62. vacuum & 400

[Johnny Bravo]

I lost reverse and 3rd so I took the trans in and they rebuilt it, new converter and all. Now not even after 100 miles its playing with me, long hard shifts, 1-2 is at 45mph+ 2-3 is at 60mph+. I can get it to shift sooner if I let up on the gas, but I have to let it rev some first.
Not sure if its a vacuum issue, the pump is putting out 20+, the vrv is putting out 10, and at the trans its 7 1/2, does this sound right for what the vacuum output should be ?
The guys at the trans place seem to be stupid and know nothing about diesels and vacuum, they bent the vacuum hard line almost shut, & switched the vrv lines. They guy says that its the vrv, but according to the trouble shooting TMs, it has to be something with the trans, it says nothing about the vrv at all, just to make sure the lines are good. I did run stright off of the vacuum pump and it did work, long soft shift. The trans also seems to be leaking a little oil from the governor cover, dont know if that means something.
Could water in the trans cause this to happen ? Am I putting out enough vacuum or is it the trans, what do you guys think ?

Thanks
John

[Robyn]

Late hard shifts are a sign of problems in the modulator curcuit.

The VRV controls the artificial vacuum source and sends a vacuum signal to the modulator telling it where your throttle is.

With the throttle wide open you should have a very low vacuum reading at the modulator.
With the throttle at idle the vacuum at the modulator should be almost the same as at the pump.

You will need to check this with a vacuum gauge spliced into the line at the modulator.
Run a temporary line from under the rig into the cab via the passenger window and to a good vacuum gauge.

As you accelerate the vaccum should drop as the pedal is depressed just as it would in a gasser.

I cant tell you exactly what the number should be but the way it needs to be is High at small throttle opening and lower at high throttle openings.

A very low vacuum at the modulator will give you a very late and harsh shift.

Be sure the lines are hooked up correctly
The pump should be sucking about 24+" of vacuum at the pump if not fix it.
Be sure the VRV is good, if not get a new one
Be sure the line to the Modulator is in good order.

If the lines are backwards it wont work right at all.


You can substitute a small hand vacuum pump to do testing if you like but I always used a gauge, T fitting and a long hose and did the testing right on the rig.

Once you get the thing plumbed right and good vacuum to the modulator things will/should work right.
Then you can tweek the VRV adjustment to get the shift feel where it needs to be.

These were never quite as nice a shift feel as a gasser with a 400 but they will work well if adjusted right.

A light throttle 1-2 shift should be about 10-15mph and the 2-3 should be at 25 or so with stock calibration in the valve body.
Not knowing whats been done in the tranny I can't say for sure but things should be close.

A moderate throttle application should give you a 1-2 at 25 andf 2-3 at 40

A hard throttle should do 1-2 at 35 or so and 2-3 at 55 plus depending on the gearing and tires

The old 400 will be a happy camper with any of the numbers above and the little 6.2 will just do fine.

The idea is to keep the sucker from screaming and you dont want the tranny shifting all over the place everytime you wiggle your foot.

Hope this helps
Robyn

[Robyn]

Ok reread your post.
The leak at the governor cover is due to crappy workmanship and most likely a bad gasket (They are cork)

Dont run with full vacuum as you tried. This will tell the tranny you have a low throttle and you wont get any throttle pressure so the shifts will be soft.
Used this way you can burn the clutches really quick.

This does tell us however that the tranny is working.
Follow what I posted above and sort out the vacuum issues and you should be fine.

Next time use a different tranny shop.
This is not rocket science getting the VRV to work right and those folks should have aced it right off.

Good luck

Robyn

[john8662]

Robyn,

Thanks for the detailed description of the 400 Operation on a 6.2L !

Helps us TH700 guys to understand.

Johnny,

moved to the Drivetrain forum.

J

[Robyn]

Except for the deep low gear and the overdrive I would not touch a 700 on a bet.
The venerable old 400 is almost bomb proof if you build them right.

The 4L80E shares a lot of parts with the 400.

It would be way kewl if you could run a 4L80 manually with manual throttle pressure and a mechanical governor.

I have thought about doing a 400 on my tall GMC Burb but I hate to lose the Deep low gear.
Th 35 inch tires really do away with the need for the OD but with the 400's higher starting ratio its not a good choice.

Most 2500 series rigs that came with the 6.2 and a 400 had 4.10 butt gears and some of the 3500's had 4.56's

A vacuum modulation system is far and away better than the sucky cable system of the 700.
The manual throttle pressure system of the 700 was always thorn in everyones side. Just a little too light on the Throttle pressure and poof you burn the box.

The TP system in the 700 is very complex and works with all the valving to accomplish the up and down shifts in a smooth fashion.

The transgo shift kits do a world of wonders to remedy the ills associated with the TP system.

Even with all the aftermarket goodies stuffed into a 700 you still have a much smaller geartrain and clutch packs than say a 400 or 4L80 or even a 350.

One of the weakest areas in a 700 (even the late ones) is the input drum and shaft assembly.

The shaft is pressed into the aluminum splines of the drum. The shaft has a myriad of holes and passageways to accomodate all the oil flow tht needs to happen.

The area of the drum that the shaft is splined into can distort over time allowing the shaft to tear the splines out.
The aftermarket fix is to machine the hub inside the drum and press on a hardened steel ring to stop the hub from distorting.
They use a slightly different clutch piston on the first set of clutches in the drum to compensate for the hard ring that was installed.

The sunshell is another ooooops. The splined hub tears out and that ends it real quick.

The aftermarket has a couple of HD units, one of which is called the "Beast"
The hub is reinforced and welded to the shell.
The 2-4 piston is the wrong size to give good firm shifts and needs fixing.
The 2-4 band sucks and has to be replaced.

There are several internal mods to the VB too that correct poor oil flow and other issues.

Adding a 13 vane pump is a good thing too.

Only the 87 and later 700's with the auxilliary VB are worth spending any $$ on.

The early 700's even in the trucks had small Input shafts that were very prone to cracking and breaking and when they did they took the whole box out, Boom all over the street, Drip squirt, clank bang thud!!!

A late 700 with the 87-88 VB that has the valves for the TCC as well as the electric solenoid are the best choice as you can run the TCC with a switch and then let the hydraulics do the work and when you dont want it you can shut it off.

I have used several manufactures kits for the 700 and I can say from experience that the Transgo stuff is the best.

Im sure someone will stump another brand, but over the years I have had very good luck with the Transgo stuff and it always seems to do what needs doing and fixes the issues that need fixing.

Another good fix for a 700 is to eliminate the TCC completely and install a HD non lockup converter with the needed retro fit kit.
You get a much stronger converter with bigger guts and more torque muliplying ability.
You have the 4 gears and none of the hastle with the TCC and all its monkey business.

I may still do that on my 91 when I swap the tranny's

Its easy when the tranny is already out.

The old 400 really only needs a good rebuild along with a transgo stage 2 kit and its ready to do battle.
Good HD cooler with an extra filter on the return line.

I have seen 400's behind some really tweeked up engines pulling some GAWD aweful loads and live for almost ever.

No electronics to go away either.

I have seen them behind 6.5 TD's as a replacement for the 4L80 when they died.

The only issue with a 400 is the center splines in the case where the center section locks in.
If the splines wear allowing the center section to rock back and forth they will eventually leak enough oil pressure and the high clutches go away.

The only fix is a new case.
I had that happen on my 86 dually.

The fellow I got the truck from had just replaced the box and it was loose in the center and only lasted a few months.
I replaced the case and used all the other parts plus some new high clutches and poof all well again.

There is also a company that makes a manual cable run modulator for the 400 that will allow the use of the 700 hookup on the 6.2.

I dont like it much as its too crude and shifts crappy due to poor throttle pressure control.
The only really good way is to set it up for sharp slightly late shifts and live with it.

The vacuum on the other hand can be tailored to work "ALMOST" as good as it does on a gasser.

Later

Robyn