TH400 shifts late and hard

[Hulkjoegan]

New to the forum, just picked up an 85 C20 Suburban, 454 with a th400, runs pretty good except the trans shifts late and kicks hard, 1-2 anywhere between 35-50mph. Don't have a tach so I don't know what rpm. I was told this was a common Chevy problem.  The previous owner said he'd changed all fluids except trans. So I'm gonna changed fluid and filter soon, but I wanna make sure I don't have to dig into the trans multiple times. I unplugged the kick down  solenoid to make sure it wasn't stuck in passing gear, but it still kept up the problem. Could the vacuum modulator or governor be causing my problem?

[SkinnyG]

Vacuum modulator controls part-throttle shifts.  If it has failed, lost its vacuum connection, or has cracks in the vacuum hoses (or heck - even plumbed to the wrong place), the engine will shift as if it is at or close to full throttle.

[bd]

To reiterate, check the condition and connections of the manifold vacuum line that runs to the transmission vacuum modulator.  Verify that full manifold vacuum is actually reaching the modulator.  Unplug the vacuum line from the modulator and rotate the modulator body so the vacuum nipple is pointing straight down.  If any ATF drips out of the vacuum line or modulator nipple, replace the modulator valve.

[Hulkjoegan]

Thanks for the info, finally got around to really digging in. Hose on the vac modulator appeared to be a little dry rotted, replaced that and it seems to have fixed my problem.