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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Socal_GMC on September 13, 2009, 02:15:27 pm

Title: 350 trans gears??
Post by: Socal_GMC on September 13, 2009, 02:15:27 pm

Hi i need a little help with some issues i have with my trans. First off i have a 77 Gmc c25 3/4 with a 350. How many gears does the 350 trans have? I just changed my old one with a rebuilt one and it shifts 3 times but it seem when im cruising at 40-45mph the rpms are to high. is this normal? Also ever since i swapped it out my MPG when down about 4mpg? Sorry about all the questions but i dont know much about transmissions. One more question my Speedometter is off by 8-10mph i have 33 inch tires but it was never off this bad. Thanks for the help i really appreciate it..

Title: Re: 350 trans gears??
Post by: zieg85 on September 13, 2009, 03:33:58 pm

3 gears in a TH350.  There are different plastic gears to drive the speedometer cable so you just have to get the right one for your gear/tire combo.  As far as RPM's in 3rd, we would have to know gear ratio of the rear end.  If you went from a TH350 to a TH350, nothing should have changed RPM wise.  BTW Welcome to the site...

Title: Re: 350 trans gears??
Post by: VileZambonie on September 13, 2009, 08:47:41 pm

Are you sure you had a 350? Was there an electrical connector on it?

Title: Re: 350 trans gears??
Post by: Socal_GMC on September 14, 2009, 12:15:33 am

electrical connector? i dont believe so

Title: Re: 350 trans gears??
Post by: Irish_Alley on September 14, 2009, 01:44:28 am

sounds like a 350, it does only have 3 so your rpms might be a little high. if you knew the rpms it might be a little better to help you out

Title: Re: 350 trans gears??
Post by: Socal_GMC on September 14, 2009, 10:35:54 am

is there a way to change shift points??

Title: Re: 350 trans gears??
Post by: zieg85 on September 14, 2009, 05:38:01 pm

There are some adjustments that can be made however when you are in high the rpm's won't change no matter what you do.  Make sure your rebuilt trans isn't slipping in 3rd.

Title: Re: 350 trans gears??
Post by: Irish_Alley on September 15, 2009, 12:54:54 am

i know my 350 would shift into 3rd but if you gave it too much gas it would slip into 2nd. there is a way to change what rpms it shifts at but i haven't don't it so i cant tell you. but if i can remember right it was a screw/bolt on the side of the trany. your rpms being high would effect your mpgs

Title: Re: 350 trans gears??
Post by: rick_c on September 19, 2009, 10:21:40 pm

You can turn the screw inside the vacuum modulator on the back of the trans. by the pan. Turn it in (to the right) to stay in gear longer. you need a very small flat head screwdriver.
Mine runs at 3000 rpms at around 80mph but I have a stall, so I think that has something to do with it.

Hope that helps...

Title: Re: 350 trans gears??
Post by: Irish_Alley on September 20, 2009, 01:55:56 am

i thought a stall just changes the rpms when the t/c kicks in

Title: Re: 350 trans gears??
Post by: rick_c on September 20, 2009, 10:49:41 pm

You are probably right about that, I'm not sure.

Title: Re: 350 trans gears??
Post by: malibu795 on September 22, 2009, 09:35:08 am

Quote from: Irish_Alley on September 20, 2009, 01:55:56 am

i thought a stall just changes the rpms when the t/c kicks in

stall change with the t/c goes into fluid coupling. and hold the engine.

if you are sliping in 3rd gear direct clutch is bad
2nd gear intermediate clutch
1 forward clutch

th350 shifting 3 times... did you get a th350"C"?