1987 Suburban SM465 to TH350

[danno676]

For marital reasons I am going to convert my Sub to an automatic. I know most of what will be required, but have a few concerns.
A friend gave me a like year'd 350 engine/trans/transfer case, so that makes it simple.
It's an 87, so it's fuel injected, what do I do about the computer (idle compensation and so forth), would I just need to get a prom for an auto truck?
Crossmember?
Driveshafts?
Guess that's it.
I plan on leaving as much of the manual stuff in there as I can so I can return it to what it "should be" easier so I will most likely be buying a quality floor shifter.

Thanks for you help.

 

[shady]

I'm in for semi similar reasons... I'd like to switch to auto too.

So the wife can drive it lol

 

[obijuank5]

If it's from a similar year truck it should be a 700r4 and shouldn't need new driveshafts. If it is a th350, everything is pretty much different.

 

[danno676]

Yeah, my wife says that every time she drive the truck "that damn transmission fights her about everything", I tell her now she knows how I feel.

The engine/trans/transfer that I was given came from a 3/4 suburban new enough to have TBI but old enough not to have a serpentine belt set up and it's a TH350. I've seen the 3/4's come with TH350's and 400's. I find it hard to believe that a blanket statement like everything is different to be correct. I'm sure there will be drive shaft length differences, which I am fine with correcting. Otherwise??? Anyone??

 

[diesel4me]

From what I have read & seen,a TH400 will bolt in place of a 700R4 and you dont need to modify the drive shafts,the overall length is "close enough" ...the T-case for a TH400 has a 32 spline input and a 700R4 one has a 27 spline,but if your getting "everything" as a unit that is no concern to you...

A TH350 is shorter than a 700R4 by an inch or more,they sell a spacer that lets you put one in place of a 700R4 and use the same drive shafts..

What I'm not sure of is how much different a 4 speed setup is in overall length,than a TH350 setup....someone else here likely knows..

I think the 4 speed will bolt in place of a 700R4 and not need driveshaft mods or vice-versa..

When I put a SM465 in place of a TH350 in my '72 K5 I had to get the tranny,transfer case,all the clutch parts,pedals,flywheel--after I had it all installed the rear drive shaft still fit OK,but I had to get a different u-joint yoke for the t-case ,the 4 speed t-case had a bigger u-joint (probably off a 3/4 ton ?)...the front drive shaft no longer fit,but I was able to find a used one from a 4 speed truck at a salvage yard..

 

[danno676]

Good info. Did your cross member still line up correctly? That's my main concern, that and the computer. I know the newer cars will compensate idle speed when the vehicle is shifted from in/out of gear with automatics.

 

[diesel4me]

My swaps didn't require moving the cross member,on both the '72 K5 when I put in a SM465 to replace the TH350,and my only other swap was a '77 GMC that had a NP-203 full time t-case with a TH350--that t-case was "skipping" upon take off,like it had a loose chain,so I put a NP-205 gear drive case I had that had a TH350 behind it previously in it instead..that required a much shorter front drive shaft,but the rear one still bolted right up..

Keep in mind these were much older trucks than yours--GM had at least a dozen different drive train combinations,and at least 3 or 4 different cross members for later 4x4 trucks --some had "double humps" for the exhaust,some were flatter,others were different heights where the t-case "foot" bolted to it..others here like SweetK30 will probably have better info on that ..

A friend recently swapped his 4 speed '86 K20 to a TH400 so his other half can drive it..he had to make a spacer to go under the t-case foot in order to keep the original cross member,but as far as I know he didn't have to swap or modify the drive shafts,but I didn't help him or get all the details the swap entailed..it only took him a few days in his driveway lying on his back..wish I could still do that..

He did get lucky that his radiator had the transmission cooler in it--most standard tranny trucks dont have one,but anything 30 years old has likely had another radiator swapped in it by now..

I'm no help regarding the computer issue...I never owned a square body new enough to have a computer--and probably wont!..others here will have to give that advice..

 

[danno676]

I kind of figured it would boil down to luck of the draw on a few things that can probably be solved with some good measurements. My major concern is the t body. I can't say I love it, but it is nice and reliable as the whole truck only has 77k miles. My wife used to drive a carbd k5 but with 2 kids in tow her flexibility is getting more limited, I'm just trying to get together a backup/snow/Holocaust vehicle.