How to change Transmission Fluid and Filter?

[ClydesdaleKevin]

Okay folks...any special advice or suggestions I might need before I tackle this project, which I'm going to do soon?

It looks like I'm lucky enough to have a drain plug on the transmission pan on our 89 P30 chassis, so it won't be all that messy.

Just looking for some good tips so I can do the job right and not have any problems like leaks or anything later on down the road.

I know I should use a new gasket...should I also use gasket cement?  Both sides of the gasket?  What type to you recommend?

I'll be putting in a new filter too.

How about transmission fluid?  I know I have to use Dextron III or better, and a lot of it (I have the capacity written down somewhere), and I'm not going to bother draining the torque converter...not enough fluid in there to worry about really.  So, any tips on making sure I don't overfill it from the get go?  How many quarts should I put in before starting it up?  I know it has to be warmed up in Park for an accurate reading...can it just be warmed up while sitting stationary?  Will it warm up with just engine temp warming the fluid via the radiator, or do I really have to break camp and take it for a test drive?  Can I just put it in drive with my foot on the brake and let it warm up that way?

I'm going to do this job in the next week or two, and reinstall the OEM external transmission cooler in the stock location piggybacked to the new cooler...and want to get some feedback first so I'm not going into the job blind.

Thanks ahead of time everyone!

Kev

[chicknnhead]

I want to say it takes somewhere close to 11quarts, i know this doesn't help much..lol...i also am running two coolers, oem in the radiator and one external. I have been looking at guages for tranny temp..hoping to find a vintage looking electronic one to put in, in july

[Froggy1936]

Hi Kev i reccomend any gasket material except cork. No cement neccessary (but if you want to use it use aviation type The can with the brush in it) The most important step is to straighten the pan gasket surface @ the bolt holes with a hammer  on a hard surface (they will be raised from the bolt) When removing the pan leave two bolts 1/2 way in on one end (opposite the drain plug) so the pan hangs down let it drain as long as possible the converter will only drain 1/2 way out . (takes a long time) When refilling put in at least 4 qt before starting check between ea additional qt fill as fast as possible and kick off high idle as soon as possible (Do Not Overfill) Final check fluid level when trans temp gauge is at its normal range (idleing in gear will heat it sufficently) Then ck it again when you fill fuel tank on your next trip. Frank

[ClydesdaleKevin]

Thanks Frank!  Didn't even think about straightening the pan bolt holes, which is exactly the type of advice I was looking for...lol!  I'll have to find some aviation gasket stuff, and make sure its not a cork gasket, check.

Transmission capacity on this thing is around 11 quarts according to the owners manual that came with it, so I'm figuring if I start with 9 quarts before startup, I should be safe enough.

This should be a simple enough project, especially since I already have the front of the RV raised pretty high on Lynx blocks to level it in the campground we are in, and since there aren't any chiggers or fire ants here in Eminence, KY...lol!

I'm very happy to find out from you that warming it up in gear will be sufficient...having to break camp would be a royal pain in the butt!

I just changed the engine oil 700 miles ago, but I will have to grease the fittings while I'm at it.  And check the rear diff fluid...which won't be easy, since the front is jacked up, and the back sits low....

Thanks for the tips guys!

Kev

[ClydesdaleKevin]

Okay...I'm going to be getting the parts I need tomorrow to change the gasket and filter and fluid, and had some more questions.  I want to make sure I get the right parts!

I crawled under the rig to check out the transmission, and it doesn't say what type it is...the only thing written on the transmission pan is Hydramatic div of GMC.  Its an 89 P30 chassis with a 454.  Anyone know what this transmission is called? 

On the very bright side, there is indeed a drain plug on the pan...I don't know if its stock, or if it was added by a previous owner, but I do know that its going to make the job a lot less messy!

Thanks a lot in advance, guys!

Kev

[ClydesdaleKevin]

Okay, I'm still not sure which transmission I have, but I'm guessing its either the 400 or 475...but I have no idea which one, so hopefully it will show up on the Napa computer based on the year of my chassis. 

It is pretty neat that they made a 4 speed overdrive transmission in later years, but it requires a computer to control it.  Hmmmmm...I wonder how hard it would be to upgrade to one in the future if this transmission ever goes out?

Kev

[TommyM]

Hi, Kevin,

Your transmission is either a TH400 or TH475.  It's probably the TH475, which is externally identical to a TH400.  The two are easily confused, but it doesn't matter unless you're actually ordering internal/rebuild parts.  The parts stores might list only the TH400, but the filter and gasket kits are the same so it doesn't matter.

The 4L80E, which was introduced in the 1991 model year, is based on the design of the TH400, with similar strength, etc, but with electronic controls and an overdrive gear.  The 4L80E uses a different case, and does not look the same as, a TH400. 

I have a 1990 3/4-ton 4x4 Suburban with a 5.7L TBI (throttle body injection) engine, that came with the TH400.  I bought a junked-out 1991 3/4-ton 4x4 Suburban that also had the 5.7L TBI, but came with the 4L80E, specifically for the purposes of swapping the transmission into mine.  In order to swap the transmission, I had to also change the driveshafts, the entire engine wiring harness, the ECM, and a bunch of little bits and pieces.  I also had to rearrange the pins on one connector under the dash.  It was frankly a fairly straightforward swap, since I had the two otherwise-identical Suburbans sitting next to one another in the yard, with all the pieces available.

If your motorhome had the TBI 454, you could probably find a suitable donor truck and do the swap the same way I did.

But since your 454 is carb'd, the easier way to control the 4L80E would be to either get a stand-alone trans controller from an early-90's Diesel truck, or buy an aftermarket trans controller.  You'll have to figure out the crossmember (it may be as simple as moving yours back) and you might need to shorten your driveshaft, because the 4L80E is one or two inches longer than the TH400.

Having said all that, it is a very do-able swap.  There are a lot of things to do, but none of them require an engineering degree or super-special tools or eye of newt and magic potions.  So if you (or anyone!) wants to tackle it, I'm happy to share my experiences!

Tommy

[ClydesdaleKevin]

Great advice Tommy!  If my transmission ever starts to go out, or if I find a really sweet deal on a newer or rebuilt 4L80E, I'll definitely keep it in mind!  Overdrive would be a great thing to have to save some gas on the road.

The transmission filter and fluid change went along well, although there were hornet nests after all...had to spray them down before getting to work.  The hardest part was cleaning the outside of the pan down to shiny metal again with all the old road gunk and oil caked onto it. 

Draining the trans was a breeze since a previous owner had installed a drain plug...sweet!

I made sure I captured all the fluid, and then transfered it into 2 one gallon milk jugs to see how much drained out.  Just over 1 and a half gallons.

So when it was all clean and put back together with a new filter and gasket (nitrile, not cork), I added 1 and a half gallons to the transmission before starting it up.

I let it warm up to a transmission temp of 150 degrees by keeping it in drive with my foot on the brake, and then checked the fluid.

It was almost perfect, down just a little over a pint, but I had also reinstalled the OEM transmission cooler , so that accounts for the extra bit I had to add.

All in all a very easy job!  No leaks either.  One of the back bolts was a bit hard to get a wrench on, but once I figured out it was reachable with a long 1/2 inch box wrench by reaching through the rear transmission mount, it went fast from there.

So yay!  All my mechanical projects are done for the trip to New York!

Kev

[Lefty]

Glad you got it done Kev,
It's a little late, but I was going to say I use a product called Permetex gasket sealant and adhesive. It comes in a spray can, and sprays out cherry red. You spray it on the gasket, and the surface the gasket is mating to, the allow to get tacky. It helps form a good, pliable seal, and I've never had a leak on anything I've used it on. And if you ever take it back apart, it wipes clean with carb cleaner and a rag.
As said, the biggest issue is the bolt holes will always need checking, and when you tighten the bolts up, do them in an alternating pattern a little at a time till they are snug. Trans bolts only need about 15in/lbs which I guesstimate by holding my ratchet by the head and not the handle and just tighten till its snug. If the gasket squeezes out, your getting it too tight (which dents the bolt holes... D:oH! ).

Another tip... never,ever use silicone on transmissions.. if it gets in the transmission, it will get into the valve body and wreck havoc... been there,done that

[ClydesdaleKevin]

Thanks Lefty!  That is pretty much just the way I snugged the bolts.  The bolt holes were still nice and flat, so I didn't have to flatten them out.  The nitrile gasket was nice and thick, so I also didn't bother with any sort of sealant.  The main reason I cleaned the pan so thoroughly was to be able to detect leaks, and so far no leaks at all.  Sure looks shiny under there compared to the rest of the under-rig....lol!

Kev

[BrandonMc]

I just changed the tranny oil pan filter and gasket. I made sure to get those screws super snug, and noticed the gasket squish and kinda torn near a few of the screws.


would you recommend I replace the new gasket before filling the oil?  i??  Or will it be okay and not leak? I had something of a felt type of gasket in the kit which recommended dipping in the atf oil before install, which I did.
thanks for any reccomendations! 

[SLEETH]

time to replace

[M & J]

If you squished the gasket by tightening very hard you should replace it before adding fluid. When off, check the areas around the bolt holes for dimpling. Use a straight edge and make sure there are no high or low spots so it compresses the gasket uniformly around the sealing surfaces.

[Rickf1985]

Yes, take the pan back off and then tap all of the hole in the pan so they are flush and smooth with the flange. I have never heard of dipping the gasket in fluid, that will just cause it to squeeze out. And do not overtighten the bolts. Just snug them up. One hand on a 3/8 drive ratchet twisting, not pulling, is more than enough. If you see the gasket starting to squeeze out STOP. If it leaks you can always give it just a little more later, as a matter of fact it is a very good idea to check it after a week or so because you probably will have one or two bolts that need just a little more tightening. Once you go to far it is too late so better to start light and sneak up on it.

[BrandonMc]

I'll order a new one, and check today the condition of the pan. I'm learning to be a mechanic by working on this beast, and with the help of forum members here.


once again, thanks!

[BrianB]

I just changed the tranny oil pan filter and gasket. I made sure to get those screws super snug...

The torque specification is 12 ft-lbs. If you plan to do a moderate amount of your own repairs, a torque wrench is a good addition to the tool collection.

You'd be surprised how easy it is to over-torque bolts just by tightening "by feel". In this case, you have a compliant material (the gasket) which means when it feels tight, it is too tight.

[Rickf1985]

Are you sure that is not 12 inch lbs.? I haven't looked it up but 12 foot lbs. is a lot for a cork gasket with a 5/16 bolt and no load spreaders.

[BrianB]

Cork gasket? Transmission filter kits for the TH475 these days come with a rubber gasket - at least the one Autozone sells (Duralast TF206)

I know this is right, because I just came in from installing my new filter/gasket tonight. 12 ft-lbs is what I used and it "felt right" and the gasket did not squeeze out.

Also, I installed a drain plug kit in mine.

Helpful note - you need to take off the nut for the rear mount and jack up the back of the transmission to get the back-most bolt out/in (and use a hand wrench on it).

[Rickf1985]

Cork, Nitrile, Rubber. They all have the same type of compression.

[SLEETH]

affirmative

[legomybago]

Just another word of advice when doing transmission pans.......ONLY USE A 1/4
" RATCHET. Helps the rooks and everyone else too from over tightening. After your pan is flat, all you need to use is a Spray Tack Adhesive for gaskets. Applying permatex to a tranny pan gasket is only asking for it to slip and bulge out during the tightening sequence.

[Rickf1985]

Just another word of advice when doing transmission pans.......ONLY USE A 1/4
" RATCHET. Helps the rooks and everyone else too from over tightening. After your pan is flat, all you need to use is a Spray Tack Adhesive for gaskets. Applying permatex to a tranny pan gasket is only asking for it to slip and bulge out during the tightening sequence.

Good advice.

[BrianB]

ONLY USE A 1/4"

I disagree. Use a torque wrench and apply the correct torque value.

[M & J]

If the lad dint know not to squish the gasket then doubtful he has a torque wrench. The other's  advice is sound given that.

[SLEETH]

I personally  use a 1/4 inch torque wrench