'89 3500 Th400 Tranny Issue

Dirty Chris · June 22, 2009

Please bear with me...

I am borrowing an '89 GMC 3500 2WD with the 454 and the TH400 tranny. I'm getting it ready to haul a slide-in camper for the summer.

My brother-in-law rebuilt the tranny a couple months ago but the truck has had very few miles since then (maybe a few hundred miles). I've driven it a few times and noticed what seemed like a slight slip in 3rd under acceleration but nothing obvious.

Yesterday, I take off from the house with the truck cool and at normal thottle. The tranny slips and shutters and the dash shakes and no acceleration from the driveway. I put the tranny in second and everything seems fine. First gear seems fine as well.

Today, I put in a new filter/gasket and Advance Auto Parts ATF and go for a test drive. The old fluid seemed a little dark and there was a lot oif gunk on the pan magnet but nothing that seemed indicative of any issues, especially considering what are assumingly new clutches breaking in.

After the fluid/filter change: At first, the tranny issue seems fixed. I park the truck at home, leave it running and check the fluid level since it's hot now. It's 1 1/2 quarts low, so I add more of the Advance ATF. Take it for another test drive and the slip/shutter is back, full on.

When starting in first gear: Under normal acceleration, all's good. Under hard acceleration, when I manually shift to second, there is a shutter (dash shakes, pickup jumps) and a hard shift, with the tires breaking loose slightly (hard acceleration).

When starting in second, everything seems fine. But, under hard acceleration, the same slight slip happens during the 1 to 2 shift.

When starting in Drive (3): There is NO acceleration, the engine revs but the truck shakes horribly and feels like the tranny is going but nothing happens.

When shifting manually: Under normal acceleration, everything in first and second seems fine. In third, the tranny just slips. The inly time third doesn't slip is under heavy throttle and even then, it feels like the tranny is barley keeping up wioth the engine.

Now, I'm not sure what fluid my brother-in-law refilled after the rebuild. (It was probably whatever was cheapest; like the Advance stuff). I put in the Advance (store brand) ATF, which says it's formulated for all pre-2005 domestic vehicles and specifically mentioned Dexron II and III on the bottle. The owner's manual calls for Dexron II. It's my understanding that III and the the subsequent versions were just improvements so...the fluid SHOULD be okay, right?

My only hesitation on the fluid is the Advance compter called for Dexron VI, which I assume is just the latest Dexron version...true/not true?

Ok smart guys, what's the issue? And thanks a ton in advance.

bill190 · June 22, 2009

Might want to poke around here...

http://www.bobistheoilguy.com/forums/ubbth...d=16&page=1

kuhl_530 · June 23, 2009

As a counterman I can tell you three things for certain. One: Advance may seem like a good enough store, and stuff is pretty cheap... exactly... cheap... you get what you pay for... but the fluid shouldn't matter what brand it is, just as long as it's the right formula. As long as it has the ase approval stamp on it, you're golden. Second: The majority of guys who work at advance have no idea what the hell they're talking about (notice I said MOST, so before anyone goes jumping down my throat about "their cousin's, brother in law's best friend who works there", i was indeed speaking in generalities.) I can't tell you how many people have come into my store with another part from advance that took a dookie.

And bringing up that point moves me on to my third item of knowledge: whoever told you it took dextron VI is full of ...poop... You would be correct on the truck taking dextron, probably II, and also correct in the fact that III is just a supercession of II and is compatable, and the same is correct for VI, but you don't need to waste your money on the VI, II/III will suffice.

As far as the transmission trouble, it sounds like rebuilding/installation error in the transmission itself. I may be wrong, but I can't bring myself to blame a serious issue like this on something like dextron II/III over VI

Dirty Chris · June 23, 2009

Might want to poke around here...
http://www.bobistheoilguy.com/forums/ubbth...d=16&page=1

Copy that and thanks. I have also since spoke with a mechanic and he thinks (without looking at /driving the truck) that it may be a governor because it sounds (over the phone) like the tranny's not shifting down. I may be on the right track cause this seems right since the tranny seems to be ok when shofting it manually...

I'm handy with cars but tranny's are my weak point.

Any other guidance?

Dirty Chris · June 23, 2009

As a counterman I can tell you three things for certain. One: Advance may seem like a good enough store, and stuff is pretty cheap... exactly... cheap... you get what you pay for... but the fluid shouldn't matter what brand it is, just as long as it's the right formula. As long as it has the ase approval stamp on it, you're golden. Second: The majority of guys who work at advance have no idea what the hell they're talking about (notice I said MOST, so before anyone goes jumping down my throat about "their cousin's, brother in law's best friend who works there", i was indeed speaking in generalities.) I can't tell you how many people have come into my store with another part from advance that took a dookie.

And bringing up that point moves me on to my third item of knowledge: whoever told you it took dextron VI is full of ...poop... You would be correct on the truck taking dextron, probably II, and also correct in the fact that III is just a supercession of II and is compatable, and the same is correct for VI, but you don't need to waste your money on the VI, II/III will suffice.

As far as the transmission trouble, it sounds like rebuilding/installation error in the transmission itself. I may be wrong, but I can't bring myself to blame a serious issue like this on something like dextron II/III over VI

Thanks. I too wa a parts guy for probably 5yrs and I hear exactly what you're saying...and agree 100%. Just as a point of clarification, it wasn't the parts GUy who said Dexron VI, it was his computer. I went with the cheaper "ATF" cause the bottle itself said it was compatible. That and the VI was $6/qt vs. the $2.50/qt for the ATF and the fact that Advance didn't have any II/III.

So....anyone with any input on the "governor" issue/diagnosis?

Thanks again.

doverarjim · June 23, 2009

I feel so dumb because I can not remember the name of the part that I think you need. It is on the side of the trans and has a vaccum line attached to it that goes up to the intake. It will some times fail and not allow the trans to shift correctly. One way to check it is to pull the line off and see if you get trans fluid to come out of it. As I typed this I remembered that I have one in my tool box from my 69 El Camino it was brand new and I never installed it so went out and looked and the box has rotted off!!! Some other smart guy on her may pipe up and give us the answer.

I should have looked it up first it is the Modulator Valve. They were cheap so you might give it a try. As I remember one 9/16 bolt is all that holds it in, you pop it out make sure the gasket comes out with it, oil up the new gasket and put it on the new one put it in, bolt it up hook up the vaccum line and you are off to the races. Hope this helps.

Dirty Chris · June 23, 2009

I feel so dumb because I can not remember the name of the part that I think you need. It is on the side of the trans and has a vaccum line attached to it that goes up to the intake. It will some times fail and not allow the trans to shift correctly. One way to check it is to pull the line off and see if you get trans fluid to come out of it. As I typed this I remembered that I have one in my tool box from my 69 El Camino it was brand new and I never installed it so went out and looked and the box has rotted off!!! Some other smart guy on her may pipe up and give us the answer.

I should have looked it up first it is the Modulator Valve. They were cheap so you might give it a try. As I remember one 9/16 bolt is all that holds it in, you pop it out make sure the gasket comes out with it, oil up the new gasket and put it on the new one put it in, bolt it up hook up the vaccum line and you are off to the races. Hope this helps.

Right on, maybe I'll try that? You think it's just coincidental that it failed this close to the rebuild?

Update: I drove the truck again a few minutes ago and as far as operating it manually: When in second, and starrting from a dead stop, there is a slight shutter. Second gear is not "fine" as I thought it was. I know in some trannies, when 2nd gear is selected manually, the tranny actually starts in second. I'm not familiar enough with the TH400 to know if this is the case. If it's NOT supposed to start in 2nd when second is manually selected, then there must be a downshifting problem, right? Like, it's not going to first unless First is manually selected?

Dirty Chris · June 24, 2009

After some more research, it sounds like the vacuum modulator and governor only regulate UPshifts and kick-down. If this is the case, then it would have to be something internal since my tranny only seems to have issues DOWNshifting on its own, correct?

To clarify, the tranny is not downshifting on it's own during normal driving. This is to say, when slowing for a stop, the tranny will not downshift to second or first...it stays in third (Drive) all the time. Even when first puting the tranny in gear.

UPDATE: I replaced the vacuum modulator with a new one and...no fix.

Mike G · June 25, 2009

Have you confirmed that it did this after your brother-in-law rebuilt the trans or is it a new symptom? I know there are a lot of things on my old vehicles that I take for regular activity that may be of concern to others...

--Mike

Dirty Chris · June 27, 2009

Have you confirmed that it did this after your brother-in-law rebuilt the trans or is it a new symptom? I know there are a lot of things on my old vehicles that I take for regular activity that may be of concern to others...
--Mike

Yes. My father-in-law had my bro-in-law "rebuild" the tranny because it would no longer go into Reverse. Apparently, that was the only problem and these new issues are exactly that; new. I have since learned that my bro-in-law only replaced some gaskets and did only what was necassary to fix the "no-Reverse" issue. Common sense tells me it would have been just as easy and better to rebuild the entire thing while it was apart...[/rant]

We're looking inot a new or verifiably rebuilt tranny now.