I have several of these with the following behaviors:
Full heat all the time
Won't turn on AC
Won't turn on heat
The one in my car does a pretty good job but reacts slowly.
Seems like they fail all sorts of ways.
I'll send you my box of spares if you want. If you can make 2 of them work
right send me one and we'll call it good. If you can only make one work right
keep it for yourself. If you can't make any work, add them to the scrap pile.
-Curt
Date: Thu, 4 Aug 2011 08:25:51 -0700
From: Jim Cathey <j...@windwireless.net>
To: Mercedes Discussion List <mercedes@okiebenz.com>
Subject: [MBZ] 201 HVAC
Message-ID: <09ceacb8-beae-11e0-9000-000502d9a...@windwireless.net>
Content-Type: text/plain; charset=US-ASCII; format=flowed
Anybody have any in-depth experience with the early ('86) 201 HVAC
controller? Mine refuses to turn off the water valve so it's
always heating, and I've 100% proven that it is the HVAC box
that's not activating the vacuum solenoid, everything downstream
of there works perfectly if it's jumpered manually. The only
other symptom is that the automatic AC compressor setting is
equivalent to off, you have to use the ON/DEF AC button to turn
it on. Other than that the system seems to be working properly.
I've followed my own checklist to ensure that all the inputs to
the unit are reasonable, specifically the four analog inputs
(3 temps and the blend flap feedback pot) appear to be OK.
A junkyard unit (spare) does _exactly_ the same thing.
Coincidence? Unrecognized external problem? Common failure?
I've resoldered it, and dug far enough into the thing to prove
that it's not a fried output. (If I ground the "-" input of
the op-amp that ultimately drives the water valve it will engage
the valve and turn off the auxiliary pump, as it should. It's
a control problem of some sort. On the bench the blend flap
and AC control outputs work, and react to the external inputs
and the control dial. Just no heat-off ability.
-- Jim
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