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?

I'd recommend not worrying about it. They are running motors hotter lately to get better efficiency out of them. If you are running hotter than your t-stat rating lowering the rating don't do squat. Once it's open it is open.

Running lower than 100deg C is just fine with me, but I drive a BMW and they are controlling their t-stats electronically and so they don't worry folk the temp gauge reads exactly in the middle from 170degF to 230deg F. They consistantly run around 110deg C.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Yes, I remember the normalized temp guage on my 740i. That was a neat feature, and I'm sure prevented lots of people's OCD from kicking in.

Still, aren't I losing some power via retarded timing at 212f vs. the 170 or so that the car ran at when it was new? I'm not that concerned about efficiency, just longevity and power. I guess the reason I'm concerned in the first place is that it has crept up with time...when it was newer, it seems like it always hovered around 85c, and as it has aged it's picked up 15c.

If you say it's fine, then I believe you. You guys have a great reputation. Actually I've been debating making the haul over to gvl. to let you get to the bottom of my a/c problems. The refrigerant pressures are normal, all the fans etc. are working, but the damned thing just doesn't cool. The dealer keeps printing me "No Fault" reports, and I've had it to two indys who can't figure it out either.

My IR thermometer on the 500 reads out 48f-52f vent temps on a 90 degree day when the car is stopped at idle. On the 600, I'm lucky to have vent temps under 59f-65f under the same conditions. It's just not sufficient, especially with the pano roof on.

Oh ya, but one more thing, what temp would this car have run when new?

Because BMW has this electronic control of their T-stats they also have a lot of faults from it. I have moniotored control while driving and it is amazing how differtent temperatures exist during a simple varied test drive. First thing BMW has temps sensors at t-stat outlet and radiator outlet. During run up to 110 deg C the outlet at radiator runs 50-60 deg. As the electronic is added the temp drops to 95-100 at the t-stat but the radiator outlet goes close to 90.

The point is that the temp varies a lot (these reading are taken from a scanner while driving) by drive condition and where you are reading. MB states that temp can go near the red in city or mountain driving. I prefer it to stay below 100deg C. With a 87 deg t-stat I would expect up to 95 deg normally.

Another issue is the AC. With the fans usually running more for the AC than engine temp, the condition of the AC can affecty engine operating temp.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician