Date: Sun, 7 Jun 2015 02:58:09 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: A/C wiring warning
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I just had occasion to trace out the A/C circuitry for '89-up and was
rather dismayed at what I found. There is unfortunate interaction
between the A/C wiring and the vehicle wiring. Tl;dr: I suggest
putting indicators on that will warn if S1, S42, or S23 should blow.
as they can back-feed each other through the radiator switch and the
mid-speed fan resistor when the A/C is operating.
'89-up, the A/C switch gets power from fuse S12 which also powers the
heater blower. That fuse is supplied by the X line in the panel which
is the output of the load reduction relay and is powered when
ignition is on but you are not cranking.
That switch supplies the first two A/C blower speeds directly. When
you switch to the third speed it activates one of the two internal
relays in the A/C double relay, which is sourced from 50 amp fuse S23
coming directly from alternator B+.
When you switch to speed four, it activates the evap blower relay
which feeds the blowers directly without any intervening resistors.
This relay is also sourced from S23.
If the other side of the A/C relay is not activated (next post) these
are the only connections.
The A/C switch also powers the A/C thermostat. When the balance of
the thermostat setting and the temp sensor II (not the one on the
engine) calls for it, the thermostat activates the other side of the A/C relay.
This closes two circuits. The first one supplies power to the
normally-closed side of the A/C pressure switch, thence through the
normally-closed ambient temperature switch and the normally-closed
A/C clutch relay to engage the compressor clutch (and incidentally
inform the idle stab controller that compressor is engaged).
The second circuit connects S23 to the normally-open side of the A/C
pressure switch. When the pressure switch operates, this section
activates the stage II radiator fan relay (position 5 in the
fuse/relay panel) which runs the fan at mid-speed; and the other
section dumps the compressor clutch.
The second circuit also connects S23 to the low-speed end of the
radiator fan resistor, which causes the fan to run at low speed. And
here is where it gets tricky. Any time the first-stage switch in the
radiator is closed (first stage nominal 95C, second (really third
because of the stage II relay) stage 102C) it cross-connects S23 with
30-amp S1 through the radiator switch which is not meant to handle
thirty amps. So supposing that S23 were blown, for example, its
functions would be supplied with reduced overload capacity by S1
through the poor groaning radiator switch.
And when the radiator 102C switch closes, it dumps the compressor by
activating the A/C clutch relay, and activates the radiator cooling
fan stage III relay, powering the radiator fan directly from fifty
amp S42. And at the same time adding yet another cross-connection,
back-feeding S23 and S1 through the mid-speed section of the fan resistor.
So there is a great deal that can happen that should not if S23 is
blown (that should be the unhoused fusible link in the back somewhere
on a relay board -- left pillar?), always supposing that one or both
radiator switches are closed.
Yours,
David
--
David Beierl -- dbeierl@attglobal.net