Is this a problem of the A/C not working at low speed? Or the heat not working at high speed? Your description isn't very clear...

If the A/C isn't cold enough at low speed, you have a bad condenser fan and/or fan switch. Or the condenser fins are all bent closed from years of small debris hitting it.

If the heat isn't hot enough at high speed, your coolant thermostat has failed. They almost always fail in the open position, resulting in an engine that runs too cool (and no heat on the freeway).

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1998 E300 turbodiesel

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