The Tranny Switch

amd103 · February 22, 2002

Hi folks,

The quest to replace my '90 1/2 ton 4x4 Suburban with a "newer" '90 Suburban continues. Basically I need to find one that is a copy of mine without the rust and exceptional miles (250k). The latest option: a 1990 3/4 ton Suburban with 80k on it.

Here's the problem: it does not have OD. It was setup with the 350 engine (like mine), 4x4 (like mine), tow package (like mine), but does not have overdrive. This could dishearten some, but I know my old Suburban has OD...can you see where this is going?

Question: How bad an idea is it to take the tranny off my 1/2 ton '90 with 250k on it, rebuild it, and place it on the "new" 3/4 ton truck? Will the 700R4 tranny I currently have simply bolt onto the other's 350 without problem? The only issue I see is the beefier drivetrain etc. that the 3/4 ton has. Is this even an issue? I'm assuming that these old trucks wouldn't have any computer issues with this switch...is that correct?

Any help or comments welcome.

Thanks!

DuckhunterInTN · February 22, 2002

For the cost and trouble of all of that, you might could add an overdrive/underdrive unit (like from Gear Vendors, etc) and come out cheaper and stronger. Just a thought.

amd103 · February 23, 2002

Now that's an interesting idea. I'm not too familiar with these units, are they simply an additional gear box that gets placed in front of the transfer case and changes the tranny's output ratios?

Have people had pretty good experience with these?

Thanks

95 Silverado · February 23, 2002

the aftermarket overdrive units are pretty pricey and require many drivetrain modifications to install them. If the 3/4 ton suburban has a turbo 400 transmission, the transfer case has a different input spline than what's on the 700R-4. you could either swap your tranny and transfer together, or use the input gear from your transfer in the other one to make it work, if the 3/4 ton has a turbo 350 tranny, the transfer case will fit without any modifications as it has the same spline count as the 700R-4. both vehicles probably have the same type of transfer case.being that you will have both vehicles to work from it shouldn't be a bad swap. the only other difference would be the wiring for the lock-up converter on the 700R-4 might not be on the non overdrive sub you are swapping it into, but you could get it from the donor sub.