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Old, but any experts on TH350 / TH400 ?

This is a discussion on Old, but any experts on TH350 / TH400 ? within the Automatic Transmission forums, part of the Drivetrain category; Over the course of time I've repaired or rebuilt many of these but I've got something running around in my ...

  1. #1
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    00 LS1, WS6, V8 Gremlin
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    Old, but any experts on TH350 / TH400 ?

    Over the course of time I've repaired or rebuilt many of these but I've got something running around in my mind I can't quite sort out.

    Yes, this is about the OLD TH350 3-speed automatics and it isn't in an LS-anything.

    OK, here's some background. Earlier this year I ran into a deal I couldn't turn down. 1978 Chevy G30 1 ton 125" wheelbase van, 400 4 bbl small block, TH350 and the 14 bolt rear end. Only 63,000 original miles for $700 and running great.

    There were of course a few fine tuning items to be done but rebuilding the old Quadrajet and replacing the power steering hoses was cheap, topping off the R-12 Freon charge was a bit expensive as it has a rear AC unit as well.

    Anyhow I'm not really happy about the rear end gearing and/or the shift points of the transmission. According to the books it should have come with a TH400 but best I can tell the one that's in it is original.

    Changed the fluid/filter even though it was still bright red. Changed the vacuum modulator as preventative and it's dialed in at 14 psi but here's the deal. The 1-2 shifts seem to occur just fine, but it tends to think a bit longer than I'd like to go from 2 - 3. I'm familiar with changing the modulator springs/weights to raise shift points but I'd like to lower the 2-3 and the only springs out there are for raising them.

    I haven't measured it but the rear end is really low, typical for a big vehicle from the age when the speed limit was 55 so you can imagine I'd rather the engine wasn't revving so high to go 80 - which it will do in a heartbeat. :-)

    If I change the rear end gearing it will obviously be going faster when the governor commands an upshift but I'm still afraid that 2-3 is going to still be late.

    Vacuum level is fine, used the red stripe modulator as this thing has no EGR, absolutely -0- emissions stuff except for the PCV, big trucks were exempt at the time.


    Anyone have experience at making lowering the 2-3 shift point without bothering the 1-2 much at all?

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    Senior Member Orcus79's Avatar
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    Nop sorry I don't, you sure its TH-350 and not a mistaken 400? I think you can change the shit points in the valve body with a tuneable shift kit but I ahven't really looked into it too much. Do you have a tach hooked up to watch for RPM fluctuations at the shift point?

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    00 LS1, WS6, V8 Gremlin
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    No doubt it's a 350 due to the pan shape and the way the governor is mounted. Why this thing came with the huge engine, rear end etc and only got that tranny is a mystery but it may be because it was originally a passenger van and not a service one.

    I've been meaning to hook up a temp tach but it doesn't act or sound like it's hunting at all, just waiting like they will if there's a vacuum leak but I have none.

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    Senior Member Orcus79's Avatar
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    hmm, well thats where my knowledge stops I'm afraid. I would think that you could change it some how, but i'm not really sure as i know you are aware that these transmisions are operated hydrolicly. Call a performance trans shop to see what they say.

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    I have done a few 100 turbo 350 rebuilds.Try this,take the vechicle out on a flat road and slowly accelerate from 0 up to 40 mph at very light throttle.Post back the eact shift points.Two things effect the shift points in this transmission,one is the governer weights and springs and other is vacuum modulator.Have you checked the line pressure with a gauge and adjusted the modulator to specs?I had a trans do something very simliar to this and the problem was carbon build up in the intake manifold.It would shift 1-2 fine but 2-3 was very late.I went through the entire trans and found nothing wrong.I finally hooked a vacuum gauge up right at the modulastor and took it for a ride.The vacuum would drop so low it didn't want to shift.I pulled the line off and couldn;t find anything blocked or broken,then i looked into the fitting at the intake.It was full of carbon and was blocking the line at times.Funky for sure.

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    00 LS1, WS6, V8 Gremlin
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    Thanks, I'll try that, and it would make sense since it's under more load in 2 than it is in 1.

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    What mph did it shift at in the light throttle test?

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    00 LS1, WS6, V8 Gremlin
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    Haven't had a chance yet, it's parked at my office near downtown and I'd have to get away from the traffic and 5,000 stoplights before I could attempt it.

    Hopefully I'll be taking it on a short road trip Sunday to go look at a similar van a guy is scrapping, I need an undented rear bumper, couple of pieces of the chrome trim and a few cosmetic pieces for the interior. It may be easy finding old Chevy truck parts but difficult for these old huge vans.

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    00 LS1, WS6, V8 Gremlin
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    At almost mimimal aceleration's worth of throttle1 - 2 is smooth as silk around 15 mph and 2 - 3 is a lot firmer around 32 mph.

    Under average acceleration (average for a van, not an LS1) 1 - 2 is still smooth and occurs around 25 mph, 2 - 3 is very firm and doesn't happen until around 45 or 50.

    In the above condition, any time I briefly let up above 35 it will shift into 3rd and stay there after I reapply the gas.

    It's not so much the shift points as it is the way it seems to hang before making the 2 - 3 shift. The RPM has to have leveled off for a few long seconds before it decides to shift.

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    I know what you are saying about the hanging.I've seen that and to honest with you its been so long ago i can't honestly tell you how change it.I'm sure i know what it though,all the springs in the valve body and acumulators are setup for different applications.Example a truck would use different calibration than a car.I'll bet maybe the 2-3 acumulator spring is broken or something like that.Its a easy transmission to go through and cheap to fix too.They work so much nicer freshened up anyway and while you're in there you may find sometime obvious. BINGO i just remembered something.......these 350's use tiny screens under the valve body to protect the pump and governer from debris.I'll bet they are almost blocked with dirt,this will cause weirdness.Just thought iof somethiung else too...worn sprags clutches etc can all add up to a sloppy shifting trans.So it may be a bunch of small things.I'd dig into it.

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    First thing I'm going to eliminate is the potential low vacuum problem you mentioned by comparing it to a different vacuum port on the manifold.

    Filter screens and accumulator springs are easy enough to get at so that'll be my next check. I also still can't rule out a governor that's slightly sticky in that area.

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    To get to the screens in mentioned you need to remove the valve body,one is in the rear in the port going to the governer and the other is much larger in the frt going into the pump.Don't forget about the check balls that will drop out of the case when you remove the seperator plate from the case.

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    00 LS1, WS6, V8 Gremlin
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    Oh, I remember those balls well. Rebuilt a few TH400 ages ago, about went cross-eyed looking at that maze. If I recall I opened the valve body upside down then fished the balls out with a magnet.

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    Some use rubber balls and they all go in the case above the plate.You can drop the VB without messing with them,but if you remove the plate you will need to use grease to hold them in place.To clean the screens you will need to remove the plate.Bon appetit

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    00 LS1, WS6, V8 Gremlin
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    Thanks. Found out what gears are in that 14 bolt rear end, 4.10 so almost a 4.11 which explains why it revs so high cruising the highway.

    10-1/2" ring gear, bet that thing can take some torque. Only disadvantage is you've got to pull the axles to get the brake drums off.

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