Yet another transmission mystery. TH400

[Stitz Love]

I searched but didn't see anything like this.

Here is the symptom specifically;
I select ‘Drive’ and proceed out to the street. I feel it go from 1st. to 2nd. To 3rd by about 45 mph, suddenly the engine is racing as truck seems to have gone to neutral. I take my foot of the gas, begin to slow down. I tentatively apply gas and seem to be in 1st or 2nd but feel as if it keeps slipping out (into neutral). I creep along as transmission has trouble staying engaged (in ANY gear) to the corner turn around and stop. From the dead stop I head back to the driveway, same thing occurs. 1st. 2nd. 3rd. -45-ish mph, SLIP – gone… creep back to the drive way, stop.
I stay in first gear easing up to the garage, which is about 200 yards of dirt/gravel/grass.
Reverse works as it should.

Here is Equipment info;
This tranny sat in a garage for 4 years or more. It has 75,000 miles on it.
The Transmission is a TH400 (deep pan) on a 5.7 TBI in a 1990 Chevy C1500 2 wheel drive.
The engine is from a 1992 Caprice (Police Cruiser) which had an electronic transmission.
(I bought the truck this way. It had been sitting at friend’s garage for a few years. Prior to purchase I was able to drive it up and down the driveway in first gear.)
There was a pipe threaded brass plug in a hole in the intake manifold behind the TBI. We put a like threaded brass bolt with vacuum nipple in its place and ran ¼ vacuum hose to the diaphragm on the tranny. Diagrams I have seen of this show a metal vacuum tube connected like this using just about 2” of hose to attach it at each end. I am looking for one from a parts truck or maybe I will make one from brake line or fuel line… is the diameter of that vacuum that important? If so, can anyone tell me the correct diameter?

I have checked and added fluid level as/per instructions procedure and it does read full.

Any thoughts on why it’s slipping when put to use and suggestions of things to check before I go to a Transmission shop will be greatly appreciated. And of course, "Thank you."

[75K30]

Welcome to the site.

This is a tricky question, and there could be several problems, and perhaps we can try to start with the simple ones.
The first thing that came to mind is that the trans is trying to downshift once it reaches speed? Why would it do this, I have no idea, but it is what came to mind when I read and re-read this question. Depending on the year of the 400, the downshift command is done via 12VDC.
The first question is whether or not this kickdown is hooked up and if this posibility is even possible.

As far as the vacuum line, it is entirely possible that the long vacuum hose is pinching down under engine vacuum. See the long hose can colapse and restrict flow.
I am not confident that this is resulting in any problems, but a steel line is probably a better idea. 1/4" tubing or even 3/16" tubing will be just fine. The size here is not critical since you are only capturing vacuum and not flow.

[scooter18155]

have you checked your linkage from your column to your transmission? if this linkage isnt tight or properly adjusted perhaps it isnt fully engaging into drive and popping out into nuetral. ive never seen your specific problem myself so im just guessing. have you changed the filter or modulator valve or check the line that goes to the modulator valve? I seen the line pop off that modulator valve before and was causing shifting issues. i dont remember if it just wouldnt shift from 1st to 2nd and so on or if it acted like the tranny was in nuetral and slipping it was quite a long time ago it happened. but i remember when it happened it was like wtf is going on it was just fine. got under and checked and the hose was ripped right where it connects to the modulator and cut the bad part out and pushed it on and all was well.

[Stitz Love]

Ah, I see I forgot a couple things.
The Trans. is the original trans. of the 1990 C-1500 (Cheyenne).
A little history on it. My friend bought it from the state vehicle aucton with a blown engine (thrown rod?) He later purchased the 1992 police cruiser 350 TBI to put in the truck, to go with that he (from yet elswhere) purchased a high powered electronic trans. to go with the police cruiser engine. He drove this for about 1,500 miles... and bought the truck of his dreams during this time. Still later he put the cheyenne up for sale, but because of the high performance trans. his price was too high for local tastes. He then removed the high performance trans. to sell it seperately, 3 years have gone by, I kept pestering him to sell the chevy truck to me... finally, he put the original tranny back in... I drove it up and down the driveway impressed with it's spunk in first and reverse. Un benounce to both of us, he did not have a vacuum line connected so the truck stayed in first while zipping up and down the long driveway.
I bought it, put a plate and ins. on it and took it out on the street... that's when we saw it wasn't shifting at all. We put the vacuum tube on it, and now it shifts as I described. We got the engine up to Op. Temp. and checked and leveled up the fluid. Still doing this.
To clarify, it MAY be down shifting but it doesn't feel like a down shift, it feels like Neutral and then while rolling, trying to engage when gas pedal applied.. only to slightly engage maybe 2nd. or 1st... but then rev as if in nuetral. Hence the creeping along to the turn around or back to the driveway. A distance of about 1/2 mile.. (1 mile round trip.)
Also, the left front disc brake is stuck on, which we are planning to repair Monday or Tuesday... I didn't think that has anything to do with the trans acting up so I didn't mention it.
Am I forgetting anything? Oh yes, he gave the original engine away to another friend for parts, long long ago.

[z07]

This is just a swag...

Based on the symptoms, I'd bet on a TC stator that is spinning. You'd think that something called a stator would not rotate right? I suppose that in the original TC design for the early autos the stator didn't spin at all. But later designs called for it to spin in one direction only. I think yours is spinning both ways.

[75K30]

I still think that this downshift theory is something to look at.
Here is why. With it engaged the trans can and will shift, though it may shift late. Upon light pedal the thing may just drive like normal. When you reach high gear, it will engage, but it is waiting to downshift. The governer has as much to do with its as anything.
Ahhh heak, I may be full of hot air! Im just trying to isolate the problem starting with the simple things first.
Maybe just check the wire on the side of the trans. You can actually disconnect it to make me feel better.

[skyline2250]

Does the engine bog when you try to floor it? If it does, you might want to check out the fuel pump. I had a 96 tahoe do the same thing.

[Stitz Love]

The engine roars when I stomp on it in these instances... the couple times I tried that out of frustration,hoping to inspire it to shift.. well, to kick back into 3rd. but no.. it revs as if in neutral. Though as pointed out, it my be in 2nd or 1st and RPM is so high for those gears, no forward thrust is evident, only slowing down as if I put it into neutral and yet am reving the engine.
Hoping to have some time Monday and or Tuesday evening to go through the suggestions and ideas here. I sure appreciate it, and heck keep 'em coming.
BTW, Does anyone think the fluid could have gummed something up by sitting for 4 to 6 years in the trans. not attached to the vehicle?

[scooter18155]

Originally Posted by 75K30

I still think that this downshift theory is something to look at.
Here is why. With it engaged the trans can and will shift, though it may shift late. Upon light pedal the thing may just drive like normal. When you reach high gear, it will engage, but it is waiting to downshift. The governer has as much to do with its as anything.
Ahhh heak, I may be full of hot air! Im just trying to isolate the problem starting with the simple things first.
Maybe just check the wire on the side of the trans. You can actually disconnect it to make me feel better.

sorry 75k30 but i dont think anyone was trying to discredit or discard your theory i wasnt anyways. your theory may be 100% correct i was just giving my .02 worth of info and giving another idea or something to look at.

[75K30]

Originally Posted by scooter18155

sorry 75k30 but i dont think anyone was trying to discredit or discard your theory i wasnt anyways. your theory may be 100% correct i was just giving my .02 worth of info and giving another idea or something to look at.

Hope i did not come off wrong. I like the suggestions above.
Many times I just think out loud and type. No disrespect here.
None implied or taken.
Its all cool........