How hot is too hot?

[Lance Allison]

On my 87 300SDL, the temp runs right around 85-90C when driving, possibly a tad higher with the AC on.

That sounds about right. But...when the outside temp is above 85 degrees with the AC on, the temp gauge rises to over 100, almost to 120 at times when the car idles for longer than 3-5 minutes, like when sitting in heavy traffic. Once moving again, the temp falls back to "normal" range around 85 or 90C.

As I indicated above, this only occurs when it's relatively hot outside and when the AC is running. This morning, I sat outside the office idling without the AC on for about 15 min. It was about 65 degrees outside and the temp gauge stayed around 90C.

The owner's manual states when the car sits idle, the temperature gauge may climb and be close to the max range indicated on the gauge and this is normal. I'm concerned with this. Just what exactly is normal? Given the aluminum head is sensitive to high temperatures, I really don't want have to replace it anytime soon.

Also, which is the auxiliary fan? I'm assuming it's the one that is visible when the hood is raised. Is this correct? I don't believe I've heard it run...I've got the W126 MB CD, but haven't had the chance to look through it yet.

Thanks in advance for help.