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Old 10-01-2018, 11:47 AM   #1
GreyGhost85
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TH350 Intermittent Engagement Questions and Troubleshooting Steps

Good morning, First time poster and been lurking for about a month now. So long story short, I recently purchased a 71 c10 305 with a th350. The owner that I purchased from swore up and down that the trans was good and he seemed pretty legit. I had it trailered to the house so that I could begin working on it and getting it running. The problem is that the th350 is not always engaging properly or may take a couple of seconds to engage. It seems to get better the more I keep messing with it but then it just goes to no gears, no reverse, no drive,d1, or d2. The trans had been sitting for a while but connected to motor.

Here is my troubleshooting:

1. Changed trans fluid and filter. Saw a couple of leaks with old pan and Speedo cable so I changed pan, filter, orings for bullet, and cooler. No leaks.

2. Started her up again, poured 4 qts of fluid in and let her run for about 5 minutes and kept checking fluid to make sure it was within the hot crosshatches. And it was even after shifting thru all gears. Tried to drive and it moved forward only with a bit of revs and reverse as well. Drove around the block and no gears at stop sign. Pushed back to garage.

3. Drained pan and pulled filter and there was some sludge , I assume from sitting for a while, put new filter on and repeated the fill shifting through gears. This time I put the rear end on jack stands. I had strong reverse, strong drive, d1, and d2. Wheel spinning away. When appoint brake I put it in park and left it running, checked fluid and it was perfect. Hopped back in and put it in reverse and go nothing. No drive either???

4. I started engine with transmission cooler line off and trust me, itís pumping, made a mess all over floor. Repeated the rear on stands process with fresh fluid. Popped it in reverse and bam, wheels turning hard through all drives as well. After going through gears once and back to park, I get no gears again while on jack stands.


Is this th350 toast??? Is a break in process for a th350 that has sat? There are 0 leaks, Iím just confused as to why it able to go in gear on second and then 0 gears the next.
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Old 10-01-2018, 01:07 PM   #2
Chevyrestorerman
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Since you just bought the truck and most likely wanting to drive it I would just go ahead the pull the transmission out of the truck and go thru the damn thing. Most likely you will spend more time messing with it not knowing what you are doing and wasting time. In a hour after the truck is on jack stands you could have the transmission on the bench. Rebuild it and put it back in the truck so you do not have to worry about it leaving you on the side of the road. Itís what you want out of a truck, spend a few bucks and get it right.
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Old 10-01-2018, 01:37 PM   #3
GreyGhost85
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Originally Posted by Chevyrestorerman View Post
Since you just bought the truck and most likely wanting to drive it I would just go ahead the pull the transmission out of the truck and go thru the damn thing. Most likely you will spend more time messing with it not knowing what you are doing and wasting time. In a hour after the truck is on jack stands you could have the transmission on the bench. Rebuild it and put it back in the truck so you do not have to worry about it leaving you on the side of the road. Itís what you want out of a truck, spend a few bucks and get it right.
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One more question, if I put trans in reverse, it should only spin one way correct?
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Old 10-01-2018, 02:53 PM   #4
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

Just like if you put it in drive the output shaft turns clockwise. I would hope if you put the transmission in reverse the output shaft will turn counter clockwise.
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Old 10-01-2018, 03:53 PM   #5
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

If it's been sitting for more than a few years it will need gone through most likely. There is a possibility that crud settled out in the VB and the valves are sticking. Try refilling and add a bottle of Trans Medic and running through the gears. Then if it's a no go, let it sit a day and try again. If it's still a no go, it will need torn down for overhaul.
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Old 10-01-2018, 04:06 PM   #6
GreyGhost85
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Just like if you put it in drive the output shaft turns clockwise. I would hope if you put the transmission in reverse the output shaft will turn counter clockwise.

Sorry if I wasnít clear.

The rear end is on jack stands. If I put the trans in park, I cannot rotate driveshaft either way.

If I put the trans in drive, and if the trans is not engaged, I can rotate driveshaft manually either way. Running or not.

If I put the trans in reverse, I can rotate driver shaft manually either way running or not.

I put some hydra trans in there and shifter through gears and same symptoms.

Trans seems to only engage for roughly the first 30 seconds or so before no gears. If I start the truck, and immediately put it in reverse or drive, wallah! Wheels spin. Go to another gear after brakes, 0 gears. Maybe time to pull.
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Old 10-01-2018, 04:07 PM   #7
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If it's been sitting for more than a few years it will need gone through most likely. There is a possibility that crud settled out in the VB and the valves are sticking. Try refilling and add a bottle of Trans Medic and running through the gears. Then if it's a no go, let it sit a day and try again. If it's still a no go, it will need torn down for overhaul.
Yea when I drained transfluid again, there was a lot of black sludge, not metal though.
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Old 10-01-2018, 05:55 PM   #8
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

Transmission is a sealed unit, how in hell can anything get into it...its not like a engine where the oil gets dirty....if trans oil is "dirty" is because its coming apart from within.... BLACK sludge ? how long was this sitting 10 years ? So if the vehicle sits for one year the ATF dont go bad....have you checked the date on a ATF bottle to see if it says MUST USE BY THIS DATE.....I dont think so. Like I said in previous post....pull the trans, and rebuild it...you will be happy you did. Enjoy!
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Old 10-01-2018, 06:00 PM   #9
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

I'm guessing the black sludge is clutch material. Probably has stuck parts thus slipping horribly and ruining clutches.
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Old 10-01-2018, 06:05 PM   #10
GreyGhost85
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Transmission is a sealed unit, how in hell can anything get into it...its not like a engine where the oil gets dirty....if trans oil is "dirty" is because its coming apart from within.... BLACK sludge ? how long was this sitting 10 years ? So if the vehicle sits for one year the ATF dont go bad....have you checked the date on a ATF bottle to see if it says MUST USE BY THIS DATE.....I dont think so. Like I said in previous post....pull the trans, and rebuild it...you will be happy you did. Enjoy!
Pulling now!
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Old 10-04-2018, 11:20 AM   #11
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Good morning folks, pulled the transmission and discovered that I have a TH350C. My question is are there any internal difference between the th350 c and regular 350 during rebuild process? I know you can wire a switch to the electrical connection to lock up during crusuising and highly cruising, but i plan to plug thread the connection. And thoughts before I order the rebuild kit?
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Old 10-04-2018, 11:22 AM   #12
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Here is sludge in pan.
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Old 10-04-2018, 06:12 PM   #13
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

That looks like the TCC is coming apart to me, if indeed its a TH350C.....

Input shaft is different, VB gaskets will be different as well. I would replace it with a 700R4 if it was mine. I just installed a 700R4 in my 72 c10 swb with 3.42 posi and it works great...
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Old 10-04-2018, 06:49 PM   #14
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That looks like the TCC is coming apart to me, if indeed its a TH350C.....

Input shaft is different, VB gaskets will be different as well. I would replace it with a 700R4 if it was mine. I just installed a 700R4 in my 72 c10 swb with 3.42 posi and it works great...

Interesting info, would you recommend further tear down of the transmission or have a shop inspect the tc? I have my mindset on rebuilding the 350 and replacing the tc right now. I found one pierce of metal in the valve body and about a peas size worth of sludge. Input shaft looks good, no wear or ground off splines.

I donít plan to use the lockup feature and if I did I would use a switch. I know the trans will run slightly hotter with this feature disabled but Iíll accomadate a bigger cooler.
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Old 10-04-2018, 10:56 PM   #15
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

I'd run it with lockup, might as well gain mileage on roadtrips. Rebuilding is the same for all of them, clean thoroughly, inspect all hard parts, follow instructions, and reassemble. Get an ATSG manual, if there are updates or mods that should be made it's noted. I would run the lockup through a switch, an led in the dash and the brake pedal switch. I believe the original setup would have been vacuum switch and brake switch. Then you can turn it on off as you please and the led would remind you.
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Old 10-05-2018, 12:31 AM   #16
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I'd run it with lockup, might as well gain mileage on roadtrips. Rebuilding is the same for all of them, clean thoroughly, inspect all hard parts, follow instructions, and reassemble. Get an ATSG manual, if there are updates or mods that should be made it's noted. I would run the lockup through a switch, an led in the dash and the brake pedal switch. I believe the original setup would have been vacuum switch and brake switch. Then you can turn it on off as you please and the led would remind you.
I dot really see myself taking roadtrips with this build but I am considering a switch
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Old 10-05-2018, 08:35 AM   #17
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

GM offered different pressure switches, you can get a pressure switch to engage the TCC at a higher speed, so it would not engage the TCC to early. So in town driving would be better on the TC, no one wants a TCC locking and unlocking all the time while driving in town.
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Old 10-05-2018, 10:51 AM   #18
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So during the continued tear down of this transmission I have discovered the following so far:

1. The accumulator spring is completely missing. I heard that this mod can cause harder shifts but also burn up the transmission.

2. The vacuum modulator valve is completely missing (the modulator gold vacuum connection is there). Whatís the point of having the vacuum connection without the line?? Beats me

3. TH350C has 5 check balls in all of the stock locations. I donít think a shift kit was installed.

I will be pulling pump and gears soon to inspect and start the rebuild.
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Old 10-05-2018, 02:02 PM   #19
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

Transgo shift kit will instruct you to install 5 checkballs for the TH350C transmission which all 5 checkballs go into the case of the transmission. I have provided a pic of the TH350C transmission separator plate. In the pic will show you drill requirements along with checkball locations. Replace the TC and make sure you clean out the cooler and the cooler lines real good. The black stuff in the pan is pieces off the torque converter clutch. You can call it sludge or whatever but itís the TCC coming a part
The TH350C torque converter is different than the t350. Make sure you buy the correct one. The TH350C will have the pilot type of input shaft.
Inspect all the hard parts for wear. Buy a master kit with steels, shift kit by transgo, intermediate band, and filter. This transmission likes its bushings replaced along with the thrust washers. Itís ok if the spring was missing in the accumulator. If you feel you need it. Put the spring in.
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46 total vehicles in my collection:
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3 - 67 corvettes, 1 - 67 chevelle, 5 - 68 chevelles
2 - 67 Nova SS, 1 - 57 nomad, 1 - 57 belair
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Old 10-05-2018, 05:20 PM   #20
GreyGhost85
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Transgo shift kit will instruct you to install 5 checkballs for the TH350C transmission which all 5 checkballs go into the case of the transmission. I have provided a pic of the TH350C transmission separator plate. In the pic will show you drill requirements along with checkball locations. Replace the TC and make sure you clean out the cooler and the cooler lines real good. The black stuff in the pan is pieces off the torque converter clutch. You can call it sludge or whatever but itís the TCC coming a part
The TH350C torque converter is different than the t350. Make sure you buy the correct one. The TH350C will have the pilot type of input shaft.
Inspect all the hard parts for wear. Buy a master kit with steels, shift kit by transgo, intermediate band, and filter. This transmission likes its bushings replaced along with the thrust washers. Itís ok if the spring was missing in the accumulator. If you feel you need it. Put the spring in.
Thanks a ton. I would have never guessed that a shift kit was in place. Iíll try to get some more pics of the internals to post just in case this thread helps someone else in the future.
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Old 10-05-2018, 05:26 PM   #21
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Transgo shift kit will instruct you to install 5 checkballs for the TH350C transmission which all 5 checkballs go into the case of the transmission. I have provided a pic of the TH350C transmission separator plate. In the pic will show you drill requirements along with checkball locations. Replace the TC and make sure you clean out the cooler and the cooler lines real good. The black stuff in the pan is pieces off the torque converter clutch. You can call it sludge or whatever but itís the TCC coming a part
The TH350C torque converter is different than the t350. Make sure you buy the correct one. The TH350C will have the pilot type of input shaft.
Inspect all the hard parts for wear. Buy a master kit with steels, shift kit by transgo, intermediate band, and filter. This transmission likes its bushings replaced along with the thrust washers. Itís ok if the spring was missing in the accumulator. If you feel you need it. Put the spring in.
Are all transgo kits going to be the same?they are pretty cheap on eBay like 30$
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Old 10-05-2018, 10:00 PM   #22
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

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Old 10-05-2018, 10:04 PM   #23
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I took everything apart, all I found was a spot of rust and diss regraded paper gasket behind the center support. Roller sprays were a little beat up and I plan to replace them both during the rebuild. Iím really thinking the it was the tcc and simply replacing that would have maybe resolved the issue. But since everything is apart, might as well rebuild it strong for years to come.
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Old 10-07-2018, 12:53 AM   #24
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Re: TH350 Intermittent Engagement Questions and Troubleshooting Steps

Quote:
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Are all transgo kits going to be the same?they are pretty cheap on eBay like 30$
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If you are needing a shift kit for your TH350C than the SK350 transgo kit will work for you. I have asked before on this forum which shift kit to use and the transgo kit seems to be a poplar choice. There are other companies that offer shift kits like B&M, Superior, and Sonnax. I am not sure which one is the best one for you, I think its how you plan on using your vehicle. If the engine is stock it seems that the Transgo kit is used in a lot of shops to recalibrate the VB in not only TH350's, but also in 4L60E and 4L80E's using the correct shift kit for each type of transmission. Every shop I talk to installs a shift kit in their rebuilds.
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46 total vehicles in my collection:
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3 - 67 corvettes, 1 - 67 chevelle, 5 - 68 chevelles
2 - 67 Nova SS, 1 - 57 nomad, 1 - 57 belair
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Old 10-08-2018, 12:26 PM   #25
GreyGhost85
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If you are needing a shift kit for your TH350C than the SK350 transgo kit will work for you. I have asked before on this forum which shift kit to use and the transgo kit seems to be a poplar choice. There are other companies that offer shift kits like B&M, Superior, and Sonnax. I am not sure which one is the best one for you, I think its how you plan on using your vehicle. If the engine is stock it seems that the Transgo kit is used in a lot of shops to recalibrate the VB in not only TH350's, but also in 4L60E and 4L80E's using the correct shift kit for each type of transmission. Every shop I talk to installs a shift kit in their rebuilds.
One more question chev, you have been a tremendous help btw, I did some research on this and could only find a few items that said it was normal but wanted to confirm. During cleaning of the transmission housing/case there are holes that appear to be cut out or punched out. Is this normal? I threw a pic of the pic in there as well.
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