I have an 89 300TE and the automatic heat control keeps turning the heat up to full blast no matter what the knob is set at. I cannot tell if the knob is broken or is the interior temp sensor isn't working. Does anyone have any thoughts?

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'05 Ford Escape 100,000 miles
'87 560SL 92000 miles
'89 300TE 199,000 miles
'02 Audi TT 100,000 miles
'00 Ford Excursion V-10 121000 miles
'92 Mercury Capri 100000 miles
'02 BMW 325XI 60000 miles
'92 230CE 160000
'87 BMW 535SI 160,000 miles
'93 Rinker Captiva 209
5 Kids
2 Dogs
1 Wife

The heater valve is pulsed when controlling hot water flow, except in the max position. In "Min" position the valve should be full closed. There are two wires to the valve. One should be battery voltage when the key is on and the other should be ground in the "Min" position. Verify that. If that is the case and you have heat then the valve is bad.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

If your problem is the interior sensor, take it out and blow the dust off it. Could be all it needs. Another problem may be the aspirator(spelling?). This is a little fan usually locted behind the glove box insert. I have not found mine yet but I know it's in there somewhere. The aspirator draws a constant flow of air into and over the interior sensor that monitors interior temp.

As suggested in this forum, to test the operation of the aspirator without removing the glovebox, tear off a small piece of paper. Gently place the paper over the vent of the sensor.(This will work only if the sensor is in the overhead.) If the aspirator is working, the paper should be held in place as you pull your hand away.

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