A343F reverse noise

[uscmas412]

Another bump. Same issue as OP and others, grinding noise when shifting into reverse on an incline. Noise is momentary and then reverse works fine. My engine is being pulled soon so it's a good time to address this transmission issue.

Has anyone ignored this problem and then had a complete transmission failure?
Any updated fixes?

 

[lumbee1]

U-joints?

I had a similar issue when reversing. It sounded like a cat's meow. I pulled my rear driveshaft and one of the caps had rusty dust where the needle bearings used to be.

 

[iptman]

Another bump. Same issue as OP and others, grinding noise when shifting into reverse on an incline. Noise is momentary and then reverse works fine. My engine is being pulled soon so it's a good time to address this transmission issue.

Has anyone ignored this problem and then had a complete transmission failure?
Any updated fixes?

Did you drain some ATF and see if it looks burnt?

 

[BillyGoatMTB]

Do a mud search if you're going to do your ATF fluid do the flush at the cooler in front of the radiator it's easy to do and I put 7 gallons through it. You can also drop the pan measure how much food was in the pan while looking for debris and put that much fluid back. Make sure to use the FSM to check level proper level should only be checked when it operating temperature and that doesn't mean the engine at operating temperature you got to drive it and go through all the gears.

On to the noise, after my experience it's not happened since however I did a whole bunch of mud reading and found it's most likely a check valve in the reverse assembly solenoid. I never finished following up on the serial number and date code of my transmission to see if it received the upgrade from the factory.

I I've done a number of very difficult trails since and have not had an issue.

The complete replacement of fluid resulted in a much crisper and cleaner shifting 80.

Hope this helps.

 

[YngTLCOwner]

Bumping this thread since it seems I'm having the exact same issues. Just happened recently while trying to back up on a trail with slight incline. Shifted into reverse fine but as I applied gas truck wouldn't move. Finally it did start to reverse but also encountered a very loud clunk sound. Horrible sound to hear on trail. I do have the basic trans go shift kit and under normal road conditions/driving there isn't any issue. Has there been any definitive fixes besides doing full trans replacement?

 

[BillyGoatMTB]

Bumping this thread since it seems I'm having the exact same issues. Just happened recently while trying to back up on a trail with slight incline. Shifted into reverse fine but as I applied gas truck wouldn't move. Finally it did start to reverse but also encountered a very loud clunk sound. Horrible sound to hear on trail. I do have the basic trans go shift kit and under normal road conditions/driving there isn't any issue. Has there been any definitive fixes besides doing full trans replacement?

I haven't had the issue since it happened to me a few years ago. I'd already changed the fluid. I've heard rumors that there's a ball check valve in that reverse valve body that gives on some trannies.

I have yet to understand exactly what it is.

 

[jpoole]

I don't know exactly what it is either but I was able to learn a little more about this noise, which is fortunately rare in my 80, and will share in case it's a useful clue.

A year or so ago I drove a few hours and then needed to back up a fairly steep driveway in reverse. I think I was towing/backing a trailer but don't recall exactly. When I stopped on the steep driveway and then hit the gas somewhat hard to get rolling in reverse again the noise would occur and the 80 barely moved. This happened two or three times in a row and was very concerning. I then eased onto the throttle and gently/slowly accelerated and there was no noise and the 80 moved in reverse normally.

The way this felt was as if there were two different forces at play that need to balance out properly to avoid this noise. One force seemed like a hydraulically actuated clutch, clamp, braking band or similar that needed rpms to go up while in reverse to drive the transmissions hydraulic pump and increase pressures that then tighten this clamping force. The other force seemed like drive power from the engine to the wheels. When the drive power was too high and too quick it overpowered the clutch and caused it to slip which triggered the noise and lack of reverse motion but when the rpms slowly increased the necessary hydraulic pressure could be applied to the clutch as the power from the engine increased, maintaining the balance.

It seems somewhat relevant that this happened after a few hours on the interstate as that likely had warmed the trans fluid and thinned it out a little. That should lower the operating pressures in the transmission a little and shifted the balance between the clamping force and the driving force to the point where the slip/noise occurs.

The above fits with the trail riding scenarios because I assume that the transmissions are typically pretty warm in those cases and that there is the need to apply a decent amount of power to get rolling on the uneven terrain.

I checked and made a few adjustments to the transmission related cable(s) around the throttle body if I recall correctly but more importantly I now ease into reverse acceleration and I've not heard the noise since the incident I described above.

 

[Malleus]

Bumping this thread since it seems I'm having the exact same issues. Just happened recently while trying to back up on a trail with slight incline. Shifted into reverse fine but as I applied gas truck wouldn't move. Finally it did start to reverse but also encountered a very loud clunk sound. Horrible sound to hear on trail. I do have the basic trans go shift kit and under normal road conditions/driving there isn't any issue. Has there been any definitive fixes besides doing full trans replacement?

I would not think replacing your transmission would solve your problem, since I don't think it's a transmission problem. I think it's most likely a rear differential problem, and I'll explain why.

The transmission is, in theory, always in gear. There are four primary accumulator valves which assist in, and help lessen the shock of, changing gears. Two of these are directly involved in the 1st/reverse selection. They also do other things, and the the other two valves do nothing while you're in 1st/reverse.

There is no reverse solenoid, valve or assembly, per se. Unless it's shifted into another gear, the transmission is always in 1st/reverse. If your output shaft speed is less than the equivalent 10-15 mph, you're in 1st/reverse (unless you've selected 2nd start; I'm assuming you didn't).

There are three driven shafts in the transmission; the first is directly (sort of) connected to the engine through the torque converter. The third is directly connected to the transfer case, through the output gear train. The second one floats between the two.

At no time is anything rotating the way a manual gearbox does with side-by-side gears in or out of mesh. Everything in the A343F is connected inline, either by a shaft-in-shaft joint or by direct friction contact, inline with the shafts (there's one exception, I'll explain below).

The first accumulator valve is always "on" (that is, fluid is always backing it up), unless you're in overdrive. This is what connects the first shaft to the rest of the transmission. The first and reverse gears share a planetary, so the determining factor deciding which is engaged is how the planetary is held. I won't go into the theory behind planetary gear trains (you can look that up if you're interested), but very simply, if you hold the ring the output shaft spins in one direction. If you don't hold it, the output shaft spins in the other direction. This is how 1st/reverse is selected. The 2nd coast brake band is responsible for holding the ring. It has a clutch lining on the ID and it surrounds the ring; when it's called for, it tightens around the ring and you get reverse.

There's really no way this action could produce a clunk, or any sound other than a screeching, from the band ID slipping on the ring OD.

There are one way clutches (like the seatbelt clutches) which also help to lock the 1st/reverse in place, but those also will not make a clunk if they fail.

The second (center) shaft is connected to the first and third shafts by another one-way clutch, through a planetary gear train. It's always engaged; it can only slip in one direction, which cannot generate any clunk, or grab in the other.

There are two additional areas which could singly, or in combination with each other, cause a loud clunk or bang: the propeller (drive) shaft and the rear differential. The rear differential assembly is driven by a ring and pinion, which is a spiral bevel gearset. Manufacturers adopted the spiral bevel because it is capable of transmitting a great deal of force (in one direction only) without generating a lot of noise. They have a weakness, though. Their load transmitting capability is significantly less in the opposite (coast) direction. Ask anyone who has tried to use a 40 to pull a stump while in reverse.

The result of this design drawback is that the wear on the coast side can cause a loud clunk when it's engaged quickly. This is due to a lot of factors, but it's mostly wear in used gearsets. In new gearsets, it's the result of poor setup. Combined with loose universal joints, this could create a very alarming sound.

In another life, I built quite a few of these and we had infrequent complaints from customers claiming this very problem. Although to be fair, theirs were setup problems. We didn't find the wear related root cause until we shipped a bad batch of axles and had to compare them to operating field units with many hours on them.

I'm also curious as to whether or not the reported "clunk on reverse up an incline" problem trucks also have limited slip rear differentials.

 

[JDUBinCO]

This is really good info. Thank you for taking the time to explain. Mine more recently developed this sound, particularly when backing a trailer up the driveway and if I "stab" the throttle. If I ease onto throttle I don't get the alarming clunk/squeal. So I had my wife do it while I lay next to the vehicle and it really sounds like its coming from the prop shaft area. I also get a "ting" sound when switching F to R or vice-versa under no-load.

Is there anything in the prop shaft that would cause this? The sound that I'm hearing is consistent with high friction metal on metal, and I feel a clunk, like a u-joint that has eaten through the needle bearings, moves through space (loudly) and comes to rest on the bearing cap before transferring torque. Though when I check the u-joints they seem tight.

Or could it be from the transfer case?

 

[YngTLCOwner]

Thank you, Malleus! Gives me something else to look over to determine what could potentially be going wrong. Going to get under the truck tomorrow.

 

[Kernal]

Been discussed before but FWIW: IME one cause of a screech/slipping/shudder noise in reverse with the A343F ATM is if the incorrect transmission fluid has been used and/or if any additives have been dumped in (which may make the symptoms worse). This may be related to the Tech Service Bulletin discussed earlier or may occur irrespective of that TSB, IME.

@JDUBinCO : IMHO if the vehicle is new to you and you don't know what type of ATF or additives are in your (A343F) transmission and it has the screech/slip/shudder in Reverse one option would be to perform a complete ATF fluid exchange using a DEXRON II/III compatible ATF.

The "ting" sound when shifting between gears (vehicle not moving) IME could be from dry/worn U-joints.

Edited for clarity

 

[Malleus]

This is really good info. Thank you for taking the time to explain. Mine more recently developed this sound, particularly when backing a trailer up the driveway and if I "stab" the throttle. If I ease onto throttle I don't get the alarming clunk/squeal. So I had my wife do it while I lay next to the vehicle and it really sounds like its coming from the prop shaft area. I also get a "ting" sound when switching F to R or vice-versa under no-load.

Is there anything in the prop shaft that would cause this? The sound that I'm hearing is consistent with high friction metal on metal, and I feel a clunk, like a u-joint that has eaten through the needle bearings, moves through space (loudly) and comes to rest on the bearing cap before transferring torque. Though when I check the u-joints they seem tight.

Or could it be from the transfer case?

If your U-joints appear to be tight, I'd look at the slip joint. It is a maintenance item, just like the U-joint bearings.

Have you tried backing up with the transfer case in low and high range, for comparison? I doubt your problem is in the transfer case, but it won't hurt to check.

 

[Malleus]

Here's the schematic representation of the A343F operation (from the A343F repair manual):

The drawing convention used is any component that is engaged is outlined in bold and the transmission case is represented by the angled lines at the top and bottom of the diagram. The important things to get from this are:
» the engine output direction (left side open arrow) is the same as the first and second shafts (the little stub shaft in the center of the diagram)
» the open, dashed arrow (marked "X"), indicates the optional direction of rotation for the output shaft
» "F2" is the 1st/reverse one-way clutch, which is actually inside the planetary hub
» the determining factor for 1st or reverse gear is the C1/C2 accumulator engagement; C2 locks the 2nd coast brake band (B1) onto the 1st/reverse ring gear

Physically, the B1 brake band is connected to the inside wall of the transmission case. just like the steels. It can't rotate. The plunger, accessible from the outside of the case (the large round plug with the X in its face), is responsible for its activation.

The theory of operation is explained on pages AT-2 through AT-8, if you're interested. There are other diagrams there, representing the operation of the other gear selection lever positions, but they aren't important for understanding what's going on at very low speed.