trans or rearend??

[jameswoodlee]

hey everyone I have a 77 c10 it was a manual but whoever had it before me swicthed it to a automatic. truck has no power and running 55 mph it tachs upto 3000 rpm!! so mileage is shoot. I was going to get some different gears installed in it but someone suggested maybe a 700r4 trans would be better. This thing has no power at all everyone is like smoke the tires and im like im trying but i cant lol. I need some help on this one what do you recommend or have you done?
Thanks in advance

[Captkaos]

Need more info..
What gears do you have now? What trans? What size tires?  What is your budget?  What is you mechanical level?

[1980c10]

Also what engine? carb? any info that you know will help. Is your transission slipping?

[jameswoodlee]

truck is a 350 it has 373 gears no slippage at all has a 350 turbo trans i have NO mechanical skill level budget not much of an issue but obviously cheapest way is the best. tires are 295 50r 15's it runs great like I say but has no power on takeoff and once you are upto 55mph its tacking 3000rpms and sucks some gas down. I have a 85 c10 with a similar setup it has I think 3:42 gears or 3:43 gears not sure but it is a beast off the line. I just want to be able to drive it and it no be horrible on gas and be able to burn some rubber whenever i choose. Sorry for the dealyed reply i didnt realize I had any responses yet. Please let me know any info you have.
Thanks again

[Grim 82]

It almost sounds like the trans is stuck in 2nd or something, like it's too high for smoking tires and then is too low for higher speeds, causing the high rpm's. From what you described it should easily put on a decent smoke show.

[jameswoodlee]

yea I Would hope so but it isnt yet. You can hear it changing into 3rd gear but it just tach ups so high. It cost me $20.00 a day back n forth to work in it and I live 15 min from work lol. not to easy on the wallet.

[Grim 82]

Have you tried manually shifting it through the gears? Otherwise the engine runs fine?

[jameswoodlee]

Yes it shifts through all the gears fine and engine runs good. just tachs up real high

[1980c10]

You could check your vacuum modulator, it is mounted on the transmission probably somewhere near the pan and would be connected to a vacuum line to you intake or carb. make sure it is hooked up and working. Maybe someone here can post a little more info on this as I only have owned only manual trans C10's. Google it to see what it looks like.

[jameswoodlee]

yea ill get that checked out tomorrow at work I have no idea what that is.........
thanks

[1980c10]

Your problem could also have something to do with your "kickdown cable". This cable monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body. (make sure it is hooked up)
The Vacuum Modulator monitors engine vacuum by a vacuum line which is connected to the engine.  Engine vacuum reacts to engine load with high vacuum produced when the engine is under light load and diminishing down to zero vacuum when the engine is under a heavy load.  The modulator is attached to the outside of the transmission case and has a shaft which passes through the case and attaches to the throttle valve in the valve body.  When an engine is under a light load or no load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft.  As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift later and more firmly.
hope this helps you understand why you should check this. btw a lot of modulators also have an adjustment screw on them which changes spring tension to adjust how the modulators react.
when the modulators go bad they will leak trans fluid; so remove the vacuum hose from the modulator, and if there's trans fluid, it's definatley bad. Also check the vacuum line that connects it to the engine.
You could also have a vacuum leak elsewhere on the engine

[Russ130]

With 3.73's powered by a 350 you should have no problem getting rubber. Timing? Vacuum leaks? Fuel problem? Cap and rotor? Wires? Plugs?

[jameswoodlee]

yea it just tachs up to 3000 rpm at 50 mph and sucks some dang gas!!!!

[Psycho71]

well, with a 27" diameter tire, 3.73's, and a 1:1 final drive, the rpm calculator shows about 2500rpm at 55mph. To get it to show 3000rpm, I had to run the ratio down to about 4.4:1. That's using 27" tire height for the 295/50/15, which i googled. Any chance someone swapped the gears in the past, before you got the truck? The numbers just aren't adding up for 3.73's, and everything operating correctly.

[Grim 82]

Agreed. Having a lower rear gear than what you think is in there could explain the higher cruise rpm, but that would tip the scales the other way as far as curing the symptom of it being gutless on takeoff. Somewhere between the engine rpm and the road something has to give, and the only thing that's coming to mind is that the trans is slipping. Since I don't know, I'll throw this out there for anybody who might: Is it possible that having the wrong torque convertor could cause these problems?