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'78 GMC Jimmy 3/4

VW_Factor

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Woodburn, Oregon
Not that this is going to be a "build thread". Simply a way of introducing myself, and my father's truck.

This truck has been around for quite some time. Trying to do the math, but I can't really recall how old I was when he first purchased it. Not really important I suppose, but it was nearly brand new.

400sbc, TH350, NP205 I believe (it operated like an all-wheel drive, putting the shifter into the traditional 4H or 4L would lock the front and rear, otherwise it operated like an open diff). Its since been switched out for a more tradtional TC, unknown at this time to me. Still has the original Posi unit in the rear. No idea the gear ratio rear or front, but the truck came with a tow package. Engine oil cooler, trans cooler separate from the rad.

Many many years later, it certainly doesn't look new anymore. Eventually, the looks became unimportant, function was key.

It was purchased in California and eventually made its way to Oregon Coast. When the smog equipment began to fail it was simply removed. Exhaust system began to rust into nothing, the muffler and cat were falling off. My buddy and I got some Hedman headers for my fathers birthday and installed them. So it ran open headers for a couple of weeks until we could afford some mufflers hah.

We got around to installing a 3 inch body lift. Back when I was younger, I wanted to see it be huge and all. My father wanted 33's and still be easy to climb into.

A/C failed and being on the Oregon coast, decided to remove it.

First wreck I ever had was in this truck. Hit some washboard on a gravel road went sideways into some trees. Tore up the running boards and bent a wheel. Removed the running boards, and got "new" wheels.

Original trans suffered a break down around this time. Splines striped off the input shaft. Replaced the trans, which just so happened to be manual shift for some reason. My father hated it for years until he came to find it to be beneficial in certain situations to start off in 2nd.

Years of being on the Oregon coast, took its toll on the sheet metal. The tail lights are screwed into the inner panels, as the outer portion of the rear fenders have fully rusted away. Things such as that. There are dents and war wounds all over in various areas.

17 years later, my father had ended up back in California. he did some small things to it, like replace the window regulators that had rusted away, and put in a good electric fuel pump. The ignition switch was completely busted so he went with the switches and momentary push button start.

Never drove it much at all, it sat still, dormant for 6 or 7 years at this time.

I finally drove down to pick it up. Being what I would consider a modern redneck I have no plans to restore the body or attempt to make it look pretty. Simply function. The flexplate is chewed up to hell for whatever reason (looks like the starter is brand new). Took off the inspection cover so I could move the flexplate to where the teeth are better. I wouldn't normally consider this, however the QJet carb looks like he replaced it maybe a few years back and it starts up immediately. I don't think I've ever seen an old tired engine of any sort fire that quickly. (Still the original 400ci engine). Holds maybe 3 to 5 psi at idle and maybe 10 to 15 spun up to 2000rpm. I understand the 400 has a tendency to oval out its mains reducing oil pressure.

Runs super strong, quick throttle response. Has tons of blowby evidence, but not much smoke if any escapes down the exhaust end of things.

I've worked on this very truck when I was a "kid". It introduced me into the Chevy/GMC truck world. This one has over 300k miles on it, and as far as I am concerned, still rocks. Dependable as ever.

So, immediate plans are to replace that stupid flexplate (lord only knows how it got so friggin chewed up), and replace the front crank seal (popped and started blowing oil halfway on the trip). Remove the rest of the interior carpet in the rear cargo area, and begin rhino lining. Rust be damned!
 
400sbc, TH350, NP205 I believe (it operated like an all-wheel drive, putting the shifter into the traditional 4H or 4L would lock the front and rear, otherwise it operated like an open diff).

You are describing an NP203, not an NP205.

Martin
 
You are describing an NP203, not an NP205.

Martin

I don't really recall the difference of them. Just know they look similar and operated differently and do (or did) not remember which was which.

At any rate, the TC operates normally now so I know my father had switched it at some point. He had kept the original trans and TC for years which I remember. I did not find any of that when I was retrieving things. I did find in sets of pictures that he took some images of one of these TC's apart and he was doing something with it.

Too bad I'll never get to find out what.
 
A 1978 Jimmy would have an NP203 from the factory if it was an automatic.

Martin
 
Here is a good site for identification and general info on the NP T-Cases:

off-road.com/trucks-4x4/tech/chevy-sfa-transfer-case-guide-18978.html
 
Reading through that, it is apparent that I do not know the model numbers compared to operation conditions.

Remembering a bit more clearly reading those descriptions, I'll say for sure that the truck came with a part time unit when my father purchased it.

I also remember him playing with it, to find out how the 4WD worked, by removing the rear drive shaft and driving it round a bit as a FWD.

Upon the trans change some years later it came with a TC. (The truck has Warn hubs btw ever since I can remember) Getting it in the truck and buttoned up. Putting it in gear got us no where. Truck would not move regardless of gear. (This was 2wd H position). Putting the shifter into 4W H or L truck would move, but he felt this wasn't right. Took about 15 minutes to figure out that it was a full time operation unit and locking the hubs was mandatory in getting the truck to move with the shifter in the 2WD H or L range.

I'd be willing to guess that my father at some point then has switched the TC back our for another unit as it is again a standard part time unit. Hubs unlocked it operates just fine in the 2WD H or L.

I'll get some pictures later perhaps figure out whats in it now. Reading though it looks like the 203 and 208 are the TCs that are similar in size (regarding driveshaft lengths).

Guess it would be good to know whats in there. Really wish I had talked to my father more about this stuff before he passed. Then I wouldn't be quite as confused as to what equipment he's switched in/out over the years.
 

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