The HVAC system in my '83 300D was doing weird things, even after
I replaced all the units, pushbutton, temp controller, and fan speed
controller units.
Having had bench experience with the weird things low voltage can cause
a solid state system to do; (especially "old" solid state systems), I used a
relay and fed 12v directly to the fanspeed control module even though
voltages were normal at various points in the system. Power to the
pushbutton module and the temperature module remained the same.
The system has been working well with no problem for several months.
I ran a piece of 14ga drop cord from the 12v bus bar on the right fender
throught an existing grommet in the firewall to power the fan speed
controller
and put an inline fuse holder next to the buss.
The headlight relay coil connects to the power supply from terminal
#8(16amps) on the fuse module and the 12v line from the buss and the
positive line from the controller connect to the relay contacts. Drop cord
has a very tough covering and I've found it to be ideal in running extra
wiring; especially through the firewall since one size of grommet fits it
snugly.
If your system is anything like a 123, splitting the power supply might be
worth trying.
Gerry
'83 300D and 240D
---------------------------------------
From: "Jim Cathey" <j...@windwireless.net>
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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