trany shifts funny

[87suburban]

on my suburban, i had the transmission rebuilt last summer. now it doesn't do this all the time, but it seems like when i get on the highway the trany seems to constantly downshift/upshift multiple times. now when it starts doing that, i give it more gas an it quits. it seems to do this more when i'd be going up a grade. could be something that needs to be adjusted, such as the modulator?

[1979C20]

More details can get you an answer quicker, like which transmission you have and what year, model, weight class vehicle. All you gave us was 87 suburban. I think I remember reading on a different thread that there is a switch on the brake pedal and if it is not adjusted correctly then when the brake pedal shakes, it opens and closes the switch and cause the transmission to downshift, then up shift. But, that's only for certain transmissions, I believe.

[bake74]

      Does your transmission have a TV cable ?  If so sounds like it needs adjusting. 

[VileZambonie]

If it's a TH400 then replace the vacuum modulator and make sure it has a good vacuum supply. Adjust the modulator to your liking.

[87suburban]

Its the 87 TH400 trany with the 350TBI, an i had the trany rebuild last summer because reverse was the only gear that worked. The forward gear was gone, the only way it'd go forward was with the help from gravity. anyway, it weird that it only does this at certain times.  when it does it, im usually gettin the highway goin up a slight grade. i noticed comin home from work last night, when i took off from a red light, it upshifted an downshifted multiple times. it'll keep doing that, till i give it more throttle then it'll quit. just thinking, would it help out if i put a tach on it, an made a video of it when it started to "yo-yo"? that way, maybe it will help explain what its doing? also to, when i had the trany rebuilt, i had the trany already out of the truck. now, would i have to make any adjustments to it?

[87suburban]

also i was lookin in a trany book that i got from summit racing for the TH400, an lookin at wiring diagrams for "detent solenoid circuits 1988 and later". can any of y'all show a picture an the location of a "downshift control relay"? an where there would be a "micro-switch" on the throttle pedal?

[VileZambonie]

What are you trying to accomplish? If you think it's the detent circuit just unplug the connector on the trans.

[87suburban]

well i was tryin to figure out why my tranny would downshift/upshift multiple times, when i'd be gettin on the highway or anywhere. then i was lookin in a book on that tranny, lookin at the troubleshootin charts tryin to see if it could point me an any direction. then i came across the wiring diagram for the "detent solenoid circuit", an was wondering if that would have anything to do with my tranny.

[Irish_Alley]

If your speed or rpms drop that can cause the trans to downshift to give more power cause it thinks the engine needs it to keep its speed up. And it will downshift when you give it gas to give you that same power as if you where passing someone every style of trans will act different if your not used to a th400 its going to feel different than a 4l60e or what ever you are used to

[87suburban]

my suburban is the only one that does that, our vette has the TH400 trany an it doesnt do that. as well as my other 2 cars they dont do it either, but they have the TH350 trany.

[VileZambonie]

So if you suspect the detent solenoid circuit is malfunctioning, simply unplug the connector on the side of the trans and see there is any change.

[wildcatjason]

It sound vacuum related to me. I'd make sure you have good line and has the modulator been replaced?

[87suburban]

ok, ya the trany was rebuilt just last summer. now could it be possible that the modulator needs adjusted?

[1979C20]

Instead of asking what it could be, why dont you try troubleshooting it and actually try some of the advice that we have suggested? Itll get you a lot farther in your diagnostic. Try unhooking the detent solenoid plug from the trans and see if its any different. And try unhooking the vacuum line from the modulator and plug the hose and see if that makes any difference. And try adjusting the modulator. Report back with your findings and we'll go from there.

[87suburban]

sorry i havent give an update on my suburban, between work an weather just havent had the time to try out a few suggestions y'all told me about. anyway, i have some new findings. the first thing i did was unhooked the detent solenoid plug, which it made no difference with the tranny shifting. then i unplugged the modulator an drive it, an it made a difference, it wouldnt upshift/downshift  multiple times between 2nd an 3rd gear. i also got it on video with my cell phone doing it, so if y'all want i can try to put it up on here so y'all can get a better idea of what i'm talking about. however, the video isnt that good, but it may help.