TH350 Vacuum line HELP - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bouncytruck · 08-20-2003, 11:49 AM

HELPI rebuilt my TH350 at the end of last summer and it never shifted into 3rd properly. Last night was the first time I've had a chance to really test it out (we had a baby in January). It appears to be shifting into third if I'm not giving it any gas. When I step on the gas it shifts down to 2nd and won't shift up again until I start coasting again. I think it might be a vacuum line issue. right now, the vacuum line to the tranny is going to a port in the Edelbrock Performer intake manifold - the same one as the power brake vacuum. I switched it there because someone told me that is where it should be hooked up. Before the rebuild, it was attached to the carb (quadrajet). Where should the vacuum lines all be hooked up? I don't have a vacuum gauge. I really want to drive my truck again before the weather turns to rain

jose · 08-20-2003, 11:56 AM

Sounds like you have a vacuum leak or the rebuild wasn't done properly .

tom hand · 08-20-2003, 12:03 PM

Have you ajusted the detent cable properly?????????

jhow66 · 08-20-2003, 12:47 PM

Make sure the vac line is hooked to vac. modulator-if not modulator is bad.

tom hand · 08-20-2003, 12:48 PM

If the modulator was not getting vacuum it would never shift out of first

jhow66 · 08-20-2003, 12:48 PM

Sorry that should that should read "if it is".

bouncytruck · 08-20-2003, 01:17 PM

Vacuum modulator is brand new. It appears to be working because I adjusted the shift point last night and it is shifting sooner. The detent cable is disconnected from the carb right now because I thought it was stopping the tranny from shifting. The line out from the vacuum modulator is a rubber line. I have it zipped tied away from the exhaust. I made sure it doesn't have any sharp bends or kinks in it. I know the truck is shifting into 3rd when I let off the gas because i can put it down into 2nd and feel it drop down - same going into 1st. It is just holding in 2nd when I am on the gas - even slightly.

**COBALT** · 08-20-2003, 02:43 PM

Vacuum should be full manifold vacuum, so in other words you have it connected correctly.

You know this may be a governor problem. Did you rebuild the governor when you did your rebuild? Remember that it helps determine shift position based on its counter weights, and how fast it is spinning (RPM).

As far as the rebuild goes...

When you re-assembled your valve body did you make sure to include all the right seals? There are some pressure checks you can do to verify that the appropriate fluid pressure is being applied whenever the gear selection has been changed. You probably should have done that before it went back into the truck. Whenever RPM is high enough modulator vacuum is supposed to help shift into 3rd and not hinder it. Almost sounds like the vacuum is trying to prevent the 3rd shift. This would lead me to believe that the valve body isn't translating the vacuum-to-fluid-pressure to indicate a 3rd shift correctly, or a bad seal is allowing fluid pressure to drop...

Just thinking...

bouncytruck · 08-20-2003, 03:14 PM

Cobalt, thanks. Not what I wanted to hear, but thanks. I guess I may have to drop the tranny/t-case again. I may just have to find someone to do this for me. I was hoping that it was just a simple vacuum line issue, but it sounds like it may be my inexperience in rebuilding the tranny. Anyone know a good (inexpensive) shop?

**COBALT** · 08-20-2003, 03:31 PM

Good question. I've been see-saw-ing back and forth on whether or not I want the 400 out of my daily driver taken to a shop, but I think I'm going to rebuild the one I got from WES and the one I got from breeh before I tackle my daily driver transmission...

Let me know how things work out...

Longhorn Man · 08-20-2003, 04:01 PM

Quote:

If the modulator was not getting vacuum it would never shift out of first

This is incorrect. It will shift, but not untill you hit 5500 or so RPM. The tranny has a govenor that forces a shift regardless if the vac line is on, or even if you have the shifter in first gear, you spin it high enough, it WILL shift. (asuming the govenor is in proper working order)

bouncytruck, try this. Drive down the street up to where you think it should be going into 3rd, then go about 5 or 10 MPH faster. Then take the shifter, and place it into 2nd, and told it there for a couple seconds. While maintianing the same speed, shift it into 3rd. It shopuld grab 3rd, and stay there untill you slow down. My vac line came off on my mod last winter and I discovered this trick. I drove it for a week untill all the slish was off the road before crawling underneath.

You have it plumbed properly, but improperly. (confused?)
The tranny mod DOES need full vac to shift early, but the brake booster is taking all the vacume.
Try moving the tranny line to another port on the intake, or if none are avalible, put it on one of the lowest ports on the carb. (below the throttle plate) It should shift fine after that.
If no ports are open for this hose, you can place it on an upper port, but shifting will occur at a higher RPM. You can compinsate for this my adjusting the modulater screw to shift earlier.
LEt us know what happens.

bouncytruck · 08-20-2003, 05:01 PM

longhornmail - thanks, I'll give that a try tonight. Hopefully that will fix the problem!

**COBALT** · 08-20-2003, 06:58 PM

Hey that is a cool trick. It at least allows you to isolate the modulator out of the equation, which is what's standing in the way of determining if your rebuild is suspect.

Governors are easy to rebuild by the way. 4 bolts and the thing will turn out of the side of the case. It's at least worth checking to see if those counterweights gets stuck the farther it spreads out, which would indicate it's not operating properly at higher RPMs, or where 3rd gear might be!

Good luck.

Longhorn Man · 08-20-2003, 07:34 PM

I stumbled on that trick about 2 years ago when the hose came off. I had been shifting it manually and the hose popped off, so I had thought I was holding it in 2nd. I looked down and it was in drive...I don't know why, but I popped it down and back up and WOW. It shifted.
Went home and saw the hose off.

crazy longhorn · 08-20-2003, 08:01 PM

Another thought, you do need rubber hose on ea end, but should be running a steel line (all but 6" ea end)......you may allready be set up this way.... good luck,crazyL

bouncytruck · 08-20-2003, 08:07 PM

I'm running a brand new all rubber hose. Should it be a steel line? What would the difference be? The kit did come with a short, pre-bent rubber line to run out of the vacuum modulator. I thought it was just so there were no kinks in the line. Should I use that and run steel lines up to the manifold?

Longhorn Man · 08-20-2003, 08:07 PM

Good point Al, if you have rubber, it can get sucked shut...actually collapse from the vacume inside. The longer the hose, the more likely it is to happen.
The stock hard line shoud start close to the back of the engine, and then the bottom end should only be about 6 inches of rubber line.

bouncytruck · 08-21-2003, 11:47 AM

I'll pick up a new piece of line and give that a try too! Anything is easier than dropping that transfercase!

**COBALT** · 08-21-2003, 02:08 PM

...hmmm. I thought of this too when I threw away the horrible disfigured, and bent steel line I had feeding my modulator, but I had some reinforced rubber line small enough so I just rigged up some of that.

I might want to flare up a piece....

**COBALT** · 08-21-2003, 04:12 PM

Oh and another thing. A member reminded me of this site that has a pretty good message board for transmissions. I'm a member, but haven't been there for a while. Good place to ask questions about problems, rebuilds, and modifications:

Netgearhead.com

bouncytruck · 08-22-2003, 12:23 PM

Thanks again, I haven't had a chance to get out there and try any of the suggestions yet. Hopefully this weekend!

08-20-2003, 11:49 AM	#1
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	TH350 Vacuum line HELP HELPI rebuilt my TH350 at the end of last summer and it never shifted into 3rd properly. Last night was the first time I've had a chance to really test it out (we had a baby in January). It appears to be shifting into third if I'm not giving it any gas. When I step on the gas it shifts down to 2nd and won't shift up again until I start coasting again. I think it might be a vacuum line issue. right now, the vacuum line to the tranny is going to a port in the Edelbrock Performer intake manifold - the same one as the power brake vacuum. I switched it there because someone told me that is where it should be hooked up. Before the rebuild, it was attached to the carb (quadrajet). Where should the vacuum lines all be hooked up? I don't have a vacuum gauge. I really want to drive my truck again before the weather turns to rain __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

08-20-2003, 11:56 AM	#2
jose Registered User Join Date: Feb 2001 Location: Salem , Oregon 97301 Posts: 1,796	Sounds like you have a vacuum leak or the rebuild wasn't done properly . __________________ 68 GMC 4x4 SWB jose6772@yahoo.com

08-20-2003, 12:03 PM	#3
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	Have you ajusted the detent cable properly????????? __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

08-20-2003, 12:47 PM	#4
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Make sure the vac line is hooked to vac. modulator-if not modulator is bad. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

08-20-2003, 12:48 PM	#5
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	If the modulator was not getting vacuum it would never shift out of first __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

08-20-2003, 01:17 PM	#7
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	Vacuum modulator is brand new. It appears to be working because I adjusted the shift point last night and it is shifting sooner. The detent cable is disconnected from the carb right now because I thought it was stopping the tranny from shifting. The line out from the vacuum modulator is a rubber line. I have it zipped tied away from the exhaust. I made sure it doesn't have any sharp bends or kinks in it. I know the truck is shifting into 3rd when I let off the gas because i can put it down into 2nd and feel it drop down - same going into 1st. It is just holding in 2nd when I am on the gas - even slightly. __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

08-20-2003, 02:43 PM	#8
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Vacuum should be full manifold vacuum, so in other words you have it connected correctly. You know this may be a governor problem. Did you rebuild the governor when you did your rebuild? Remember that it helps determine shift position based on its counter weights, and how fast it is spinning (RPM). As far as the rebuild goes... When you re-assembled your valve body did you make sure to include all the right seals? There are some pressure checks you can do to verify that the appropriate fluid pressure is being applied whenever the gear selection has been changed. You probably should have done that before it went back into the truck. Whenever RPM is high enough modulator vacuum is supposed to help shift into 3rd and not hinder it. Almost sounds like the vacuum is trying to prevent the 3rd shift. This would lead me to believe that the valve body isn't translating the vacuum-to-fluid-pressure to indicate a 3rd shift correctly, or a bad seal is allowing fluid pressure to drop... Just thinking... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-20-2003, 03:14 PM	#9
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	Cobalt, thanks. Not what I wanted to hear, but thanks. I guess I may have to drop the tranny/t-case again. I may just have to find someone to do this for me. I was hoping that it was just a simple vacuum line issue, but it sounds like it may be my inexperience in rebuilding the tranny. Anyone know a good (inexpensive) shop? __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

08-20-2003, 03:31 PM	#10
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Good question. I've been see-saw-ing back and forth on whether or not I want the 400 out of my daily driver taken to a shop, but I think I'm going to rebuild the one I got from WES and the one I got from breeh before I tackle my daily driver transmission... Let me know how things work out... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-20-2003, 05:01 PM	#12
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	longhornmail - thanks, I'll give that a try tonight. Hopefully that will fix the problem! __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

08-20-2003, 06:58 PM	#13
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Hey that is a cool trick. It at least allows you to isolate the modulator out of the equation, which is what's standing in the way of determining if your rebuild is suspect. Governors are easy to rebuild by the way. 4 bolts and the thing will turn out of the side of the case. It's at least worth checking to see if those counterweights gets stuck the farther it spreads out, which would indicate it's not operating properly at higher RPMs, or where 3rd gear might be! Good luck. __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-20-2003, 07:34 PM	#14
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	I stumbled on that trick about 2 years ago when the hose came off. I had been shifting it manually and the hose popped off, so I had thought I was holding it in 2nd. I looked down and it was in drive...I don't know why, but I popped it down and back up and WOW. It shifted. Went home and saw the hose off.

08-20-2003, 08:01 PM	#15
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	Another thought, you do need rubber hose on ea end, but should be running a steel line (all but 6" ea end)......you may allready be set up this way.... good luck,crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

08-20-2003, 08:07 PM	#16
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	I'm running a brand new all rubber hose. Should it be a steel line? What would the difference be? The kit did come with a short, pre-bent rubber line to run out of the vacuum modulator. I thought it was just so there were no kinks in the line. Should I use that and run steel lines up to the manifold? __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

08-21-2003, 11:47 AM	#18
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	I'll pick up a new piece of line and give that a try too! Anything is easier than dropping that transfercase! __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L



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08-21-2003, 02:08 PM	#19
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	...hmmm. I thought of this too when I threw away the horrible disfigured, and bent steel line I had feeding my modulator, but I had some reinforced rubber line small enough so I just rigged up some of that. I might want to flare up a piece.... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-21-2003, 04:12 PM	#20
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Oh and another thing. A member reminded me of this site that has a pretty good message board for transmissions. I'm a member, but haven't been there for a while. Good place to ask questions about problems, rebuilds, and modifications: Netgearhead.com __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

08-22-2003, 12:23 PM	#21
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,049	Thanks again, I haven't had a chance to get out there and try any of the suggestions yet. Hopefully this weekend! __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

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