Exterior Temperature Gauge Weirdness
 

On my new 95 E300D, sometimes the exterior temp gauge would work, sometimes it wouldn't. But when it DID work, it consistently read 17F lower than the actual temp. So as long as I could add 17 to any number, I was fine.

Now when it works it's reading the correct temp.

What could account for it being off by 17F....and then not?

The temp gauge thing is a pretty simple little setup.

You might have a sensor that is going bad. I bought replacements for the sensor (havent actually got around to it yet but the part was only $2-3)

Your temp display might need to be recapped. I've done a few units and was able to repair them as such.

Go to Radio Shack and pick up a couple of resistors (yes you still can find those there buried in the corner in the back behind the r/c cars)
Get a 10 k, a 3.3 k, and a 1k

Open up the fuse box and carefully undo the 6 screws and lift the fuse assembly up, be careful not to short anything out. In the spaghetti pot of wires there is a 2 pin connector with a brown and green wire. This joins the sensor at the bumper to the instrument harness. Unplug the harness connector and apply each resistor to the instrument side of the harness. 10 k should be about 50 c or 120 F. 3.3 is about 20 c or 68 F. 1k is about 0 c or 32 F.

Measure the other side of the connector going towards the probe. Based on temperature the resistance ought to be about 5 k for this time of year.

FSM 54-6320 has all the details and the temperature/resistance chart.

Based on this you should be able to tell if the display or the probe is bad.