modulator valve bad?

[SuperCab]

Just bought a 1975 Chevrolet Suburban Scottsdale K20.

350V8, TH350 Trans, NP203 Transfer Case.

It won't shift into high gear.

The PO had the trans rebuilt before he parked it 9 years ago, but it's not working now.

It runs fine from first to second, but won't shift into high. I'm thinkng the modulator wasn't replaced when it was rebuilt and now it's sticking and once I replace it, it'll be fine.

Please tell me I'm right. I don't want to think I just bought a rig with a bad tranny...

Also... Does anyone know the valve backlash spec for this? Thanks.

[VileZambonie]

Engine valve lash? There is an adjustment procedure. Stock one full turn past zero lash.

Make sure there is vacuum to the modulator and pop the governor out to check the gear.

[SuperCab]

Yeah engne valve lash. Thanks.

The governor could do this too? Thanks. Didn't know that.

[VileZambonie]

yes. Governor gear is a weak link and tends to apple core.

[SuperCab]

Ok. I changed the modulator valve and now it shifts into high, but when going over a hill, it shifts down as soon as you're over the hill, ie when the load is off it.

Could the governor cause this too?

To remove the governor from the trans, what is involved? Are there gaskets that must then be replaced? Do you have to drain the fluid?

Thanks for all the help.

[VileZambonie]

Do you have it connected to manifold vacuum? You can also try adjusting it with a pocket screwdriver.

AS far the governor goes just pop the cover off. It will lose a small amount of fluid. Then just slide it out. To reinstall it use a little RTV on the cover.

[SuperCab]

Thanks man.

[SuperCab]

Ok. I took the governor out today and the gear is fine. If i adjust the modulator valve could it stop the downshifting problem?

also, it's not upshifting consistently. Sometimes it will, sometimes it doesn't...

I'm kinda stumped. Any help is appreciated.

[VileZambonie]

Do you have it connected to manifold vacuum? You can also try adjusting it with a pocket screwdriver.

[SuperCab]

ok.

[VileZambonie]

You still didn't answer the question. Do u have manifold vacuum present at the modulator diaphragm?

[SuperCab]

Yes. Manifold vacuum at the modulator.

I tried adjusting the modulator and it seemed to fix the problem of not upshifting, but the more I drove it, it wasn't upshifting anymore and was still dowshifting down hills.

Also, it seems to freewheel down the hills...

I'm thinking maybe there is a stuck valve in the valve body? Or something?

Why is it so inconsistent? All help is appreciated guys.

Thanks.

[VileZambonie]

Is the detent cable adjusted correctly?

[SuperCab]

I don't know. How do you adjust it?

[Russ130]

I always thought if the modulator was bad you got no upshift and where stuck in first. You have to make sure you are getting full manifold vacumn to the mod. Unplug it when its running and put your finger on the end if it will stay without holding it then you should be good to go if not you don't have enough vacumn.

You say when you come to the top of a hill it downshifts after cresting it but you also say it feels like it is freewheelin downhill. Those are contradictory to each other.

Does it downshift when you mat it at about 25 -30 mph?

What state of tune is the engine in? Have you hooked up a vacumn gauge to verify the valve timing is ok? Have you hooked up a timing light to verify the ignition timing is ok?