Transmission Slipping - The 1947 - Present Chevrolet & GMC Truck Message Board Network

nroden · 09-10-2004, 10:43 AM

My truck doesn't seem to want to shift into 3rd, the RPMs go really high and then you have to keep on letting off and it'll finally go from 2nd to 3rd. Then after you get up to about 45 MPH it slips (more you press the gas the faster the engine goes but the truck isn't going any faster). I hope this is nothing major, but it may be time for a new transmission. Its a TH-350. How much do you think a rebuild would cost at AAMCO or someplace like that? The current one has approx. 109,000 mi on it.

Longhorn Man · 09-10-2004, 11:29 AM

Do this quick test. Go drive your truck around the block. Shift it manually. Start off in first, and go untill the engine should shift at light throttle, then wait for just a second or two longer, then pop it into second.
Do the same thing for third gear.

If it seems fine when you do this, then you need to inspect/replace the rubber ends on the vacume line for the tranny modulator. You can see a hard line on the back of the engine...on top of the intake on the passenger side. Look follow that to the carb and inspect the rubber hose, then same thing underneath. (It may just be poped off)
If all this is good, and the tranny is still reacting the same way, replace the modulator. (cheap/easy fix)

old gm blues · 09-10-2004, 03:31 PM

Did you check the oil? My Th350 was leaking from the rear through the front tranfer case seal,so I never noticed any leakage, until I checked the oil lever from the transfer case and got a face full of dextron.

**68 Stepside** · 09-10-2004, 07:07 PM

If the truck doesn't wanna run over about 45 mph....but the engine still gains RPM's, the trans is definitely slipping. Providing the fluid level is correct, I would change the atf/trans filter, and hope that fixes the problem.

Let us know what you figure out.

71Rat · 09-10-2004, 08:03 PM

when u change the fluid if it is burnt and brown/black it is bad. sometimes if it is that bad and u change it, the problem will be worse.... sounds like u need a rebuild...... i do them at our school and the rebuild kit is only like $50... but i dont know what a trans shop would charge.... i am assuming ur looking around $800.

Best of luck
Rat

Sweet72 · 09-10-2004, 11:32 PM

The no 2-3 upshift until you let off the accelerator is a vacuum related problem. As for the slipping, sounds like you have a internal problem, maybe as simple as a stuck valve in the valve body. Do what Longhorn Man said and take it for a run, then change the fluid and filter. Good luck

ZOOMY · 09-11-2004, 12:23 AM

With these old trucks, sometimes the valve body is gummed up. I put in a pint of "Sea-Foam transmission additive", & drive it for about 100 miles before changing the fluid. Works sometimes; sometimes not.

09-10-2004, 10:43 AM	#1
nroden Registered User Join Date: Jun 2001 Location: Springfield, MO Posts: 1,098	Transmission Slipping My truck doesn't seem to want to shift into 3rd, the RPMs go really high and then you have to keep on letting off and it'll finally go from 2nd to 3rd. Then after you get up to about 45 MPH it slips (more you press the gas the faster the engine goes but the truck isn't going any faster). I hope this is nothing major, but it may be time for a new transmission. Its a TH-350. How much do you think a rebuild would cost at AAMCO or someplace like that? The current one has approx. 109,000 mi on it. __________________ Nick Rodenberg 72 Cheyenne 10 V8 350/TH350 white/ochre/white/ochre, ochre/saddle interior Retired and in the dry... still runs. I now drive Red '01 Ford Focus... better MPG!!!

09-10-2004, 03:31 PM	#3
old gm blues Registered User Join Date: Nov 2002 Location: Montreal, Quebec, Canada Posts: 30	Did you check the oil? My Th350 was leaking from the rear through the front tranfer case seal,so I never noticed any leakage, until I checked the oil lever from the transfer case and got a face full of dextron. __________________ everyone is intitled to my opinion 1968 chevelle 1972 k-5 4x4

09-10-2004, 07:07 PM	#4
68 Stepside huh? Join Date: May 2000 Location: Washington, Illinois Posts: 5,692	If the truck doesn't wanna run over about 45 mph....but the engine still gains RPM's, the trans is definitely slipping. Providing the fluid level is correct, I would change the atf/trans filter, and hope that fixes the problem. Let us know what you figure out. __________________ Someday when I'm lonely, Wishing you weren't so far away, Then I will remember Things we said today. RIP El Jay

09-10-2004, 08:03 PM	#5
71Rat ran out of money Join Date: Apr 2003 Location: northern, NJ Posts: 925	when u change the fluid if it is burnt and brown/black it is bad. sometimes if it is that bad and u change it, the problem will be worse.... sounds like u need a rebuild...... i do them at our school and the rebuild kit is only like $50... but i dont know what a trans shop would charge.... i am assuming ur looking around $800. Best of luck Rat __________________ Modify everything, leave nothing alone!~

09-10-2004, 11:32 PM	#6
Sweet72 Registered User Join Date: Nov 2003 Location: Alexandria, VA Posts: 1,534	The no 2-3 upshift until you let off the accelerator is a vacuum related problem. As for the slipping, sounds like you have a internal problem, maybe as simple as a stuck valve in the valve body. Do what Longhorn Man said and take it for a run, then change the fluid and filter. Good luck __________________ '72 GMC SWB C1500 Custom, frame-off in progress. 383 SBC, 9:1CR, Comp Cams XE262H, Scat internal balanced crank, Eagle SIR 6" rods, Keith Black dish pistons, Dart Iron Eagle 72/180 heads, Weiand Stealth intake, Stewart stage I water pump, Holley 4bbl vac sec, TH350 with B&M Shift Improver Kit. 12 bolt positraction.

09-10-2004, 11:29 AM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Do this quick test. Go drive your truck around the block. Shift it manually. Start off in first, and go untill the engine should shift at light throttle, then wait for just a second or two longer, then pop it into second. Do the same thing for third gear. If it seems fine when you do this, then you need to inspect/replace the rubber ends on the vacume line for the tranny modulator. You can see a hard line on the back of the engine...on top of the intake on the passenger side. Look follow that to the carb and inspect the rubber hose, then same thing underneath. (It may just be poped off) If all this is good, and the tranny is still reacting the same way, replace the modulator. (cheap/easy fix)

09-11-2004, 12:23 AM	#7
ZOOMY Senior Member Join Date: Feb 2003 Location: WIS Posts: 1,752	trans With these old trucks, sometimes the valve body is gummed up. I put in a pint of "Sea-Foam transmission additive", & drive it for about 100 miles before changing the fluid. Works sometimes; sometimes not.



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