Date: Sat, 20 Jul 2013 22:55:32 -0700
Reply-To: Craig Forney <craig@OPUS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Craig Forney <craig@OPUS.COM>
Subject: Re: Idle Control Issues
In-Reply-To: <CAN5Ep7QS5OjwY5xN9ayz5ZU3oNyjp7nQYa8aNOYuh8wtU6JzJQ@mail.gmail.com>
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Sorry for the repetition, but I was trying to send this out from my phone,
and it got screwed up. Here is the complete message:
I have spent a lot of time with Vanagon Idle Stabilation Control Modules
(ISCMs) and thought it might to explain the workings to help those with
idle problems diagnose and fix them.
Under certain conditions, the ISCM (installed behind the right, rear
taillight) adds additional air into the intake plenum, bypassing the
throttle body. This increases the engine RPMs to compensate for the
engine being cold, air conditioning, power steering and some other
conditions needing additional power.
These functions are in addition to trying to maintain an idle of about 880
RPM. If the idle dips below 880 the air flow past the throttle body is
increased to provide a stable idle.
The additional air to the intake plenum is regulated by the Idle Air
Control (IAC) Valve (cylindrical object on top of the engine). It is
controlled with a pulse width modulated (PWM) 5 volt signal (from the
ISCM) that is going from 0 to 5 volts and back to 0 volts about 8.6 times
every 1/1000 of a second, basically opening and closing the valve each 0-5v
transition. The duration of the 5 volt signal determines how much time the
valve is open and how much air is getting redirected.
When working properly wirh a good ISCM, the IAC should hum when the key is
on.
The AC compressor engaging sends a signal to the ISCM and increases idle
speed a bit when the throttle is closed (assuming the throttle position
switch is actually closed). When you step on the gas, it increases the
flow even more so that the engine doesn't stumble. If the TPS is not
giving a closed throttle signal to the ISCM, the idle speed will jump up
typically to 1500-2000 RPM. Sometimes "blipping" the gas pedal will bump
the TPS switch enough to reduce the idle towards 880 RPM. This sounds like
Casey's problem. It appears the IAC is working just fine, as it has no
problem increasing the idle speed.
Craig in Cupertino
1987 Syncro GL
1987 Syncro Westy
On Sat, Jul 20, 2013 at 12:35 PM, Zeitgeist <gruengeist@gmail.com> wrote:
> So, I just got back from a quick hop to the local Quickee Mart, and I
> decided to experiment a little. I dialed down the AC temp switch which
> cycles the compressor on and off a lot more. The idle issues would appear
> and then disappear with the compressor, which seems to be more of a relay
> issue than something with the valve--no?
>
>
>