Thanks for the "intro", raidersadie.
John,
I'm right in the middle of "the upgrade" on my '83 P'up. These transmissions were weak to begin with, and on top of that--they require motor oil instead of the usual 90w gear oil. That's a fact that most shops are ignorant of, so the bearings fail prematurely. I don't know if that's the case with yours, but I'm pretty certain that's what happened to mine at about 85k miles.
According to our resident Isuzu expert (on the IsuzuPup.com forum), if you use 5th gear below 45 mph, or lug it at all, it really contributes to early failure.
Isuzu redesigned this transmission for the '86 model year, and it's quite a bit stronger. The output shaft is noticeably larger, and some bearings are also larger. These later transmissions (diesel through '87, and gas all the way through '95 models) can be retrofitted to the earlier trucks with very minor modifications. I'm using one from an '87 Impulse RS Turbo, because it had a higher 5th gear.
We have a
LONG topic on the transmission upgrade. It's in the "how-to" section of our forum, but it's really just a long topic that lots of people chimed in on. Eventually, I'll re-write it into one step-by-step "article". But that's AFTER I finish my own upgrade.
Basically, assuming your donor transmission comes from an '86 or later gas Iisuzu P'up or Pickup, you need 1) a good diesel 5-speed bell housing (you should have that already), and 2) a slip yoke that fits the new transmission's larger output shaft AND fits the old U-joint. There's a list of Isuzu vehicles this could come from, and we keep having to revise the list because we discover donors that we missed, or we learn that some do not fit. But I pulled three at a salvage yard, and I have two left to sell ($50 shipped). They're all from '86 P'ups and are guaranteed to fit.
I'm told the "upgrade" transmissions should last the life of the truck, if they haven't been abused.
Jack
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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.comIsuzuPup.com