I know this must hace been covered before, but i was not able to find anything for my issue when I did a search.

Here is the deal. The car will not overheat (runs at about 85-90ºC) while moving with or without the AC on the highway or around town or without the AC when stopped or in traffic. When I turn on the AC and am either stopped or in stop and go traffic the temp will creep up to over 100ºC pretty quickly. I can generally put it in neutral and rev it to about 2-3K and it will come back down to normal relatively quickly as well.

Heat will not get hot out of the vents unless the engine is reving? Otherwise if the engine is above idle the heat works great.

Good fluid level (MB coolant) with a flush last summer. No oil in coolant. radiator hoses are soft while running and in the morning. No smoke and such, the engine seems to run perfect so I don't think the head is cracked.

The electric fan does not come on at any temp up to about 105ºC (as high as I let it get)? I can jump the low speed switch at the AC dryer and it will come on. I can jump the high speed three prong switch (new and replaced with the grey one) and it will come on.

I can stop the clutch fan with a newspaper while the engine is at idle. The fan clutch does not engage when the temp goes over 100ºC. So I think this is a good place to start, but...

Why does the electric fan not come on? I think the red switch on the dry may be bad, but I just changed the three prong switch, so that one should still give it a high speed signal when the temp gets up there. Unless there is something else going on.

Any help appreciated.

G