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Seeking some advice - 1990 K3500 6.2HD Utility truck

1396 Views 27 Replies 7 Participants Last post by  benthecomputerguy93
New guy, first post. If this thread is in the wrong subforum then I apologize. Also if I happen to leave out anything important here then please let me know and I will add it.

Starting at the beginning - I recently purchased a 1990 K3500 with 40k original miles. As I understand it, this truck was originally a special order for the fire dept. in a small town around here, but they ended up not using it, so it was then sold/sent to the water dept. in that same town where it was then used for 25 years (thus the super low miles). The first thing I did after purchasing was take it by a local shop where I get some work done, and have one of their guys look it over. This individual co-owned a chevy dealership for about 30 years so I figured he would be able to tell me more about what I had. His answer surprised me a little, and that's why I'm here. He claims to have never seen this exact configuration before, which could have something to do with it being a fire dept special.

Here's what we have:
  • 4x4 but its a split diff with a skid plate (according to him, it should normally have had a solid axle)
  • 16" 8-lug wheels (he believes that stocks would have been 19")
  • 159.5 wheelbase - (this is where he explained to me the difference between a cab+chassis configuration, versus a simple "bed delete" option on a normal truck, with the different frame-rail spacing, axle width, etc. He was a little bit perplexed because he couldn't tell which one it actually was. Also said he thought this length was common for a crew cab, but not a single cab)
  • He ended with saying that because this was the body-style change year for the 3500, this was the first year of these and it could be some one-off deal due to that.
I realize that this guy could just be old and confused. He could also simply be full of crap. Thats why I'm here.

So what do I want? Two main questions, at least for now.
  • I bought this not realizing it might be "special", just hoping to adapt it to pull a camper. I'm curious if there is even anything I can easily fit on the back, to replace the utility bed that is there currently, since thats not really convenient for my intended purpose. I have no idea where to look for such information, and even if I did, I'm willing to bet that without knowing exactly what I have, it would lead to some pretty confusing results. I thought about trying to fit a 8' regular bed on it, but then that leaves a ~3-foot gap between the bed and cab. I've seen this configuration a time or two, with some sort of frame-mount basket or something fitted in between to take up the space so it doesn't look stupid. Any advice or suggestions, first-hand experiences are much appreciated.
  • Whats the deal with this engine? I've read some pretty mixed things about the 6.2HD, but most say that its made more for economy than for pulling, even though this is a 1-ton truck. If this is the case, then I may be better off trading it off for a cummins or something. Again, any advice is appreciated.
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First of all, welcome to the group! I would say, anything that old and with only 40k miles is already special!
Don't let the special/custom stuff bother you, all parts will probably be a bit difficult to find.
That axle is different, maybe some pictures would help us help you.
And the engine, it will pull the doors off anything you might want to tow. I suspect it has the proper axle ratio to use the engine's power.
Post a picture of the build codes sticker and we can help decode the build info.
The sticker should be on the glove box door
Well, how big is your "camper"? In my part of the world, a camper is a soft/hard top tent trailer, or a slide in.

Your truck is going to have low gearing in the rear axle (that's why I asked for build codes) and it will probably cause the engine to run around 3k rpm, this should be enough rpm to maintain road speed (unless your load is real big).
OK, the GT5 is a 4:10 to 1 reduction gear set

The G80 is a "standard" GM locking rear diff assembly, noting special there

I suspect the axles are full float, sounds pretty standard to me.

And, I guess a skid plate is good.
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Beach, the M40 is his transmission code, a manual 3 speed

The LL4 is the engine code

No overdrive in the transmission, so the 65 mph revs will be kind of high

Unless, the rear axle your buddy was talking about is a 2 speed axle (might be the reason for the skid plate (vacuum operated)).

The 2 speed codes are FA1 and FA2, doesn't appear you have a 2 speed.

But, it could have been changed after it was built.

Ben, do you have a knob fastened to the shift stick?
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Wait. I didn't catch this yesterday. This truck is an automatic, and there is no hole in the floor to suggest that it ever was a manual, unless they did a really good professional swap. I suppose I could check it out from the underside. So I wonder what transmission I have then?
My bad!!!!

The M40 is a 3 speed automatic, the Turbo 400

Sorry Ben and Beach, must have had too much rum when I checked that code.
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Hmmm, practical!!
None of this is practical, a big truck to haul an even bigger trailer.
When you consider the purchasing costs, the insurance costs, the towing costs and the camp ground fees; none of it is practical. You can stay at a very nice hotel resort for the cost of "camping".
So be it, none of it is practical; but it sure is fun!!!
So, it will cost a little more for fuel, at this point, so what.
My 2 cents
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Well, let's try to figure it out, we know the rear ratio, the trans gears, all we need is tires.

Ben, could we have the rear tire size?
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