Is my TH400 dying?

[pmndlt]

Hey All, 1988 GMC C3500 RCSW. Truck has 91KMS 5.7L TH400. Recent tune up; plugs, wires, cap, distributor. Truck runs ok but recently have noticed when driving at constant speed the Transmission hesitates/surges... I think. Into second gear at 40KMS (+/-25MPH) the truck RPM jumps without shifting. I've got a Snappy scanner (MT2500) and the increase is consistent at 300RPM increase/decrease. It eventually shifts and the engine speed evens out. At higher speeds there is no issue at all, highway and pushing it, it runs well and shifts fine. Only happens at slower speeds. No noticeable shift noise or delay just the fluctuating RPM. It's mostly a nuisance. When I got the truck last year I changed the fluids and gasket but have had no issues until recently. Heard the TH400 was bulletproof and really don't want to blame it. Any thoughts? TIA.

 

[618 Syndicate]

So it's flaring when it shifts?

 

[pmndlt]

@618 Syndicate thanks for the reply. TBH I read up on previous posts and Im not sure about terminology. It sounds like it though. While maintaining constant throttle it surges speed (300 RPM) until I apply more pressure and the truck shifts. But at higher speeds it does not. Is this what you refer to as flaring?

 

[618 Syndicate]

In a normal shifting auto, hold the throttle steady and the rpm drops when it shifts. Flaring is an increase in rpm before that drop, even with no increase in throttle.
Does that make sense?

 

[618 Syndicate]

This is a bit extreme, but you get the point.

xc_hide_links_from_guests_guests_error_hide_media

 

[pmndlt]

@618 Syndicate Thank you - appreciate the info and video. Yep I would say it’s flaring, occasionally 2-3 time prior to shifting.

 

[618 Syndicate]

I did some quick googling and found this, seems like a good place to start.

Th400 transmission problems

www.dieselplace.com

 

[pmndlt]

interesting read. I was leaning towards a bad TC. I will have a closer look at the Vacuum lines and see if I can pinpoint the VRV. Thanks @618 Syndicate

 

[Schurkey]

Doesn't sound like shift flare to me. Sounds like the engine doesn't run right, surging before the shift.

I'd want to drive the thing to be sure.

Shift flare happens AS the trans is shifting--not before, not after. You get to the "top" of whatever gear you're in, you'd expect a shift to the next gear, and instead you get several hundred extra RPM you don't expect, for just an instant...and then it drops into the next gear.

 

[pmndlt]

Thanks @Schurkey for the response. If you were closer I’d swing by so you could test drive lol. Any suggestions on what to look at engine wise while Ive got it inside checking the transmission?