Date: Tue, 4 Sep 2007 16:38:55 -0400
Reply-To: "T.M." <tinkerman007@YAHOO.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "T.M." <tinkerman007@YAHOO.COM>
Subject: Re: It's time to hit that idle Stabilizer on the head!!!
Volks, a few warnings before we proceed:
*** Warning ***
1. Be VERY CAREFUL while doing the procedures I describe below from the
moving parts in the motor bay while the engine is running!!!
2. Be very careful not to short any adjacent pins while doing the tests
described below so as not to cause damage to your van and it's parts.
3. Everything you do is at your own risk, take all necessary precautions!
OK, now that we've cleared that, I'll describe my progress.
Since I haven't been able to locate a replacement Idle Stabilizer Control
Unit (ISCU) for testing, I have done further investigation:
1. I have tested my Idle Control Valve (ICV) and it seems to be working
good. How? Very simple: I connected a variable low-power voltage source (a
small wal-wart, with switch selectable voltages in the range of 3-12V,
also might be possible to use 4 AA batteries, just don't use a high power
source such as the car battery so as not to burn the ICV coil) across the
ICV terminals (with crocodile jumpers) while the motor was running.
Connecting the voltage surged the RPM up a bit, which proved the ICV was
working. The polarity didn't matter, but AC didn't work.
2. I checked the continuity of wires from the ICV to the ISCU socket
behind the right-hand rear lights. Tested OK. I also checked that power
was actually getting there with the ignition switch turned on. So the ISCU
must be the culprit!
3. I connected 12V directly to the ISCU (+12V to pin 15, -12V to pin 31)
and checked the outputs (St1 & St2) - they seemed dead (no output drive to
a test lamp). My ISCU draws about 100mA at idle.
4. I opened the ISCU again, and started "sniffing around" for signals. On
the LM2902 IC (the one on the main board, near the external connector) pin
14 I found a live signal! (166Hz audio buzz, with a cheap-man's
oscilloscope, I high impedance earphone...:-)
5. Non of this signal was reaching the output (the buzz we should hear
from the ICV in a properly working system), so I suspected that one or
both of the power transistors (BD438) on the main board were bad.
6. I removed the 2 power transistors (without damage - old proficiency
pays...), they were OK (the double diodes test, and no conduction
otherwise). No luck!
7. I started decoding the two output pins (St1 and St2) circuitry: St1
comes from the closest transistor's collector (the two power transistors
are interconnected in some way), while St2 connects via the big 0.43 Ohm
resistor to ground, i.e. the ICV is driven by the transistor from +12V to
ground via the 0.43ohm resistor. That explains why both burn when the ICV
gets stuck and draws excessive current.
8. Now I need the list's help: I need voltage measurements from a known
good ISCU unit, to try and find where the problem hides. It's not very
difficult - all you need to do is connect a low power 12V source (wal-wart
seems safer than 12V from a big battery) to pins 15 & 31 as described
above, and tell me what you measure at all the other output pins. Also, I
need you to connect a low-power test light across the outputs (St1 and
St2) and tell me if it lights or not (it should IMO) or even a small
loudspeaker to know if it's a variable signal.
** be very careful not to short any adjacent pins so as not to cause
damage **
9. The braver types can open up the ISCU and give me internal readings.
Just be careful when you split the boards - the ribbon cable that connects
the two boards was flaky in mine (one track broken), so check yours too.
Luckily the two outermost tracks are redundant - both are GROUND (and
that's exactly what broke on mine, so it shouldn't have caused a problem).
10. I've also discovered that all opamps on board operate at about 8.3V,
generated locally on the board. Pin 4 is Vcc, 11 is GND.
Folks, we are near to busting this animal! And I think the method I
described to test the ICV before replacing it unnecessarily is very easy
and will help many. Give me your feedback.
Re VanagonWiki, I tried posting there but the text turns up garbled, so
I'll wait for this to be solved before I invest more time on that site.
And I still haven't decided where it's best to post all those pictures,
now that VanagonWiki doesn't seem like the best option.
