Trans won't shift under load - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chrismoore701 · 03-27-2016, 05:40 PM

Seems like the Trans will only shift when I take my foot off the gas. I had it gone through and the gaskets redone about 3 years ago. It has been pretty much sitting with little use for that time. It is a turbo 350 with a vacuum line hooked up. Behind a 350. In my 79 c10. Anything I can check before I take it to the shop? I did run the motor a couple times when it was low on fluid. Not dry just low

**old Rusty C10** · 03-27-2016, 05:42 PM

did you check the modulator and the vacuum line?

chrismoore701 · 03-27-2016, 05:56 PM

Did not. How do you check this? It is hooked up. That's all I know

chrismoore701 · 03-29-2016, 07:23 AM

Anybody know how to test the modulator? Sounds like I disconnect the vacuum line going to the carb. Fire up the truck and put a vacuum gage on the line. see if the truck pulls vacuum, this will determine if my line or hoses are split. Then connect the gauge to the modulator and pump up to 20 inches and see if it holds. sound right?

cadillac_al · 03-29-2016, 10:17 AM

Yes, verify the vacuum at the modulator with a gauge. If that is good, you can try to test the vacuum can or just replace it ($10). While you have it out, pull the valve out too to make sure it isn't sticking. If it does the same thing after that, then you can look elsewhere.

chrismoore701 · 03-29-2016, 10:42 AM

I thought the valve was part of the modulator, no?

chrismoore701 · 04-01-2016, 11:07 PM

My vacuum hose from the carb to the hard line was not tight on the hard line. Now it will pump up to 20 inches at the carb with the gauge on the Trans line not the carb. So I took it out for a test drive and it winds up pretty good and then slams into gear. I have a B&M shift kit in it. But I bet it gets up to 3000 rpm before shifting. Is this normal. I have a tach I can wire up and check later on. What are you guys thinking?

Jaysonb · 04-02-2016, 02:08 AM

I would just replace the modulator, they are cheap. Tripple check the line from the carb, to the modulator. If it's shifting ar a real high rpm, and slamming into gear, then it's shifting off the governor. The modulator is most likely adjustable as well. Take the line off the modulator, and look for a small Allen head screw in the nipple . It will adjust how early or late it shifts.

RodsGreen10 · 04-02-2016, 07:59 AM

All good advice also check the TV cable and make sure it is adjust properly as well. Also when adjusting the modulator turning it counter clock wise makes it shift sooner and softens the shift turning it clock wise makes it shift later and firmer.

03-27-2016, 05:40 PM	#1
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,326	Trans won't shift under load Seems like the Trans will only shift when I take my foot off the gas. I had it gone through and the gaskets redone about 3 years ago. It has been pretty much sitting with little use for that time. It is a turbo 350 with a vacuum line hooked up. Behind a 350. In my 79 c10. Anything I can check before I take it to the shop? I did run the motor a couple times when it was low on fluid. Not dry just low __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

03-27-2016, 05:42 PM	#2
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,313	Re: Trans won't shift under load did you check the modulator and the vacuum line? __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

03-27-2016, 05:56 PM	#3
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,326	Re: Trans won't shift under load Did not. How do you check this? It is hooked up. That's all I know __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

03-29-2016, 07:23 AM	#4
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,326	Re: Trans won't shift under load Anybody know how to test the modulator? Sounds like I disconnect the vacuum line going to the carb. Fire up the truck and put a vacuum gage on the line. see if the truck pulls vacuum, this will determine if my line or hoses are split. Then connect the gauge to the modulator and pump up to 20 inches and see if it holds. sound right? __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

03-29-2016, 10:17 AM	#5
cadillac_al Registered User Join Date: Oct 2013 Location: Maine Posts: 2,376	Re: Trans won't shift under load Yes, verify the vacuum at the modulator with a gauge. If that is good, you can try to test the vacuum can or just replace it ($10). While you have it out, pull the valve out too to make sure it isn't sticking. If it does the same thing after that, then you can look elsewhere. __________________ 76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10

03-29-2016, 10:42 AM	#6
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,326	Re: Trans won't shift under load I thought the valve was part of the modulator, no? __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

04-01-2016, 11:07 PM	#7
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,326	Re: Trans won't shift under load My vacuum hose from the carb to the hard line was not tight on the hard line. Now it will pump up to 20 inches at the carb with the gauge on the Trans line not the carb. So I took it out for a test drive and it winds up pretty good and then slams into gear. I have a B&M shift kit in it. But I bet it gets up to 3000 rpm before shifting. Is this normal. I have a tach I can wire up and check later on. What are you guys thinking? __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

04-02-2016, 02:08 AM	#8
Jaysonb Registered User Join Date: May 2009 Location: Good Thunder Mn. Posts: 955	Re: Trans won't shift under load I would just replace the modulator, they are cheap. Tripple check the line from the carb, to the modulator. If it's shifting ar a real high rpm, and slamming into gear, then it's shifting off the governor. The modulator is most likely adjustable as well. Take the line off the modulator, and look for a small Allen head screw in the nipple . It will adjust how early or late it shifts.

04-02-2016, 07:59 AM	#9
RodsGreen10 Registered User Join Date: Jun 2015 Location: Richmond,KY and Knoxville,TN Posts: 1,584	Re: Trans won't shift under load All good advice also check the TV cable and make sure it is adjust properly as well. Also when adjusting the modulator turning it counter clock wise makes it shift sooner and softens the shift turning it clock wise makes it shift later and firmer.



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