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Automatic transmission vacuum modulator
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Jarzki
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PostPosted: Mon Jul 01, 2024 2:18 am    Post subject: Automatic transmission vacuum modulator Reply with quote

I have some trouble, it seems that my gearbox only uses 2 first gears. Might need more atf oil...now between low/full mark ? Other I am thinking is the vacuum modulator, there ia somekind of set screw, how big difference can it make ?
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vwsquare2
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PostPosted: Mon Jul 01, 2024 9:24 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

There's a set screw in the vacuum modulator but before adjusting it, be sure the modulator is getting vacuum. What you describe could be the vacuum hose to the modulator is loose or not connected.

I had the same symptom and I was going to try adjusting the set screw. I went to pull off the vacuum hose and the old cloth covered hose fell apart in my hand. I pulled gently on the hose to trace it back to the engine and the other end of the hose pulled out of the engine. It had broken in the engine bay near the "T" that provides vacuum from the intake manifold. I replaced the hose and third gear was back.

If you had third gear earlier and it stopped working, check first to be sure you have vacuum to the vacuum modulator before trying to adjust it.
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Jarzki
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PostPosted: Mon Jul 01, 2024 12:04 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

vwsquare2 wrote:
There's a set screw in the vacuum modulator but before adjusting it, be sure the modulator is getting vacuum. What you describe could be the vacuum hose to the modulator is loose or not connected.

I had the same symptom and I was going to try adjusting the set screw. I went to pull off the vacuum hose and the old cloth covered hose fell apart in my hand. I pulled gently on the hose to trace it back to the engine and the other end of the hose pulled out of the engine. It had broken in the engine bay near the "T" that provides vacuum from the intake manifold. I replaced the hose and third gear was back.

If you had third gear earlier and it stopped working, check first to be sure you have vacuum to the vacuum modulator before trying to adjust it.


The car is running for the fist time in maybe 25 years or something. When I bought the car the vac modulator was blocked and while we serviced the engine we put new hosesand connected everything properly. Today we mostblikely found the reason for not engaging third gear... the membrane inside vacuum modulator was ripped...yes I know it is not should be opened but now it is open and membrane fixed. Lets see tomorrow what happens
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Dfasty
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PostPosted: Tue Jul 02, 2024 10:38 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

There is a vacuum modulator on ebay
Right now.
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Jarzki
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PostPosted: Mon Jul 29, 2024 10:04 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

I bought TM-39 modulator for 20€ and my dad nodified the threads so it fits to VW gearbox...have not tested to drive nyself yet but acc. to dad it worked atleast equally well as the old repaired one.
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Multi69s
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PostPosted: Sat Aug 03, 2024 6:54 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Jarzki wrote:

The car is running for the fist time in maybe 25 years or something. When I bought the car the vac modulator was blocked and while we serviced the engine we put new hoses and connected everything properly. Today we most likely found the reason for not engaging third gear... the membrane inside vacuum modulator was ripped...yes I know it is not should be opened but now it is open and membrane fixed. Lets see tomorrow what happens
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That my friends is old school mechanics Cool . Its the same way with smaller farms and farmers. You do whatever it takes, even if it is outside of the box.
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DQDan
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PostPosted: Sun Aug 04, 2024 9:14 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

I wish one of you guys would help this guy out. He's been searching for this for years... on craigslist

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Bobnotch
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PostPosted: Mon Aug 05, 2024 10:00 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

DQDan wrote:
I wish one of you guys would help this guy out. He's been searching for this for years... on craigslist

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You might want to send him an e-mail to check e-bay or facebook. I know some don't like e-bay, but then some parts are just getting harder and harder to find. 20 years ago, you could buy the whole AT trans for a 100 bucks, now it'll cost that much just to ship it, if not more.
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Tram
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PostPosted: Mon Aug 05, 2024 11:46 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

DQDan wrote:
I wish one of you guys would help this guy out. He's been searching for this for years... on craigslist

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Is this someone you know?
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Typ3nut
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PostPosted: Mon Aug 05, 2024 6:30 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Info gathered for the Type 3 fully automatic vacuum modulators
part#
003 325 391 (no letter) - dual carb
003 325 391 A (yellow stripe) - F.I. : 1968 thru 1971
003 325 391 C (brown stripe) - F.I. : 1972 & 1973
Color stripe signifies the years used.
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reay
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PostPosted: Wed Aug 07, 2024 10:59 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

I have a VW available. I can't remember the letter, but I found a NOS unit for a dual Carb and used that. (I have a 68 type 34).

PM if interested.
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DQDan
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PostPosted: Sat Aug 17, 2024 2:21 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Tram wrote:
DQDan wrote:
I wish one of you guys would help this guy out. He's been searching for this for years... on craigslist

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Is this someone you know?


It is NOT. I just see his wanted advert for months..
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dickie
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PostPosted: Thu Sep 12, 2024 6:32 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

I just found an automatic transmission for my 71 fastback, which was missing when I bought it. I can't find clear information about how it hooks up...the shift cable connector and connection....and the vacuum modulator on the side. How does it connect, where does it connect, how? Would anyone be able to post a photo or two or three that might illustrate these for me? Thanks. I have no other references. Any info would be great, thanks.
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Bobnotch
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PostPosted: Fri Sep 13, 2024 1:01 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

dickie wrote:
I just found an automatic transmission for my 71 fastback, which was missing when I bought it. I can't find clear information about how it hooks up...the shift cable connector and connection....and the vacuum modulator on the side. How does it connect, where does it connect, how? Would anyone be able to post a photo or two or three that might illustrate these for me? Thanks. I have no other references. Any info would be great, thanks.


Do you have the Brown Bentley? If not you should get one. It covers the AT very well, in fact that same trans was used in the type 4 (411-412) and some buses.
I don't know if the parts book in the technical tab would cover how the cable and bracket are st up or not, but the brown Bentley does.
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dickie
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PostPosted: Sat Sep 14, 2024 5:27 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Ok, thanks...i will check that out.
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Jpeter
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PostPosted: Wed Oct 08, 2025 7:57 am    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Hey everyone,
I have a 1990 westy, stock 2.1l and automatic transmission. When hot, shifting from second to third requires the foot of the accelerator and depressing it again, which causes som sort of reset and the third kicks in smoothly.
I think it is a vacuum issue, because it only happens then. No leaks, nos smoke, no smells, ATF looks great.
I was thinking that the vacuum modulator valve could be the issue? But why only when engine is hot and does not happen any other time?
The vacuum lines are connected, and all seems well. (well, the central door locking system does not work, which I guess uses the same vacuum?)
I appreciate any comments or ideas on how to start a problem solving plan.
the vacuum line that goes from the engine to the tranny loos good, not cracked
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Multi69s
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PostPosted: Thu Oct 09, 2025 11:43 pm    Post subject: Re: Automatic transmission vacuum modulator Reply with quote

Jpeter wrote:
(well, the central door locking system does not work, which I guess uses the same vacuum?)


Don't know what that is, but if it uses the same engine vacuum, make sure that it is leak free.

Before you start investigating the transmission issue, make sure that the engine has a good tune up and that the valves are adjusted properly.

Why does it only do it when it is hot? Its kind of a guessing game: parts expand, ATF gets a little thinner, etc. When you let off the accelerator pedal, there is a higher vacuum signal going to the modulator, telling it to shift. If this is a relatively new occurrence, time to do a little checking, and testing. Check every vacuum hose to make sure that they are in good condition, AND that they are a snug fit at each connection. To test the modulator, see if you can get a hold of a little vacuum pump like this.

Image may have been reduced in size. Click image to view fullscreen.


It doesn't have to be this fancy, a cheap HF one will work just as well. Take out the modulator and connect it to the pump with a small piece of hose and give the pump a couple of squeeze's. You don't need to build up a lot of vacuum 5 -10" is fine. What is important is that the vacuum will hold steady and not bleed off. If the vacuum holds steady, the modulator is fine.
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69 road Bug 2110
73 Squareback - 2L, T4, Automatic W/ AC
Gone, but many fond memories 69 Baja Bug 2010 - 5 Rib Bus Transaxle
Gone but not forgotten 72 Baja Bug 2010
My builds
T4 into Squareback http://www.thesamba.com/vw/forum/viewtopic.php?t=458944&highlight=
Auto Trans Rebuild http://www.thesamba.com/vw/forum/viewtopic.php?t=516066&highlight=
AC in Squareback https://www.thesamba.com/vw/forum/viewtopic.php?t=...highlight=
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