TAC Tech => Mechanical => Topic started by: SoupMan on May 08, 2021, 09:46:46 AM
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Hoping someone here has some TH350 knowledge.
The TH350 was rebuilt 4 years ago. It has been good to me to date but last year I started getting a grinding noise when going from P or D to Reverse. This doesn't happen when the transmission is cold, only when the oil is hot. I drove it all summer (over 3000 miles) just not caring if it was going to fail.
Before I pull out the transmission and bring it to a shop, I'd like to hear if anybody has an opinion or knowledge on what might be wrong.
I'll be honest, if I'm going to pull the TH350, it's probably not going back in. I'll be looking to get a 4L80 with manual valve body. Oldschool's comments on another forum were very convincing and the cost in my opinion isn't that high. I figure it's going to run me $1200 to $1500 if I go with a known working trans and probably closer to $2500 if buy a built performance transmission.
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I used to work on these early to mid 70’s, so my knowledge is pretty rusty. Off the top of my head I can’t offer good suggestions especially without hearing it.
First have you checked the fluid level, this should be done on level ground with the engine running and warmed up. The fluid should be transparent with a red tint. There should not be any metallic sparkle under sun or bright light. Brown is not good and burnt smell is bad.
Other than the noise, how does the transmission drive? Are shifts sloppy?
Rebuild cost could depend on what it needs. If just seals, bushings and clutch plates, it shouldn’t be too bad. If hard parts like planetary gears, clutch drums, and pump need replacing it gets more expensive. Sometimes it’s cheaper to find a decent core to use for parts.
Honestly I can’t think what would cause a grinding noise, but a pressure regulator can make a noise that could be interpreted as grinding.
Switching to a different transmission will definitely be more work and expense. The cost of a different drive shaft, cross member, possible throttle linkage or electronic shift controls all add up. If you are looking for a trans with overdrive that can handling high horsepower that may help justify the extra cost and labor.
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5th t/a,
Thanks for helping. I'll follow your advice and double check all fluids although this is something I did repeated times last year in hope is was that simple. I'll try to get a recording of the sound along with it's exact location while on the hoist.
Otherwise the transmission feels pretty good.
What is nice with the 4L80 is that I can get the overdrive for better highway cruising and still put a bigger gear. I was planning to get a trutrac carrier and go from 2.73 to 3.23 (maybe higher)
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I don't see how there's anything there that can grind. I wonder if the stator clutch in the torque converter is bad. I hate to ask if it seems slow.
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Have you checked to see if the linkage is loose or misadjusted?
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The Park pawl can make a grinding or ratcheting noise, but the car would have to be rolling.
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right, it's definitely not that.
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Car does not need to be rolling for the parking pawl to make noise. Just running.
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Weather is good here today. If work can slow down I'll go out for a spin to warm it up and record the sound while on the hoist. It will tell me exactly where it is. Since I first posted weather has been crap.
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Car does not need to be rolling for the parking pawl to make noise. Just running.
I can't imagine how such a thing could happen.
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With the engine running, parts in the trans still can turn or operate.
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71455formula,
I don't see any way the parking pawl could make noise without the driveshaft spinning, The driveshaft yolk slides over the transmission output shaft. The output shaft is directly connected to output ring gear where the pawl drops into to keep the car from rolling. I shouldn't have even mentioned the parking pawl except Soupman mentioned he heard the noise when going from Park to D or Park to reverse. So if the car was rolling the pawl could make noise. As Firebird parts mentioned the noise would more likely be caused by something in the front end of the transmission. The front pump gear turns at engine RPM. Possibly front pump problem, torque converter or something in the front end. As mentioned before, I have heard the oil pressure regulator make a hammering noise that might sound like grinding. I don't ever recall hearing any automatic transmission make a grinding sound while sitting still, but it's been 40 years since I have rebuilt a transmission.
Here is a picture of the output shaft and ring gear.
(https://live.staticflickr.com/65535/51172667396_e6ca996748_k.jpg) (https://flic.kr/p/2kXXibU)IMG_8033 (https://flic.kr/p/2kXXibU) by Lawrence Alexander (https://www.flickr.com/photos/188552644@N06/), on Flickr
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Sorry. I was reading that backwards. D/R to park. Have had so many vehicles here where i work have that problem. Things are blurring together. I'll ask a few guys tomorrow and see what they think.
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Finally got out and drove it to check everything that was mentioned.
- Oil level a little high. I pulled a quart out now I'm low. So call it half a quart over
- I looked at the oil. Looks alright to me for over 3000 miles wear but you tell me. I included a video
TH350 screech from under. Only really hear it once and it's short lived
https://youtu.be/S8jgCzJhNbc (https://youtu.be/S8jgCzJhNbc)
TH350 screech from just above the motor
https://youtu.be/SGb7diZZq_4 (https://youtu.be/SGb7diZZq_4)
Oil.
https://youtu.be/sVMkZ9pe-OA (https://youtu.be/sVMkZ9pe-OA)
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Just out of Curiosity have you checked to see if the torque converter bolts are tight?
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I think one of the sprag clutches is slipping for just a short moment. If that's true, then mechanically it's the kiss of death. I have never heard that sound, though. I'm not at all saying this based on experience.
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Kiss of death. Sounds kinky.
I don't mind the trans stopping on me as long as it's not devastating to the motor or car in general. I'm making calls to start getting a 4L80 setup with manual valve body. Thinking I might just drive until is dies and then swap.
I took it for a rip tonight. 30 mile drive. I always beat it pretty hard on it since that's what I own it for. It doesn't slip and I get no noise while drive. It's just such a good transmission outside of this issue which is why it would be a shame to pull it if it's something silly. Pump pressure is 40 to 50psi when driving and the temp is 160 to 170.
If time allows, I'll drop the oil and look at the filter and residue in the pan. I'll double check the bolts for the torque converter while under there. What has me baffled is that I drove this car over 3000 miles with this problem last summer and it hasn't gotten worst.
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Before you pull the pan check the torque converter bolts. Back in the day I have seen this problem many times. With load on the converter there is no noise. But with no load the converter can shift back and forth on the flex plate causing a rattling noise.
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I think drive it till it dies, after you tighten the converter bolts, might be a good strategy. It may last for years. If it's pulling hard, then the converter internals must be okay at least for now. I was thinking the sprag clutch inside the converter, which is on the stator, might be the one that was making the noise. If it starts freewheeling backward you'll get no torque amplification there and so that is very diagnosable. And fixable, too, without a full teardown.
It if all comes apart then all you'd have to worry about for long term consequences is transmission shrapnel being in the transmission oil cooler.
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That's all good information guys. Thanks a lot!
I'll check the torque converter bolts. Didn't have time yesterday. Had time for a drive though :)
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With noise questions such as this, would be nice to have a video with the sound of said noise. You could get a better, or more accurate opinion.
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tajoe,
See one of my replies. You will see a video.
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Sorry, I missed those. I did put a couple comments for you.
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Just want to update everyone.
Not long after the last response I managed to get to the car. One of the bolts for the torque converter was loose by 1/2 turn. I wanted to drive to confirm this was truly the issue since sometimes the screech wasn't present. I can almost with 100% certain say the screech is gone.
Mechanically, what was happening in the torque convert to cause this screech?
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Hey Soupman,
I was wondering if you had a chance to check the converter bolts. If I recall they need to be torqued pretty tight, something like 30 foot pounds, but I don't know if you could get a torques wrench in there. I used to have a wrench that would grab the flywheel teeth and hold the converter/engine from turning. You could also carefully use a vise grip being careful not to ruin the flex plate/ring gear. I used to just use a long box wrench. Once all the bolts were snug, go around again to all three bolts hitting the end of the box wrench hard with the palm of your hand to really snug them down. Kind of has the same effect as an impact wrench.
The noise is caused by load and no load on the converter bolts shifting back and forth in the steel flex plate. The flex plate acts like a bell amplifying the sound. You called it a screech, but it also can sound like a rattle. It's been well over 40 years since I worked on these, but the video clip jogged my memory. I have run into this problem before. When you said the transmission drove OK and it was recently removed and replaced that also made me think the converter bolts might be lose.
Glad to hear it's resolved.
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I would assume the same. If the flex plate could bump into the converter and the bolt head it might happen repeatedly in a short time and sound like a screech, I guess.
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Well done "Souperman", and thanks for the follow-up. You're one of a few.
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It's minimum I can do for the guys on this forum that might need it. Give and take.
Thanks again! Worry free driving for me :)