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At 1;30 monday morning the TH400 in my 88 K3500 started acting wierd with my son driving. He had been making 100 yd pushes in 20+" of dense powder when he got what he thought was transmission slip in reverse when he went to back out of a pile. He let it set for a few minutes to cool off, thinking maybe it was getting hot, and then it backed out normally. He had been working that site alone and I showed up at that point and sent him home rather then keep working the truck with a posible problem.

Cold fluid level seems ok and when I tried it yesterday it seemed to engage and move fine, but something was definitely "different" in how the shifter felt to me. It seems like it's out of synch with the transmission detents. To just shift it to the stops sometimes it's not engaging in one direction or the other.

Is it possible for the transmission to not get shifted fully and not engage properly? I haven't looked over the linkage, but are there any bushings in there that could have broken and made the slop I'm feeling come on quickly?

I held it with the brakes this afternoon and throttled it and there's no apparent difference in engine load/speed apparent between forward and reverse.

I'm nervous about having a possible cripple at a bad time, but I'd also like to get past this month without a major expenditure. My plan for right now is to get it flushed and filter changed, and put in a temp. guage so we can monitor it. My biggest account has a payment due in February and I'd like to shoot for then to do a swap if needed.

A goodwrench rebuilt was quoted at $1,485 in the crate, comes with a 3 yr warannty that way. Truck has a new heart and body is sound so putting a tranny in it is worthwhile. Truck has 153K so I'm not feeling bad about doing the repair.

Opinions needed here. I don't want the truck down when it's needed but I also don't want to throw a big repair at it right now. What would you do?
 

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I would check the frame to linkage mounts for the shifter. If it is anything like my K-10 linkage there is a white plastic/nylon bushing across from the tranny where the linkage is supported.

I believe there is a spring there as well which pushes the linkage into the tranny lever. It is possible that bushing fell out, and would then cause slop in the shifter and linkage.

If so the linkage might well allow the part known as "Manual Valve" not travel fully into proper position.

This "Manual Valve" is visable to some extent with the pan down.

I have a Haynes book in hand, and am trying to understand what I might do, but I certainly am no expert, or final word of advise on this topic...

I might add Ti's grim to hear of a problem with your 400 tranny at the moment...

I will re-read your post twice more and if I find a clue, I will post what I find. As for the Haynes book, it tends to intermix tranny's some which makes it difficult to determine just what is said..

Mac
 

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Mac is correct,check the linkage,especially the cross rod with the spring and convex bushing.That is most likely your slop,and maybe why it wouldn't go into gear fully.

$1500.00 is pretty steep for a TH400 rebuild,even if it's a GM crate tranny.Those things are so simple,any good trans shop should be able to fully rebuild it,and hop it up a bit for a third of that price,maybe even less.My last TH400 rebuilt was only $400 CDN.
 

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According to Haynes:

A pressure gauge with apx 10 feet of tubing can be used to test for oil pressure. The gauge must reach 300 psi.

Apx 4" behind the shifter linkage on the 400 tranny at the first webbing on the case back there is a fitting ( appears as a bolt in a pic) This bolt is at the top of the casting web, and behind that are more bolts.

Again from the shifter back, there is a ID tag riveted to the case and then a casting webbing, where this bolt is at the top on the casting webbing. Behind that are 2 more bolts 1 larger and higher, and one just below and rearward slightly..

If you aquire the testing tool, you want the bolt at the casting web..

My skills with the written word are not great... Consider me the dummy, but I don't want you to take my words and get into living hell..

All tests are at 1,000 rpm, and all tests are with the brake applied.

There is no listing for Park

neutral 55 to 75 psi
Low and D2 130 to 160"
Drive 60 to 90"
Reverse 85 to 150psi
 

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I would start with the linkage- I had to replace mine this year. The plastic bushing cracked, and the spring broke, and when I crawled under for the diagnosis I also noticed that the linkage was all rusted solid. So I went to my local dealer and $57 later replaced the linkage from the column down. Have had no problems since. If you are getting a fluid change, they will be able to check the linkage, and possibly let you look as well. I would also check the pan when they drop it for any particles that would be above the norm. You will have some "sludge" in the bottom, but it should be fine particles no large pieces- this is what I have found on all my fluid changes on any high mileage tranny's that I have owned ( quite a few- but I'm still young and starting out!!).

While they are installing the temp gauge I would have them throw in a drain plug it's only a couple of $$ more, and makes it muc easier to change in the future.

Good luck and let us know how you make out!!

Bill
 
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