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Th400 questions

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dixiechevy01

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My 87 k5 has a th400 with a np208. I'm trying to get all the problems fixed with the trans. It had a 700r4 in it and the guy before me swapped the 400 into it. I don't think they hooked up everything the right way but not really sure. The modulator valve has a rubber hose instead of a steel line, there a prong on the side of the trans that isn't hooked up to anything. There's also a hose at the top front of trans not hooked up, not sure what these should go to.

The trans seems to shift ok now. Both the output shaft seals are leaking. I pulled the front cover off and it looks like the input seal is leaking also. I've had the trans out once before and replaced the flex plate, torque converter, and output seal. I don't remember if we replaced the input seal or not. A buddy of mine said he had a problem with the valve body in his th400 creating to much pressure and blowing seals out all the time. Is this possible and if so is there a way to test the pressure?
 
prong on the drivers side is for the kickdown which is electric on the 400, the hose at the top of the case is the vent and does not get hooked to anything.
 
Oh ok thanks Greg Ducato. I was thinking that might be what it is for. Well where is the wire supposed to go from the there? And do you know if there is anyway to check the valve body pressures to see if there to high?
 
the kickdown was wired to a gas pedal activated switch.
the pass front, just above the pan, modulator has a vac line to the intake manifold
the vent may be plugged causing the seals to leak

"neutral, brakes applied, 1k rpm: 55-70 psi
D, idle (idle "set to spec"): 60-85
D, brakes applied, 1k rpm: 60-90
Intermediate or lo, brakes
applied, engine at 1k rpm: 135-160
R, brakes applied, 1k rpm: 95-150
D, brakes applied, 1k rpm,
downshift switch activated: 90-110


this is for the oil pressure gauge mounted on the test port on the driver's side of the case."
 
If your pressure is high, shifts will be extremely firm, if the seals, especially the front pump, are not pouring fluid while the vehicle is running, you have no pressure leaks.
 
Ok thanks for the pressure specs. Where would I be able to get a gauge to check this pressure and is there any special fittings I will need to hook it up?

I had problems with the modulator valve hose popping off before. I actually put a new modulator valve and hose going up to the intake on it a few years ago but later found out its supposed to have a steal line.

As far as the kick down switch what does this affect? Cuz its always shifted up and down ok
 
Well its not pouring out but I always noticed I had to top the trans off but thought that was from the leaking output seals
 
Well I just looked at every thing again tonight. I'm so sure it's my input shaft seal now cuz most of the ATF is around the oil pain and not actually up in the bell housing. The torque converter is also completely dry. I'm thinking maybe the pan gasket is leaking but I've replaced this before already, any idea what would make it leak again as long as it was a clean surface?

I also saw the test port plug you said about, looks like I could rig something up with pipe thread fittings, a pressure gauge and some hose

Also looked around for wiring going to the kick down switch and couldn't find anything or a switch at the pedal, so I guess they didn't swap that part. Any ideas on the kickdown switch?
 
The selector shaft seal is a very common leak point on this trans, it will drip down to the pan rail and collect along the edge at the lowest point.
The easiest way to restore kickdown function will be to find the factory parts
 
Is the selector shaft seal hard to replace if that is what's leaking? And do you know what year k5's or pickups had the th400 in from factory? Would not having the kick down switch hooked up affect anything? Cuz like I said it does downshift ok
 
dixiechevy01 said:
Is the selector shaft seal hard to replace if that is what's leaking? And do you know what year k5's or pickups had the th400 in from factory? Would not having the kick down switch hooked up affect anything? Cuz like I said it does downshift ok
Click to expand...

the kick down is to switch into passing gear
 
There were no K5's with the factory 400, it was used in lot's of other square body trucks from 1973 on up with big block or towing package options both two and four wheel drive. There is a special tool required to remove and replace the selector shaft seal, it is done externally and is very easy with the tool. The tool is available at most parts stores
 
Well it downshifts still without being hooked up. I dong know if it shifts at the right rpm but it will downshift into 2nd from 3rd
 
it can do that on the vacuum drop at certain speeds depending on the trans governor but you will not get the full potential from the trans without the passing gear switch operational
 
Oh ok thanks for the info. My buddy has a military diesel k5 and it had the th400 from the factory but ill have to look around for a truck most likely. Be easier to find anyway. Well I'm gonna double check the selector seal and see if that is what's leaking. The modulator valve had some fluid around it but I don't know if it was just from everything being wet. Is even ATF around the seal for the modulator valve? If so us there a seal to replaced there?
 
Is there a chance that could build to much pressure in the valve body if its not downshifting the kickdown function?
 
Yea I'll have to look at his to see what im looking for if I can find anything in the junkyard. does the o-ring seal atf in? If so that may be wahts leaking then and going to the lowest point of the pan lip
 
Ok thanks, I was thinking there wasn't fluid there because the modulator doesn't have fluid coming out if you take the hose off.

Has anyone does the rear output shaft seal and bushing on these before? It has an np208 and I just need to know what involved in getting the bushing out
 
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