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Date:         Sat, 12 Jan 2002 15:30:58 -0800
Reply-To:     Todd Last <Rubatoguy@MINDSPRING.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Todd Last <Rubatoguy@MINDSPRING.COM>
Subject:      Re: Idle stabilizer
Comments: To: John Clavin <jc@AUSTIN.RR.COM>
Content-Type: text/plain; charset=us-ascii

According to the Bosch fuel injection book, you are correct. The idle stabilization unit is always active.

From the BOSCH book page 26

"The opening of the idle-speed stabilizer depends on the percent dwell - the on-off ratio - of the digital control signals from the control unit... The greater the on-off ration the further open the stabilizer and the more air will bypass the throttle. For a warm engine at idle, the stabilizer is open part-way with a typical dwell of 32%. You can use a dwell meter to measure the on-off ratio...

On the other hand, the stabilizer can also avoid an RPM drop when switching on the A/C. The input signal to the control unit may be one or more switches: Driver's actuation of the A/C switch, closing of the magnetic A/C clutch circuit, pressure increase in the high pressure side of the compressor..."

It also looks at things such as the power steering pressure switch. If you look at the wiring diagram on page 97.138 (Air Conditioning) You will see the A/C compressor cut-out relay is wired to the Idle stabilizer valve.

John Clavin wrote:

> Per the wiring diagrams in Bentley and my own observations on my 1985 > Vanagon, the idle stabilizer is connected only between the Hall sender in > the distributor and the Hall control unit mounted on an aluminum plate on > the side wall next to the idle stabilizer. Three wires connect the Hall > sender to the Control unit. It is these three wires that are routed through > the two plugs on the idle stabilizer. > > Having hacksawed open my defunct idle stabilizer, I found a moderately > complex circuit board with several integrated circuits (IC). Tracking down > the part numbers (MC14538) on some of these ICs revealed them to be > monostable multivibrator circuits. The others were all basic logic gates. > > Typically, multivibrator circuits are used to create free-running or > triggered oscillators. My guess is that in the idle stabilizer they are > triggered by the sender unit to send the actual ignition pulses to the Hall > Control unit. The reason for this would be to provide some signal > conditioning that evens out the timing between individual, erratic pulses > from the sender. It also would provide some wave form shaping of the pulses > so as to make them more uniform, and hence more reliable, as seen by the > Hall Control unit. The net effect would be to take a jittery stream of > poorly shaped pulses and replace it with a more uniform stream of evenly > shaped and spaced pulses. > > Disconnecting the idle stabilizer by connecting the cables to it together > merely sends the raw sender signal to the Hall Control unit. If the > electrical signal from the sender is "clean" because the sender produces a > good signal and the engine is running smoothly, then it is likely that you > can run without the stabilizer. > > An examination of the circuit board reveals that the three wires involved > are power, ground, and signal. I doubt the idle stabilizer itself knows > much about what the temp senders or O2 sensor are doing. Decisions based on > those outputs are probably performed in the ECU. > > In addition to the wire from the idle stabilizer, the Hall Control Unit > connector also has wires which connect to the ignition coil and the ECU. I > suspect this control unit functions much as an electronic relay, switched > on and off by the Hall signal, and handling the higher currents involved > with the coil. The heat sink it is mounted on suggests it has some high > power transistors doing this. The wire to the ECU is probably an output > which tells the ECU what the ignition is doing, although it could be some > other type of control signal from the ECU. > > If the above analysis is correct, then the idle stabilizer is always "on" > and performing its function when plugged in. I could see no way that the > ECU or Hall Control Unit could tell it to turn off for a warm engine. I > suspect that it may seem to turn off because its effects under those > running conditions are not as critical or noticeable in terms of engine > performance. > > Regards. > John Clavin > > At 12:00 AM 1/10/02 -0500, you wrote: > >Here's a new question for you though: > >Is the idle stabilizer only active when the idle switch is activiated?


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