What could cause that? 1990 300e

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88 260E 92K
90 300E 143K
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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

When the ignition key is set to on and the engine is not started the a/c blower works perfectly can be ajusted with the controls, and will blow at high speed.

Then after engine start the a/c blower will only blow at low speed regardless of where the controls are set. Some times after start the blower will remain on high until you drive a little(maybe 1/4 to 1/2mile) then drop to low with no control.

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

You have to monitor the voltage at the yellow wire at the 3 prong plug on firewall, behind Brake booster when condition exist. That is the variable trigger voltage input to blower regulator..if that voltage changes and blower speed responds , the problem is upstream from there [ Control section of circuit]..if blower changes speed and that voltage remains constant, the problem is down streams from there ..[ reg/blower section of circuit]
You also can check that same 3 wire plug for constant 12v across blk/red wires ..if those drop below system voltage, you could have either a bad Ign. Sw/ poor ground/ bad connection at fuse ..[ they are infamous for hairline cracks/poor connection]

OK Voltage on yellow

Key on Engine not Running/Fan High 9.76vdc|Fan Auto 4.5vdc

Engine Running/Fan High 10.98vdc | Fan Auto 4.5vdc

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

Need blk to red reading

Black to red 12.21vdc key on/ 13.57 engine running

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

So as Arthur said, the problem is downstream of the connector. Either the blower motor regulator or the motor itself.

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Saumil S. Patel

I don't understand that, because the only difference is that the engine is running. If it was a regulator/blower motor why would it make a difference if the engine was running or not? Why wouldn't the lack of control be constant?

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

RPM Control is the function of the regulator, so if you have constant supply voltage to both the reg and blower, then you have an intermittant problem , possible in the regulator b/c running voltage is higher with charging system.
Or, you have a bad ground at regulator,.

The test will be to jumper the blu wire to ground at the blower..this bypasses the regulator and gives fan full power [ high speed].
If the motor still slows down with constant voltage , you have a blower problem..if not, then I suspect regulator

Thanks for the replys, it'll be awhile until I can test and or repair it. I've got a biz trip tomorrow and a family reunion this weekend. but thanks again for the response.

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