Automatic transmission...throttle too sensitive?

[hawairish]

Lately I've been taking a few longer highway trips with the truck, and my transmission has been keeping higher RPMs under certain uphill conditions that seem odd to me. I'm trying gauge whether my observations are normal.

 

For starters, this is the first AT I've owned personally. Always an MT guy. But over the years, I've also driven about a hundred or so various AT cars (business travels), so I'm no stranger to different vehicles and transmission types, but what I'm experiencing seems more reminiscent of a CVT, and I'm not okay with that.

 

So here's what's happening...

 

At 65-75 (speed limit here is 75), I can keep my RPMs between 2500 and 3000 on flat surface. When I start an incline and if I'm attempting to maintain speed (manually or with cruise control), it downshifts and revs to about 3500-4000, which is not entirely unexpected. The problem is that once the road starts to level off, it doesn't upshift and return to a lower RPM unless I completely back off the throttle or cancel cruise. Even then, I sometimes have to very very gently touch the throttle to avoid it spiking to a high RPM again. It seems too sensitive to some load, mechanical or throttle, but at that point, I can't see how there's additional load.

 

Most of the time, I am deliberately trying to keeping RPMs down by going 5-15 mph slower on inclines. I'm certainly not gunning it uphill. If I've got cruise control on, it just stays at a higher RPM for longer than I feel is necessary, so I've been canceling it. I feel I could make it up the hill at a lower speed and RPM, if it would just let me.

 

So I believe the problem is that I can't get it to just stay in overdrive (or return to OD when I think it should) at low throttle response. I don't know if that's a problem per se, or just normal. Everything else about the transmission feels excellent, though I honestly feel like the throttle is just too sensitive. Over the weekend while off-roading, I had a similar observation...while in 4L, it just seem to surge more than I expected. Perhaps it's just the shear brute of the VQ?

 

Any suggestions?

[jyeager]

You have described my truck since I have installed 33's. It's simply that the decreased effective gear ratio has increased the torque required to cruise at a given speed.

The transmission behaves according to throttle position and load (MAP sensor reading)

That's what you are experiencing. I assume your truck is lifted (increased aero drag) and has larger tires (decreased leverage).

[Kingman]

Mine holds 3rd for a long time as well before finally settling back down into overdrive. If you want it to instantly shift, flip the overdrive on/off switch real quick.

[hawairish]

You have described my truck since I have installed 33's. It's simply that the decreased effective gear ratio has increased the torque required to cruise at a given speed.

The transmission behaves according to throttle position and load (MAP sensor reading)

That's what you are experiencing. I assume your truck is lifted (increased aero drag) and has larger tires (decreased leverage).

 

Well, I have lift and larger tires, but I've also re-geared.

 

Stock is 255/65/16 (29") with 4.363, but I bought it with 245/75/16 (30.6") and 4.363. This is about a 5% increase in load over stock. Never drove it stock, but also never did any hilly highway driving, so no baseline.

Changed gears to 4.636 on the same tires, which is about a 1% decrease in load over stock. My experience is from a pair of trips to/from 7500 ft, as well as Saturday morning on lower-grade short climbs between 1K and 2K ft in elevation. This should have made slight grades a tiny bit easier at low RPMs.

On Saturday night, went to 265/75/16 (31.6"), which is about 2% increase in load over stock. Did the return drive from the morning, pretty much same experience.

 

So really, I'm not too far off from stock load, even with larger tires. But I think the throttle position and load is probably the best answer. But man, I was barely even touching the throttle...any less and I was off it. I have a wifi OBDII dongle and the DashCommand app, and I was watching both throttle percentage and load, and it just didn't seem reasonable for it to be revving that high. Plus, I'm not sure the load value corresponded with a mechanical load.

 

Mine holds 3rd for a long time as well before finally settling back down into overdrive. If you want it to instantly shift, flip the overdrive on/off switch real quick.

 

That's what I found myself doing, actually, but with varying results. Sometimes I could get it to drop down a little sooner than normal by switching OD, but for the most part still needed to completely let off the gas and coast a bit before giving it throttle again. But usually, this would still rev a little high (lower than 3500) before dropping back to cruising RPM. I'm sure there are a number of factors that play into this, though.

 

But it's already sounding like this is normal...

[Kingman]

The first drop is overdrive, the second is torque converter lock up.

 

Interesting yours doesn't shift the moment you flip the switch like mine does. Ever since I've had this rig I've thought it holds 3rd too long on a downshift but then I was thinking to myself just how much I hate autos that dive right back into overdrive which forces you to mat the gas again then repeat. So I'll take it.

[Karmann]

You and I have very similar trucks, even in terms of mods, and the situation you're describing seems to be 100% normal. I've taken mine over the grapevine a million times, done several cross country trips, and have taken it up to high elevation several times, so I'd like to say I'm familiar with how the transmission behaves, and I've noticed that trait every time. My trick is usually just to let off the gas when going back down hill for a few seconds, and it'll eventually settle into OD. Or if you manually switch it out of OD when climbing the hill, and then turn OD back on, it seems to swap gears marginally sooner. But I think you already know both those things.

 

You might be due for a throttle body cleaning, I know I was when I took it all apart. Afterwards it does seem to have less jumpy throttle response, which is a good thing around town and on the trail. Other than that, you might want to get yourself a scan gauge or something to otherwise monitor the tranny temps, and check the fluid frequently of course, but mine hasn't grenaded and it's been doing that shifting pattern for the past 65k. I think it's just a programming issue, I really wish they came with a 5th gear, or lower overdrive just to conserve fuel and quiet it down some, but it's 13 years too late now.

Edited September 30, 2015 by Karmann

[hawairish]

The first drop is overdrive, the second is torque converter lock up.

 

Interesting yours doesn't shift the moment you flip the switch like mine does. Ever since I've had this rig I've thought it holds 3rd too long on a downshift but then I was thinking to myself just how much I hate autos that dive right back into overdrive which forces you to mat the gas again then repeat. So I'll take it.

 

Yeah, there's definitely a delay on mine no matter how I spin it, except if I'm off the throttle.

 

I don't mind having the power/efficiency when I need it, but once the objective is met (getting up the hill, or getting back to cruising speed), that's when I want it to subside. Any other AT I've driven has been fine in this regard...but perhaps it's a 4-speed vs 6-speed transmission thing.

[hawairish]

You and I have very similar trucks, even in terms of mods, and the situation you're describing seems to be 100% normal. I've taken mine over the grapevine a million times, done several cross country trips, and have taken it up to high elevation several times, so I'd like to say I'm familiar with how the transmission behaves, and I've noticed that trait every time. My trick is usually just to let off the gas when going back down hill for a few seconds, and it'll eventually settle into OD. Or if you manually switch it out of OD when climbing the hill, and then turn OD back on, it seems to swap gears marginally sooner. But I think you already know both those things.

 

You might be due for a throttle body cleaning, I know I was when I took it all apart. Afterwards it does seem to have less jumpy throttle response, which is a good thing around town and on the trail. Other than that, you might want to get yourself a scan gauge or something to otherwise monitor the tranny temps, and check the fluid frequently of course, but mine hasn't grenaded and it's been doing that shifting pattern for the past 65k. I think it's just a programming issue, I really wish they came with a 5th gear, or lower overdrive just to conserve fuel and quiet it down some, but it's 13 years too late now.

 

That's what I found odd...there were times when starting to go downhill (say, right after peaking on an uphill climb), that the RPMs would still stay high, even at low throttle. It's as if just by maintaining some throttle (very little, albeit), it just didn't want to shift.

 

I actually cleaned my TB a few months ago when having some idling issues, as well as it stalling once the engine was warmed up. I got all that sorted, though.

 

I do have a scanner setup (described previously) that I usually keep up and running when driving on longer trips. In fact, the first 7500 ft elevation climb the other month was really the first extended drive I've taken in the truck, so I had general concerns about the trip and was watching the readouts constantly. All seemed fine, though.

 

Mine also didn't come with a cooler, so I've been thinking about adding one for safe measure, at minimum to offset whatever extended use the higher RPMs have been doing to it.

[greenda3]

Mone also has a delayed shift. From2-3 that i have to back off the throttle to make the shift. Is that galling into this same thought? Mine did it stock and now on 33s its not as bad. My AT knowledge is minimal at best.

 

Sent from my LGMS395 using Tapatalk

[jyeager]

I don't know all the factors that the engineers used in deciding the shift profile, but I always assumed that it was holding the downshift so long because that helps to minimize the number of up and downshifts when you hit a hill and then another one in quick succession.

There have been a number of tactics by engineers to increase the lifespan of transmissions and this may be one of them.

 

I hate it because I live in rolling hill country and on the interstate I can't leave overdrive on unless I want the car downshifting constantly. I have to just hold a certain throttle position and then it won't downshift in the first place (but I have to accept the truck slowing).

This became noticeably annoying when I went from 31s to 33s and lifted 4". But I haven't regeared.

 

I see my solution to this as decreasing my final drive ratio through new gearing.

It might also help if I can get the computer to see less load on the engine so it doesn't downshift. That just means keeping it clean and tuned unless/until I dig in to the engine and can give it a new cam and reworked heads. Bumping up the power should mean that the MAP reading stays better under moderate loads.

 

[hawairish]

I don't know all the factors that the engineers used in deciding the shift profile, but I always assumed that it was holding the downshift so long because that helps to minimize the number of up and downshifts when you hit a hill and then another one in quick succession.

There have been a number of tactics by engineers to increase the lifespan of transmissions and this may be one of them.

 

I would think that prolonged use at higher RPMs would be detrimental to the transmission's life span, as well as the engine's. That's why I'd prefer it to stay at a lower RPM (or rather, return to a lower RPM ASAP), but there are also mileage benefits. I just stays locked too long for my liking, but I accept that at it sometimes needs to do it in the first place.

 

With an MT, it was always easy to determine whether a downshift was necessary to climb. At some point, you could upshift, maintain speed, and get a low RPM. That's what I want to replicate. I'd rather struggle a little going uphill (or at least going up slower) than having the engine race. But in order to do that, I would need it to stay in OD I would think. This all gets me thinking about an MT swap!

[Kingman]

Load has more to do with it wear and tear than RPM.

[hawairish]

I agree, for the most part. For my MT scenario, if the increased load at low RPM is causing the clutch to slip or the engine to work too hard (truck is slowing down with more throttle), then absolutely. But that goes hand-in-hand with knowing when to downshift.

 

But for the AT scenario, we're replacing load with heat, which can't be good for wear and tear either, even if mechanically the systems are running more efficiently from the downshift. In my case, I've seen the coolant temps jump 20° in a matter of seconds, and some inclines have taken over a minute to pass. Revving at 3500-4000 RPMs for a prolonged period can't be good.

 

Again, sounds like this is all normal, and that's what I wanted to be sure of. No bad shift solenoids, controller, etc. I don't have any surging or clunking in gears, so the transmission seems great otherwise. I just have concerns that this may not be best for the transmission, and I've not really ever experienced it before on any other vehicle. I'd feel a whole lot better if it hovered closer to 3000 RPM on inclines and not 3500-4000.

[01Pathmaker]

One thing you guys might be overlooking is the increased rolling weight of the larger (and possibly higher load range, ie. thicker carcass) tires. This factor increases the actual load on the drivetrain more than everyone thinks, it's not just a difference in size.

[pattersin]

Oh man, i've been plagued by this for 2 years now. I was hoping for a solution but at least i can find solace in that it's due to my tires and not something major. Toggling the OD button works about half the time for me, the rest of the time I'm playing with the throttle. I also find that it shifts hard into second and kinda lags into third. It makes a lot of sense that the extra weight and resistance of the tires would alter the shift points though.

[NovaPath]

Mine does this as well. It seemed to start when I ran the larger tires, as I don't remember my first couple road trips having this issue. I was concerned it was a sticking solenoid or early sign of failure, releived to hear that everyone else's is doing the same thing. I can also confuse mine and make it seemingly(it isn't really, it's just confused) slip in second gear by accelerating hard, letting off, and then getting on it mid way as it is trying to go to third. It really seems to hate that.... Mine has 150k on it, not sure how long these things typically last?

 

That being said, it is bloody obnoxious to drive when it does it. I really wish there was something easy to adjust to fix it.

[jyeager]

Mine does this as well. It seemed to start when I ran the larger tires, as I don't remember my first couple road trips having this issue. I was concerned it was a sticking solenoid or early sign of failure, releived to hear that everyone else's is doing the same thing. I can also confuse mine and make it seemingly(it isn't really, it's just confused) slip in second gear by accelerating hard, letting off, and then getting on it mid way as it is trying to go to third. It really seems to hate that.... Mine has 150k on it, not sure how long these things typically last?

 

That being said, it is bloody obnoxious to drive when it does it. I really wish there was something easy to adjust to fix it.

 

Don't know what the typical transmission lifespan is, but mine has 330,000 miles without any problems so far.

[hawairish]

So after taking a long road trip last week, I think I officially hate our transmission. It's way too sensitive under load. By load, I mean that the ECU calculates load as a percentage (observed using a OBDII adapter and iOS app while driving) based on vehicle speed and throttle position, and surely other factors. But even under very minimal throttle, enough headwind caused loads to peak, shooting revs up to 4K+ at times...and staying there. If it'd had just stayed in a higher gear, I have no doubt it would've been far better off at 3K rpm than 4K.

 

Granted, I had severe headwinds at times, constantly around 20mph (25-40mph gusts), but I also had a lot of problems getting it to downshift appropriately when wind was less of the factor. I feel the transmission is still very healthy, but the TCM (trans computer module) must really be in a safe mode of some sort. Major performance robber...I only got about 10 mpg over about 900 miles.

 

Starting to wonder what our options might be for getting transmission with more speeds from maybe an FX35. Would be awesome to know if they bolted up.

[bushnut]

keep in mind that the peak output for the 3.3 is 170hp@ 4800rpm (according to edmunds.com) so maybe higher RPMs might be a good thing.

I recently got an "Ultraguage" and while watching over the past few weeks. HP vs Torque vs Speed/RPM vs % engine load this engine normally chugs along @ 55KPH @2200ish rpm with aprox 120fps of torque and 68hp @ 79% of peek engine load.

I'll be posting a video of this soon. while I understand that this is a calculated hp/torque/engine load. I was very surprised at how few hp this engine makes at average (normal) RPM. According to Expedition Portal, the new Range Rover diesel at almost the same displacement produces roughly twice as much Hp and torque at almost half the RPM.

I also understand that my "UltraGuage" just might be out to lunch.

[TowndawgR50]

I have the same experience and it is obnoxiously noticeable when towing. Utilizing the draft of a semi or fellow hauler while my truck is pulling our 18ft trailer it performs great. Anything other than substantial grades it holds overdrive with ease when drafting. As soon as I loose the draft it wont hold overdrive without some footwork. Takes a consistent manipulation of throttle to maintain low RPMs.

[hawairish]

Good thinking, but torque is what matters here. For the 3.5L, max torque is 265 ft-lbs at 3200 RPM. But I'm sure this isn't a factor in the shift logic by the TCM, instead all the load calculations.

 

I had actually started checking the FSM to see if the TCM could be reset, for whatever it might be worth. Didn't see anything along those lines.