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?

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

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.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech