TheSamba.com :: View topic - Automatic transmission Idle Control Valve

Sun Nov 08, 2009 8:20 am

Automatic owners...do you have an idle control valve installed? In the photo from Bentley, it shows an adjustable vacuum operated control valve. I believe it's purpose is to bump up the idle in Drive. Looks like it just pushes on the throttle. I get a 250 rpm drop in idle between Park and Drive. To prevent my bus from stalling at stop signs the Park idle needs to be set about 1100 rpm. Of course with this high idle I get a lurch when engaging Drive. Just wondering if I'm missing this device. The picture from Bentley does not look like it's from a bus.

Sun Nov 08, 2009 8:47 am

How are you setting your idle speed now?

Sun Nov 08, 2009 1:20 pm

i was wondering what that did on my new 75 auto

can anyone post where the vac line connects to

to make frozenbutts question more specific (i think he also has a75)
is this a 75auto only part
or
is it also on the 76-79 automatics

Sun Nov 08, 2009 1:29 pm

My '77 auto does not have this part. I believe it is '75 only.

Sun Nov 08, 2009 5:27 pm

You set idle on my 75 just like any other FI bus...big slot head screw in the picture. The idle control valve has a vacuum hose attached and pushes the throttle open a little bit when in Drive with the brake applied. Apparently this eliminates the 200-300 rpm drop in idle rpm that automatics have when going from Park to Drive. I just measured my vacuum and the vacuum in Drive is lower of course because the rpm is lower. If you push the throttle open a bit to increase rpm, the vacuum increases back to where it was. I don't see how the use of this adjustable valve wouldn't cause rpm oscillations? Maybe that is why they only used it for one year. Does someone know how this is supposed to work?

Sun Nov 08, 2009 5:33 pm

The D-jet powered T4's had a throttle dash pot, probably pretty much the same as being talked about here. It just worked off of manifold vacuum. When the vacuum dropped the dashpot opened the throttle a bit and the vacuum picked back up. The system definitely could pulsate if things were not adjusted quite right.

I have the FI off of a '76 bus installed on my 411 with an automatic. I don't have a problem with excessive rpm drop. Don't know what the difference would be between a '75 and '76.

Sun Nov 08, 2009 5:42 pm

vw76westy wrote: i was wondering what that did on my new 75 auto

can anyone post where the vac line connects to

to make frozenbutts question more specific (i think he also has a75)
is this a 75auto only part
or
is it also on the 76-79 automatics

It Tees into any vacuum line. I think the one for the fuel pressure regulator works.

Sun Nov 08, 2009 5:46 pm

Does the ECU on the 76 1/2 - 79 correct the RPM drop where as the 75 -76 1/2 it is mechanically vacuum corrected RPM??????

The '75 throttle body has the pad which the idle control pushes on and I have not seen it on any other year.

Sun Nov 08, 2009 6:19 pm

Quote: I have the FI off of a '76 bus installed on my 411 with an automatic. I don't have a problem with excessive rpm drop. Don't know what the difference would be between a '75 and '76.

75 is a 1 year only automatic only fi system
the decel valve is electric & the egr(both manual & auto)works & connects different
the throttle body for 75 manual has 2 vacuum ports(76-78 has 1)
the TB for 75auto has 3 ports ,the 3rd one is a vacuum line that T's
to plenum & then runs to transmission

im just starting to learn the automatic system
1st thing that im learning is reconizing what are the automatic parts that differ from a manual
& what 75auto parts differ from 76-79 auto

i have read that for 77-78 the only difference from man & auto is the decel valve,
which would lead me to assume that the idle control valve,temp valve & dual vacuum disy
are part of the 75 only

id like to see a pic of of the throttle body(plenum & s-boot also) on
someones 76-78 automatic to see how the system is setup to
compensate for not having the idle controll valve

Sun Nov 08, 2009 9:41 pm

vw76westy wrote:
id like to see a pic of of the throttle body(plenum & s-boot also) on
someones 76-78 automatic to see how the system is setup to
compensate for not having the idle controll valve

I have the '77 automatic and as far as I have been able to learn, there is no difference between it and a manual transmission of the same year except for the decel valve. The ECU has a different part number but according to several sources they appear to work interchangeably.

The plenum, s-boot, and throttle body are all identical to the manual.

Basically there is no compensation. I think I read somewhere that the stall speed on the torque converter is different on the later models. You set the idle to about 950 in neutral and it drops to maybe 800 when you put it in drive. Not too bad.