What could cause that? 1990 300e

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88 260E 92K
90 300E 143K
97 Suburban 2500 154K
98 Chevy 3500 155K
65 John Deere 3020 ?

I've installed a new CCU and it did not change the problem.

There must be some relay that changes when engine is running vs just having the key in the on or acc position.

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88 260E 92K
90 300E 143K
97 Suburban 2500 154K
98 Chevy 3500 155K
65 John Deere 3020 ?

Check charging voltage at battery when running,...

Does this happen when car is cold ?

The only control for high speed operation of aux fans seem to come from CCU. The only control for low speed operation of aux fans seem to come from the freon high pressure cut-off switch. So if the CCU is set for no a/c operation and the car is cold, you should not have aux fans come on at any speed. If it comes on at high speed when the switch is on run position, since you have already replaced the CCU, I would think that your coolant temp sensor is signalling a wrong signal, a signal that is equivalent to car being hot. BUT I have no idea why the temp sensor signal is suddenly being interpreted differently once the car starts, strange.

If the fans come on at low speed when the car just started from cold, that means there is a very high freon pressure sensed by the pressure sensor. If the a/c has not been turned on for this test, there is no reason for the pressure to be high, so I would think that your pressure switch is bad.

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Saumil S. Patel

He is talking about the Blower.

Oops !! That was not clear to me, actually I dont understand the problem then. For what blower motor setting this problem is occurring ? Give us more detail of the test condition.

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Saumil S. Patel