Erratic shifting/Hunting for Gear TH350

[Ovals_and_Bowties]

Hi y'all I need help diagnosing an interesting shifting problem.
[A bit about the truck]
Okay I have an 85 C10 Silverado, 305 stock other than a 1406 carb with secondaries disabled (couldn't get ahold of a good Q-jet), a TH350c lockup unplugged, and limited slip 2.73 rear. I should probably mention I intend to keep it relatively stock, it's being used as a work truck.

[The symptoms]
The tranny is very confused about what gear it wants to be in and when. Both at around 20 MPH and 30 MPH under light acceleration or sometimes cruise it'll upshift (often not completing the shift), then immediately downshift, then immediately upshift again. It will do that on repeat until it's either picked up more speed and finally upshifts and holds 3rd or you slow down enough for it to stay in 2nd.

I think it wants to upshift too soon and is failing to complete the shift. It's grabbed 3rd gear at 22-25 MPH before putting the engine around 700-800 RPM which seems a bit too aggesive to me. The engine sounds to be lugging a bit trying to drag the truck around like that. I thought it was slipping at first but it holds fine once it finally picks a gear. The truck shifts normally if I keep it in L2 until 35 then shift to drive or in drive under medium-light accel to a speed 35 or higher.

What I've tried so far:
- 2 different modulators a single red stripe and a purple stripe both from ATP. Same symptoms.
- Double checking the kickdown cable adjustment since this occured AFTER installing the Edelbrock.
- Pulled out and inspected the governor no wear to speak of on the lands, valve inside moves when the weights are moved. Though I could use some pointers here, like how far is the valve supposed to move, etc.
- Put a pressure gauge on the main line tap
With the moduiator dissconnected all ranges were about 120-160 PSI with reverse being the lowest. Pressure tended to bounce rapidly +/- 10 psi though.
With mod connected at idle: P 60, R 100, D 65, 2 90, 1 90
During the gear hunting at around 20 MPH it'll bounce between 60-90 in time with engine RPM.
30MPH in 3rd was either 60 or 90 I think 90 but I need to retest I don't trust my dyslexia.
A short drive with the modulator unhooked held 150 PSI steady.
- Replaced part of the vacuum line to the modulator.

Any ideas where to go from here? And thanks in advance for any help!

 

[75gmck25]

Replaced part of the vacuum line?

Do you have a steel vacuum line with a short section of rubber line on each end to connect to the modulator and manifold vacuum? Too much rubber line may cause it to collapse under vacuum or wherever there is a kink in it. It should connect to the intake manifold vacuum fitting, not the carburetor.

 

[Ovals_and_Bowties]

Replaced part of the vacuum line?

Do you have a steel vacuum line with a short section of rubber line on each end to connect to the modulator and manifold vacuum? Too much rubber line may cause it to collapse under vacuum or wherever there is a kink in it. It should connect to the intake manifold vacuum fitting, not the carburetor.

Yes it's a metal line, I replaced the section of rubber line at the modulator. I didn't replace the rubber section by the engine. It looked fine and I figured if there was a problem with that part I would've seen it the last time I had my leak smoke test machine hooked up to the intake.

 

[Hunter79764]

I'm no expert, but the detent cable shouldn't matter here. Modulator could cause some issues, but typically if that is the problem, you get high, firm shifts from my understanding. I think there's some kind of valve body issue, and probably safe to say that it's beyond a driveway fix over the internet, unless you wanted to try throwing a shift kit in it and see if that inadvertently fixes the issue?

 

[Ovals_and_Bowties]

I'm no expert, but the detent cable shouldn't matter here. Modulator could cause some issues, but typically if that is the problem, you get high, firm shifts from my understanding. I think there's some kind of valve body issue, and probably safe to say that it's beyond a driveway fix over the internet, unless you wanted to try throwing a shift kit in it and see if that inadvertently fixes the issue?

Yeah I didn't think the detent should matter. But that does remind me if I recall correctly it was for some reason adjusted to open before WOT. When it still had the Q-jet for whatever reason. My first suspicion was that the modulator is modulating the pressure down too low or maybe there's something wrong with the governor I'm not seeing. I suppose I could pull the valve body out disassemble and inspect it.

I wonder if someone was using the detent to artificially boost the line pressure?

 

[Ovals_and_Bowties]

I should've mentioned at the start I don't know the full history of the truck. I know it had a rebuilt engine put in sometime in the 90s because there's a rebuilder's plate on the engine. It could well not be the original trans either but I haven't a clue how to tell if that's the case.

If it's not the original I suppose the gov could be mismatched to the rear. I haven't a clue how to tell one gov calibration from another though.

 

[Hunter79764]

My understanding, and again, I'm no expert, is that the modulator controls the RPM and firmness of shifts until the detent gets activated. When the detent is pulled, it takes over and commands essentially full pressure/firmness shift and send the shift timing responsibility to the governor, which uses weights and springs to "figure out" when the engine is at redline and go to the next gear. Which is why I think you have a valve body issue, something in the circuit is taking the information given by the modulator (because you likely aren't in the detent control range), then trying to shift but fluid is going to the wrong places and you end up in both or neither gear for a bit. All of that is either valve body (which is a hydraulic computer if you've never seen one) or internal damage to the case itself (less likely in my mind, but who knows?). There's some transmission guys on here that may know much better, see if Nick has a thread on the TH350 that might help or reach out to one of the experts on here.

 

[Ricko1966]

You can drive without a modulator they just shift very late and very hard,you can drive with no detent cable and they just won't downshift. Modulator and governor control upshift,detent cable and selector lever control down shift. I'd make sure my detent wasn't hanging up anywhere first,and then take a test drive with it disconnected. I'd also make sure my fluid was not to full or too low. If it still hunts under those criteria,I'd assume internal transmission.

 

[Ovals_and_Bowties]

You can drive without a modulator they just shift very late and very hard,you can drive with no detent cable and they just won't downshift. Modulator and governor control upshift detent and selector lever control down shift. I'd make sure my detent wasn't hanging up anywhere first,and then take a test drive with it disconnected. I'd also make sure my fluid was not to full or too low. If it still hunts under those criteria,I'd assume internal transmission.

I took it for a short drive without the modulator vacuum hooked up it held first at least up to 20MPH. I didn't go any faster I didn't go any faster than that because cylinder 1&2 crank journal is out of round and I didn't wanna break the loctite holding the bearings in place.

I've pulled the detent cable a dozen times everything seemed free to move and triple checked the adjustment.

Fluid is hard to tell someone replaced the dipstick with a random one cut down to fit at some point and didn't bother labeling where full should be. It's about an 1.5" up the stick.

For internal what do you think it would be? Broken 2-3 spring, crossleak in the valve body or elsewhere? It really seems like something control side to me valve body, mod, or gov. I'd hate to take it out, rebuild it, put it back in and still have the same problem.

 

[Ricko1966]

Okay now hook the modulator back up and go drive it with the downshift cable disconnected.did you check the fluid? Not over or under full?

 

[idahovette]

Fluid level looks to be a problem with his description of the dipstick?

 

[Ovals_and_Bowties]

Okay now hook the modulator back up and go drive it with the downshift cable disconnected.did you check the fluid? Not over or under full?

Okay with detent disconnected same problem.

Fluid is about 1-1/4" up the stick idling in park. Sadly somebody (previous previous owner I think) cut a random dipstick down and didn't bother to label full.
Local scrapyard said it looked like a TH400 stick to him maybe. I've had the fluid an inch lower than it is now and it would still move and I've had it much higher before too. Pulled the stick after a drive and from the fluid on the stick it didn't look to be aerated.

With how light the throttle is when the erratic shifting occurs, I highly doubt the stud is anywhere near the end of the slot on the detent to even begin pulling on it. I hooked my DMM up to use as a tach and we're talking under 1500 RPM here.

 

[Ovals_and_Bowties]

I think full is supposed to be near where the pan meets the case. Could someone confirm or deny that?
Guess if I pull the dipstick tube out and it starts puking fluid everywhere then it's over-full?

 

[Rustisbest]

I've always been told level with the pan rail on TH350 and TH400 at operating temp. Dont know about any others. Best to verify, the new aftermarket one on my 400 was off 1/4"

 

[Ricko1966]

Full is even with the top of the pan.