1989 300te- Heat is turned on, blower on (working nicely)...When you idle, the heat turns to cool coming from the vents. As soon as you start rolling (almost immediatly) the vent air heats up again and I have a toasty car. I fear getting caught in traffic during a cold spell.

Its either the auxilary water pump or the monovalve. The aux pump circulates water at idle. The pump is a usual suspect, you can feel if the pump motor is spinning or not. The monovalve could stick in the closed position and the water pressure at idle not enough to overcome the friction. At higher RPM the monovalve would work OK. But first check your aux pump.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

Classic circulation pump failure.

Unplug it before it toasts your climate control brain.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

OK...now you have me concerned. What is a circulation pump? I am familiar with a water pump (obviously not that)...where is it? How does it get disconnected? Is this the Aux. Pump that is mentioned earlier?

The auxilary water pump (some call it circulation pump, same thing) is located near the passenger side fenderwell below the coolant reservoir, hard to miss. You will see the hoses going in/out to the monovalve which is next to the battery.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

How can I confirm that this pump has failed? I am lame with a meter...is there anyother way to establish if it is functioning? Does it start at any particular time that I can check it? I would hate to replace a working part.

As the man said before....

"The pump is a usual suspect, you can feel if the pump motor is spinning or not. "

1) Try using a length of radiator hose as a stethiscope and place one ond on the Circ Pump and your ear to the other end...you will hear if the pump is turnong when the Heat switch is engaged.

2) Place your fingers on the outside of the Circ. Pump...you will feel the vibes if its working.

Thank you, kind Sir. The internet has been good to me.

OK...it's humming away. I can feel it vibrate. NOW, how do I know that it is pushing H2O? Or is feeling the 'vibes' good enough?