Replacing vacuum lines and possibly the modulator...

[2HIJOS2126]

Am thinking about replacing all my vacuum lines today and seeing if that helps my shifting issues out that Ive been having as of late... My tranny doesn't seem to want to shift normally, it shifts way late from 1st to 2nd... When it does finally shift its a little hard, not a huge klunk, but not smooth either... I am also thinking of replacing the vacuum modulator, is that a good idea to do as well or should I leave it be??? Also Ive read that you can purchase the modulators at Advance Auto, but unfortunately I don't have one of those nearby... All I have nearby are Autozone and Oriellys... Does anybody know if they carry this part and if so does anybody know a part #??? Thanks in advance...

 

[Warthog]

Before you start replacing parts why not do the checks that are in the TM?

Check the vaccuum pump, check the hoses, check the adjustment of the throttle position valve, etc

If you do need to replace the modulator, the GM part number is in the parts Tech Manual. Any parts house

 

[Durango_USMC]

Dont forget to check the fluid level too.

 

[2HIJOS2126]

Did check fluid level and hoses... Fluid level is good, but some of the rubber hoses were cracked... That's why I am replacing them and was going to use the replacement of these hoses as a starting point in solving my shifting issues... As Ive read on many threads that some other members had similar issues and replacing such hoses remedied their problem... I was just inquiring about replacing the modulator because theoretically the part is 28yrs old and might be worth replacing since its easy and cheap...

 

[IdahoPlowboy]

The vacuum lines were bad on both my trucks and replacing them solved all my shifting problems. If your modulator is bad I have one off of my old transmission that should work.

 

[2HIJOS2126]

So anyways, replaced the old cracked rubber hoses... Solved the semi hard late shifts but arose the opposite problem... It now shifts a little early and seems a bit on the sluggish side... Pulled the modulator and it does not have an adjustment screw on the inside... I do indeed know about the TMs and how to use the search function, so away I go into the depths of my computer screen...

 

[Keith_J]

Check the throttle vacuum valve. The purpose of this valve is to trick the TH400 into thinking it is behind a gasoline engine. At idle, you need 17" of mercury vacuum, dropping off to 0" at wide open throttle. A cheap vacuum gauge and a few assorted vacuum tees will go a long way to diagnosing your problems.

The throttle position valve will leak vacuum so it has a little polyurethane filter to protect the vacuum system. This filter loads with dirt, eventually decomposing and causing the valve to fail. Simply teeing into the TOP port with the vacuum gauge and then going on a test drive will show if the valve is at fault. Also, you need 18" of mercury or more vacuum at the vacuum pump.

 

[Shankem-Deep]

I'm having the same problems. Subscribed.

 

[2HIJOS2126]

Throttle vacuum valve... Got it... Thanks... I'll get some tools in order and do some more troubleshooting... Also Im assuming the throttle vacuum valve and the throttle position valve are one in the same???

 

[Keith_J]

Yes, the valve on the injection pump that has the top hose to transmission and bottom hose to the vacuum pump. There is a special gauge block to adjust the valve but you can make it yourself. Or just trial and error.

I found a genuine AC Delco part on that website named after a South American river .

 

[2HIJOS2126]

I appreciate all your input Kieth_J... I have been reading other threads where you have shared such knowledge as well... THANK YOU... Gonna try adjusting probably this weekend at my buddies shop, he's got digital calipers and other fancy tools to aid in my efforts if I decide to make my own gauge block... I guess I'll just have to keep driving my Chevy Cruze to work... Boo Hoo... Poor me...

 

[MarcusOReallyus]

I found a genuine AC Delco part on that website named after a South American river .

It's perfectly okay to mention Amazon and even eBay!


The rule is against posting LIVE AUCTION links.

 

[Warthog]

Here is how to adjust the vacuum valve.

http://www.steelsoldiers.com/showthread.php?50568-CUCV-Throttle-Vacuum-Valve-Adjustment

Not much on the CUCV that hasn't been covered.

 

[2HIJOS2126]

Thanks a ton Warthog... I found this thread last night and have it saved on my favorites... After looking at your last pic showing you tightening up the bolts I went outside to take a gander at the position of my valve... Looked at it and the **** thing is almost sitting straight up (so fully towards the firewall)... If I remember correctly I think I read on that thread that moving the valve towards the fan makes it shift later, so once I can get a gauge block made I should be good to go... I'm also gonna replace the modulator while Im at it, like I previously stated, the one that is installed now has no adjustment screw... I also read that the replacements are the ones that are adjustable for the fine shift points... Thanks again to all who have posted ideas and info on this thread and also to the ones that shared their knowledge on the various other threads I came across...

 

[2HIJOS2126]

Adjusted the vacuum valve and replaced the modulator and no change at all on my shifting issues... It seems like Im starting out in 2nd gear cause it only upshifts once at around 1700rpm, but the shift is weird... When the shift happens the truck kinda slows down a tad and then its a little sluggish gaining speed... Gonna get me a vacuum gage and do some trouble shooting...

 

[Easypatriot]

From personal experience, I too replaced the vacuum modulator with no change in shifting (high rpm and hard shift from 1-2) and in the end it was adjusting the modulator that solved the problem. IIRC, clockwise increases shift RPM and harshness, counter clockwise lowers and softens. I went counter clockwise 4.5 turns on the modulator, and it shifts like butter.

 

[2HIJOS2126]

Replaced the vacuum valve and adjusted it to "about right"... That was my problem (aside from cracked rubber hoses) it shifts like its suppose to now... I'm still gonna get a block made and get the specified adjustment made on it and also tweek my modulator a bit to dial it down to where its suppose to be... Thanks again to all who offered their info and ideas...