Author Topic: TH400 Dragging in 3  (Read 540 times)

Offline Big Dee

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TH400 Dragging in 3
« on: April 07, 2022, 11:09:13 PM »
85 silverado c30 with 454 and th400 seems to be holding back in high gear. shifting into neutral at 40 mph she frees up. back into 3 seems like 2. mileage is very bad. fliud and filter is new. original trans with 125K miles. could sure use some help diagnosing. :-\
85C30...

Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #1 on: April 08, 2022, 04:13:41 PM »
Are you sure it's the tranny and not the motor choking?

When you drop into 2nd what does it do?

It's just a dumb animal waiting for torque from your big mama 454.

What type of fuel delivery system does she have?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #2 on: April 09, 2022, 11:05:36 AM »
JP, running an edelbrock 1400 because the 100K thermoquad looks complicated. hindsight what it is, that was the wrong decision. however i have improved at tuning eddies. 
Downshifting to second at 20 mph revs up and holds back. typical of a downshift but look at the speed.
4.10 gears / 265/75 tires.
Choking motor?
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Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #3 on: April 09, 2022, 06:36:24 PM »
JP, running an edelbrock 1400 because the 100K thermoquad looks complicated. hindsight what it is, that was the wrong decision. however i have improved at tuning eddies. 
Downshifting to second at 20 mph revs up and holds back. typical of a downshift but look at the speed.
4.10 gears / 265/75 tires.
Choking motor?

Not enough information  :-[ Please define 'holding back' in 3rd gear.

You said you go into N and drop back in at 40, seems to free up. What does that mean?

Maybe a video?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #4 on: April 09, 2022, 07:44:51 PM »
JP,
when coasting in neutral around 40 mph. then shifting to Drive, slows down like the brakes are on. engine revs up until reaching 30 mph then cruises good. :-\
Feels like driving around in second but counting all 3 shifts.

85C30...

Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #5 on: April 10, 2022, 01:16:31 PM »
Got it!

You're dragging your motor from idle to the RPM needed for the two to sync.

Try bringing up the RPM to 15-2000 before you shift into gear.

The two will sync much faster. 
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #6 on: April 11, 2022, 10:28:44 AM »
I will try to explain the problem another way.

Gear #1 = idle rpm - 2500 rpm. to 10 mph
Shift #1 Gear #2 = 2500 to 3500 @ 20 mph
Shift #2 Gear #3 = 2500 to 4000 rpm @ 40 mph
50 mph 5000 rpm
60 mph 6000 rpm
5 mpg  :(

The shifting points should be around 4000 rpm?
Cruising @ 50 mph should be around 3000 rpm.


85C30...

Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #7 on: April 11, 2022, 10:36:33 AM »
That's helpful, thanks!

Has it always been this way? Does it slip at any point

If it has always been this way, it sounds like your diff is geared really low.

If not, and it doesn't slip,  I would suspect your torque converter has failed.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #8 on: April 11, 2022, 10:41:18 AM »
Has not always been this way, does not slip.
So this is a failed torque converter ???
any way to test to confirm?

Thanks JP.
85C30...

Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #9 on: April 11, 2022, 10:44:15 AM »
Yeah, try to take off with your foot firmly on the brakes, see how high your RPM gets before it overcomes the brakes.

How many miles on the transmission? 
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #10 on: April 11, 2022, 10:54:13 AM »
Will try that on next test drive. and get some exact numbers.
Have tried to do that before, can't overpower brakes below 5000 rpm.

125,00 gentle miles on drivetrain
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Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #11 on: April 11, 2022, 11:07:35 AM »
Wow 5000 RPM something is wrong.

Last ditch-when was the trans last serviced, in miles? As in fluid and filter?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Big Dee

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Re: TH400 Dragging in 3
« Reply #12 on: April 11, 2022, 12:31:43 PM »
Did a test. overcomes brakes at 2000 rpm.  :D

Serviced at 124,000 miles. May 2021. myself.
valvoline dex/merc, ac delco filter.

old fluid was new at 100,000 miles in 2008 and looked and smelled o.k.




85C30...

Offline JohnnyPopper

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Re: TH400 Dragging in 3
« Reply #13 on: April 11, 2022, 01:18:23 PM »
So when you last tested this you had to run it up to 5000 RPM?

Now it's 2000?

I don't know Big D, maybe wait for some other comments from the Brain Trust.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Shifty

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Re: TH400 Dragging in 3
« Reply #14 on: April 11, 2022, 01:41:15 PM »
Does the fluid smell burnt/what color?  Maybe the seal on the filter is sucking air, or the filter dropped in the pan?
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed