cooling fan speed 07 impala 3.5 flex fuel

[smantas]

when turning either the defroster or a/c on it takes about 15 seconds and the fans go on full high speed ...there is no medium...turn defroster or a/c off then 15 seconds later goes to low speed and stays there until car is shut off...recently replaced a/c pressure sensor wiring connector due to 2 corroded wires causeing fans to stay on high spped constantly and it was done properly...coolent temp sensor checks out ok..is this normal?...what should I look for?...thanks

[rhandwor]

The older cars had two fans one for the radiator and the other for the air conditioner. When you turned on the air conditioner the other fan started. The defroster uses air from the air conditioner.
Probably the radiator was made smaller and both fans are for the radiator. The higher speed is used for the air conditioner.

[j cAT]

Originally Posted by smantas

when turning either the defroster or a/c on it takes about 15 seconds and the fans go on full high speed ...there is no medium...turn defroster or a/c off then 15 seconds later goes to low speed and stays there until car is shut off...recently replaced a/c pressure sensor wiring connector due to 2 corroded wires causeing fans to stay on high spped constantly and it was done properly...coolent temp sensor checks out ok..is this normal?...what should I look for?...thanks

the GM vehicles with 2 radiator fans primary / secondary are used to cool the engine down and to provide proper ac cooling. the primary fan comes on in ac mode which is when every the ac compressor is commanded on. it will also come on when the ac is off and the engine temp drifts too high usually when stoped and no air flow thru radiator. the secondary fan runs when things get tooooo hot or the ac refrigerant pressures get toooo high.. so on a hot day you first turn on your properly charged and working ac the pressure will surge very high at first this will then cause both fans on.. after a period of time the sec fan will shut off.. lets ay its 100 deg f and your in traffic stopped then the engine temp can get tooo high like over 230 deg f then the both fans come on. the ect could be bad or the connector to it corroded pins etc very common in the rust belt this can effect proper fan operations as well as other engine failures like poor cold start and poor gas MPG... the pressure sensor of the ac system is not usually an issue but could be .. then you have fan relays that when old do fail.. fan relays due get heat damaged over time and can cause over heating poor engine temp / fan control.