Climate Control

Stuck on "Hot"

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 the plug, & 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!

Ian H.
80 Euro 928S

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