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1990 GMT400 TH400: Slipping when cold.

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FRANKENBURBAN
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Theres not much you can do other than the pan filter and filter seal. The other filters are inside the valve body itself.
 

Road_Case

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On the way to the keys for the weekend today. Get the truck up to temp, on the way to get gas. Stop for gas, now reverse won't engage at all. Stop by a nearby Autozone, and get a quart of Castrol Transmaxx dex/merc, and add about half the quart after looking at the trans fluid level.

I was hoping this might restore my reverse gear at least a little bit, but it still won't engage reverse at all. While I was at Autozone, I bought two of the 24oz Lucas transmission fix bottles, but didn't want to add either of them to the transmission if I didn't need to. Clearly at this point, the transmission is going to need some work. Should I try the lucas trans fix, or will it do more harm than good?

As a side note, all three forward gears work normally once the truck has been driving for a while. Thanks again for any advice.
 

Will L.

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Any transmission not engaging or slipping- unless there was a leak and you are way low on fluid-
adding new fluid will only make it worse as the new fluid has detergent in it for cleaning the carbon and silicates out and getting them to the filter. The problem is that clutch material and band surface that is beat up and deteriorating on the micro level- the parts that are barely getting the traction to hold now get “washed” out by the new detergent. Especially when slipping occured because the burning that occurs is turning the lining material to high carbon like when you heat wood. And the detergent is designed specifically to attract and break down carbon.

If you are nursing a dying trans and it is low- that is when you add the GOO products like lucas wrench in a bottle stuff IF you are going to try it. Drain some out and add the GOO if you already bought it. Draining 1 qt. Is hard, harbor freight sells this hand pump (use this coupon) to suck it out the dipstick tube. You need to get 6’ of hard tube to attach to the rubber hose it comes with it to reach down there and have pump where you can operate it. Pump into empty bottle to Measure how much you take out.

But imo don’t spend much money trying to nurse a dying trans- save nickles for a complete rebuild or new crate trans.

C4A619D3-35E9-4A26-90FB-842DBCAA4F59.png

That pump also works ok for gear oils and to pump bad fuel out of a fuel tank. About a month ago we used it to empty tank and draw out last of bad fuel even through the fuel filter, lp, and ip (with key on) of a hmmwv that was broke down. Then pulled in the new fuel much faster than cranking the manual fuel pump. Well worth the $5 to own.
 

Road_Case

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Thank you for that Will. That makes sense. After driving for a while earlier today, after adding the half-quart, we arrived to a store and the truck was able to engage reverse in its slippy manner again.

I think Dexron VI is the reason my transmission is not happy anymore. It would make sense, because these problems have gradually worsened over the years.

About 2 years ago we were making this same Keys trip, and I noticed the floorboards getting really hot. For one reason or another, I purchased a quart of STP Dexron VI and probably added about half. Maybe a little less than half.

Roughly 2-3 months ago, I tried to remedy these issues by adding the rest of the Dexron VI that was in the STP bottle.

I think adding it and leaving it has allowed it to wear away at some internals and now I'm left with the issues I have today.

Hence me being ready to pull the trigger on Dexron VI earlier in this thread. Just figured I would share more of my brainstorming. I wonder if a fluid/filter change is still a good idea.
 

Road_Case

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I've been doing a lot of reading, a lot of watching videos, and lot of researching and I've decided I'm going to take a crack at a rebuild on the 3l80 myself.

The only thing I'm unsure of is what kind of rebuild kit to go with. After quite a bit of window shopping, I've found a kit from an ebay seller, and I'm just trying to discern if this will be a good fit. Photos below.

The main thing I'm thinking about here is of course longevity and trouble-free service, however I also want to make the right choice here for the truck's intended purpose. We want to one day slap a big honkin' slide-in truck camper on her and go traveling. We like looking at used Lances. But that's for another thread, another day.

If I'm going to put my time into this trans, I want to build it up good for the possibility of hauling that heavy payload. I am also looking into an additional trans fluid cooler to supplement the pass-thru cooler inside the side tank of the radiator, because I read everywhere that heat is the killer of trans fluid, and subsequently trannys.

Please let me know if this seems like a decent rebuild kit choice, or if you can suggest a better option? Thanks :D



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FRANKENBURBAN
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The BEST advice ANYBODY can give you is to NOT buy online, ESPECIALLY EBAY! There is way to many on ebay selling counterfeit Chinese copy parts, there is no way I would trust one. 2nd, when building an auto, there is almost a 100% chance you will run into a problem, and need to swap something out. I've built several 4l60e's over the years, just did one last week, and ended up making 3 trips back to the trans shop. 1 for bushings that were bad right out of the box, then for a clutch pack that didn't stack up right, and finally another for an upgraded servo I forgot to order at 1st. It happens, and unless you've built quite a few and have extra selective steels and such on hand, you're bound to have to swap something out. And finally, buying your parts from a respected trans shop/supplier can pay back in dividends with knowledge if you run into a problem. There's only so much info you can get out of an ATSG or similiar manual, and thats where the knowledge of an experienced builder can be invaluable in helping you with the small details. Many times its the little details that can make the difference between a rebuild that craps out after 5k miles like the one I just built, or one that outlasts the factory unit.
 

Road_Case

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Understood. That's super solid advice. That's why I try to refer to this awesome forum before I make choices lol.

I suppose I will be on the hunt for a trans shop willing to sell me parts. Not sure if there are any trans suppliers nearby. Thanks again.

I am planning on using an ATSG manual, glad to hear you reference it. Whenever this actually happens I will document and share on here in hopes I don't totally botch it! 😂

Thanks gentlemen, for now the to-do list only includes getting drunk in Islamorada and playing cards.
 
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