Anyone who has been following my transmission posts will quickly conclude that I'm not too fond of lock-up transmissions,especialy the first two years 1978 and 79.They had many drivability and reliability problems that took several years to partaily solve.Some of the many problems these trans. were cursed with included;Lock-up occured at 20-25 MPH,The L-U module would rapidly cycle between L-U and non L-U causing a shudder or bucking,out of spec parts could cause L-U to occur when shifting in reverse stalling the engine,and slow mushy shifting in general. I usauly throw these in the scrap pile content with the $5 I'll get for it. Lately,scrap aluminum prices are up,and I didn't have anything to do last weekend,so out came the air ratchet. Most of the differeces I discovered wern't a surprise,but one thing was.




The first pic shows some lower springs for the accumulator, I removed the orange spring from this trans. Also shown are the yellow spring common to most S/6 904s, A dark spring common to V-8 904s and most 727s, and a blue spring common to HP and 3/4 ton-up trucks. A stiffer spring causes 2nd gear to shift firmer and helps 3rd gears slightly too.




The second pic shows a 78-79 L-U module on the left,and a 80-up one on the right. Notice the 80-up one has an extra passage,this solves the bucking/shudder problem, It also has a stiffer L-U controll spring,causing L-U to occur at higher speeds.This can be installed in the 78-79 valve bodies.



WHAT! No rear servo cushion spring? Nope this trans didn't have one,shown are two springs that would normaly be installed. I believe this is either an assembly line mistake,or was taken out to "fix" a slow shifting reverse by a dealership mechanic when the car was new. It's common for full-race only shift kits to include a solid spacer that replaces the spring,and causes the servo to engage,and disengage quicker. Removing the spring,like shown here,would make it act solid,but I don't recomend it. Most of the adjustment for the band adjusting screw would be used up,making future adjustments impossible. Back to the two pictured cushion springs-the dark one is often found in late 60s 904s and is best for a performance build.



OK,I saved the best for last,this trans. had 4 clutch discs in both clutch drums.I know truck L-U 904s always had better parts,but this was a surprise.It also had the clearanced rear pump half and the longer splines on the rear clutch drum. Both of these are needed If you want to use a 5 clutch front drum. Shown in the top of the pic is a pre-78 non L-U V-8 rear drum, most people get in trouble if they try to use a 5 clutch front drum with this rear drum with shorter splines. This is the first L-U car trans I've torn apart,so I don't know if it is a fluke or if they are all like this. unfutunatly the car is long gone,so I can't check if it had a towing package or other HD options,It was a station wagon with a super 6, The P/N was 4028812,date code 5927 2322. Maybe since these trans. were designed to shift so soft,the extra clutch disc was added to make them last longer.

The final verdict; The bad, These transmissions in stock form are the bottom of the barrel just like I thought. The good,literaly ANY junk 904 will have better parts,so it would be easy to upgrade. Find a 5 clutch drum from a pick-up V-8 904,install a stiffer accumulator spring,and increase the line pressure,and you'll have a strong transmission!

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V8?? We don't need no stink'n V8!!

Interesting! Thank you.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

Cool writeup! Want to build me a tranny or two??

Lou

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Home of Slant6-powered fun machines since 1988

Do you have any latter model electric lockup valve bodies? I believe they are 86 and up

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Nick
86 d150 turbo /6.
64 valiant 4 door

Bump for ted

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V8?? We don't need no stink'n V8!!

bump for sam

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V8?? We don't need no stink'n V8!!

Do you have micrometer readings on the various springs? Are the colors always indicative of the application, or firmness? Also, if I supply a photo of the numbers on the side of a transmission can you ID it?

Sam

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bump for slanty6

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V8?? We don't need no stink'n V8!!

There used to be a chart where you could look up Mopar trannies. That is how I ID'd a 65 BB cable shift. Darned if I could find it.

Just put the serial number into Google and see what pops up!

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

Try this web site

http://www.mymopar.com/downloads/transguide.pdf

Richard

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Part of Tyrde-Browne Racing

bump

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V8?? We don't need no stink'n V8!!