Climate Control

Stuck on "Hot"

I've just fixed a problem which caused my climate control to be stuck on "hot" all the time,
regardless of where the temperature lever was set.

The fault turned out to be in the control loop, rather than the valves or vacuum system. This
consists of the temperature control lever, the inside temperature sensor in the dash, and the
outside temperature sensor, all wired in series. The connections to the outside temperature
sensor had corroded, leaving the loop open (high resistance); this corresponds to the
"maximum temperature" setting.

The temperature sensor is in front of the left front wheel (i.e. passenger side on my RHD car).
To get at it, the left headlamp needs to be removed from its carrier:

1. Pop up the headlights

2. Unscrew the 3 Philips screws holding the left-hand side headlamp cover on & remove it.

3. The bottom of the headlamp is secured to the aiming adjustment cable. Push the little plastic
circlip off the joint, then the ball-end of the cable will pop out if you pull.

4. Unbolt the two 8mm bolts on either side of the headlamp, detach theplug, & remove the lamp
from the carrier.

5. Pop down the headlights.

You can then see into the left front wheel well, there's a flexible duct (taking cooling air to the
alternator), and a silver metal sensor sticking out of it. On my car the leads from this were joined
via two bullet connectors. After cleaning these up, I measured the resistance across the sensor
at 1.2K ohm (at 5-10 deg C outside temperature). I guess it probably varies between 500-2000
ohms. I then reconnected one
of the leads and joined the other two up via a multimeter switched to measure current, just to
check that nothing else in the loop was broken. With the engine on & heater controls both at
mid-position, this read (IIRC) 1.2mA. When it was reassembled, the heater was restored to full
operation!


Cheers
Ian H.
80 Euro 928S