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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: SuperCab on March 15, 2009, 10:14:53 pm
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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.
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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.
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Yeah engne valve lash. Thanks.
The governor could do this too? Thanks. Didn't know that.
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yes. Governor gear is a weak link and tends to apple core.
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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.
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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.
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Thanks man.
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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.
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Do you have it connected to manifold vacuum? You can also try adjusting it with a pocket screwdriver.
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ok.
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You still didn't answer the question. Do u have manifold vacuum present at the modulator diaphragm?
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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.
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Is the detent cable adjusted correctly?
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I don't know. How do you adjust it?
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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?
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I know that sounds contradictory; let me try to explain...
After cresting the hill, the engine revs as if it's in neutral and seems to coast, but if you really push the gas, it goes in 2nd gear.
There is ample vacuum at the modulator.
I havn't checked the vacuum with a guage, I should do that today.
Havn't really checked the timing, I'll check that too...
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The modulator diapragm acts upon the modulator valve. The actual modulator valve is in the valve body. I'll try to simplify:
Governor pressure is proportional to road speed. The more governor pressure rises it will force an upshift.
If modulator pressure which is proportional to engine load over comes governor pressure it will force a downshift or delay an upshift.
The detent cable is directly proportional to driver demand. When throttle pressure is high it will force a downshift.
And for beavis and butthead "the angle of the dangle is directly proportional to the heat of the meat"
Make sure all of those basics are set correctly first and then we can move on. Adjusting the detent cable is done at the carburetor. Depress the tab and push the plunger toward the firewall. Now actuate the gas pedal to the floor. This will give you a base setting.
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Vile I think you are talking about a tv cable adj., the detent or kickdown cable should be about halfway in the slot at idle. Now correct me if I'm wrong but isn't the only way a th350 is going to downshift going downhill is if there is a crack or porosity somewhere in the tranny?
Myself, I'd get my money back or at least get a couple hundred knocked off to cover a used tranny if the engine checks out ok.
He could just disconnect the kickdown cable and avoid any kickdowns caused by the cable at least just to see if it still happens or not.
Vile - I don't mean to step on your toes with the cable thingy but it just sounded like you forgot that it was a kickdown cable and not a tv cable.
Supercab - That may not be the right way to adj. the kickdown cable its just where mine is and it works perfectly for my truck.
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Why would you think that? When driver demand (detent cable) is high it forces a downshift or delays an upshift. TV cable does essentially the same but varies throttle pressure regulating line pressure in 2004r, 700R4. The base line adjustment is the same.
No, a cracked case or porosity is not what can only cause his problem. As a matter of fact that's a rare issue unless there's been physical abuse to the driveline. That's usually an issue with a new or reman unit not functioning correctly.
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So you set a detent (kickdown) cable the same way as a tv cable?
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Some detent cables have the same design as what you would see with modern TV cables. Others have a spring and retainer clip that you can pop up, adjust and lock back into place. I'm going to look at that blazer right now but if you google detent cable I'm sure you'll see plenty of pix.
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Wow! Finaly found the problem!
The governor was worn out.
The spec on the governor lands is .800 inch. Mine mics at .793.
Replaced the governor, works great now.
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glad to hear.