: The famous TH400 hydramatic



7.0,hellyeah
10-09-04, 01:49 AM
I love the transmission on my caddy. The TH400 is very good at its job. Although a little lacking in getting all the power to the rear wheels it is adequate. I recently have been taking advantage of the drive-drive-low feature. Low=1st, drive2=1st+2nd and drive=1st+2nd+3rd. I've been keeping it in 1st around town so that I have more acceleration up to 25 and 30mph. Recently I took a trip up the valley to napa(a 30min drive) and it wasnt going into third. Usually the RPMs arent too high at 55-60mph. I noticed however that it was staying in what I thought was 2nd gear. I tested it tonight by going 50mph and putting it into 2nd gear. It should have been in third, I think, but it didnt downshift. Has a wire been disconnected or something that would cause this or is it a much bigger and much worse problem? thanks for any help! :banghead:

majax
10-09-04, 04:35 AM
I have no clue, but just to say my dads '77 CoupedeVille lost its transmission at about 140,000mi. It went out in a huge kinda crash clink grind.

Night Wolf
10-09-04, 02:01 PM
the TH400 is vaccum operated, not electric...

... My '79 will push 3rd gear while going 30mph... up a hill.... the 425 dosn't have hp, but it has nice off-idle torque...

I had a hard time understanding your problem... what exactly is wrong?

I heard somewhere that these trannys were built to slip on purpose... for a very smooth shift, and that it actually wears them out much faster, and that a shift kit helps prolong the life... can anyone add to this?

7.0,hellyeah
10-10-04, 03:12 PM
What is wrong is the fact that it is NOT going into 3rd gear. The only cause I could think of is the fact that I've been using it like a manual a lot lately. Maybe the column shifter is funky and I'm not putting it in to gear, I have no idea.

Night Wolf
10-10-04, 11:06 PM
manually shifitng it woudln't cause a problem

maybe the linkage is not set correctly.... does park, neutral, reverse etc.. still work correctly?

the only other thing i coudl think of, get a fluid and filter change done... my Olds was not shifitng out of gear at all.. only when the enigne woudl redline (even under part thortttle) turned out the filter was clogged and the line pressure was great enough to shift... only shifted when the emergency override would go on... at redline...

DaveSmed
10-11-04, 12:26 AM
another possibility is the kickdown switch might have become shorted. Unplug it or pull the trans fuse and see if it comes back.

cadillacdeville
10-11-04, 03:35 AM
well there is an electric downshift switch on the drivers side of the carb two wires that plug to the switch try checking those or the fuses also try getting up to around 60 to 70 she if maby shes just being stubborn

barge master
10-11-04, 05:57 AM
well there is an electric downshift switch on the drivers side of the carb two wires that plug to the switch try checking those or the fuses also try getting up to around 60 to 70 she if maby shes just being stubborn
No CDV, I chased that same switch thinking it was the downshift years back. The one on the carb shuts off the A/C under full throttle, the trans kickdown is on the gas pedal.

7.0,hellyeah
10-12-04, 01:08 AM
Is there a drain plug on the tranny pan? I got a new B&M pan that adds 2qts compared to the original pan and I need to install it. The bottom of the old one is really caked with oil because my rear intake manifold gasket just failed so I cant see anything including a possible plug.

caddyladdy
10-12-04, 06:57 AM
Not as stock, but one could have been put in. It is fairly simple to do and I am going to put one on my 74 coupe deville when I put in the new transmission.

Azriel
10-12-04, 08:14 PM
Hey 7.0,
How does it feel in first and second, does it seem to grip well, or do you feel slipping? Also, when it shifts from first to second, does the shift feel right, or does it seem sluggish?

Usually when a trans dies, it goes from the top down. First thing I lost in my Mercury was overdrive, then 3rd, etc...until I just had 1st and a fragment of second...I remember limping into the trans shop..lol.

I'll tell ya, running at 30 in first is scary dude, everything's spinning way too fast for it's own good. First gear's going 3 times as fast as it should, there's NO reason to do that, bad for engine and trans. That likely put 'er down to an early grave.

But hey, definetly had it checked out, you never know, could be a valve body problem, could need to be cleaned up. Had an Audi that was hesitant to go into 4th, and it was a dirty valve body, expensive as hell to fix though, but that's how imports are. American trans are cheap to fix.

Good luck man.

Broughamadrosis
10-12-04, 10:01 PM
Check the vacuum modulator, a cylindrical thing about the size of a cupcake, mounted to the side of the trans - there's a vacuum line going to it from the manifold. If the line is off/cracked/broken, it will delay the 1-2 shift, and never hit 3rd...

...or...

If the diaphram in the modulator has gone bad, there'll be trans fluid in the vacuum line...

B&M sells an adjustable version, which would be the hot ticket for your situation, you can adjust the shift points.

The TH400's do shift smoothly by design, but I wouldn't call it slipping...just one of the best trannys ever built.
(Rolls Royce bought TH400's from GM because of their shift quality...)

Night Wolf
10-12-04, 10:41 PM
I'll tell ya, running at 30 in first is scary dude, everything's spinning way too fast for it's own good. First gear's going 3 times as fast as it should, there's NO reason to do that, bad for engine and trans. That likely put 'er down to an early grave.

heh...how about 50mph in 1st?

seriosuly, my car will shift into 2nd around 53mph... if i am doing 40 or so and floor it, it'll downshift into 1st and pull like crazy... gotta love the 2.28 rear end...

davesdeville
10-13-04, 12:51 AM
Check the vacuum modulator, a cylindrical thing about the size of a cupcake, mounted to the side of the trans - there's a vacuum line going to it from the manifold. If the line is off/cracked/broken, it will delay the 1-2 shift, and never hit 3rd...

I had the vacuum modulator line disconnected for awhile and it would still hit 3rd.



heh...how about 50mph in 1st?

seriosuly, my car will shift into 2nd around 53mph... if i am doing 40 or so and floor it, it'll downshift into 1st and pull like crazy... gotta love the 2.28 rear end...

If I'm easy on the gas mine shifts at under 20, floored it shifts at about 25 and with the vacuum modulator disconnected it only went to 30. I held it in first to 50 once, it didn't like that at all and it was definately at the end of the powerband. Might have the 3.08s after all.

Night Wolf
10-13-04, 04:15 PM
Heh, my car will push 3rd going uphill at 30mph... not taxing the enigne either....

... funny with the big block, but since I cleared the cat out, she seems to pull the best around 20-50... in 1st... there is an actual top end now...

7.0,hellyeah
10-14-04, 12:26 AM
So the answer is very simple; the shifter is a little worn from so much use. The stick is loose and it's very difficult to find the correct place to get it into 3rd. After a few seconts of playing around its possible to get it right. I need to take the cat off mine too. Just have it on now. I think just a glass pack will do fine.

Night Wolf
10-14-04, 12:39 AM
I don't think the shifter is the problem... maybe the linkage is not set right....

... I wanted to leave my cat on, that way it is visually there.. I just drianed the cataalyst beads.... now I need to find a piece of metal and try to JB Weld the hole... it is loud.. and not a really cool loud and sweet sound like my 4.9 running open exhaust right before the cat, it is like a really loud exhaust leak.. after drivign for 10minutes it starts to bother my head...

davesdeville
10-14-04, 05:17 AM
it is like a really loud exhaust leak..
That's cause it is a really loud exhaust leak :yup:

With my collapsed lifter and hole in the Cat the car sounds bad ass, like it has a real big lopey cam. It's pretty funny, but it's off the road until I sell it to my friend... hopefully he'll get the money soon. Hopefully it will see a cammed, nitrous fed 500 with an edelbrock or small sheetmetal single plane, a rebuilt th400 and a posi 3.42 or so. But I'm really off on a tangent now..

barge master
10-14-04, 03:46 PM
So the answer is very simple; the shifter is a little worn from so much use. The stick is loose and it's very difficult to find the correct place to get it into 3rd. After a few seconts of playing around its possible to get it right. I need to take the cat off mine too. Just have it on now. I think just a glass pack will do fine.
Down on the frame is a little nylon donut that the Z-bar pivots on. If that donut and its spring are worn/missing the shifter will get really sloppy.