If the fan is coming on when the AC is turned on, then it is most likely the fan resistor. DC voltage can't be transformed, so it must be stepped down via resistors. Low speed fan= resistor in circuit, high speed fan = no resistor. It seems like your fans are only coming on at high speed, thus preventing a serious overheating although the gauge might be showing in the red- no hoses have melted.
There are a couple of ways to fix this. 1) replace the cooling fan resistor, and check the wiring to it. We have seen damage to the resistor plug-in in some cases.
2) Bypass the low speed fan relay so that the fans will come on high speed instead of low when the computer powers the low speed fan relay coils.
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