Author Topic: Question For TH350 Folks - Trans 'Hops' in Reverse  (Read 7365 times)

Offline wilbilt

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Question For TH350 Folks - Trans 'Hops' in Reverse
« on: October 25, 2019, 06:37:37 PM »
So I have this 1971 GMC C2500 I bought for $200 a decade ago. 350 V8, TH 350 trans, 'Super Camper' with 4.10 gears.

A few years back, the trans decided to lose 3rd gear on my way home from work. I got it home by keeping it under 25 MPH in second. My fellow commuters were not amused.

I bought a reman trans kit from TCI, including the trans, torque converter, cooler, TV cable, cooler and even the fluid for a decent price online. TCI is a reputable outfit from years back. I was impressed with the documentation that came with the trans. Signed inspection tags, fluid pressure printouts, etc.

The only issue with this transmission is that it 'hops' in reverse. Not a smooth application of power, it is like it shudders. The problem is exaggerated if backing uphill.

I called TCI and was told the issue was likely caused by trapped air behind a reverse servo piston or something and that it would work itself out. Nearly two years later, it still hops. Granted, I don't drive it much. It might have 250 miles on it since the trans install.

Thoughts?

Offline oldnslo

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #1 on: October 25, 2019, 08:25:55 PM »
I'd be making friends with the tech support line at TCI.

Offline Uncle Buck

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #2 on: October 26, 2019, 09:07:58 AM »
Short of turning it over to a local shop at great cost about all you can do is talk to TCI again.
You boys better hold on cause I'm gonna have to stand on it!

Offline bonneyman

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #3 on: October 26, 2019, 10:23:00 AM »
Your truck is old enough this advice might not apply, but consider thinking "outside the box".

My 2004 Astrovan had this "slip" intermittently when taking off from a stop. No rhyme or reason, not all the time, just sometimes you'd go to take off from a stoplight and the engine would speed up and the truck would sit there. Then it'd drop in really hard - and be fine for a week or 10 days or whatever. Tranny service, inspection, check the computers, it'd never act up at the mechanic shop. They thought it was in my head.
Some time later the ABS light came on, brake shop said it was the brake module. $1200 to replace it, and then they could track down any sensor or wiring issue. Thought that was bass ackwatds so I repaired the thing myself. Several months later I realized the tranny wasn't acting up anymore.
How could that be? The brake system and automatic transmission aren't connected? Well, actually they ARE! On 4 wheel drive models each tire has an ABS sensor. On 2 wheel drive models like mine only the rear wheels have ABS - and the sensor is in the tranny! I think the old module was faulty and sensing a signal from the tranny even though things were OK, and messed around with it's operation.

Sorry for the long winded response. Hopefully it'll help someone.

Offline Uncle Buck

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #4 on: October 26, 2019, 11:06:46 AM »
A 71 GMC will have none of that ABS stuff, It does not apply to anything that old.
You boys better hold on cause I'm gonna have to stand on it!

Offline bonneyman

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #5 on: October 26, 2019, 12:18:56 PM »
A 71 GMC will have none of that ABS stuff, It does not apply to anything that old.

Yeah I figured that, but was just recommending he think outside the box.

 

Offline wilbilt

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #6 on: October 27, 2019, 09:43:45 AM »
Short of turning it over to a local shop at great cost about all you can do is talk to TCI again.

Probably. I sure as heck am not going to pull it back out and ship it to TCI in Mississippi.

I may put some more miles on it and see if it corrects. I only drive it when I need to haul something. 9 MPG.

Offline ron350

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #7 on: October 30, 2019, 09:13:40 PM »
 Here is a thread that sounds similar to your problem.

https://www.jalopyjournal.com/forum/threads/turbo-350-clunck.822131/

If your 1-2 or 2-3 shift is firm or too soon you can adjust your vacuum modulator to lower line pressure and reduce harsh reverse apply.


Offline stokester

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #8 on: October 30, 2019, 11:19:17 PM »
If you not already have one I bought this book when rebuilding a TH350 a number of years ago in school.  It has an extensive troubleshooting section as well as rebuild.

You are welcome to it for postage, I have no more use for it. 

PM me.
Nick
Yorktown, VA

Offline pep

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #9 on: October 31, 2019, 07:43:04 AM »
My .02

First not understanding the hoping explanation/discription, not ever heard that term. 
If it were air in the system, why is it only in reverse?
 Valve body maybe, some spring or piston loose not holding position/sealing in reverse. Most likely could be replaced with just dropping the pan.

One thing for sure waited to long, not driven it enough, warranty long gone. Is the drive shaft 2 piece, something going on there?

Hoping, as in going in and out of neutral, shifter linkage loose, not totally at home when reverse is selected?

Anyway back to the valve body, if my GUESS is somewhat close. Some magic goop, trans repair in a bottle from our friends at autozombe.

Lucient goop could offer a quick troubleshoot, it is hyped as a seal and general lubricant and expander. Dump some of that in, if the problem changes, you may need to look closer at the function of the valve. That book shown is a good reference, used the same.

As I think and not a auto trans guy to be clear, that valve assembly is common and a key component to all.

BTW have a TCi & B&M, would not buy a TCI again, its ok but just that ok.

Pep
1776 ................... what happened!

Offline wilbilt

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #10 on: October 31, 2019, 10:05:59 AM »
My .02

First not understanding the hoping explanation/discription, not ever heard that term. 
If it were air in the system, why is it only in reverse?
 Valve body maybe, some spring or piston loose not holding position/sealing in reverse. Most likely could be replaced with just dropping the pan.

One thing for sure waited to long, not driven it enough, warranty long gone. Is the drive shaft 2 piece, something going on there?

Hoping, as in going in and out of neutral, shifter linkage loose, not totally at home when reverse is selected?

Anyway back to the valve body, if my GUESS is somewhat close. Some magic goop, trans repair in a bottle from our friends at autozombe.

Lucient goop could offer a quick troubleshoot, it is hyped as a seal and general lubricant and expander. Dump some of that in, if the problem changes, you may need to look closer at the function of the valve. That book shown is a good reference, used the same.

As I think and not a auto trans guy to be clear, that valve assembly is common and a key component to all.

BTW have a TCi & B&M, would not buy a TCI again, its ok but just that ok.

Pep

It is like it jerks or shudders while backing up. It is a 2 piece driveline that has been rebuilt. No issues when moving forward.

I'll have to check the warranty, but probably only a year. It isn't a shift linkage issue, as the gear select detents are on the shift shaft in the transmission itself, not the shifter.

I am hesitant to pour any kind of 'goop' or additive into it. I have always avoided that kind of stuff.

Offline fatfillup

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #11 on: October 31, 2019, 10:12:34 AM »
My neighbor the Saab tech hates snake oil, but does recommend Lucas Transmission goop.

Offline wilbilt

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #12 on: October 31, 2019, 03:57:04 PM »
My neighbor the Saab tech hates snake oil, but does recommend Lucas Transmission goop.

Noted.

I checked the warranty. 90 days from purchase date. Since it took them over a month to ship it to me after I bought it, I guess the warranty was almost over when I received it. Live and learn.

Offline pep

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #13 on: October 31, 2019, 11:00:25 PM »
I would think/expect, the date of delivery begins the warranty period. No company (example) builds stuff, with a 1 year warranty sticks it on the shelf.

Six months later it goes out the door, and the warranty. Expires 6 months from date of transfer to the customer.

  They would not last very long. I think that is what is called a scam, for sure a crook.

Pep
1776 ................... what happened!

Offline chevy302dz

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #14 on: October 31, 2019, 11:55:39 PM »
I have a PowerGlide that acts like that when backing up uphill. Never did figure out why and it's not frequent or severe enough in my case to really worry about but maybe there is something in the troubleshooting for PGs that might help.

Offline wilbilt

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #15 on: November 01, 2019, 06:43:24 AM »
I would think/expect, the date of delivery begins the warranty period. No company (example) builds stuff, with a 1 year warranty sticks it on the shelf.

Six months later it goes out the door, and the warranty. Expires 6 months from date of transfer to the customer.

  They would not last very long. I think that is what is called a scam, for sure a crook.

Pep

The warranty specifically states it is from the date of purchase. I contacted them after purchasing and asked why it was taking so long to ship. They told me they build to order and don't have them sitting on the shelf. It really doesn't matter at this point, as the 90 days are long gone.

Offline TexasT

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #16 on: November 18, 2019, 08:38:04 PM »

The only issue with this transmission is that it 'hops' in reverse. Not a smooth application of power, it is like it shudders. The problem is exaggerated if backing uphill.

I called TCI and was told the issue was likely caused by trapped air behind a reverse servo piston or something and that it would work itself out.
Thoughts?

When you stated you bought a kit I thought of the pieces and part to assemble one but it appears by the rest of the thread the trans came assembled. I would have to conclude you had a torn seal in the direct(i believe the direct also applies the reverse) , or it has a problem in the pressure control circuit. Usually the low/rev has a way that produces additional pressure for reverse and especially when backing up a hill. Did the pressures noted on this documentation note that when in low or reverse the line pressure was elevated compared to 2 and d?
Rich

Offline Heiny57

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Re: Question For TH350 Folks - Trans 'Hops' in Reverse
« Reply #17 on: December 22, 2019, 07:55:24 PM »
We need pictures of that classic!
MAGA

If you can’t fix it with a hammer, it must be electrical.