Transmission Slams out of overdrive under certain conditions

Hi All,

I have a 2015 Tacoma DC v6 auto with about 30k miles on it. The only thing on the vehicle that is not stock is the tires, Goodyear Duratrac's 265/70R16, which I replaced the stock rugged fails at about 5k miles.

The problem I am experiencing is the truck occasionally slams out of overdrive when on the highway. It is only under certain conditions, such as high wind, or steep grades, but the truck will go to downshift and violently slam back into 4th gear. I am not towing or hauling when this occurs, and it is infrequent. I have brought the issue up to the dealer, and they could not find anything wrong, and suggested the tires may be at fault.

Has anyone else experienced this? I just want to ensure that this isn't a sign for issues down the road, and if it is, that it is resolved while the vehicle is under warranty still. Also, does it sound plausible that the tires are at fault? This seems like a stretch to me, but I am new to Toyotas and how the transmission calibration is done.

 

If it were my truck I would check that the transmission fluid level is correct first.

 

Ask them to explain how the tires are causing that. Post the dealer's answer here.



In the meantime, find a different dealer.

 

The autos in these trucks can shift hard. They're reliable, just a little rough around the edges. Nothing wrong with having it checked though.

 

The dealer may be assuming the tires are larger than stock. A larger diameter tire will rob engine power and result in more frequent downshifting. But Toyota offers that size as an option and while slightly wider, are the same diameter as the more commonly seen 245/75/16.

The fact that the truck is downshifting under those conditions is perfectly normal. Doing it violently does not seem normal. Are you sure it is going to 4th? It sounds like you are really getting on it and the transmission is dropping to 3rd in order to maintain speed. Under the right conditions that would be normal too.

 

Is this your first automatic transmission? It's quite normal for the trans to downshift on the highway under those conditions.

If you use cruise control on really hilly or windy highways, expect it to happen frequently since the cruise control can't "see" the road ahead. It just tries to maintain your set speed, but if you suddenly start going uphill at a severe angle, then the cruise control will tell the ECM to shift down and the revs will increase to get you more power.

This is largely why I stopped using the cruise control on hilly highways. Too many downshifts like this cause premature trans wear.

 

I had my 2006 out for the first time on a hilly drive up towards Donner summit in the Sierra and was surprised at how "roughly" it downshifted. I too at that point decided to take it out of cruise and do my own modulations. Works great on the flats.

 

Yeah, Check transmission fluid.....

 

I know what you meant, however tire size has no effect on engine power--it affects the overall efficiency of the engine's power being applied to move the vehicle, but as I'm sure you know the engine's actual output is unaffected.

One other issue would be where the OP lives and drives--you lose a bit over 7 HP for every 1000 feet above sea level:

HPcorrected = HPsealevel * [elevation in feet] * 0.03 * 0.001

My daughter found this out when she moved from here (12 feet above sea level) to Show Low, AZ. Her base model 2015 KIA Soul, never a powerhouse by any measure, became nearly undriveable at 6300 feet...

 

Check your transmission fluid!!!! When I changed mine out and ensured the correct level of fluid was in the transmission the harsh shifts went away. Shifts much smoother altogether. Mine was about a quart low!!