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Author Topic: TH400 not shifting till 4,000 rpm  (Read 18405 times)

Offline Dr_Snooz

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TH400 not shifting till 4,000 rpm
« on: December 29, 2010, 06:58:52 pm »
This just started the other day. The shift points all moved up to 4,000 rpm. Upshifting is quite violent and downshifts also happen at higher rpms. I've replaced the vacuum modulator and unplugged the kick-down switch and the problem remains. Any thoughts?
« Last Edit: December 29, 2010, 07:22:56 pm by Dr_Snooz »
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline VileZambonie

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Re: TH400 not shifting till 4,000 rpm
« Reply #1 on: December 29, 2010, 07:49:20 pm »
Do you actually have vacuum to the modulator diaphragm? If so check the governor gear
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Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #2 on: December 31, 2010, 01:25:23 am »
Checked the governor today. The spindle is scored, but the gear is fine and the weights and springs are all intact. The modulator is getting vacuum. Any other thoughts?
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline VileZambonie

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Re: TH400 not shifting till 4,000 rpm
« Reply #3 on: December 31, 2010, 08:28:15 am »
Is the governor valve moving? Have you adjusted the modulator?
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Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #4 on: December 31, 2010, 11:03:58 pm »
Quote from: VileZambonie on December 31, 2010, 08:28:15 am
Is the governor valve moving? Have you adjusted the modulator?

There's a valve? In the governor or the case? Sorry, I don't know domestic stuff so good.
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #5 on: January 06, 2011, 10:38:18 am »
I hooked up the vacuum modulator to a handheld vacuum pump yesterday. I pumped it down to 22", went for a drive and...     no change. Then I wired up the kickdown solenoid to battery voltage and put a handheld switch in the wire. I hit it with battery voltage a few times and went for a drive. I got a clean shift from 1-2 at the right rpms. 2-3 tried hard to shift, but couldn't manage it.  Then it returned to shifting at 4,000 rpm. I can hear the solenoid inside the case working, but the kickdown mechanism does not work properly when driving. I was hoping to avoid dropping the pan again, but I'll have to take a look at the kickdown mechanism. If that looks fine, then I guess I'll pull the valve body. Stupid truck.
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Grim 82

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Re: TH400 not shifting till 4,000 rpm
« Reply #6 on: January 06, 2011, 10:58:25 am »
Quote from: VileZambonie on December 29, 2010, 07:49:20 pm
Do you actually have vacuum to the modulator diaphragm? If so check the governor gear
One step at a time...  You can check the governor without removing the pan.
http://www.dragracingonline.com/technical/vi_9-bm-1.html
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Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #7 on: January 07, 2011, 08:08:45 pm »
I bought an ATSG book today in anticipation of a valve body rebuild. Reading up on the governor assembly, I finally learned that there is a valve inside the spindle. That didn't move around when I was inspecting the governor, so I'm assuming it's stuck. Is that a safe assumption? How freely should the valve move? Should gravity alone be enough or does it require hydraulic pressure? I'd be very happy if it was an easily accessed $25 part.

I also bought a valve body rebuild kit. I may do that too just to have it done and to get the last of the fluid leaks fixed. I'm tired of keeping stacks of cardboard under it.
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #8 on: January 08, 2011, 08:50:21 pm »
Governor works great. Valve body rebuild is next on the docket. If I don't find the problem in there, then I don't know what happens.
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #9 on: January 12, 2011, 06:51:36 pm »
I pulled the valve body today. The kickdown solenoid looks like this.



The seal is on the left. The screen is in the middle and the solenoid is on the right. I'm assuming the seal failed, opening the circuit at all throttle positions. If it's not that, then I'm completely at a loss. The trans looks really good inside. The valves all move smoothly. There's no scoring or any real signs of wear. Knowing that this truck hasn't had a lot of love, I'm surprised it looks so good inside. The governor screen was completely clean.

Anyone have any suggestions for getting the retaining pins for the shift valves out (below)? They appear to be pressed in and I'm not real sure how to get enough leverage on them to get them out.



Pulling the valve body is giving me a chance to get most of the leaking seals in the trans. The thing makes an unbelievable mess when parked in any one place for more than about 4 minutes. I'm hoping to participate in a volunteer crew building a home next month and I need this thing to pull a trailer full of tools. I hope this fixes the problem. Then I hope it passes smog.
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline VileZambonie

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Re: TH400 not shifting till 4,000 rpm
« Reply #10 on: January 12, 2011, 07:46:32 pm »
You can use a pick tool or small screwdriver to push them back or a pair of needle nose or small dykes to work them out.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #11 on: January 17, 2011, 09:27:16 pm »
Anyone know where this piece goes?



It exploded out of the transmission when I removed the valve body along with a number of other pieces. I've figured out all the rest, but I can't find any diagram showing this one. Any help is greatly appreciated.

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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline Da67goatman

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Re: TH400 not shifting till 4,000 rpm
« Reply #12 on: January 17, 2011, 09:39:32 pm »
I saw that bushing. it goes at the frame and the shift linkage goes into it.  Should go right accross from the linkage on the trans, but at the frame.
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Offline Dr_Snooz

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Re: TH400 not shifting till 4,000 rpm
« Reply #13 on: January 17, 2011, 11:07:47 pm »
When you say "frame," do you mean the frame rails? Outside the transmission case?
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1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

2009 Chevy Silverado 1500 WT 4WD, 4.8L, 4L60

Offline beastie_3

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Re: TH400 not shifting till 4,000 rpm
« Reply #14 on: January 18, 2011, 01:28:29 am »
Yes. there is a rod that comes down from the steering column, then across to the transmission. that is your shift linkage. The rod pivots in that plastic washer.
« Last Edit: January 20, 2011, 02:27:41 pm by beastie_3 »
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