Excessive DownShifting

[SnowBuck]

2005 Yukon XL 170K miles
Recently noticed on the slighted grade, it would down shift. On a flat highway, pressing the pedal at all, like to pass, it would down shift ?!? Would a tranny service help ? Is it from a lack of power? Is it just old age?

 

[B-train]

My guess is there is another issue causing the engine to sense higher load. On electrically controlled transmissions, there are inputs from throttle position and other engine load parameters that will tell it, or ask it, for a downshift.

Potential causes:
1. Dirty air filter = more throttle input to maintain desired power.
2. Coked up throttle body = restricted airflow at low throttle opening
3. Crappy fuel
4. Worn spark plugs - incomplete burning of fuel.
5. Throttle pedal that is getting bad and has voltage inconsistencies

Check the color of the ATF. It should be bright red. What has been the maintenance history of this truck? Share what has been done to this point with 170k and that will help as well.

 

[strutaeng]

What @B-train suggested.

In addition, check the tire pressures. You can also remove the belt and spin every accessory by hand. My 99 Silverado had a noisy tensioner. Fired it up and after locating the culprit, the tensioner wheel was very stiff and hot.

 

[Marky Dissod]

Clean your MAF sensor.

 

[Joseph Garcia]

@NickTransmissions

 

[NickTransmissions]

2005 Yukon XL 170K miles
Recently noticed on the slighted grade, it would down shift. On a flat highway, pressing the pedal at all, like to pass, it would down shift ?!? Would a tranny service help ? Is it from a lack of power? Is it just old age?

Does this happen at all vehicle speeds and in all gears (ie it will downshift with the smallest amount of marginal throttle input while in 4th, 3rd or 2nd) or is it just happening at one speed range or in one gear?

A priori, sounds like you have a C3 problem (communication, command and control) between the PCM and transmission...The above replies have covered most of the bases but I'll add throttle position sensor and vehicle speed sensor (check your speedo to make sure it's reading accurately) to the list. You can drop the pan to continuity test the solenoids with a DVOM but based on what information you provided, they are likely not the source of your issues.