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Old 04-17-2024, 11:37 AM   #1
JohnIL
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Transmission Dilemma - 700r4 no 2nd Gear

We have a project crisis on our hands. My teenage son recently bought a '77 C10. One of the reasons he chose this particular truck is that a previous owner had swapped in the drivetrain from a CMT400 (5.7L & 700r4). We were hoping that would keep us from needing to do a drivetrain swap for a while. We're not afraid of the work. We've done it before. But, we were hoping he could drive it, as is, for a few years first. What we didn't realize is the transmission has a serious problem.

Now that we've had a chance to drive the truck a little bit, it's pretty obvious the 700r4 is skipping 2nd gear. All the other gears are strong. It pulls hard through 1st gear, all the way up to 30ish mph. Then, it shifts (slams) directly into 3rd gear. The 3rd-4th shift appears to be normal. Not harsh, not soggy either. Once it's in overdrive, it cruises down the road just like it should.

I'm out of my depth here. Transmissions are full of witchcraft and sorcery. I know engines pretty well, but I've always left transmission guts to the experts.

I stopped by a local transmission shop and described the condition. They theorize that the 2nd gear band is probably toast. The problem is, a full transmission rebuild would cost almost as much as the truck itself. My son can't afford that right now.

So, I hit the interwebz looking for more information. I found references to identical problems on several different forums. I read several posts about possible problems with the 2nd/4th servo. Stuck servo, bad seals, and short pins. None of that is expensive or difficult to work on. I have ordered a new servo, seals, and pin in hopes this will get the transmission back in the game. I'm not terribly confident in our odds.

Here's where I need your help. Does any of this sound plausible? Could the problem be in the servo or am I just wasting time and money? Do any of you have any ideas that might lead us in the right direction? We appreciate any wisdom you can share.

Thanks.
John
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Old 04-17-2024, 06:44 PM   #2
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Did you do the easy things first like make sure fluid level is correct and the TV cable is adjusted correctly?
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Old 04-17-2024, 06:49 PM   #3
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Probably not the band. The band is engaged for 2nd and OD.

Does he not have 2nd from OD and D shift positions ?

Does he have manual 2nd ?

Does it down shift from drive (3rd) to 2nd ?

Hopefully you bought a corvette 2nd gear servo assembly. They're cheap and an easy upgrade. The 1-2 accumulator pistons are also known for wear. Also cheap and accessible if you drop the pan.

Last edited by Grounded63; 04-17-2024 at 08:19 PM.
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Old 04-17-2024, 08:34 PM   #4
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Re: Transmission Dilemma - 700r4 no 2nd Gear

https://www.youtube.com/watch?v=YfPgBUH6Ru0

Year of the transmission helps.
What does the transmission do when it's forced into 2nd gear with the gear selector?
Do you have compression breaking in all gears when selected?
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Old 04-18-2024, 08:57 AM   #5
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Thank you all for the responses. I'll do my best to answer your questions.

The transmission is a 1991, according to the stamping number.

Fluid level is full and we reset the TV cable.

I ordered a new corvette servo, seals, and pin.

I'm afraid I can't give confident answers to the manual shifting questions because I only drove it once, and I was concentrating on up shifts at the time. I did attempt to manually downshift from 4 to 3 to 2, but I was at low enough speed that it was difficult to fell the downshifts.

Tonight, we will try to do some more scientific testing. We will install a temporary tach so we can keep an eye on RPM's to confirm shifts. Then, we'll manually run it up and down through the gears to see what works and what doesn't. I'll report back with test results.

Thanks again for your help.

John
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Old 04-18-2024, 11:30 AM   #6
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Re: Transmission Dilemma - 700r4 no 2nd Gear

I have the same transmission - same year, even - in my 1981, and I had the same problem. It was the bands, alright. The above recommended tests will quickly confirm it.
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Old 04-19-2024, 04:24 PM   #7
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Sorry guys. No new information today. We got hammered by thunderstorms all evening. We weren't able to do any test driving. Hopefully tonight or Saturday. I'll report back when I have more information.

Thanks.
John
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Old 04-20-2024, 06:04 PM   #8
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Test Results and More Weirdness

I have test results.

I think we have 2nd gear after all. We temporarily installed a tach on the hood and went for a test drive. At first, I couldn't select all the gears manually. The column shifter would only go down to the the 2nd gear position. Back to the garage. After some adjustment and a little shift linkage customization, we're now reliably getting all 7 positions, from park all the way down to 1st gear. Off on another test drive.

Once we got outside of town, I came to a complete stop, then drove forward under moderate throttle. The engine wound up to 3200 rpm before the first shift. That was around 30 mph. This is what I assumed was 3rd gear. But, the tach said we were at 2000 rpm. I could be wrong, but my gut told me that was too high for 3rd gear at 30-35 mph. My brain told me that must be 2nd gear.

So, to test that theory, I stopped again and pulled the shifter all the way down into 1st, like Grounded63 suggested. I wound it up to 3000 rpm, then moved the shifter to the 2nd position. It did not shift until I got to 3200. That's second gear, at 30 mph and 2000 rpm. I wound it up again to 3000 and moved the shifter to the 3rd position. At 3200, it shifted again. That's 3rd gear at 60 mph at 2000 rpm. Keep in mind we're on a country road in a fairly untested truck. I moved the shifter to the OD(4th) position and poured on a little more coal. I lost my nerve at 75 mph and the engine was only turning about 2800rpm.

Now I'm thinking we probably have all gears, but the transmission is shifting WAY late. Based on the later model GM pickups I've owned, by 75 mph, we should have been well into overdrive and turning about 1500 rpm. I think the shift points in this thing are just really really high. I've already reset the TV cable and it doesn't seem to change the shifting characteristics.

Is it possible someone put a shift kit in this thing that has raised the shift points dramatically? That would make sense for a drag car, but not for a stock-ish half ton pickup.

Any other ideas?

As I said before, I'm way out of my depth. I know how a stock 700r4/4L60 acts, but I don't know much else about them. Thanks for any advice you can share.

John
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Old 04-20-2024, 09:44 PM   #9
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Re: Transmission Dilemma - 700r4 no 2nd Gear

You didnt feel it shift to 4th (O.D.} ? make dam sure cable is tight no slack at idle, tight as a hell at WOT .Do this with engine off . You can give the cable a click or 2 tighter at idle I set mine up with a press. gauge. I have a 327 with 3;73 rearend 28 " tall tires and it runs 2800 rpms at 80 mph
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Old 04-20-2024, 10:03 PM   #10
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Is the engine induction method stock ? Like, was this a TBI engine and trans that was swapped. Is it still a TBI. Or was it converted to a carb. And, it may not have the correct throttle arm bracket on the carb. The TV cable needs to travel through a specific arc and range. I would check, confirm this first. Before anything else is done.

When/if you install the 2nd gear servo. Check the actual amount of band engagement. Before you completely disassemble it. You may need to hit the outer cover with a soft mallet to break the seal. Or just remove the retaining clip, pull the cover out enough to remove the o-ring seal. Replace the cover and retaining clip. Then depress the cover, and measure the distance it moves. You'll probably need to find a way. To pry or clamp against the cover to depress it while in the vehicle.

Assuming that the TV cable, throttle arm is correct. I would be look at possibly a sticking/stuck TV piston/plunger ( inside the valve body ). Or same issue with the governor ( accessible outside of the case ).

Last edited by Grounded63; 04-21-2024 at 01:21 PM.
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Old 04-21-2024, 03:11 PM   #11
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Re: Transmission Dilemma - 700r4 no 2nd Gear

One step forward, two steps back.

Grounded63's suggestions led me to go take another look at the throttle linkage and bracketry. I've looked at it at least 50 times and I never realized the geometry was wrong. It turns out that the previous owner didn't bother to install a TV cable correction bracket on the Edelbrock Performer carburetor. That will obviously throw off the TV cable adjustment. I managed to find that part (Edelbrock 8026) at a local O'Reilly.

After installing the bracket, it's now obvious that the previous owner didn't use the right throttle cable/TV cable bracket on the back of the carburetor. Now, the TV cable is offset out of alignment by about 1". There's no way that's going to work right. So, I ordered a different bracket, specifically for the Edelbrock Performer/AVS carburetor. It's from Jegs (part# 157313). It won't arrive until the end of the week. So, now we wait...

I don't know for sure if fixing the geometry will fix the shift points, but it makes logical sense. The way it was, the TV mounting point on the throttle bracket was about 3/4" too far forward. That would put way too much tension on the TV cable as the throttle travels forward. Logically, that would raise the line pressure significantly and cause the transmission to shift very late.

Fingers crossed that this fixes the problem. I'll report back when the new throttle bracket arrives. Thanks again for your advice.

John
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Old 04-21-2024, 07:00 PM   #12
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Problem Solved!!!

Thanks to your advice, I think we found (and solved) the problem.

After my last post this afternoon, I was so disgusted with the prospect of waiting a week for a new throttle/TV cable bracket to arrive that I broke out my junior mad scientist fabrication set (angle grinder and welder) and customized the old throttle bracket. I'll save you the gory details because they are a little bit embarrassing. But, after a few hours of cutting, welding, cutting again, and welding again, I managed to adapt the existing bracket to move the throttle cable and TV cable 1" toward the driver side. This put the TV cable in straight alignment with the new TV correction bracket on the carburetor.

I reset the adjustable TV adjustment and pulled the carburetor to full throttle. The cable did not actually adjust, but there is light tension on the cable at rest and it moves to full extension at full throttle.

With everything buttoned back up, we went for another test drive. I knew were on to something good when it shifted into 2nd gear at 20 mph, before we even got to the end of the block. When we got outside of town, we went through the full battery of tests. Under moderate throttle, it shifts into 2nd at 20-25 mph at 2500 rpm. It shifts into 3rd at 30-35 mph, again at 2500 rpm. It hangs in 3rd up to 55-60 mph, around 2700 rpm. Once in overdrive, it settles down at 65 mph at around 1900-2000 rpm.

So, I think we're in pretty good shape. My only concern is that I would expect it to be in overdrive a little bit earlier, around 50ish mph.

Based on these numbers, is there anything else we should look at? Does all of this sound relatively normal?

Again, I can't thank you guys enough. I was completely lost. You all showed me the way back home.

Thank you, thank you, thank you!

John
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Old 04-22-2024, 07:09 PM   #13
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Re: Transmission Dilemma - 700r4 no 2nd Gear

Glad to hear your making some positive, forward progress !
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