Need advice for th425 in a 77 gmc RV
December 12th, 2020, 09:59 AM
#1
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Need advice for th425 in a 77 gmc RV
Hello this is my first post. I have a 1977 GMC RV with the olds 403 connected to the th425 trans.
i drove it from california to maryland a couple months ago. It gave me trouble then. It would downshift from 3rd to second and we struggled to keep it from doing that. We would have to shift to 2nd then back to drive and that would often work.
once home I changed the fluid and filter. There were very fkne metal specks but nothing that looked like shavings. Once I had it back together it worked better but still it would slip a bit before going into gear. Once aarmed up it was normal operations. That is until this past week. Even warmed up it was requiring revving up to go into gear. Imagine having to rev up to 3500rpm before it would start to lurch forward.
yesterday was the last gasp. It stopped going forsard in anythin but low and even then it would barely break 25mph. High rpm did little to help. Actually it had more grab at lower rpm.
despite all that the trans would shift properly when we reached certain speeds. It just seemed to me like the torque converter was not doing its job.
On the other hand reverse gear works great!
this morning i was checking it out again. I can only move it forward if I put it in low and feather the gas but keep it at low rpm. It makes a groaning sound around the bell housing and it sort of bumps around but eventually I get some torque. And I can move at low rpm only and i cannot go up any type of incline no matter how small since this RV is 11,000 lbs.
I want to know if there is any way it could just be a bad torque converter and also want to know if this transmission can be removed without taking out the engine.
i drove it from california to maryland a couple months ago. It gave me trouble then. It would downshift from 3rd to second and we struggled to keep it from doing that. We would have to shift to 2nd then back to drive and that would often work.
once home I changed the fluid and filter. There were very fkne metal specks but nothing that looked like shavings. Once I had it back together it worked better but still it would slip a bit before going into gear. Once aarmed up it was normal operations. That is until this past week. Even warmed up it was requiring revving up to go into gear. Imagine having to rev up to 3500rpm before it would start to lurch forward.
yesterday was the last gasp. It stopped going forsard in anythin but low and even then it would barely break 25mph. High rpm did little to help. Actually it had more grab at lower rpm.
despite all that the trans would shift properly when we reached certain speeds. It just seemed to me like the torque converter was not doing its job.
On the other hand reverse gear works great!
this morning i was checking it out again. I can only move it forward if I put it in low and feather the gas but keep it at low rpm. It makes a groaning sound around the bell housing and it sort of bumps around but eventually I get some torque. And I can move at low rpm only and i cannot go up any type of incline no matter how small since this RV is 11,000 lbs.
I want to know if there is any way it could just be a bad torque converter and also want to know if this transmission can be removed without taking out the engine.
December 12th, 2020, 02:01 PM
#2
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Join Date: Jan 2010
Location: central Indiana
Posts: 5,273
The Th425 is basically a TH400 trans with a big chain between the input shaft and forward clutch. The diagnostic and clutch application chart is pretty similar.
If the converter was causing the delayed engagement, it would also be delayed also. The forward clutch is engaged in every forward gear, including manual low. Since it backs up fine, chances are the direct clutch, reverse servo and band are all functional. That leaves the low/reverse roller clutch or the forward clutch as your likely fault.
What does the fluid look like? If the pan was full of crud, chances are the trans will need rebuilt. Did you install new o-rings on the filter tube? It’s common practice to install 2 o-rings as added insurance against the filter sucking air instead of fluid.
I have no clue how the trans comes out of a motor home chassis. I’m going to guess it’s a similar process as a Toronado, Eldorado or Riveria. Other than the chain, the 425 and 400 trans are almost identical inside. Many of the parts will interchange.
If the converter was causing the delayed engagement, it would also be delayed also. The forward clutch is engaged in every forward gear, including manual low. Since it backs up fine, chances are the direct clutch, reverse servo and band are all functional. That leaves the low/reverse roller clutch or the forward clutch as your likely fault.
What does the fluid look like? If the pan was full of crud, chances are the trans will need rebuilt. Did you install new o-rings on the filter tube? It’s common practice to install 2 o-rings as added insurance against the filter sucking air instead of fluid.
I have no clue how the trans comes out of a motor home chassis. I’m going to guess it’s a similar process as a Toronado, Eldorado or Riveria. Other than the chain, the 425 and 400 trans are almost identical inside. Many of the parts will interchange.
December 12th, 2020, 02:37 PM
#3
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Join Date: Dec 2020
Posts: 3
Matt,
I only put the one o-ring back in so i wonder if there was any of that air issue going on. All the way across country.
I have not taken out the current fluid to see how it looks. I am sure this trans has possibly 200k miles on it. I need to check the maintenance logs.
In any case it seems clear that we are talking about a rebuild here so I am going to start looking into what it takes to remove the trans. With luck the engine can remain in place. I won't mess with the trans myself. I am looking at taking it to a local shop that was recommended by a trusted race engine builder.
I only put the one o-ring back in so i wonder if there was any of that air issue going on. All the way across country.
I have not taken out the current fluid to see how it looks. I am sure this trans has possibly 200k miles on it. I need to check the maintenance logs.
In any case it seems clear that we are talking about a rebuild here so I am going to start looking into what it takes to remove the trans. With luck the engine can remain in place. I won't mess with the trans myself. I am looking at taking it to a local shop that was recommended by a trusted race engine builder.
The Th425 is basically a TH400 trans with a big chain between the input shaft and forward clutch. The diagnostic and clutch application chart is pretty similar.
If the converter was causing the delayed engagement, it would also be delayed also. The forward clutch is engaged in every forward gear, including manual low. Since it backs up fine, chances are the direct clutch, reverse servo and band are all functional. That leaves the low/reverse roller clutch or the forward clutch as your likely fault.
What does the fluid look like? If the pan was full of crud, chances are the trans will need rebuilt. Did you install new o-rings on the filter tube? It’s common practice to install 2 o-rings as added insurance against the filter sucking air instead of fluid.
I have no clue how the trans comes out of a motor home chassis. I’m going to guess it’s a similar process as a Toronado, Eldorado or Riveria. Other than the chain, the 425 and 400 trans are almost identical inside. Many of the parts will interchange.
If the converter was causing the delayed engagement, it would also be delayed also. The forward clutch is engaged in every forward gear, including manual low. Since it backs up fine, chances are the direct clutch, reverse servo and band are all functional. That leaves the low/reverse roller clutch or the forward clutch as your likely fault.
What does the fluid look like? If the pan was full of crud, chances are the trans will need rebuilt. Did you install new o-rings on the filter tube? It’s common practice to install 2 o-rings as added insurance against the filter sucking air instead of fluid.
I have no clue how the trans comes out of a motor home chassis. I’m going to guess it’s a similar process as a Toronado, Eldorado or Riveria. Other than the chain, the 425 and 400 trans are almost identical inside. Many of the parts will interchange.
December 12th, 2020, 04:55 PM
#4
Registered User
Join Date: Jan 2010
Location: central Indiana
Posts: 5,273
I doubt only one O-ring will cause a issue. If it absolutely needed 2 o rings, GM would have done it.
Im guessing the biggest reason people double the o-rings is the overhaul kit includes a replacement for the vacuum modulator. When you buy a replacement modulator, a O-ring is included. Doubling the o-ring on the filter tube is a better use for it than simply throwing it away.
if you have 200K on the trans, I’d say you got your money’s worth!
Im guessing the biggest reason people double the o-rings is the overhaul kit includes a replacement for the vacuum modulator. When you buy a replacement modulator, a O-ring is included. Doubling the o-ring on the filter tube is a better use for it than simply throwing it away.
if you have 200K on the trans, I’d say you got your money’s worth!
January 1st, 2021, 05:17 PM
#5
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Join Date: Dec 2020
Posts: 10
Hi Sax;
Below are links to very good enthusiast FB groups for the GMC.
Tons of helpful advice, parts info, and help for on the road breakdowns.
Enjoy!
Hugh
https://www.facebook.com/groups/354604151348488
https://www.facebook.com/groups/1635356906770465
https://www.facebook.com/groups/vintagegmcmotorhomes
Below are links to very good enthusiast FB groups for the GMC.
Tons of helpful advice, parts info, and help for on the road breakdowns.
Enjoy!
Hugh
https://www.facebook.com/groups/354604151348488
https://www.facebook.com/groups/1635356906770465
https://www.facebook.com/groups/vintagegmcmotorhomes
January 1st, 2021, 09:11 PM
#8
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Thread Starter
Join Date: Dec 2020
Posts: 3
Thanks for the links!
Hugh,
Thanks for the links I will be sure to check them out!
Thanks for the links I will be sure to check them out!
Hi Sax;
Below are links to very good enthusiast FB groups for the GMC.
Tons of helpful advice, parts info, and help for on the road breakdowns.
Enjoy!
Hugh
https://www.facebook.com/groups/354604151348488
https://www.facebook.com/groups/1635356906770465
https://www.facebook.com/groups/vintagegmcmotorhomes
Below are links to very good enthusiast FB groups for the GMC.
Tons of helpful advice, parts info, and help for on the road breakdowns.
Enjoy!
Hugh
https://www.facebook.com/groups/354604151348488
https://www.facebook.com/groups/1635356906770465
https://www.facebook.com/groups/vintagegmcmotorhomes
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