is also a input for shifting.

Also the vehicle (powertrain*) is STOCK?

*just tires are changed.
Scanned when we got it and then recently. No codes.

No alteration that I am aware of.

The specs I see online for the TPS vary greatly depending on trans, year and poster. I was thinking about replacing the TPS for the heck of it. At this point I don't know what else there is to do aside from monkeying around with the TV cable in tiny amounts.
 
#18 ·
CJ7-Tim has some concerns about the TPS - that would be reason enough for me to change it, finding specs, time to test, vs just R&R.

That and maybe loosening up the TV cable a little seams like a plan.
 
#19 ·
I looked up the TPS test specs in my 2000 Factory Service Manual that has genuine Jeep Mechanic coffee and oil stains and it only has a slight variance of the WOT reading:

approximate range of .26 volts at minimum throttle opening (idle), to 4.49 volts at wide open throttle.
 
#21 ·
Where to start?? Nothing is simple. The XJ seems to shift much better now. I adjusted the TV cable while I was waiting for the new TPS and that helped. Changing the TPS helped also. On the way home I was able to get a little bit of acceleration without the trans kicking down but it did kick down when needed. It also shifted properly when floored and run up to 75. I am happy with that. Now here is something that cropped up a couple of days ago. The CL Light came on when I changed the TPS. My OBDII reader did not see any fault nor did it erase because it did not see a fault. I went to parts store and used their reader which said that three faults were stored in memory. There was the large evap system leak that I thought I fixed and cleared last week. A P0443 Purge control valve and a P0445 TPS high output. I successfully cleared them and they did not return so far. Could these be left over from the earlier troubles. If they do not return I'll be set. Looks like I need a better scanner.
 
#22 ·
Are you sure about that P0445 code?
IIRC P044x codes would be EVAP system.

I am willing to bet on CJ7-Tim hunch that TPS was the issue.
ANYWAY glad its shifting better, keep us posted on any new codes or issues.
 
#23 ·
Are you sure about that P0445 code?
IIRC P044x codes would be EVAP system.

I am willing to bet on CJ7-Tim hunch that TPS was the issue.
ANYWAY glad its shifting better, keep us posted on any new codes or issues.
My mistake. I had a Po445 and a P0443 both are for the evap system that I fixed a week or more ago and cleared. I think mu cheap reader might have not cleared them fully and that they were leftover.
The TPS High Output is a P0123 and it cleared with the better reader at the O'Rileys. I hope it is a leftover from the old one or possibly something I did while testing the old one and I won't even worry unless it comes up again.
I think I am good to go and very much appreciate the help. I am learning.
 
#24 ·
I always re-boot the PCM when I install a new OBD engine sensor. This is not required, but it does clear out any old and/or faulty data from a malfunctioning sensor.

This procedure will erase the 1996+ OBD-II "Adaptive Memory" stored inside the PCM and allow a new "Adaptive Memory" to be developed. After performing this procedure the PCM will re-learn and store into Adaptive Memory your engines performance characteristics. Perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn adaptive mode.

  • Disconnect the POSITIVE battery Terminal clamp and firmly touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).
  • Reconnect the Battery Cable
  • Turn Ignition Switch to the "ON" position but DO NOT start the engine
  • Turn Headlight "ON"
  • Turn Headlights "OFF"
  • Turn Ignition Key "OFF"
 
The PCM Adaptive memory has now been flashed, or erased from the PCM. When you start the engine it will be running off pre-programmed data tables that come with the PCM from the factory. When the engine gets up to operating temperature the PCM will start to collect and store new data for the Adaptive Memory.
 
#26 ·
Hi
here is the report:

Shifting is spot on as far as I am concerned. It kicks down if I want but does not do it too easily. The CEL has not returned so it was definitely an old stored code. We have done some long off road drives and a bunch of paved roads getting there and home and no problem at all. Thanks to all.
 
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