700R4 hits 45mph then kicks into 1st gear

[Ewhitaker0020]

I just towed a fairly lightweight trailer with my 84 K10 305 v8 yesterday. It towed pretty good, although the severely limited power output of the 305 was annoying. But, today when I drove it the to return the Uhaul trailer the truck decided to shift down into 1st gear when I hit 45 mph. Luckily it was just across town, but it was every time I got near that speed.

Starting off in first gear, perfect. Shifted into second gear, perfect. Then your 45 and shifts straight into first gear and rpms go way up. I can make it shift back into second gear by using the shift lever and going into 2.

What could cause this? It was rebuilt about 18 months ago by a reputable shop and has ran smoothly ever since. Since it shifts up into second gear so well, I would assume it's not the TV cable, but idk. I can shift into reverse just fine and the truck pulls fine in first and second gear.

I called the tranny shop to ask them what it could be and the guy called back and left me a voicemail saying it could possibly be something to do with the detent cable not being adjusted right. Is that the same as the TV cable?

Edit: I just drove it and the truck wants to hang up in first gear until way late in the rpm range. Then shifts a little hard into second. Does that point to the TV cable being out of adjustment?

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[RoryH19]

I would check the TV cable first. Doesn't take much to become out of adjustment. Also it will burn up your trans very quickly if not set properly.
Can you think of anything you may have done to the carb or that area?
Cable could also stretched or come lose inside the trans.

 

[Matt69olds]

Late hard shifts are better than slipping sliding shifts, within reason. Make sure the TV cable is adjusted correctly, moves freely, and returns to its idle position with the throttle released. If the cable is damaged and binds internally it’s possible it might shift funny.

I’d guess whatever the problem is will be related to either the governor or valve body. Luckily those can be accessed with the trans in the truck

 

[Ewhitaker0020]

I would check the TV cable first. Doesn't take much to become out of adjustment. Also it will burn up your trans very quickly if not set properly.
Can you think of anything you may have done to the carb or that area?
Cable could also stretched or come lose inside the trans.

Tv cable seems to be adjusted properly. I noticed that it seems to shift back into first when it should be shifting into third. Another transmission shop I called says it sounds like a throttle body is sticking. And he said that means something is probably coming apart internally

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[Ewhitaker0020]

Late hard shifts are better than slipping sliding shifts, within reason. Make sure the TV cable is adjusted correctly, moves freely, and returns to its idle position with the throttle released. If the cable is damaged and binds internally it’s possible it might shift funny.

I’d guess whatever the problem is will be related to either the governor or valve body. Luckily those can be accessed with the trans in the truck

The tv cable seems to be adjusted properly. It hasn't moved from where the transmission shop placed it and it moves freely. Another transmission shop I called says it sounds like a sticking valve body to them. Guess it's going back to the transmission shop that did the rebuild so they can see if it's warranty work.

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[SirRobyn0]

Is there any chance it is shifting into 3rd, but third has gone out so the RPMs go way up. It's something to consider if your not sure. I think taking it back to the tranny shop is a good plan.

 

[Ewhitaker0020]

Is there any chance it is shifting into 3rd, but third has gone out so the RPMs go way up. It's something to consider if your not sure. I think taking it back to the tranny shop is a good plan.

It's possible, well the tranny is cooked. It started slipping like crazy when I got close to home. I can smell it now too. Burnt tranny fluid smell

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[bucket]

Is there any chance it is shifting into 3rd, but third has gone out so the RPMs go way up. It's something to consider if your not sure. I think taking it back to the tranny shop is a good plan.

I've had a few 700r4's fail in that exact way.

 

[JoeR Jr]

The 3-4 clutch pack is smoked. Maybe from towing in OD, but either way that is what it probably is.
It's a very common failure on the TH700, and that is the exact symptom when it happens. If R, 1 and 2 are fine, but 3 and 4 are gone, dollars to donuts-that's what it is.
Joe

 

[Ewhitaker0020]

The 3-4 clutch pack is smoked. Maybe from towing in OD, but either way that is what it probably is.
It's a very common failure on the TH700, and that is the exact symptom when it happens. If R, 1 and 2 are fine, but 3 and 4 are gone, dollars to donuts-that's what it is.
Joe

Do I not put the truck in OD while towing? I just assumed it wouldn't shift into OD if it wasn't okay to do so

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[RecklessWOT]

Do I not put the truck in OD while towing? I just assumed it wouldn't shift into OD if it wasn't okay to do so

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no, you should leave it in D instead of OD. It will still try to shift into OD based on speed, not necessarily based on load. The OD in a 700R4 is very weak, it does not route oil through the radiator for cooling while in overdrive gear for whatever reason (I guess they assumed it would be low stress, but still a dumb design). If you have an aftermarket tranny cooler it's not as bad, I have one and will shift into OD while towing from time to time but that's only on flat(ish) highways, if encountering any sort of hilly areas you should stay in D even with a cooler.

 

[Ewhitaker0020]

no, you should leave it in D instead of OD. It will still try to shift into OD based on speed, not necessarily based on load. The OD in a 700R4 is very weak, it does not route oil through the radiator for cooling while in overdrive gear for whatever reason (I guess they assumed it would be low stress, but still a dumb design). If you have an aftermarket tranny cooler it's not as bad, I have one and will shift into OD while towing from time to time but that's only on flat(ish) highways, if encountering any sort of hilly areas you should stay in D even with a cooler.

At least I'll know for next time. Is a 700r4 just a weak tranny? I had it rebuilt last year, but I doubt they built it up of anything.

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[RecklessWOT]

At least I'll know for next time. Is a 700r4 just a weak tranny? I had it rebuilt last year, but I doubt they built it up of anything.

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They're not the greatest, but not nearly as bad as some people say they are. Not bulletproof like a TH400 or even the 350 but still can be pretty dependable overall, the main advantage is overdrive, it will get a lot better gas mileage than the 3 speeds and opens up all sorts of possibilities for gear ratio/tire size combos that would otherwise be bad for 3 speed trucks.

The 700R4 was a gas crisis transmission, they're geared really weird (first gear is really short to get you up to speed, then 1-2 shift is a huge gap, 2-3 is normal, and the OD gear is stupid tall 25-30% overdrive iirc). And they're prone to overheating while towing without an aftermarket cooler. They can be built to handle a lot of power but even stock I wouldn't specifically call it "weak", you just can't beat the **** out of it. As I said in my last post, mine is completely stock (except for the external trans cooler, I also have a temp gauge to monitor it while towing) and has ~170k ish on it it has hauled a lot of weight it's whole life (Original owner used it to tow a big boat, the 2nd owner was a sheetrocker and used to load the thing up to the ceiling with drywall, and I have towed many cars with it since I've owned it). I also put 33s on it and do not drive it gently. I have had no problems with the transmission.

 

[Matt69olds]

I went back and re-read this.

Is this a sudden problem? Usually 3-4 clutches slip and slide before failure, they don’t just instantly burn up leave you with no 2-3 shift.

Drive shell failure does fail without warning, resulting in no 2nd and 4th gear. But since there is no complaints of no reverse, I doubt the drive shell has broken.


Having said all this, I’m going to revise my prediction. If the 3-4 clutches are cooked, or for whatever reason won’t apply, once the 2-4 band releases the trans is once again in first gear. However, because the overdrive sprag should freewheel, you shouldn’t be able to feel the trans slam back into 1st gear. I’m kinda puzzled. I’m guessing if the 3-4? clutches are burnt, there is going to be other damage as well.

Let me know what your shop finds, I’m curious.

 

[Ewhitaker0020]

I went back and re-read this.

Is this a sudden problem? Usually 3-4 clutches slip and slide before failure, they don’t just instantly burn up leave you with no 2-3 shift.

Drive shell failure does fail without warning, resulting in no 2nd and 4th gear. But since there is no complaints of no reverse, I doubt the drive shell has broken.


Having said all this, I’m going to revise my prediction. If the 3-4 clutches are cooked, or for whatever reason won’t apply, once the 2-4 band releases the trans is once again in first gear. However, because the overdrive sprag should freewheel, you shouldn’t be able to feel the trans slam back into 1st gear. I’m kinda puzzled. I’m guessing if the 3-4? clutches are burnt, there is going to be other damage as well.

Let me know what your shop finds, I’m curious.

I will let you know what they say. I'm working on the cheapest way to tow the truck to the transmission shop a hour away. I'm hoping they'll warranty it. Tranny should be able to get more than 2,000-3,000 miles before it craps out.

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