Driveline Questions

[Malleus]

FWIW, that's why you can't park an 80 on a hill and expect it to stay there without using the parking brake. The only thing holding it still it the friction in the clutches and they will let go sooner or later. This is a well known phenomenon; it's why even though passenger cars, and even large rolling stock like train locomotives, have four wheel disc brakes, they use drum brakes as parking brakes. No matter how much force you exert on a clutch plate, it will eventually, and unpredictably, fail to grab at some point in the future.

 

[ThreeEyedBandit]

The clutches are not applied unless there is hydraulic pressure. What you are describing is a trans brake, used usually in drag racing.

Nope. The pawl only serves to lock the transmission into position; it doesn't prevent the output shaft from rotating.

And to be clear, there isn't really a "park" gear, it's 1st and reverse engaged simultaneously.

What do you mean the pawl only serves to lock the transmission into position?

Your saying that park brake pawl doesn't engage with the speed sensor rotor to hold the output shaft? This is definitely how the transmission locks the output shaft, hence the ratchet noise to a solid stop when rolling slightly.

 

[Malleus]

I'm sorry, but it doesn't work this way. The pawl engages with the lower (rearmost) end of the rear planetary gear unit, which has the 1st/Reverse clutches mounted on it. The speed sensor rotor is in the extension housing, outside of the transmission case. (The photos below are from a 2000 year model 100 series A343F [actually an LX470], but it's more or less the same inside as the 80 series unit)

Maybe you're thinking of another transmission?

Here's the pawl, the blocky looking part, in the lower end of this photo. You can see the "teeth" on the reverse drum just below it. This is what the pawl engages, however, the drum isn't splined to the shaft; the connection to the shaft is through the clutch pack. This is why, even though the pawl is engaged with the drum while the selector lever is in "P", the shaft can still rotate.

You can see the opposite end of the manual lever in the top of the photo below; this is what is directly connected to the shift lever, and the NSS. The clicking you hear when you shift is the detent mechanism in the shifter, not the transmission.

The speed sensor, in the extension housing, is the black plastic part, oriented at a 45° angle upwards to the left. It "sees" the rotor, below.

Here's a photo of the speed sensor rotor, on the output shaft. The lower end is in the transfer case, the relieved area above and below the rotor is all in the extension housing, and everything from the sealing ring groove, just above the rotor, to the opposite end of the shaft is inside the case.

 

[ThreeEyedBandit]

Maybe you're thinking of another transmission?

Interesting, in an A442, the speed sensor rotor is also in the extension housing as is the park brake pawl. The rotor is splined directly to the output shaft.

The clicking you hear when you shift is the detent mechanism in the shifter, not the transmission.

The clicking/ratchet/buzz noise I'm talking about while moving is not the detent mechanism. It is the pawl skipping along the rotor as it is still spinning, this is by design.

In your pictures it looks like the brake pawl is engaging the 2nd brake drum between the 1st/Rev and the 2nd brake clutch packs?

 

[Malleus]

To be clear, this is the A442F parking lock mechanism:

Unlike the A343F, the A442F has a "parking gear", mounted on the output shaft in the extension housing. Although this "gear", actually a toothed wheel (it doesn't mate with anything to effect power transmission and doesn't have a tooth profile that would allow this anyway), is splined to the output shaft, the shaft itself is mounted similarly as the A343F output shaft, within the 1st/Reverse gear subassembly.
This is the (80 series) parking lock mechanism:

The engagement is essentially the same. I'm getting ready to rebuild an A442F next month. I'll post photos of it when I tear it down, for comparison.

I don't think there has been a complete A343F or A442F teardown/build thread. There is one for an A440F, but that's not really the same transmission.

 

[ThreeEyedBandit]

The parking gear(speed sensor rotor in the FSM <- just for clarity, same part inside A442) is splined to the output shaft, this is where the pawl is engaging the square teeth of the 'gear' holding the output shaft against the case when in park.

I've been inside a couple A442s and this is how the parking brake works in them.

 

[Malleus]

IMHO, these discussions are very good for the general population. Most people can describe how an engine works, and some can find almost any part on one, but very few people have even looked inside an automatic transmission case, let alone actually rebuilt one.

These contrast/comparison discussions about the variants used in our trucks are especially good, since they give an impression of the characteristics of the various generations of builds Toyota sold and allow people wanting to buy or maintain one to know more about what they're getting into beforehand.

I think the more you know about how the various systems in the truck actually work, the better owner you'll be.

 

[Idaho Savage]

Update to the thread:

I think what we’ve got here is the classic simultaneous issues causing confusing symptoms.

I had a minute today (literally) so I got under the rig and just started at stuff, as one does. After scootching around on my back, I craned my head and saw that one of the exhaust junctions where the Y pipe meets the manifold pipes was missing both bolts. I found a few 10mm fine bolts in the bolt bucket and tightened it down. No more noises or growl, just light vibration remains.

Not sure if they removed those bolts when they regeared my truck ( not sure why you would) but that was odd. Regardless, one problem solved.

Next step is to isolate the drive line in the front by reinstalling the front and removing the rear DL. I’ll report back.

 

[sbman]

I appreciate this thread and the discussion around the parking pawl action. As a result of this thread I did a pretty deep dive into the A343F and the A340 (lighter duty version, not the LC trans) and learned a lot. The A340 has a 'parking gear' that is splined directly to the shaft and that the parking pawl engages on. The A343F omits the parking gear and uses that same space for the 1st planetary assembly which has the 'teeth' that the parking pawl engages on. As @Malleus says, the planetary assembly is not splined directly to the output shaft, it is connected to it via various gears, so the trans is free to turn unless other systems hold it in place. I assume the A343F omits the parking gear to make more room for heavier duty internals in it's stack-up.

I had no idea that the A343F was so different in this respect and I'll respect the nature of this design in the future for sure... I had no effective parking brake on my cruiser for many years and only recently did I get it functional (broken brackets, cable issues, etc....). I park on steep hills regularly and find this info to be pretty valuable.

Thanks @Malleus for providing good info on this design.

 

[Idaho Savage]

So now I have to tighten my parking brake, too? Sweet.

 

[COS80]

So now I have to rebuild my parking brake, too? Sweet.

FIFY