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Old 11-09-2007, 07:40 PM   #1 (permalink)
pontiac59
 
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Is this worth getting into the middle of?

I'm driving a well-worn '89 Suburban that has fairly low miles, but is rusty - a 350 TBI truck with a 700R4.

I found a 1985 3/4 ton Suburban with an excellent body being sold surplus by a town highway department. It has the 6.2 diesel, and apparently a bad transmission - it's described as having no high or reverse, although they don't note if it's a TH400 or 700R4. The truck shows around 98,000 on the odometer. In theory, it should have been properly maintained all this time.

So I had the idea that if I can get this cheap (being sold through an auction) I could just swap in the 700R4 trans from my truck, since the '89 was a little better anyhow, and run the diesel truck as a driver and occasional car tower/parts hauler. From some digging I've done, it looks like I'd need to retain the diesel torque converter, and possibly swap out the governor (assuming it's not what's causing the problems). I think I can probably handle that, as long as I don't have to get into the trans case itself. In the process I'd do a fluid and filter, put new steel lines on, and plumb in an auxillary trans cooler.

I figured the Diesel has the added benefit of around 20 MPG highway, which would be a big improvement over the 14 or so I'm getting now. (The Fed Govt Milage website has it rated with the revised figures for 22, but it might be overly optomistic).


So as the title says, is this worth getting into, or am I just heading for trouble down the road? As crappy as my truck is now, it's never let me down, and while the body is rough the floors are good. I just hate the gas milage, when I have to keep most of my project cars 75 miles from where I sleep it gets expensive.

I suppose I could swap the drivetrain from my truck into the other body, but that would be a long involved project if I keep the TBI - and I probably would, it's good for a little better milage than a carb is. But it means a fuel tank swap, wiring harness swap, etc. etc. - instead of that, there's an auction the end of the month with a 1-ton van that is a 350 TBI, I'd probably just buy that.

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Old 11-09-2007, 10:36 PM   #2 (permalink)
EngineBuilder
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I would see if you can get ahold of the maint. records from the county/city maint. shop prior to purchasing the truck; this would give you an idea of what problems the truck has had, (if any) and what parts have been replaced.
The diesel engines in these trucks are pretty solid, even at higher mileages; if the odometer is to be trusted, I would say that the engine, at least, has a goodly number of miles left in it.
The trans swap you are considering is not a very involved procedure; and it seems as if you've already done your homework on it- you will need to retain the diesel torque converter. (assuming that the '85 has a 700R4- I believe the TH400 has a different, incompatible converter?)
If the 85 does indeed have a TH400, you will also need to transfer over the TV cable assembly- not difficult, but a critical component in the operation of the 700R4.

If you had the VIN number of the 85, I believe there are resources available that would tell you which transmission it had from the factory.

But as to your main question; is it worth it? IMO, yes. Fuel economy right now is offset by the difference in fuel costs, true; but you can mix your own biodiesel at home for virtually nothing- you can't refine your own gasoline at any cost.
The reliability of the diesel engines rivals the tried and true chevy 350, and I would venture to say that the diesel maintains its power longer than the 350 does.
There are inherent differences in the cost/mile ratio; a diesel ends up being a little more expensive due to the expense of parts/materials as compared to the gasoline engines, and this is more a product of scale rather than a direct cost issue; there are more gasoline engines on the road, therefore more parts suppliers make parts for them, keeping the prices low. Your ability to work on the truck yourself will make up for much of the expense, and with the knowledgeable members' backing from web forums such as this one, you should be able to tackle most any problem that may arise.

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Old 11-09-2007, 11:52 PM   #3 (permalink)
pontiac59
 
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I have the Krause Standard Catalog of Chevy trucks and according to it I can decode the motor, but not the transmission. It seems to imply that you could buy the two together, but that may just be the writer's opinion.

The town is actually selling two trucks, this one, and a gas 4x4 that appears to have been a fire department truck previously; it also has trans problems but they note it has a new carburetor. I don't need four-wheel-drive really and it takes the milage down to 10-12 if you're lucky, so I didn't consider that truck.

Thanks for the info -
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Old 11-10-2007, 12:37 PM   #4 (permalink)
High Sierra 2500
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Not a bad plan if the price is right.

The transmission swap isn't too big of a deal. If the diesel truck has a TH400 you will have to install a TV cable and hook up the torque converter lockup. Not a big deal. If it already has a 700R4 it really isn't hard to set up. Keep in mind that when you have the trans out you could get it rebuilt if funding is available. Just a thought. The 700 isn't the longest lasting transmission.

Do not reuse the torque converter from the diesel truck if it has the 700 already. Buy a new diesel converter. Using a converter from a transmission that failed on a good transmission is a good way to create another junk trans.

Anyway if the price is right I say it sounds reasonable.
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Old 11-10-2007, 02:58 PM   #5 (permalink)
redlinediesel
 
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The 400 trans is bomd rated the 700r4 is junk. Although I hear you can install a 400 pump on a 700 and make it pretty reliable. The pump usually is the reason that the 700 fails. If you do this to the 700 go inside because I would bet that the bands are all burnt and like Siera said you will ruin the new parts.
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Old 11-11-2007, 05:19 PM   #6 (permalink)
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The 700R4 can be bomb rated it the correct parts are used to rebuild it. I don't recall the correct parts exactly but any competent builder knows. I had one rebuilt that out lasted the two trucks it was in. The TV cable is what kills most of these. It gets out of adjustment and the tranny burns up because there isn't enough line pressure to hold the clutches for the amount of power going to the tranny.

The advantage of the 700R4 is one more gear and the truck is probably already set up for it. No drive shaft change, no vacuum system to dink with.
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Old 11-12-2007, 01:37 AM   #7 (permalink)
4doorTAHOE6.5TD
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Reverse & direct clutch failure indicates the trans is a typical 400 failure . Also I doubt the 700 being an available option on an 1985 2500 4wd. The 700 is the trans to put in. Cruising MPG could be as much as 5MPG more than the 400 3 spd. The engine simply runs30% less RPM in OD. If you don't need the 4wd just get rid of it,installing the 2wd from the other truck. However since it is from a gas rig all control componets from a Diesel will need to be installed in the gas unit,plus the flexplate or redrill the one from the 400. Not one thing from the 400 will be of use.This used to be a common change over made in the 80's,including my own 1990 2500 6.2 suburban & up to mid 90's in my automatic trans shop. Personally I would look for a 700 equipped 6.2 Diesel donor ,trans & all the componets on the truck for the change over parts.Finding someone to master all the changes from gas to diesel in the trans & truck will depend on finding a competent person that knows the different Diesel components & where they're at. Donor that is complete will make it simple in relation to finding all the parts here & there. One thing that help you make the decision is the fact the trucks 400 is DOA as purchased,so you have to fix/replace the 400 anyhow. Your just looking at the differential in parts cost basically & if you can find a healthy diesel 700 2WD trans & donor parts in one spot,that will be the jackpot & just sell your present gasser to recover the cost !! I'm a dealer & The 85 is only worth $200$/300 as it sets IMO.
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Old 11-14-2007, 12:49 AM   #8 (permalink)
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Well, assuming the town approves the bids, I own the truck - for less than $200. Which isn't bad considering the one I have now shows a tad over 4500# on the title, and depending on where you are and what time of year it is the scrapyards have been giving as much as $150 a ton for cars and trucks with engines. So it's always going to be worth what I paid for it, even if it's only good for parts.

Just to make things clear, this truck is a 2-wheel drive. The 4-wheel drive truck that was also offered, is a gas truck, and I didn't bid on it. I thought about it, since it would be easy to just put the 2wd trans in and do away with the 4x4 stuff, but to eliminate the 4x4 front axle for a 2wd setup requires changing the front end from a leaf spring setup to A-arms and coils - I wasn't sure the 4x4 would even have provisions to put that stuff on the frame, especially since on the one I drive now I had to actually cut holes in the body to bolt in barn doors and replace the shot tailgate it came with, it did not have provisions for both.

I have three different you-pull-it yards within 40 miles of me, so if one of them gets a diesel truck with the 700R4 I should be able to get all the parts I'd need, fairly cheap that way. So about the only remaining question is just what years I can use - gas trucks go to an electronic speed sensor instead of a mechanical speedometer drive in 90 or 91, and eventually the whole trans becomes computer controlled. I think with the right parts you can convert one to use a speedo cable, though.

To be honest, I used to know more of this stuff, I went to school when these changes were fairly recent, but I've never really worked on late model cars and just kind of forgotten it all playing with antiques instead.

In the meantime, if I just want to patch it up and make it go with a TH400, does that need to be from a diesel as well?
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Old 11-14-2007, 02:43 AM   #9 (permalink)
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Probably should add that I do plan to occasionally tow - flat tow automobiles, trailer them when that's not possible, up to around 5000 lbs on a steel open deck car trailer. Normally about a maximum of 100 miles, and if it averaged once a month that would be a lot -
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