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Skoda external air temp sensor - more

Skoda Octavia Elegance TDI 2 litre year 2008

I've posted before - so to recap: starting mid November the external temperature intermittently reads far too low - down to about -18 C when the actual temperature is nearer +10 C. The indicated value varies apparently randomly. It can read sensibly for a few days at a time.

Local garage replaced the sensor - no better.

With sensor disconnected indicated temperature shows --.- and the radiator cooling fans run at full speed.

Multimeter across sensor at room temperature (18 deg) shows 945 ohms (same both directions). Put sensor in freezer for 10 minutes - now measures 2.7k ohm. This is consistent with:

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Today the fault is present: so measure at connector. One contact 4.98v. The other about 11mV - so this is presumably intended to be ground. Measure resistance into this contact: one way about 25ohms - the other about 8 ohms. Suggests a corroded connection (which looks like a poor quality diode) - or possibly the input to the measuring system. So does anybody have a circuit diagram of how the sensor and its measurement system works, please?

Connected a 470-ohm resistor across the contacts. Dashboard display shows about -8 C; so fault seems to be with wiring or control unit rather than the sensor.

I followed the wiring. The sensor is adjacent to the left foglamp. The wires from the sensor enter a thin corrugated plastic tube for about 6 inches, then this tube joins other wires from the foglamp and horn, and these three bundles of wire enter a larger protective plastic tube which goes vertically upwards a few inches where it is fixed to the metalwork near the back of the left headlamp. Here a thick brown wire from an earth point enters the bundle together with more bundles of thin wire. The bundle then runs under the air filter and fuse-box towards the bulkhead in front of the passenger seat.

This bundle looks clean and undamaged - where I can see it. There are no connectors in-line.

But at some point there must be a connector where it enters the control unit. Can anybody explain where this is, and how I would get to it to check for corrosion?

Is this control unit replaceable?

Reply to
Graham J
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I doubt it's the control unit. Most likely is moisture has wicked up the cable at the sender connector and corroded it there as, IIRC, it's behind the bumper (or otherwise at the front) which is a good position to get soaked and a poor quality seal to be shown up.

Reply to
Scott M

The connector for the sensor is clean and dry, and not corroded.

I imagine the wires are crimped into the back of it, but crimping is supposed to be gas-tight as well as waterproof.

Reply to
Graham J

The connectors are crimped onto the wires but the weathersealing to the back of the connector is usually little rubber bungs that the wire runs through. If of poor quality then water can get past that and travel up inside the insulation, corroding the wire. Until you remove the connector pins from the connector housing there's nothing to be seen. In extreme cases the wire corrodes through completely leaving it open circuit while all still being held in place by insulation and bung.

Reply to
Scott M

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