Driveline bump going downhill

Being that I am in Southern California, I spend alot of driving time in traffic. My regular route home from work consists of a long, downhill section of the freeway that consistently has stop and go traffic going down hill. I have been noticing a consistent phenomenon and was wondering if anyone knew what it was.

When i am completely stopped while descending, I will let go of the brake at 0mph and coast up to the car in front of me. As soon as the speedometer gets to 5 mph, i feel a slight engagement type bump at the rear axel or possibly the back of the tranny. It happens everytime. I have put the truck in neutral and did the same coasting and I do not feel the bump.

I am wondering if it is the transmission engaging itself since i do not have my foot on the gas, so it can regulate how fast the truck accelerates on its own, sensing that I am going down hill. Does it engage itself at 5 mph in order to have some engine braking to prevent me from barreling down the hill accidentally?

 

I don’t know the answer to all your questions, but try putting it in S1 to see if it still does it.But I would say it may sound reasonable that it would coast up to 5 mph and then have engine braking.

 

I can try it in S mode and see what happens.

Nobody else has noticed this?

 

I will try to pay closer attention next time I’m in a position like that. Re-reading your description, has me trying to relive the most recent time I was in a similar situation. I want to say I do recall some type of engagement when letting off the brake from a stop on a hill. I wonder if it could be the torque converter engaging when the RPM of the engine, and possibly wheel speed engaging a higher idle with the ECM.

 

Sounds logical, being that this truck seems to have alot of irregular, regular noises that are considered normal. Because of this it seems to me that the engagement bump is normal, but i am just curious as i have not really seen anyone mention anything about this specific situation on this forum.

 

If you dig into the deep depths of the internet, you'll find threads of people complaining about this.

My truck (even before supercharging) did this, and sometimes if it was cold or right after it was started warm, really will clunk after I let off the pedal (but not immediately after letting off). It's especially noticeable at low speeds, and when driving on hilly roads when I use the accelerator to get up to the top of a hill then let off to not end up speeding by the time I get to the bottom of the hill. The way it does it makes me think that the transmission is engaging a set of clutches within in it, which is causing engine braking to occur; if you look at the clutch power diagrams for the AB60 it's definitely possible that this is what is happening. Interestingly, my truck doesn't always do it though.

It could also be driveline slack in general, but who knows. I've come to just accept it as one of the "clunky" characteristics of these trucks.

EDIT: Actually, thinking about it, I was messing around with switching into "S" mode and if I remember, it doesn't really occur when in S mode which was telling me at the time it must be flex-lock of the torque converter.

 

I've noticed a similar thing on my truck. At the end of my neighborhood there's a light at the bottom of a hill.
Leaving the light to turn left onto the next road, its still slightly downhill, so when the light turns green you basically just let off the brake and coast through the turn before accelerating. Every single time I hit that light the transmission clunks when I get back on the gas after the turn. I've never checked, but it's probably right when I hit 5MPH. I just figured it was the TC engaging or something since it does it every time. Either way, I guess its normal since I have no other issues with the transmission. I may try to hit it in S1 next time and see what happens.

 

It sounds like you had the noise at regular acceleration is that correct? Or was it just on hills? I had a driveline clunk on my old Ranger which was the slip yoke, this happened often when the truck was moving and i would take my foot off the gas and back on while the truck was moving.

For the tundy, i never hear it except in the situation described. I think you are right about the torque converter, thinking about how the truck does engine braking at low speeds and how it feel like it doesnt help out much you get to a lower speed would seem to fit this scenario. I still need to try it in S mode and see what happens. I would imagine it would still do it.

 

I may need to clarify, maybe clunk is too strong of a word for whats happening to me. I guess its more of a very slight hesitation, like feeling like the engine is at that point "joining" in on the movement rolling forward. There is no sound per se, but i feel an engagement and a resistance of sorts.

In line with what you said, i get a clunk when i roll up to a stock sign that is on a decline, and when i do the california roll and step on the gas again at a slow roll, it will slightly clunk at this point, it is right around the 5mph mark too so maybe when i gas it at that low speed the tranny is kind of stuck between the 2 phases and it makes a noise.

I am used to manual transmissions which never had issues like this so the sensitivity of the auto is still something i am getting used to.

 

It happens pretty much everywhere, but hills will specifically exasperate the issue.

I don't actually hear any audible noises, but there are a few scenarios in which I get a bump in the driveline.

For example, let's say I'm driving in rolling hills: if I am cresting the top of one hill and let off, I feel a "clunk" as I let off the throttle and the truck starts to descend the hill. When I get to the bottom of the hill and start to ascend the next one, and start to give throttle input, I don't immediately feel a clunk, but a few seconds later do, if I pass a certain throttle pedal angle. Interestingly, the tone of the engine also changes which made me suspect the VVTI system in the truck, but to be honest, I would have to go back out to the hills and datalog a bunch of channels to really figure out what's going on.

I'm well aware of driveline slack caused by slip yokes/splined shafts/etc; I used to have a UZJ100 that was notorious for clunking like this, but in that case it could be fixed by lubing the driveline. Unfortunately, my 2019 truck doesn't have zerks on the driveshaft or yokes, and as such, I'm kind of dead in the water if that is actually my problem. Who knows, maybe I'll just upgrade my driveshaft one day

To be honest I think I'm dealing with two separate issues though; one is deceleration clunk caused by flex lockup of the torque converter (suspected) and the other is some other driveline slack. I guess the bump on acceleration could also be the torque converter clutch disengaging. I really should go out there and datalog this.

 

It sounds like 2 different scenarios like you said. I too need to try a couple things as well. maybe I will do my drive home today in S mode and see if i can feel any difference and report back. If it is the torque converter, i would imagine it would still do it.......

 

One important note is that "S" mode should disable flex lockup. This is anecdotal, but makes sense/lines up with my assumptions/guesses as to what's going on.

 

So i tried a few things on my drive home last night.

I put the truck in neutral from a dead stop and cruised to 5mph just to make sure it didnt bump, and it did not.

In S6 mode i started from a dead stop and rolled to 5mph and it did bump.

In S1 mode i started from a dead stop and rolled to 5mph and it did NOT bump.

In D mode, i can confirm that if you let the truck roll to 5mph from anything lower than 5mph BUT higher than 0mph, there was NOT a bump.

It sounds like when you roll from 0 the tranny sort of engages itself or preps the gear clutches or something.

 

I wonder if it's shifting....

 

you are thinking that if you start from 0 in D without applying gas when its rolling, the truck is essentially in N and the tranny engages at 5mph? It doesnt feel like a normal shift.

 

couple things-
first when was the last time you lubed the drivetrain? Lube all the zerks in the driveshafts, ujoints, etc. If they are dry or have not been lubed in a while you will definitely get "bump" feelings at low speed that can occur when starting or stopping. My Tundra has not done it buy my 4runner will if the drivetrain needs lube. Some Tundras dont have zerk fittings, other do, think it is the later years that went to sealed non serviceable components. It is worth a check because it can feel exactly as you are describing.

Next, the ABS will do a test at about 5mph that you may be able to feel and or hear, a bump or thump. Again I have not noticed it on my Tundra but it has been written about pretty frequently on this site.

 

I did the hill/light/left turn this morning that I described earlier. I did it in S1, but all other conditions the same. When I got on the gas there was NO bump or weirdness. I went and got coffee and then did it again on the way back in D just to compare, and I got the bump. Turned around and did it again in S1 and no bump. So there’s something in the difference of being in S mode or D mode. Im not smart enough to know what’s going on there but maybe those with more knowledge can theorize what’s happening inside the tranny.

Side note, it was fun driving around in S mode!

 

Good thought but truck is brand new with 7k miles and the joints dont have zerks since its a newer model. I initially suspected the same type of thing, being that my old ranger had a noise associated with the slip yoke on the driveshaft.

I know about the ABS test, it isnt that, usually the abs does it only 1 time when you first turn on the car and get to 6mph, this phenomenon seems to happen all the time in the conditions. The abs test has a completely different feel and sound to it, this issue i cant really say there is a noise, you just feel it.

 

Alright. Curiosity killed the cat and I went out with my OBDLink MX+ + OBDFusion hooked up to my truck.

My truck is shifting from 1st to 2nd gear at 5-6 MPH. This is the bump that you're feeling. This is why you don't feel it in S1.

I also figured out my decel/accel clunking that I described here: https://www.tundras.com/threads/driveline-bump-going-downhill.90253/#post-2272232 It's my torque converter locking/unlocking. When you let off the pedal (or be basically at the very, very beginning of the pedal stroke) the TQ converter unlocks, but any other time, it aggressively chases lock. That is to be expected considering my truck is a 2019, first year of the no-transmission-cooler.

 

Awesome work looking into that, though i am surprised to hear that it shifts that early. So i guess all the weird bumps and clunks are normal, not surprising for this truck.