OD trans swap into an older 3500 dully

[SALTDUALLY]

I'm in the final stages of swapping a 700R4 into my '88 GMC 3500 dually to replace the THM-400-type 3L80. It seemed like it was a no-brainer until we actually got into the work. I admit that we weren't thorough enough in our homework. For example. it didn't occur to me that my recently purchased '88 would have an electronic speed sensor. When my candidate K-case 700R4 had the transfer case it came with replaced with a 2WD tailshaft housing I opted for mechanical speedo drive. Even the trans specialist who went through the 700R4 to ensure it was stout and with all the good HD pieces, figured that's what was needed.

 

Had I taken the time to crawl under the truck I would have seen that it had an electrical and not a mechanical link to the speedo/speed sensor, but having dealt with T5s of the same era, I assumed it was mechanical. Anyway, we found a correct tailshaft housing, reluctor, and speed sensor in the Chicago area, bought it and had it shipped to our remote location on the middle of Nevada.

 

In the meantime, I drove three hours West to Carson City where a Drive Line Service shop shortened the intermediate shaft 2-1/2 inches, using a yoke that matched the 350/700 slip yoke rather than requiring a "conversion" U-joint which might no be so easy to find on the road should there be U-joint trouble at some future date. I then drove three hours East to return to my pal's shop, all set to finish the installation the next day, since the correct tailshaft housing, reluctor, and speed sensor had arrived during my absence.

 

The first task in the morning was to bolt the new converter to the flexplate -- a routine task for an old Chevy guy; I'd done it countless times, with SBCs, BBCs, 'Glides, 350s, 400s -- how would this be any different; besides, we had a new lockup converter spec'd for the 700. Well, there's another converter scheme with a different bolt-circle diameter than most GM converters, and that's the converter/flexplate combo on my '88 454. A bit more 'net searching and we locate a HD, dual-pattern flexplate for the 454 (externally balanced, so needs a counterweight on the flexplate to complement the one on the harmonic damper).

 

So, while we're waiting for the flexplate (which can't be ordered until tomorrow [Monday] morning, due to arrive

Wednesday), we're relocating the transmission crossmember forward 5 inches, installing the reluctor on the output shaft, installing the new tailshaft housing, and sorting the TV cable adjustment.

 

It's been quite an experience, and we're not done yet solving problems if the scheme so far is any indication. I'll follow up on this thread with final details and fuel-mileage and performance info, but in the meantime I'd like to hear from anyone who has done this upgrade to a thirsty old 1-ton dually.

 

Mike

[95Sierra2500]

I just have one question: if your truck is a 2wd (which it sounds like it is), why didn't you just have the driveshaft shortened and install an electronic, stand-alone OD behind the old 3 speed? It would have accomplished the same outcome with alot less headache in my opinion.