1984 500 SEC
I bought a Farenheit unit on Ebay that would work on my W126. Some units are four pinned. And others are three pinned. Check which one yours is then look for same online. I removed the instrument cluster. Unscrewed the Celcius unit from the back of the cluster. Dropped the Farenheit unit in. Replaced the connections. Replaced the cluster. Done.
Take your time looking. Most important. Make sure the screen where you read the temp. is in perfect shape. Not distorted. I paid $45 w/free shipping. Good luck.
