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Th350c slips reverse

michael0584

michael0584

1/2 ton status
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houston texas
i have th350c that slips in reverse. You put it in reverse and it slips until you give it throttle and it grabs reverse and is strong. I've checked line pressure and idling in all gears is about 75psi, you shift to reverse and it drops to about 30psi. You give It throttle and it pops up to 150psi. It seems like it's loosing pressure in the reverse circuit somewhere. All gears shift fine and they all hold fine no slipping. Fluid looks good and is at the proper level. Trans was rebuilt about 10,000 miles ago about 20 years ago. Problem developed a few years after rebuild and truck was parked. Trying to get it back on the road and curious if anyone has an idea where to start looking. This is on a 85 c10 with a 305. Truck is completely stock. Thanks for any input.
 
michael0584 said:
i have th350c that slips in reverse. You put it in reverse and it slips until you give it throttle and it grabs reverse and is strong. I've checked line pressure and idling in all gears is about 75psi, you shift to reverse and it drops to about 30psi. You give It throttle and it pops up to 150psi. It seems like it's loosing pressure in the reverse circuit somewhere. All gears shift fine and they all hold fine no slipping. Fluid looks good and is at the proper level. Trans was rebuilt about 10,000 miles ago about 20 years ago. Problem developed a few years after rebuild and truck was parked. Trying to get it back on the road and curious if anyone has an idea where to start looking. This is on a 85 c10 with a 305. Truck is completely stock. Thanks for any input.
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Well I found my repair manual and it says it could be leak in the 1-2 accumulator, 2-3 accumulator, or direct clutch. So looks like I will pull the 1-2 accumulator and take a look as it's the easiest.
 
Yep, the PSI should be 235 -270 for reverse according to the info I have. So, hopefully you found your circuit leak in the 1-2 accumulator piston/rings . Otherwise you will have to check the 2-3 accumulator piston /rings at valve body for damage. If these two are OK, then you will have no choice but to disassemble the transmission , and check for worn or damaged direct clutch outer piston seal.
 
Hope its not going to croak on you--every TH350 I had that failed,lost reverse first...

I didn't take any of them apart to see what happened,knowing I'd never be able to re-assemble one successfully..I figured most of them were just "done" after being used to plow for several years...so I just bought used "good" ones and put them in..
 
MEANSTREAK K30 said:
Yep, the PSI should be 235 -270 for reverse according to the info I have. So, hopefully you found your circuit leak in the 1-2 accumulator piston/rings . Otherwise you will have to check the 2-3 accumulator piston /rings at valve body for damage. If these two are OK, then you will have no choice but to disassemble the transmission , and check for worn or damaged direct clutch outer piston seal.
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Well bad news, swapped out the 1-2 accumulator for another one I had and still slipped. Pulled the pan to take a look and found lots of clutch material, metal, and a piece of a seal. I'm assuming a piece of the direct clutch piston seal. Looks like I'll be pulling the transmission. I'm thinking about picking up a used late model 700r4 and swapping it in. Thoughts?
 
michael0584 said:
Well bad news, swapped out the 1-2 accumulator for another one I had and still slipped. Pulled the pan to take a look and found lots of clutch material, metal, and a piece of a seal. I'm assuming a piece of the direct clutch piston seal. Looks like I'll be pulling the transmission. I'm thinking about picking up a used late model 700r4 and swapping it in. Thoughts?
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Swapping in a used 700r4 might be a good idea. You will have over drive ,and a lower first gear.You probably already know this. Is the 700 the same length as a 350 trans?
 
MEANSTREAK K30 said:
Swapping in a used 700r4 might be a good idea. You will have over drive ,and a lower first gear.You probably already know this. Is the 700 the same length as a 350 trans?
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From my research a long tail 350 is 11/16 longer than a 700r4, so my driveshaft should be fine. Looks like the crossmember has to be moved back 3 in but holes are probably already there. I'll need to change the plug on my lockup wiring for the 700. Also if I get a later 700 I'll need to change it to mechanical speedo. I have a junk 700 I can rob those parts from. I hope I'm not missing anything. Hoping to get the dipstick with the 700 as that's different. Unsure on if a Tbi 700 has the same tv cable as a carb. I need to verify that but looks like my kickdown bracket will work for the tv cable.
 
This truck has awful gears in it. 2.73 if I recall. So that will need to be addressed once I add overdrive. I have a few spare axles I think with 3.42 I could swap in. Just need to verify they are still in good shape.
 
michael0584 said:
This truck has awful gears in it. 2.73 if I recall. So that will need to be addressed once I add overdrive. I have a few spare axles I think with 3.42 I could swap in. Just need to verify they are still in good shape.
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I, too have a c-10, though it is an 84. It also has a 305 and a 350c trans. I was wondering about the rear gear ratio, I knew it had to be high for me to go 65mph with no OD!!It is a Custom Deluxe that is apple red from the factory, the truck is originally from Kent Chevrolet in Mountain Home Arkansas.Here , where I live in Cornville , Illinois these trucks are all gone from rust. Every time I drive it people are wanting to buy it , and I don't want to sell, cause I want to convert it to 4 wheel drive. I plan on pics, as soon as I repaint it, I got a little over $1,000.00 in it as it sits now . So far it runs great , but it is no power house!.........Any way good luck with your transmission swap , man!
 
MEANSTREAK K30 said:
I, too have a c-10, though it is an 84. It also has a 305 and a 350c trans. I was wondering about the rear gear ratio, I knew it had to be high for me to go 65mph with no OD!!It is a Custom Deluxe that is apple red from the factory, the truck is originally from Kent Chevrolet in Mountain Home Arkansas.Here , where I live in Cornville , Illinois these trucks are all gone from rust. Every time I drive it people are wanting to buy it , and I don't want to sell, cause I want to convert it to 4 wheel drive. I plan on pics, as soon as I repaint it, I got a little over $1,000.00 in it as it sits now . So far it runs great , but it is no power house!.........Any way good luck with your transmission swap , man!
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Thanks, this one is two tone. Apple red top, and tan below the trim.
 
My '81 G10 van has a 2:73 rear axle ratio ,that I swapped in place of its original 3:08 axle,after the diff chowed the side gear bore..also has a TH350C with the lockup converter..

The only reason I chose that ratio was two fold--it was free,from a junk van a friend had,and the only others available in salvage yards were buried behind a bunch of other vehicles--they wanted 200 for one,if they had to move all those other vehicles to pull it...(Looking back I kind of wish I had bought the one with 3:42's now)...

Thought it was a dog before-!-now with a 307 V8 from a 73 Chevelle that was less HP than the 250 six in it,I can leave it in second gear and bury the speedometer at 85 mph,then shift to third...it goes a few mph faster,then levels off,thats it...it'll downshift on a long hill if your not going 55 when you hit the bottom of it..

Once you get to 65 mph,it will maintain it though,and you can pass other cars by flooring it or pulling it down into second manually...it's fun watching the look on someone's face when they hear it downshift at 65 mph..:eek:....:D:..

The van gets decent mpg,I'd guess about 14-16 around town,acceleration isnt "brisk",but its no worse than my 6.2 is as far as merging onto a freeway..(not sure what ratio the pickup has,maybe 3:23's or 2:42's)..
 
diesel4me said:
My '81 G10 van has a 2:73 rear axle ratio ,that I swapped in place of its original 3:08 axle,after the diff chowed the side gear bore..also has a TH350C with the lockup converter..

The only reason I chose that ratio was two fold--it was free,from a junk van a friend had,and the only others available in salvage yards were buried behind a bunch of other vehicles--they wanted 200 for one,if they had to move all those other vehicles to pull it...(Looking back I kind of wish I had bought the one with 3:42's now)...

Thought it was a dog before-!-now with a 307 V8 from a 73 Chevelle that was less HP than the 250 six in it,I can leave it in second gear and bury the speedometer at 85 mph,then shift to third...it goes a few mph faster,then levels off,thats it...it'll downshift on a long hill if your not going 55 when you hit the bottom of it..

Once you get to 65 mph,it will maintain it though,and you can pass other cars by flooring it or pulling it down into second manually...it's fun watching the look on someone's face when they hear it downshift at 65 mph..:eek:....:D:..

The van gets decent mpg,I'd guess about 14-16 around town,acceleration isnt "brisk",but its no worse than my 6.2 is as far as merging onto a freeway..(not sure what ratio the pickup has,maybe 3:23's or 2:42's)..
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Do you think that the lock up torque convertor helps that much more on gas mileage, compared to a regular turbo 350 trans?
 
Maybe not all that much,but it does help--but I do like the lockup mode,it helps "feel" like its more "direct drive" than a TH350 without it..it does drop the highway rpms about 200 rpms or so when it locks in..
 
diesel4me said:
My '81 G10 van has a 2:73 rear axle ratio ,that I swapped in place of its original 3:08 axle,after the diff chowed the side gear bore..also has a TH350C with the lockup converter..

The only reason I chose that ratio was two fold--it was free,from a junk van a friend had,and the only others available in salvage yards were buried behind a bunch of other vehicles--they wanted 200 for one,if they had to move all those other vehicles to pull it...(Looking back I kind of wish I had bought the one with 3:42's now)...

Thought it was a dog before-!-now with a 307 V8 from a 73 Chevelle that was less HP than the 250 six in it,I can leave it in second gear and bury the speedometer at 85 mph,then shift to third...it goes a few mph faster,then levels off,thats it...it'll downshift on a long hill if your not going 55 when you hit the bottom of it..

Once you get to 65 mph,it will maintain it though,and you can pass other cars by flooring it or pulling it down into second manually...it's fun watching the look on someone's face when they hear it downshift at 65 mph..:eek:....:D:..

The van gets decent mpg,I'd guess about 14-16 around town,acceleration isnt "brisk",but its no worse than my 6.2 is as far as merging onto a freeway..(not sure what ratio the pickup has,maybe 3:23's or 2:42's)..
Click to expand...
The 81 K10 I had had 2.73 gears and 33" tires when I got it. The 350 I put in it had some power so wasn't too bad but was originally a 305. I'm sure it was a dog. I swapped in a 3.08 axle set I had and went to 31" tires and that helped alot. This one won't get alot of use and 350c transmissions are hard to find, and I would hate to lose the lockup. 700r4 trans are easy to find here and cheap. looks like the going rate for working one out of a 88-92 pickup are $250-$400.
 
diesel4me said:
Maybe not all that much,but it does help--but I do like the lockup mode,it helps "feel" like its more "direct drive" than a TH350 without it..it does drop the highway rpms about 200 rpms or so when it locks in..
Click to expand...
I have heard people say that the lock up function of the 350c makes it less desirable for a performance application compared to a regular th350?
 
Could be,maybe the torque converter is "weaker" if your planning of drag racing one or something,due to the clutch disc and springs in it ?...I wouldn't worry about street uses or light off roading,as it only locks up in third gear above 35 mph..

This one has 180,000 miles on it,the van had a 250 straight six for the first 138,000 or so,and the only issue it has was the nylon govenor gear apple cored and stripped,so it refused to upshift out of first gear,all it had was first & reverse when I bought it for 500 bucks in 1990,with a "blown transmission"...a free govenor from a salvage yard tranny fixed it right up!..:D..

I have a spare TH350C I took out of an '86 G-10 that has lower miles,I bought that van for the 305 engine for a '79 Bonanza C-10 I had that blew up the 250 six it had,at the time...but after letting it sit for 10+ years in a shed with no dipstick tube,it may not be in exactly great condition now...I'm afraid water might have got in it,or condensation--I didn't drain it either,in hopes that might help preserve it better..
 
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