We have two R129s, one is a 1998 SL500 and the other is a 2001 SL600. The 500 runs 80-85c without fail, even on the hottest summer days with the a/c going.

The 600, however, runs much hotter. It normally hovers just below the 100c mark, unless the weather is about 70 degrees out or less, at which point it will run about 85c.

I wanted to get some input on what the operating range was on the SL600 vs. SL500 when brand-new?

I have done some digging, and it appears that the 500s use an 80c thermostat while the 600s use an 87c thermostat, so that would indicate that the 600 was designed to run a little hotter in the first place, but 95-100c still seems too hot to me, even with an 87c thermostat.

In an attempt to lower the operating temp, I had PTE over on MBWorld reconfigure a factory M120 thermostat, using an 80c element instead of an 87c element. Well, it did absolutely nothing for my temps, which still sit around 100c.

What would you guys recommend?