Help with rebuilt 1986 TH350C

[drummingrocks]

Good afternoon,

I'm hoping someone might have some advice with the transmission on my '86 Silverado. I just rebuilt the original TH350C using Nick's videos. The reason for the rebuild was that the transmission was slipping noticeably and was leaking from every orifice imaginable. I literally couldn't keep fluid in it, it was leaking so much.

This was a stock rebuild except I did install a TransGo SK-350 kit into the valvebody. The lockup function never worked before I rebuilt the transmission, and I did go ahead and install a new governor pressure switch. My problem is now the transmission won't shift out of 1st unless you rev it to the moon. If you rev it high enough, it does go into 2nd and will hold there even when you come to a stop. Unless you manually pull it back down into 1st, it stays in 2nd gear and will never shift to 3rd.

I thought I might have a sticking valve in the governor, so I pulled it back out this weekend. Sure enough, the valve did seem to be sticking, so I took the gear off the governor, got the valve out, and then cleaned both the valve and the bore. When I put it back together, the valve was moving properly and wasn't stuck. I really thought I had fixed the shifting issue, but it's still doing the same thing. I also replaced the modulator valve during the rebuild, but threw the original one back in to make sure it wasn't causing it not to shift. No change.

Is there anything in either the valvebody or with the lockup parts that might be causing it not to shift? I noticed that the new governor pressure switch was marked 44 psi, whereas the original one was 34 psi (I'm guessing the 34 psi one was the original factory one).

Thanks for any help you can provide, I really appreciate it!

 

[DoubleDingo]

I see you didn't tag anybody, that helps get answers

@NickTransmissions
@Matt69olds
@HotRodPC

There are more that have transmission experience, these here are who came to mind immediately

 

[NickTransmissions]

Good afternoon,

I'm hoping someone might have some advice with the transmission on my '86 Silverado. I just rebuilt the original TH350C using Nick's videos. The reason for the rebuild was that the transmission was slipping noticeably and was leaking from every orifice imaginable. I literally couldn't keep fluid in it, it was leaking so much.

This was a stock rebuild except I did install a TransGo SK-350 kit into the valvebody. The lockup function never worked before I rebuilt the transmission, and I did go ahead and install a new governor pressure switch. My problem is now the transmission won't shift out of 1st unless you rev it to the moon. If you rev it high enough, it does go into 2nd and will hold there even when you come to a stop. Unless you manually pull it back down into 1st, it stays in 2nd gear and will never shift to 3rd.

I thought I might have a sticking valve in the governor, so I pulled it back out this weekend. Sure enough, the valve did seem to be sticking, so I took the gear off the governor, got the valve out, and then cleaned both the valve and the bore. When I put it back together, the valve was moving properly and wasn't stuck. I really thought I had fixed the shifting issue, but it's still doing the same thing. I also replaced the modulator valve during the rebuild, but threw the original one back in to make sure it wasn't causing it not to shift. No change.

Is there anything in either the valvebody or with the lockup parts that might be causing it not to shift? I noticed that the new governor pressure switch was marked 44 psi, whereas the original one was 34 psi (I'm guessing the 34 psi one was the original factory one).

Thanks for any help you can provide, I really appreciate it!

Thanks for watching my videos, appreciate the views and support.

> Did you air check the case upon assembly? If so, did all clutch packs apply as expected? If yes, this likely rules out anything in the case. If not, it's tougher to rule out...

> Did you completely disassemble the valve body, with all the valves out of the valve body casting, clean/hot tank the casting and then reassemble it, along with the shift kit parts?

> Have you tried disconnecting your kick-down cable and driving it to see if you have a more normalized 1-2 upshift?

> Did you conduct a line pressure test prior to driving the vehicle?

Reply with answers to ALL of the above questions and we can go from there.

 

[drummingrocks]

Did you air check the case upon assembly? If so, did all clutch packs apply as expected? If yes, this likely rules out anything in the case. If not, it's tougher to rule out...

Yes, I did air check the case as shown in your videos and all clutch packs applied as expected. I did not check using an air check plate because I didn't have one, and the ones I found were all really expensive.

Did you completely disassemble the valve body, with all the valves out of the valve body casting, clean/hot tank the casting and then reassemble it, along with the shift kit parts?

Yes, I did completely disassemble the valve body. I cleaned it with solvent, cleaned one of the bores with a Bench Buddy to fix a sticky valve (I can't remember which one was sticking, but it did free up), then cleaned the valvebody all over again and blew everything out with compressed air. When I reassembled the valvebody, all valves moved smoothly and seemed to drop directly down into their bores with no sticking.

Have you tried disconnecting your kick-down cable and driving it to see if you have a more normalized 1-2 upshift?

I did not try disconnecting the kick down cable. I'm 99% sure I didn't hook it back up on the carburetor side because the Holley carb I'm using doesn't have the same linkage attachments the original Quadrajet did. I can easily check and verify this weekend, though.

> Did you conduct a line pressure test prior to driving the vehicle?

I did not conduct a line pressure test. I'm not sure I have the equipment to do that properly, but am happy to buy something if needed.

Thank you so much for the videos and for your help!

 

[NickTransmissions]

Yes, I did air check the case as shown in your videos and all clutch packs applied as expected. I did not check using an air check plate because I didn't have one, and the ones I found were all really expensive.



Yes, I did completely disassemble the valve body. I cleaned it with solvent, cleaned one of the bores with a Bench Buddy to fix a sticky valve (I can't remember which one was sticking, but it did free up), then cleaned the valvebody all over again and blew everything out with compressed air. When I reassembled the valvebody, all valves moved smoothly and seemed to drop directly down into their bores with no sticking.



I did not try disconnecting the kick down cable. I'm 99% sure I didn't hook it back up on the carburetor side because the Holley carb I'm using doesn't have the same linkage attachments the original Quadrajet did. I can easily check and verify this weekend, though.



I did not conduct a line pressure test. I'm not sure I have the equipment to do that properly, but am happy to buy something if needed.

Thank you so much for the videos and for your help!

Gotcha...Conduct a line pressure test before doing anything else...You can grab the Maddox oil/trans pressure gauge combo set at Harbor Freight. The gauge kit will come with everything you need to do the pressure test. You will thread in the pressure gauge into the line pressure port on the driver side of the case just to the rear of the bell housing. Once you have the gauge installed, fire up the engine and see what your pressures are at idle in all range positions. Then see if you have adequate line pressure rise when you step on the gas. It's best to have all four wheels off the ground when doing this test.

The attached picture shows the various pressure test port locations on the TH350. Thread your gauge in to the 'Line Pressure Tap'.
Make sure you have 15-20 Hg of vacuum at the modulator before conducting the test.

You must be registered for see images attach

P,N,D you want to see between 60-90 PSI
Reverse: 80-150 PSI
Manual 2 & 1: 120-180 PSI

those pressure ranges are at idle. When you step on the gas, you should see immediate line pressure rise on the gauge...If it's weak or none-existant, something is wrong and a more thorough diag will be required to pinpoint the cause(s).

Let us know your pressure readings once you have had a chance to do the test.

 

[drummingrocks]

Gotcha...Conduct a line pressure test before doing anything else...You can grab the Maddox oil/trans pressure gauge combo set at Harbor Freight. The gauge kit will come with everything you need to do the pressure test. You will thread in the pressure gauge into the line pressure port on the driver side of the case just to the rear of the bell housing. Once you have the gauge installed, fire up the engine and see what your pressures are at idle in all range positions. Then see if you have adequate line pressure rise when you step on the gas. It's best to have all four wheels off the ground when doing this test.

The attached picture shows the various pressure test port locations on the TH350. Thread your gauge in to the 'Line Pressure Tap'.
Make sure you have 15-20 Hg of vacuum at the modulator before conducting the test.

You must be registered for see images attach

P,N,D you want to see between 60-90 PSI
Reverse: 80-150 PSI
Manual 2 & 1: 120-180 PSI

those pressure ranges are at idle. When you step on the gas, you should see immediate line pressure rise on the gauge...If it's weak or none-existant, something is wrong and a more thorough diag will be required to pinpoint the cause(s).

Let us know your pressure readings once you have had a chance to do the test.

Thanks, I stopped by Harbor Freight and picked up the gauges today. I'll update once I have the readings.

 

[drummingrocks]

P,N,D you want to see between 60-90 PSI
Reverse: 80-150 PSI
Manual 2 & 1: 120-180 PSI

Hi Nick,

I did the line pressure test today, but first, let me mention something. The last time I test drove the truck earlier this week, I was manually shifting it down to 2 and 1 to see if that might make it shift. When I got back to the shop, I noticed that while I still could manually go from 1-2 up to D with the column shifter, I couldn't shift up into park. To make an extremely long story short, I took the pan off and discovered that I had left one of the bolts underneath the S-link loose, and me manually shifting down to 2-1 lodged the shifter linkage over that bolt head, preventing me from being able to shift back into park.

I tightened those bolts to spec, then for good measure went back and torqued all of the valvebody bolts, linkage bolts, and anything else I had access to with the pan off, to spec. I also reinstalled the original ECM that this truck had up under the dash, thinking that it might have had something to do with torque converter lockup function. With all that being said, I got it all back together and went for a drive today. The truck now successfully shifts consistently from 1-2 like it should. At one point on my drive, I think it did go into 3rd gear and it stayed there until I came to a stop. After that, though, it would shift 1-2 normally but never would go into 3rd again.

I came back to the shop and did the line pressure test, and here's what I'm getting:

In park at idle: 70 psi
In neutral at idle: 70
In drive at idle: 50
In reverse at idle: 90
In manual 1 at idle, 60. With enough throttle applied to begin to move the truck, 120
In manual 2 at idle, 60. With enough throttle applied to begin to move the truck, 100

This was with the original pump. I cleaned it during the rebuild but did not replace it or any of the internal gears. I didn't see any signs of scoring or damage, so I reassembled it and put it back in the transmission during the rebuild. Thank you again for the great videos and your help here!

 

[drummingrocks]

@NickTransmissions

I've put a few more miles on it after doing the line pressure tests, and it's driving normally, it just won't shift from 2-3. I thought about dropping the pan this weekend and using a bench buddy on the 2-3 shift valve bore. Other than that, I'm out of ideas other than just put more miles on it and hope that valve frees up?

 

[NickTransmissions]

@NickTransmissions

I've put a few more miles on it after doing the line pressure tests, and it's driving normally, it just won't shift from 2-3. I thought about dropping the pan this weekend and using a bench buddy on the 2-3 shift valve bore. Other than that, I'm out of ideas other than just put more miles on it and hope that valve frees up?

Sorry, been busy. Were your pressure test numbers with your kickdown cable conncted or disconnected?

If you drop the valve body, check the ke valve, make sure it's installed correctly, in addition to checking the 2-3 shift valve.

 

[drummingrocks]

Sorry, been busy. Were your pressure test numbers with your kickdown cable conncted or disconnected?

No worries! All tests were done with the kickdown disconnected. There’s nothing for the linkage to connect to on the aftermarket carburetor.

 

[drummingrocks]

If you drop the valve body, check the ke valve, make sure it's installed correctly, in addition to checking the 2-3 shift valve.

@NickTransmissions

Sorry, last question: what's the ke valve?

 

[NickTransmissions]

@NickTransmissions

Sorry, last question: what's the ke valve?

Typo, my apologies. Meant to type "KD" as in kick down valve.

 

[drummingrocks]

Typo, my apologies. Meant to type "KD" as in kick down valve.

Got it! Thanks again!

 

[drummingrocks]

@NickTransmissions

I got the the valve body removed today. Here's what I found:

1. The roll pin and retainer for the manual low control valve had come loose, and the spring was hanging slightly out of the bore. I found both the pin and the retainer in the transmission pan (parts 21 and 22 in the TH350C ATSG book on the valve body exploded diagram). When I found this, I was hoping this was a smoking gun for why it wouldn't shift to 3rd.

2. When I got the valve body on the bench, it had some small rust particles visible. This was a used valve body I got on eBay after the original was unusable, and I knew it was rusty when I got it, but I thought I cleaned it extremely well. I guess I missed something.

Despite the bit of dirt in the valve body, all valves still moved freely, including the 2-3 shift valve. Since I didn't like the rust in the valve body and I had since bought another, much nicer TH350C valve body used on eBay, I decided to go back together with that one.

The valve body I installed today was spotless when I got it. I went ahead and disassembled everything just to make sure it was clean and moved freely. I also installed the shift kit springs (TransGo SK-350) into this new valve body.

I went for a drive, and it still won't shift into 3rd.

One thing I noticed, and I'm just grasping at straws now: Reverse doesn't feel right. If I don't physically hold the shift lever up almost to the park position, the truck will go into reverse but seems like it slips. If I hold the shift lever as close to park as I can get it, the truck goes into reverse firmly and doesn't slip at all when moving backwards. It did this with the old valve body I took out as well as the new one I installed today.

Also, when I got the truck all of the lockup stuff was still hooked up. It still is. I'm not sure if that could somehow keep it from shifting, but I'm 99% sure the truck did shift to 3rd before I rebuilt the transmission, and all of the lockup stuff was plugged in back then too.

Also also, and this may completely be my imagination: I've run the truck up to 55-65mph in 2nd, and it seems like I hear a slight whine from somewhere, almost like how a power steering pump sounds when it's low on fluid. I'm not sure it's coming from the transmission, but wanted to mention it in case it helped with the diagnosis.

Thank you again for your help.

 

[Ricko1966]

The linkage isn't adjusted right you probably need to put in neutral to actually be in drive as far off as you are saying reverse is. Almost 1 position off. Disconnect the linkage put thr transmission in neutral manually,count the detents. Put the shift lever in the truck in neutral. Adjust the linkage until it can be attached without loosing the transmission or shifter positions.