Help troubleshoot 904? Slipping at higher RPMs

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Thanatos340

Thanatos340

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I bought a rebuilt 904 Trans from someone February. They had bought it from someone else a while before that for a project that never happened. Not sure how long the Trans sat after rebuild before I got it and I just got it installed a few weeks ago.

Trans has reverse pattern manual valve body, 5.0 tci shift lever, Low planetaries, Mopar steel deep pan.

I had a new custom converter built by Msaine that should stall around 2800 to 3000 Put in. Rear end is 8 3/4 Suregrip with 3.91 gears. Engine is 69 340 that should be making close to 400hp as built.

The problem I am having is it slips at higher RPMs in first and second. Super firm shifts but at about 3500 to 4000 RPM the trans slips or just drops out. Soon as that happens I can shift it to second, Barks the tires and goes strong to about 4500 and same thing. It does not slip in 3rd (At least that I have seen yet but I have not been out really getting on it and have driven the car very little). No slipping at all at lower RPMs.

Not sure where to start troubleshooting with this. Any suggestions, tips or opinions would be welcome.
 
May want to invest in a line pressure gauge. Find the instructions for the valve body & test the pressures accordingly.
 
That's the strangest problem I've seen. Any possibility your shift linkage could be mal-adjusted and holding the valve body just off the detent? I'm just taking a wild guess here.....
 
I think it's a strong possibility that it might be the converter, or not the trans at all.
By the time you realize it's slipping it is also burning clutches and would make the fluid dark and burned smelling. (Is it?)

The "dropping off" and then back on solid just doesn't make sense, and I think it's way more of a possibility that it's something to do with the engine instead of the trans.
Is it possible?
 
Thanks for all the suggestions.

To answer a few of the Questions..

fishy68 - Yes, I checked the fluid in Neutral.

OldmanRick - As for the Shifter linkage maladjusted, I have a new B&M Stealth Shifter in the car and I have checked the Cable adjustment at the Trans a couple times and it appears to me to be properly adjusted (I had this same thought when this first happened as it is so strange and the problem goes away soon as I shift).

TrailBeast - I have a deep Pan with drain on it so it will be easy to change the fluid. I am going to do that next to see how the fluid looks. It looks/smells fine to me on the stick but I am going to Drain it and look for metal ect.. this weekend.

And Yes, It could be the Converter or something else completely. I just don't know how to tell or what to check on that. I had the converter custom built for this car/combo by a reputable builder. It should be about 2800 to 3000 stall. I do know that it seems a good bit tighter than that as the trans engages and moves well at about 1500 RPM in first. I have not been able to really test it by Flashing it to see where it engages yet.

The engine appears to be just fine. Runs/Sounds/Revs great.

And Thanks to all of you for taking the time to respond. It is appreciated. I am still holding out hope that it is something obvious that I overlooked or an easy adjustment.
 
Could be wrong but wouldn't low line pressure not keep enough pressure on the clutches? If you just dropped in the valve body then you're assuming it's dead on from the factory. We know how that goes.
 
Jay, I think that 904 of one of my builds. In 20 + years of building them, That's a first for me hearing slipping problems like that. I can think of a couple things that could cause it. Line pressure like they said. If you drop the pan check to filter to make sure its tight. And check valvebody bolts to make sure they are also tight. I am getting old so i hope i didnt forget to tighten them? lol

I would also check to make sure you don't have a kinked trans line? Or a blockage in the cooling system? I built it for a buddy. And give a full one year warranty on all of them i build. So if you can't figure it out? We can meet up and i will open her back up and see if a problem in the trans. Free of charge of course.

I see you live in Marietta? I drive down to Calhoun Ga for the best ribs around just about every week or so. So if she has to come back out? Will be a good excuse to make a rib run. And meet you down that way somewhere to pick it up? Can email me @ [email protected] Or call or text me 423-402-6752 Just let me know... Thanks! Melvin Croft
 
Wow!! This is why I love Mopar guys. I was hesitant to contact you as I bought the trans second hand after it was built and wanted to make sure it was not something foolish I had done (Still not sure about that) as it was such an odd problem.

I have found one stupid thing I did that may be causing this. I hooked up the Kick Down (Throttle pressure) when I put it back in. Just plain didnt know any better as I never ran Manual VB before. About the time the Throttle pressure would be engaging is about when it starts slipping. I have removed that now but have not been able to take the car back out to see if this helped.

I am going to do all the checks on Friday and/or Saturday and hope to get it worked out in time for Caffeine and Octane on Sunday in Alpharetta. I will be in touch either way and thank you so much for responding.
 
Jay, It isn't a problem. Contact me anytime. Having the throttle pressure linkage hooked up wont cause that problem. That lever doesn't operate anything in the manual valvebody. To me you buying it from him is the same as me building it for you. If it's a problem I did? I want and have to make it right. I have been building and racing 904s for years and only got into doing them for other people when i knew i could build them to hold up to drag racing. If i build one for anyone I want it to be right.

On a different note, I'm like you on the guy who built your conveter. I dont know him? But I would rather open the 904 to see if it was something i messed up on? Than pointing at his converter. If you had said it was a boss hog or ger converter or other cheap brand converter i would really wondering about it being the problem? I had to go through a buddys 904 that live in Athens Ga 3 times cause he kept getting his boss hog converter replaced. You would be surprised how much trash a cheap converter can put into a transmission. If you don't figure anything out? Just give me a shout.
 
Thanks guys!

I know i will never get rich building these things, But if im taking money to build someone a trans? I want it right for them. And don't like seeing them come back unless it many many years later? lol Lost one friend a few years ago over a 727 I built for him. It worked and shifted fine. But had a noticeable whine going down the road. Figured i missed a bad bearing in one of the planets? Even offered to pull it back out and go back though it for him. 2 things still bothers me over that. It had the whine, And i lost a friend over it. Lesson leaned... Not worth losing a friend or a customer over $$$.
 
The Problem appears to be solved. It was a very minor issue. Kinked Transmission line and low fluid. I had installed new Pre-bent Lines and managed to kink one of them somehow at some point. Also the aftermarket Flex Trans dipstick and sheath I used went deeper into the pan than the factory stick so I was miss reading the stick. Replaced the bad section of line, Topped off with Fluid and no more issues (With the trans at least). Put 100 miles on the car today and loved every one of them.

I learned a lot this week with this problem, Mainly I learned who I know I will be going to if I ever need a Trans rebuilt in the future. Thank you Melvin for doing everything possible to help out.
 
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