Shifting between Drive and Overdrive - '90 suburban

[Groz's Burb]

1990 Suburban v1500 350tbi automatic

This may be a silly question, but is it bad for my rig to occasionally shift back and forth between OD and D? For example, when I need to pass somebody on the highway. I wouldn't be doing it at high RPMs; I just want to make sure I'm not doing any harm to my transmission.
I'm also aware it's important to drive in D when I'm towing something.

 

[cheavyk10]

It's 100% normal for the trans to shift between 3rd and 4th on the highway depending on the grade/ load. Consider 3rd (D) to be the passing gear and 4th (OD) to be the cruising gear. Remember the older autos were 3speeds, with top gear being 1:1, and that's all you has back then, and they had no problem cruising the highway, except for maybe spinning the engine at highish rpms.

 

[sweetk30]

Just dont let it keep cycling 3-4 4-3 3-4 as this will hurt stuff . Few up downs o.k.

If a constant problem and cant hold 4th then we need to look further at tire size and axle gearing .

 

[Groz's Burb]

Thanks for the input guys! It holds 4th no problem, just seems a little sluggish on the highway (which is expected, I'm used to driving old Wranglers). I'm happy cruising ~2000rpm at 65mph. I'm currently running 33"s.
This is my first Burb and I'm not all that familiar with the 700R4 so it's good to get some extra perspective

 

[6.2Blazer]

As mentioned above it's quite normal for the transmission to periodically shift between 3rd and 4th. 4th is overdrive and designed for low load cruising conditions and it should downshift when accelerating, passing, or climbing a hill. Heck, even downshifting into 2nd when you floor it to pass somebody wouldn't be unexpected. It's the same as driving a manual transmission and not downshifting as needed. The biggest concern is getting into a gear hunting situation where the trans is constantly jumping between 3rd and 4th.....I mean it's shifting back and forth every few seconds.....as that will cause it to get hot.

 

[Groz's Burb]

Makes perfect sense. Thanks for the input!

 

[paratrooper307]

I want to say the gear ratios available for a 1/2 ton 'Burban of that year were between 3.08 and 3.73. If your gears are still factory, anyway you slice it you're a little under-geared for 33s, which is part of the reason you feel sluggish. As others have said, it's not a concern if it's not shifting between 3rd and 4th a lot you're probably fine.