Please help me make a decision

[topper]

I am a new member on this forum even though I have been stalking for a while. I have a 1976 k20 suburban that I believe has a trans issue. The trans is a th350 and it takes a long time to shift from 2nd to 3rd plus the mpg is terrible with the 14 bolt rearend and full time 4 wheel drive. I have an oppurtunity to buy a 700r4 for 550.00. I am permanently disabled and am living on my ssi so I need to watch my pennies. Would the 700r4 bolt right up or will I need to spend a lot of money on extra's ? Thank You

 

[bucket]

You would have to swap in a different transfer case and have new drive shafts made. Plus a bunch of nickel and dime expenses. If it's a 2wd version of a 700r4, it won't work anyway without tearing it down and swapping in the 4x4 version of the output shaft.

What is your current mpg? For general use, a gasser Suburban will get similar mpg whether or not there is an overdrive gear.

 

[topper]

Thank you Topper for your response. I'm not sure of the mpg other than it's thirsty, it used an 1/8 tank driving 15 miles with half that on the expressway doing 70. I'm hoping the sending unit is bad but based off my life experiences I tend to believe it's going to be the most expensive thing possible. I do know that the vacuum modulator hold down bolt is a little bit loose but I can't get at it to torque it down. I'm kicking myself for not getting the 84 2500 4 x 4 pickup that I was considering but settled on the suburban.

 

[Bennyt]

Hate to say it but it takes a lot of gas money to pay for any gains in mpg. Best bet is to find someone parting out a late Suburban that you can get the trans and all the associated brackets/ parts out of. In my area, it isn't too expensive and I know several people parting out 4wd Suburbans.

 

[ali_c20]

Most important thing would be to check if the transmission is bad or if it's just the modulator, governor or a vacuum leak to the modulator. Transmission swap wise the cheapest solution would be a donor vehicle like Ben said. You could remove the front driveshaft and run the t case in hi lock to have only rwd. Not sure if this helps with mileage.

 

[Hunter79764]

If it is just a delayed shift, then I'd be checking that modulator out. If the vacuum line is not connected or cracked, it will slam hard shifts, and shift very late, essentially "thinking" that you are flooring it even when you are just putting around. I think most setups have a hard line that runs most of the way, but a rubber boot at the modulator and another at the carb/intake, and either of those can split in such a way that you don't see it obviously at first.

 

[Rusty Nail]

I believe your money may be better spent with a transmission shop. I'd maybe check out an inspection of some sort - have it serviced. Maybe they would have other ideas because im certain swapping to a different transmission is cost-prohibitive for your specific situation, your effort will be better spent thinking along a different line.

There could be any number of issues tanking the MPG between the driveline and the transmission.
The gearboxes, the ujoints, it could be a ton of stuff but a transmission shop may be a good place to start.

Good luck!

 

[Grit dog]

I am a new member on this forum even though I have been stalking for a while. I have a 1976 k20 suburban that I believe has a trans issue. The trans is a th350 and it takes a long time to shift from 2nd to 3rd plus the mpg is terrible with the 14 bolt rearend and full time 4 wheel drive. I have an oppurtunity to buy a 700r4 for 550.00. I am permanently disabled and am living on my ssi so I need to watch my pennies. Would the 700r4 bolt right up or will I need to spend a lot of money on extra's ? Thank You

I'd say, if it just shifts "late", but otherwise runs through the gears just fine, kick's down a gear like it should, etc, there is nothing wrong and someone may have added a shift kit or something making it hold 2nd longer than normal. Bottom line, once you're at highway speed, it's running in 3rd gear, right?
As far as saving your pennies, if it ain't broke, don't fix it, IMO.
What gears and tire size and engine do you have? What rpms are you running at 60mph, 70 mph, etc?
A truck with 4.10s and no overdrive AND stock size tires will not be friendly at 70mph or faster. My truck with 4.10s and half worn 35s runs 3000 rpms at 70mph (no OD). So, an OD trans would be an improvement if you have deep gearing, but like said above, you can buy ALOT of gas for the cost of a swap to an OD trans.
Regarding your fuel mileage, first, does it run "right." Smooth, not overly rich, no timing issues, etc?
Second, I can't fathom it uses 1/8 of a burb tank (40 gal) in 15 miles.
Have you actually calculated your mileage off of a whole tank? It's virtually impossible that you're getting 3mpg (5 gal in 15miles). Even my 454 4.10 gear truck gets about 7mpg in city driving.

 

[Snoots]

I've had that short piece of rubber line from the metal tube to the modulator get oily and go to poo. I installed a new rubber hose and it fixed my shifting issue. Just my 2 cents.

 

[topper]

I've had that short piece of rubber line from the metal tube to the modulator get oily and go to poo. I installed a new rubber hose and it fixed my shifting issue. Just my 2 cents.

I did replace the rubber hose on the modulator or I should say I had my neighbor kid do it since there is no room down there for a man sized hand. It seemed like he got it on there but I do need to check to make sure the vacuum hose is connected to the right place up top.

 

[topper]

I'd say, if it just shifts "late", but otherwise runs through the gears just fine, kick's down a gear like it should, etc, there is nothing wrong and someone may have added a shift kit or something making it hold 2nd longer than normal. Bottom line, once you're at highway speed, it's running in 3rd gear, right?
As far as saving your pennies, if it ain't broke, don't fix it, IMO.
What gears and tire size and engine do you have? What rpms are you running at 60mph, 70 mph, etc?
A truck with 4.10s and no overdrive AND stock size tires will not be friendly at 70mph or faster. My truck with 4.10s and half worn 35s runs 3000 rpms at 70mph (no OD). So, an OD trans would be an improvement if you have deep gearing, but like said above, you can buy ALOT of gas for the cost of a swap to an OD trans.
Regarding your fuel mileage, first, does it run "right." Smooth, not overly rich, no timing issues, etc?
Second, I can't fathom it uses 1/8 of a burb tank (40 gal) in 15 miles.
Have you actually calculated your mileage off of a whole tank? It's virtually impossible that you're getting 3mpg (5 gal in 15miles). Even my 454 4.10 gear truck gets about 7mpg in city driving.

The motor seems to run great, no knock, idles great etc. I can't check timing yet, I gave away what tools I had for carbureted engines a long time ago when I didn't think I would be in the situation I'm in. The engine is a sbc 400 with a th350 trans, np203 transfer case and a 14 bolt rearend. It is also full time 4 x 4 but I am having a friend install the kit for that. The gas gauge moves that much that's why Im hoping the sending unit is bad.

 

[topper]

I believe your money may be better spent with a transmission shop. I'd maybe check out an inspection of some sort - have it serviced. Maybe they would have other ideas because im certain swapping to a different transmission is cost-prohibitive for your specific situation, your effort will be better spent thinking along a different line.

There could be any number of issues tanking the MPG between the driveline and the transmission.
The gearboxes, the ujoints, it could be a ton of stuff but a transmission shop may be a good place to start.

Good luck!

I don't know if it is related but when I put it in reverse it clunks so hard from somewhere in the driveline that it sounds like something is going to break apart. I can't tell from the drivers seat but a guy at a 4 x 4 shop said it sounded like it's coming from the rearend.

 

[topper]

Thank You everybody for responding and helping me out I really appreciate it.

 

[Rusty Nail]

I don't know if it is related but when I put it in reverse it clunks so hard from somewhere in the driveline that it sounds like something is going to break apart. I can't tell from the drivers seat but a guy at a 4 x 4 shop said it sounded like it's coming from the rearend.

It cant be too hard to find a driveline shop to inspect the ujoints..I got mine done at the transmission shop.

 

[Matt69olds]

If the trans has a good shift into each gear, but the shifts are later than you like, first thing to do is inspect the vacuum line to the modulator. If it’s good, remove the hose from the modulator. Look inside the hose nipple, look for a tiny screw. If it’s there, your lucky and have an adjustable vacuum modulator. You can raise/lower shift points 2-4 mph by turning the screw a maximum of 2 turns either way. If you don’t have an adjustable modulator, they are about 20 bucks at autozone.

If the trans goes need repair, now would be the time to consider a overdrive. I wouldn’t replace a perfectly good trans in hopes of a little better economy.



If the engine has the torque to pull a heavy suburban in overdrive, without lugging the engine, most likely you will see a improvement in economy. You will definitely feel a huge improvement in get up and go from the lower first gear ratio of the 700 verses the 350.

Whatever you do, make sure whoever does the install is familiar with setting up and adjusting the TV cable. The kickdown cable brackets for the 350 will not work.