Thread: GMC TH350 issues

Tweet

All, looking for some suggestions....

I recently installed a new (rebuilt) TH350 in my 29 and have been unable to isolate the source of a leak at the pan. I have replaced the gasket and set the bolts to spec and still have a leak? Any ideas?

o No apparent housing cracks
o Cleaned gasket mating surfaces
o New OEM gasket
o Bolts toqured to 13 foot pounds
o Gasket installed dry per spec

jc

Damned o-ring on the dipstick!!!!!! i'll bet ya thats your problem!!!

No leak at the dipstick or kickdown cable (Lokar after market) any other suggestions?

jc

cracked case? i hate to go there but you know it could be....I had a 350 tranny that was leaking at the dipstick o-ring but it was in such a way i thought it was the pan, turned out it was the dipstick. just a thought.

Also check around the gear selecter shaft. It's very hard to detect, but the fluid will collect along the pan rail and drip off the edge so it appears the pan is leaking.

Get it on a hoist, clean it spotless, and watch for seepage with lots of light. Tranny oil is sneaky, it flows where you'd least expect. An "A" is probably not the easiest thing to access the trans, so inspection must be difficult. Check cooler line fittings closely.

Also check around the gear selecter shaft. It's very hard to detect, but the fluid will collect along the pan rail and drip off the edge so it appears the pan is leaking.

Does the gear selection shaft have a o-ring or seal?

jc

It has a small seal that is held in with a circular wire ring.

It will have a seal. The only time i saw an o-ring on a selector shaft was on a very early THM 400. I doubt that the 350's had them when they came out, and even if they did, chances are that your 350 is not the old. Beware that you need a special tool to remove the seal. I would be very difficult to try to knock it out and you would be risking the selector shaft as well. If you do get it out(if that is indeed the problem) insepect the shaft for ridges, rust, knicks, things that can damage the seal on installation.

id put a little bead of silicone around the pan for good measure to, and also if the pan has been on and off a few times the pan will spider web ( crack) around the bolt holes and oil will leak out from the head of the bolts. run into that a lot over the yrs. what part of the trans is leaking? if you have to drive it to make it leak that could be a front seal. if it leaks sitting still it should be dripping on the floor some where close to where the leak is on the trans. if the shift linkage is the leak i don't think it would drip off of the right side of the trans where the dip stick is. can you get us a little closer? there is a seal on the shift lever.

Update! little progress.... I think the pan may be warped also I may have the wrong gasket. Are all TH350 pan gaskets the same design?

o No apparent housing cracks
o Cleaned gasket mating surfaces
o New OEM gasket
o Bolts toqured to 13 foot pounds
o Gasket installed dry per spec
o No leak from gear selector

jc

Originally posted by 29arod
Update! little progress.... I think the pan may be warped also I may have the wrong gasket. Are all TH350 pan gaskets the same design?

o No apparent housing cracks
o Cleaned gasket mating surfaces
o New OEM gasket
o Bolts toqured to 13 foot pounds
o Gasket installed dry per spec
o No leak from gear selector

jc

put the silicone on the pan and then you can forget about that part of it.

its normanly the pan, dip stick , or kick down cable. the dip stick is hard to see, get a small mirrow up under there so you can see the dip stick tube where it goes into the case.

do not ever use silicone on a pan gasket it acts like a lubricant and make the gasket slide around first spray the trans with brake clean and blow off a residue get some spray foot powder and spray all suspected areas with in 5 min. the white foot powder will turn pink at the leak also it does not leak untill it is driven its not the pan some 350 tranys come will a pressure switch on the right side above the pan near the dip stick these are known to leak also check the pump area

"do not ever use silicone on a pan gasket" .....WRONG. cook67 dont know where you been the last 15 yrs. but a lot of replacrment parts dont even put gaskets in with the part any more they give you a tube of RTV Silicone Gasket and let you make your own. cames blue, red and black. you need to try it sometimes, works great.



Permatex® High-Temp Red
Formulated for hi-temp applications, or heavy-duty use. Replaces almost any cut gasket by making reliable "formed-in-place" gaskets that resist cracking, shrinking, and migrating caused by thermal cycling. Coat pre-cut gaskets to increase reliability. Temperature range -65˚F to 650˚F (-54˚C to 343˚C) intermittent. Resists auto and shop fluids. First generation gasket maker. Fills gaps to .25" when fully cured. Suggested applications: Sealing panels, window plats, oven doors and flues on high temperature applications.
81160 3 oz Tube - Carded - 12 per Pack
81915 7.25 oz. Automatic Tube - 6 per Pack
85915 7.25 oz. PowerBead Can - 6 per Pack
81409 11 oz. Cartridge - 12 per Pack
Permatex® Water Pump & Thermostat RTV Silicone Gasket
A non-corrosive, sensor-safe RTV silicone gasket material Highest water-glycol resistance available in an RTV silicone.

I've also been told not to use silicone on a tranny gasket. If the silicon drips off on the inside you will have major problems with the internals of the tranny. If the pan is straight you shouldn't have to use the silicone. I had the same problem you are having. I pulled the pan and eye balled the mating surface and discovered that the spaces in between the bolts was lower than at the bolts. Once I straightened it, it worked fine. Has anyone used a screw driver to pry off the pan?

Good luck!