Th350 behind 4.8ls

[dirtscoots]

I acquired a 77 step side with ls and a th350. The transmission doesn’t engage until about 3000rpm. It just revs then finally will start going. Reverse works as it should. There wasn’t a vacuum line hooked up, so I routed a vacuum line to my intake and hooked up the kick down cable to my throttle cable. What is my next step? Transmission supposedly was rebuilt. Also the speed sensor is just unplugged and puking oil out of an open hole. I have a plug ordered for that.

 

[Trucksareforwork]

Hmmm. My first thought was you have a high stall torque converter installed, but 3000 is really loose. Do you know the converter specs?

Since reverse works properly it’s probably something internal to the transmission. Clutch plates etc.

 

[TotalyHucked]

Throw that T350 in the trash and put a 4L60 behind it. I'll never understand why guys do the LS but keep a non-OD trans, that's half the benefit of doing the swap.

 

[dirtscoots]

I don’t know any specs unfortunately. And “rebuilt” is speculation realistically. 3000 rpm is a guess I don’t have any gauges. Is there any chance i coukd not be getting enough vacuum off my intake ? I have that line attached to a port off my throttle body

 

[bruisedbanana79]

Out of curiosity have you checked the fluid levels hot?

 

[Matt69olds]

Throw that T350 in the trash and put a 4L60 behind it. I'll never understand why guys do the LS but keep a non-OD trans, that's half the benefit of doing the swap.

Or use a carb on a LS!!

 

[TotalyHucked]

Or use a carb on a LS!!

Yeah, that too!

 

[Rusty Nail]

pix or no fix

 

[Ricko1966]

I don’t know any specs unfortunately. And “rebuilt” is speculation realistically. 3000 rpm is a guess I don’t have any gauges. Is there any chance i coukd not be getting enough vacuum off my intake ? I have that line attached to a port off my throttle body

That vacuum only affects 1/2 shift and 2/3 shift. Not take off.

 

[Ricko1966]

Oh I thought it was the engine that was so much better,not the superior engine management and the overdrive transmission.

 

[xm20k]

I run a 4000 RPM stall in a power glide in my truck and as soon as I put it in gear it wants to roll just from idling and as you add throttle it almost feels like its slipping, but it would feel this way in reverse as well if it was the converter.

Something in the trans itself isn't right like low fluid, not enough line pressure or the forward clutches are not fully engaging/burned or were setup too loose. My guess not enough fluid or rebuilder tossed the stock pump back in without rebuilding it and its not making enough pressure. Fix your leak and make sure it's full first.

 

[AuroraGirl]

That vacuum only affects 1/2 shift and 2/3 shift. Not take off.

the modulator, assuming problems ,would probably plummet the vacuum in the engines intake at least some, probably disrupting the fueling? or maybe not enough to affect it much?

 

[AuroraGirl]

Throw that T350 in the trash and put a 4L60 behind it. I'll never understand why guys do the LS but keep a non-OD trans, that's half the benefit of doing the swap.

my guess is some guys just have the engine or have an easy source and dont have the time/knowledge to modify the column to use a cable instead of linkage and have enough positions so its accurate (assuming they have a th350 or th400 stock column) , or they just dont feel comfortable making that leap

 

[jwroblesk]

Sounds like they got a spring and valve in backwards in the valve body.

 

[TotalyHucked]

my guess is some guys just have the engine or have an easy source and dont have the time/knowledge to modify the column to use a cable instead of linkage and have enough positions so its accurate (assuming they have a th350 or th400 stock column) , or they just dont feel comfortable making that leap

It's usually that part. It especially blows my mind when it's an old guy like my dad that'll finally get over the fear of EFI but a computer controlled trans is a leap too far lol