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Guest Message by DevFuse

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old gmc van differential questions

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4 replies to this topic

#1 matts87glsedan


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Posted 26 April 2008 - 07:05 PM

in my photo gallery you'll see some pics of a flatdeck chop that i did (some of you posted some helpful tips in my previous thread) . It was originally a 74 gmc vandura 'corsair' motorhome. due to my ignorance, i finished this project only to realise that the motorhome "frame" is not the same as a standard van/truck frame , and therefor cannot be registered 'as is' as a 3 ton vehicle. (its current door stamp lists it as 6000 lbs rear axle wieght). sooo, i have this thing that i want to get on the road, and i'm going to have to pull some shenanigans to make it happen.

shenanigans aside, i have some mechanical questions :

i have an 84 vandura for parts. my automatic 74 dually vandura revs extemely high before shifting (formerly a motorhome), and has a low highway speed. I think that i can fix my low highway speed by putting some better gears in the diff. will the gears from a 84 3/4 ton vandura fit in my '74, 3 ton diff, and will they be a better highway gearing? visually, the 3ton diff cover is asymetrical, whereas the 3/4 ton diff cover is more uniformly round. from the front of the vehicle, the diffs look fairly well the same.

overall, is a diff gear swap a feasible idea to increase driveability on this vehicle? can anyone speak to the high rev before shift ? i will load this thing up sometimes, but i am not carrying 6000lbs all the time on it, so what are my options.

feel free to ask me to specify more. thanks.

#2 Reveeen


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Posted 28 April 2008 - 01:32 PM

will the gears from a 84 3/4 ton vandura fit in my '74

No. (the "carrier" (case, or center) is different)

is a diff gear swap a feasible idea to increase driveability on this vehicle?

It depends what you mean by "feasible". The best gearing for mileage will be found in a differential from a diesel powered 1 ton (or heavier) donor. It will also be the most expensive. I suppose you could find/order an alternate gear set for the rear end, again, an expensive prospect. It all depends what you can scrounge.

high rev before shift?

Depends on the transmission installed. (identify and ask again, it could be a seized/mis-wired kickdown switch, or a stripped governor gear)

* A "motorhome* registration is not such a bad thing. If you can get it somehow resembling a motorhome you do not have to "scale" it, nor get to buy "premium" ($$$$) plates. I *think* you might find insurance is cheaper too.

"Maybe* cutting the back off of the 3/4 ton van and stealing from the old motorhome would be more suitable?

#3 matts87glsedan


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Posted 28 April 2008 - 08:08 PM

thanks much for the reply...

so a junk yard score will be the only way i will find gears for this thing then... since the whole thing is a rather low budget operation, maybe the diff will go untouched for a while.

the owners manual only lists one automatic trans as an option, and thats the turbo-hydramatic 350. (i didnt crawl under it to confirm, because i wouldnt know how)

A "motorhome* registration is not such a bad thing
i agree. the problem is this is an out of province vehicle. what i SHOULD have done, was get the thing inspected before i started massacre-ing it. now, they're talking that i need a mechanichal engineer to look at it ($$$$) before i can get it into this province. Ahh, hindsight :rolleyes: . To go back and make it look like a motorhome again would be too much for me. I'd rather like you say, chop another cab on the van frame, and then swap all my known good, low K parts onto that frame. but then it would'nt have the same weight rating anymore...

anyway, thanks again, what do you think about the trans?

#4 Reveeen


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Posted 29 April 2008 - 04:22 AM

Ok, a 350 turbo hydramatic uses a cable to "hold" passing gear, this cable is likely hooked to the carb (beside the throttle cable), it could be hooked to the throttle pedal (there's no telling without looking, motorhome chassis are made to order and can get weird).
Check the cable for free operation. (there is a way to set the cable length, check a manual, it's difficult to explain) It should pull and return freely.
There is a dome like tin cover on the side of the transmission, the governor "lives" under there, pop the cover off and pull the governor, don't mess with it, just check the plastic gear that drives the thing, they can strip.
A turbo hydramatic 400 uses an electric switch for selecting passing gear, usually located on the throttle pedal assembly, check it isn't stuck.

but then it wouldn't have the same weight rating anymore...

Kinda like Subaru cars, the GM trucks/vans are kinda like lego, at least the pre 2000 stuff (within reason), a 30 series van (1 ton) should accept, with minimal modification, all the suspension stuff from the motorhome. If the 3/4 ton van has a "real" frame (the newer ones don't) it will be the same as a 1 ton frame.

Weight ratings are kinda imaginary. I learned this fact from a department of transport officer. I had an old International 1/2 ton that scaled 11,990 lbs. (I was only going 3 blocks with this excessive load, the tires were almost flat with the weight) I was running with the plates from an International 1 ton (registered for 12,000) same colour, same year. He was NOT concerned with safety, his concern was if I had paid the road tax, I was told to "be on my way". Moral of the story: pay your tax, f**k safety. At vehicle registration time (when purchasing commercial plates) you can "bump" the weight rating of a commercial vehicle 1 category without any raised eyebrows, or questions, they will gladly take your money. A 1 ton usually registers @10,000 lbs, if you buy 3 month plates it won't be long until you get the weight where you want it.

#5 Nug


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Posted 30 April 2008 - 01:29 PM

A turbo 350's oil pan looks square, with one corner cut off.

A turbo 400's oil pan looks like Texas.

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