Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Replace TH400 with 700r4? Or should I keep what I have?

I have a 1982 GMC suburban with a 6.2 diesel. I get 31 mpg at 55 mph, but have a really poor gear ratio. I know my truck had the 700r4 in it in the past, but the person I bought it from yanked the 700r4 and replaced it with a TH400. The 400 has 3 gears and no lockup torque converter. Tonight someone gave me a working 700R4, but I’ll need to buy a new torque convertor for it since it came from a gas powered truck. Second, I’ll have to take the output shaft from the th400 and put it onto the 700 since I have 4 wheel drive and the 700 came from a 2wd. The good news is that they have the same output shaft and mounts to connect transmission to transfer case. They are also the same size and use the same transmission mounts. But I will need to to some major work to swap the output shafts and will have to buy the torque converter… that’s 120 bucks… I’d also feel better with replacing the trans filter, although the fluid in it is still red, and unburnt.

What year model is the 700R4? The early transmissions through the mid-eighties were very weak. That’s probably why it was replaced with the TH400 in the first place. I would stick with the TH400 which is the superior transmission. Have you looked into replacing the differential gear? That may give you the gear ratio that you are looking for. I’m assuming you just want more torque. Your fuel economy will suffer if you gain more torque so be prepared for that.

It will fit, of course, but you may run into some issues since this one came out of a gasoline powered truck. Concern number one will be durability issues with the torque of the diesel. I would guess the diesel transmissions are built a little more ruggedly to handle the torque, although if you don’t work the truck hard, it probably won’t be an issue. Second concern will be the programming of the transmission, i.e: when it shifts. I don’t know if this is a valid concern or not since I don’t know exactly how these things work (and welcome corrections from other, more knowledgeable contributors), but if the transmission out of the gas truck thinks it’s supposed to upshift at, say, 4,000 rpm, you will run into problems with that pretty quickly. Again, I’m not sure if this would be a real issue or not, but it makes a certain amount of sense. Final concern will be whether or not your truck originally had a lockup torque converter 700R4. If it did, you will have a square four pin harness under there to control it. If it did not and you want that function, you will need to get a kit to wire up the lockup function. They can be found and bought on the Internet. If you don’t want to buy the kit, you can get a pigtail off a donor truck or car and look up instructions on how to hardwire it into your truck. Many people run them off a toggle switch because that’s the simplest way to do it.

Missle, the 700 is out of an 89. Second, the th400 is only a 3 speed and with it my redline max speed is only 67. So I think the durability should be ok.

Mark, My suburban did originally come with the 700r4, which only offers it with a lockup torque converter. So that’s a simple thing. But the main thing is… I need live in rural SD, and the the highway which we have to take to get anywhere is 65 mph. I can’'t do 65 on it since that’s at my redline and so I always do 55. This tends to cause a traffic backup behind me.

As for changing my gear ratios, that’s a bad idea… If I did, I would start running into problems climbing hills since the 6.2 only puts about 90 hp to the rear wheels. that means I really would have trouble with hills then.

I owned an ‘88 2500 8’ bonus cab w/350 and 700R4. Bought it used- it was a farm truck from Iowa before I bought it. Doors filled with dust from the fields and so on. The trans fluid was opaque brown, completely roasted. Bought as a beater so only did several fluid changes and ran it for 6-8 years after that with the TCC disconnected most of that time to boot. That thing hauled almost 11k lbs of gear from the midwest to the east coast a couple years before I gave it away, still going strong. It also was used for snow plowing every year I owned it and pushed a full sized Western Pro-Plow.

I also had a 700R4 in an '89 Camaro that took a serious beating in the time I owned it.

No doubt the TH400 is much stouter than the 700R4 but unless you’re routinely pulling stumps with your truck, I think it will give much better performance than your current setup.

I’ve got a few cars with TH400s with the same issue- built for hole shot not expressway speeds. That gets old and sometimes dangerous when I have to ride on the expressway.

If it were me and I had the 700R4, I’d definitely spend the $ to convert. You’ll need a TV cable too more than likely and a bracket at the throttle end to control shift points. As mentioned the TCC control may exist or you can control it manually if necessary. It wasn’t really a bit hit to the mileage on my beater between being connected or not. The truck wasn’t exactly fuel efficient to begin with so it was kinda in the noise band. YMMV…

You didn’t say what you have to lose. E.g. if it takes a lot longer than you expect (which it probably will); or if everything turns into a disaster, etc., what are the stakes? Will you merely be inconvenienced? Or will you not be able to go to work, lose your job, get evicted from your house…? Do you have a backup plan to deal with the truck being down?

I wouldn’t hesitate to do it - if the risks of a mess weren’t too dire.

I’m in agreement about whether this project is going to be a major inconvenience or not. On the surface many things look simple at first but once the work begins snags have a way of rearing their ugly heads.

What would I do if I were in your shoes? Do the swap because traveling 55 MPH in a wide open state like SD would grate on me quite a bit.

Actually for the most part it will be a simple swap. I looked at some of the worst of it, and the absolute worst will be changing the torque converter. I’ll have to have one made for it since it’s not something that is kept in stock… anywhere. The other thing is the T.V. cable. The one that controls the shift points. It seems that they are the same between the two transmissions. Actually the worst part will be the transmission dipstick. I already checked about the output shaft, and can use the same one from the th400. So the torque converter is the most difficult thing since I would have to wait up to 3 weeks for it to come in. But I can still drive it as is. Thankfully I have a backup vehicle. a 2002 Hyundai Accent… although it makes me feel like I’m driving a clown car compared to the suburban. And will have another project soon. a 1953 International pickup. That will be the real fun project as it hasn’t been cranked for about 10 years. Time for a rebuild on it too.

Given that you have a car to drive & can afford to have the truck off the road for a bit then I would absolutely do it.

Looks like it’s down already, I downshifted to take off on a hill and it blew the rear end… Any advice?

Nobody here witnessed the failure or has seen the results. You’ll have to invest more time in describing what you mean by “blew the rear end” if you want meaningful advice.

That being said, it’s all relative. No matter what happened to the rear diff it wouldn’t change my intentions on making the switch. I’d be glad if it was going to go, it went now so I could fix everything in one shot…

Well, I was climbing a steep driveway, stopped right about halfway up it, downshifted manually, then heared a snap. the drive shaft is still intact, and the pinion yoke is spinning. I think what blew was the limited slip portion of the pumpkin.

Turns out that what happened is it shattered the spider gears in the rear end leading me to believe that they were already damaged to begin with. Still, it’s not a good thing.