Can I Get A Little Help Here???

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bob w

Still crazy after all these years!
Joined
Jan 2, 2008
Messages
13,151
Location
Stillwater, MN
Some of you are familiar with my Bantam Coupe build. 406 sbc with a 400 turbo trans. Maiden trip with it last fall it had a "shudder" in the trans area. If it is driven with a very light gas pedal it will get up to speed. It can be driven with very light gas pedal even at speed. Step on it and it shudders. Shift to neutral at speed and no vibration.

It feels like a converter problem but no one I know has experienced a problem like this. It is a small Hughes race converter (9")? I bought at a swap meet for under a hunnert bucks. Was probably over a thousand new. So I probably bought a "rebuildable " converter though not represented that way.

I have a 10" converter that is probably good since I got it from my brother. Am planning to put that in but it is a pretty big job for an old guy. Only to find out it is something else, like the trans. The 400th is a used, but supposedly rebuilt unit with a shift kit.

I'd hate to put the converter in and find out it is the trans.

Any of you guys have any experience like this?
 
I'm no tranny expert but here goes... first thing I check is fluid level and the sniff test for burnt smell. Since the converter is a fluid coupling, I wouldn't think it would shudder like that or vibrate, unless of course the bearings in it are toast - but you would think even if that was the case, it would still do it in neutral when you rev it. Sounds more like clutches going or a valve body problem, like not enough pressure. I'd drain the fluid if you have a drain plug or drop the pan and look for metal filings or clutch/band particles.
I hate going into automatic trannys, I always end up with a giant puddle no matter how careful I am......
 
I have never had good luck with used transmissions. Id have it rebuilt and change the converter at the same time
 
Before you go to the trouble of tearing it apart, try putting some TransX fluid in it. I had a Ford f150 that was shuddering like that, the TransX stopped it. There are several brands of fluids that say they stop shuddering, it's some kind of friction modifier I think that does the trick. I just know the TransX brand worked for me.
 
Another thought...if you started with a 400 block on the 406, make sure you have a externally balanced flex plate on the crank...
 
Agree with the guys suggestions. Any chance Bob you have a decent spare trans & converter you could toss in for a test run. I know easier said than done.
 
I don't know why I ask for advice. I'm too bullheaded to listen to any of it anyway. I'm sure the driveshaft angles are okay, having built several hot rods. I have the weighted flex plate for the 400 block. It's full of fluid. The trans is rebuilt. I bought it from a private party that never used it. That could be a problem. The $1,000 converter I bought at a swap meet for $50 is the most suspect, just because the guy was practically giving it away.

I managed to separate the engine from the trans by abusing the trans cover to make the upper bell housing bolts accessible. Made a piece to mount on the carb flange so the engine would stay level when I pulled it with a chain hoist. I disconnected as little as possible and pulled the engine forward and swapped the converters. The converter I put in is a known good unit my brother had in his daughter's drag car. Should get it all back together in a few days. Holiday weekend you know. Family obligations.
If this doesn't fix it, I have a spare 400th trans.
 
It's all buttoned up. I took it for a short ride around the block. Didn't get over 40 mph. It is much better. Definitely something wrong with the converter I removed. Still some vibration but I can chalk it up to solid motor and trans mounts and a lumpy cam. No shudder when I get on it.

Tomorrow: high speed tests.

It's quick.
 

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