The ETR is next to the blower motor under the PS kickpanel . It is a little box with a looping wire going into the evap. holder.

BTW, I recently refilled my r134a system in the 240D from empty, and I have had no complaints about the temp, even at idle so far this summer.

Of course it has been a mild one down here - high temp so far only near 95 with humidity steady at 50-75%

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT