3-4 flare at lower road speeds

[Orv]

I have a transmission issue I haven't heard mentioned anywhere else, so I thought I'd ask about it here.

On my 1981 300TD, the shift from 3rd to 4th flares badly at road speeds under about 50 mph. If I hold off the shift by keeping the lever in "S" until I'm above 50 mph, it shifts fine; likewise, if I push the accelerator most of the way down so the shift is delayed, it shifts just fine. But under light acceleration, where that shift can occur at as little as 30 mph, it flares and sometimes won't complete unless I ease off the accelerator.

My question is, is this likely an issue that can be cured with a shift kit? Or am I looking at a wear problem, maybe a bad rear pump?

EDIT: Forgot to mention that the problem still occurs with the vacuum line disconnected. All the other shifts get noticeably firmer, but the 3-4 shift still flares.

[Orv]

Link got censored by the site...could you PM it to me?

I'm thinking the vacuum modulator probably isn't the issue because it still exhibits the problem with the vacuum line disconnected, but I would be happy to be proven wrong.

[partsman225]

The 3 to 4 shift. Is in the K2 clutch drum could be worn or close to metal to metal.bump up the modulator vavle. By pulling the cap off turn it clockwise about 2 full turns in. Of still slips unhooking and pull the vacuum lone this will make the trans shift really late and hard you will need lots of room to do this if 3 to 4 is firm. The K2 drum IS okay.

[kestreltom]

for the same 3-4 shift flare problem - especially when the trans is cold. I also replaced the modulator, measured the control pressure and turned it up a bit (45psi) and changed the filter, pan gasket, and fluid with Amsoil ATF (the red stuff).
Aside from the fact that the Amsoil atf is nearly impossible to measure the proper fluid level due to foaming or ???, I did not really fix the 3-4 shift flare. It is manageable by backing off the throttle a bit to let the shift complete itself. When the tranny is warmed up all the way the problem is not that noticeable. I plan to replace the fluid with type F atf as per Doktor Bert's recommendations.
Later this year I plan to replace the tranny with a known good one. Dismantling the tranny and replacing components seems waaay too time consuming and expensive relative to what these cars are worth.

[partsman225]

When you replace.the.modulator vavle.you must at least turn it in closewise at least 5 full turns then try it .and report back.

[Orv]

Thanks, useful info. I expect the trans is quite simply wearing out but I'll fiddle with it and see if I can firm up the shifts more. I'll also check the control pressure since I think it should be pretty easy -- I already have a gauge kit for setting fuel pressures on CIS Volkswagens that should have the right fittings.

It's useful to know that a shift kit is probably a waste of time.

I assume the Amsoil is a synthetic? I'd always heard that synthetic lube was too "slippery" for these older transmissions.

[Mxfrank]

Before you do anything expensive, get a Superior shift kit and replace your K2 and K3 piston springs. This is easy enough to do, and may well solve your problem. On mine, both springs were shattered. I'm sure it's not a unique situation.

Superior Transmission Parts

[qwerty]

Originally Posted by Orv

It's useful to know that a shift kit is probably a waste of time.

You arrived at that conclusion by what logic?

I installed portions of the Superior Shift Kit on my '83 300D. It was a long ways from being a waste of time; the improvement was nothing short of phenomenal.

Originally Posted by Mxfrank

Before you do anything expensive, get a Superior shift kit and replace your K2 and K3 piston springs.

I suspect that you mean K1 and K2 accumulator springs. Even so, there is little point in replacing the K1 accumulator spring if no problem exists in the 2-3 shift.

[kestreltom]

Originally Posted by Orv

Thanks, useful info. I expect the trans is quite simply wearing out but I'll fiddle with it and see if I can firm up the shifts more. I'll also check the control pressure since I think it should be pretty easy -- I already have a gauge kit for setting fuel pressures on CIS Volkswagens that should have the right fittings.

It's useful to know that a shift kit is probably a waste of time.

I assume the Amsoil is a synthetic? I'd always heard that synthetic lube was too "slippery" for these older transmissions.

Yes the Amsoil is a synthetic product. Indeed it may not be the best formula for these older transmissions.

I have heard stories of broken springs in the valve body, but all mine were fine. BTW... if you do take that valve body apart, do a ton of research and be careful how you proceed. You don't want to lose those little check balls & valves! The directions for how to proceed with the replacement springs in the Superior kit are not clear to a first time rebuilder. So proceed with care and learn all you can off the internet first.
If the problem with your tranny is wear in the clutch packs or worn seals, then there is only so much that the shift kit can do.

[qwerty]

Originally Posted by kestreltom

BTW... if you do take that valve body apart, do a ton of research and be careful how you proceed. You don't want to lose those little check balls & valves!

Accumulator springs can be replaced without the removal or major disassembly of the valve body. The average fifth-grader could accomplish the task.

[kestreltom]

Originally Posted by qwerty

Accumulator springs can be replaced without the removal or major disassembly of the valve body. The average fifth-grader could accomplish the task.

That would be true only if you were replacing those springs accessible from the outside of the valve body... wouldn't it?

I used the Superior VB Kit part # S68165D
In my experience, I wanted the "To firm up 3-4 shift even more" option, and the "For even softer 1-2" option as well. In both cases, I had to remove and replace internal (infernal?!) springs that were located between the valve body halves.

[Stretch]

If anyone wants 722.3 valve body photos you can find them here =>

http://www.peachparts.com/shopforum/diesel-discussion/323259-722-303-bits-photo-shoot.html

I'm 99% sure that the "diesel" 722.3s shared the same valve body.

If anyone wants 722.1 valve body pornography you can find that here =>

http://www.peachparts.com/shopforum/diesel-discussion/304903-722-118-automatic-transmission-rebuild-monster-diy.html

722.1 transmissions had a couple of valve bodies for their diesel transmissions - the thread linked above concerns a vacuum only system.

[Orv]

FINALLY got around to this -- a couple other things breaking, and general busy-ness, kept me from doing it 'til now.

Hooked up a gauge to the test port and discovered the modulator pressure was only 20 psi. I believe that the modulator was actually backed all the way off, because it took a few turns of the T-handle before the pressure started to increase at all. I adjusted it to factory spec (43 psi) and the shifts are *much* improved.

The 3-4 shift still flares a little, but now the flaring stops and it shifts firmly if I disconnect the vacuum line, so I'm confident I can get it to behave with just vacuum adjustments from here. Before the modulator adjustment, disconnecting the vacuum line had no noticeable effect.

As a side note, the procedures I read neglected to note that the T-handle isn't actually attached to anything; it just slides in there like an allen wrench. I had a moment of panic when it fell out, thinking I'd broken my modulator.

[vtmbz]

Just for info Orv what causes the pressure to drop like that? And q#2: where does it squirt out afterwards? Does it just ooze out of the valve body all over?

[Orv]

The modulator on the transmission regulates the pressure, based on both its internal setting and the vacuum signal from the vacuum control valve on the injector pump. Nothing squirts anywhere -- this is all part of the transmission's internal hydraulic circult.