auto tranny shifting ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**67sss** · 10-05-2003, 01:51 PM

auto tranny won't shift automatically but will shift manually. Anybody have any ideas why?
Thanks.

Longhorn Man · 10-05-2003, 01:55 PM

Check the vac line. If it is still hooked up, then you have some very expensive problems.
The vac line is the only external thing that would cause this.

71GMC_3/4T · 10-05-2003, 06:17 PM

Yup, sounds like the vacuum line fell off the vacuum shift modulator. Been there, did that. Drove to the auto parts store to get a new modulator, crawled under it to see what style it was, and it was disconnected. Wasted trip!

Spoof · 10-05-2003, 07:40 PM

It may have been a wasted trip but I bet you were still happy!

jorgensensc · 10-05-2003, 09:26 PM

I had the same exact problem. Did the same thing ' crawled under the truck to see what modulator' and my vacuum was still hooked up. I unhooked it to look at the adjustment screw and found a little trash. Blew it out and ooked it back up. Problem solved.

72shadetree · 10-05-2003, 10:43 PM

i agree with the vacum

**67sss** · 10-05-2003, 11:00 PM

So here's scoop. It works now. First I took the line off the carb and sucked on it to make sure the modual was working and it was. Then I hooked the other end to a different port on the carb. and it worked. Now I know to check the vacuum port of carbs B4 I go any farther.
Thanks for the info.

10-05-2003, 01:51 PM	#1
67sss Registered User Join Date: Nov 2002 Location: Mt. Vernon, WA.. Posts: 2,324	auto tranny shifting ? auto tranny won't shift automatically but will shift manually. Anybody have any ideas why? Thanks.

10-05-2003, 06:17 PM	#3
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	Yup, sounds like the vacuum line fell off the vacuum shift modulator. Been there, did that. Drove to the auto parts store to get a new modulator, crawled under it to see what style it was, and it was disconnected. Wasted trip! __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

10-05-2003, 07:40 PM	#4
Spoof Registered User Join Date: Jul 2000 Location: Victoria,BC, Canada Posts: 1,300	It may have been a wasted trip but I bet you were still happy! __________________ 71 GMC 1500 LWB

10-05-2003, 09:26 PM	#5
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	I had the same exact problem. Did the same thing ' crawled under the truck to see what modulator' and my vacuum was still hooked up. I unhooked it to look at the adjustment screw and found a little trash. Blew it out and ooked it back up. Problem solved. __________________ 1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE

10-05-2003, 10:43 PM	#6
72shadetree KEEP ON TRUCKING Join Date: Sep 2000 Location: martinsville va Posts: 2,563	i agree with the vacum __________________ 72 cheyenne 2wd lwb 71 k20 sons bassett va

10-05-2003, 01:55 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Check the vac line. If it is still hooked up, then you have some very expensive problems. The vac line is the only external thing that would cause this.

10-05-2003, 11:00 PM	#7
67sss Registered User Join Date: Nov 2002 Location: Mt. Vernon, WA.. Posts: 2,324	So here's scoop. It works now. First I took the line off the carb and sucked on it to make sure the modual was working and it was. Then I hooked the other end to a different port on the carb. and it worked. Now I know to check the vacuum port of carbs B4 I go any farther. Thanks for the info.

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