Why so many fan control fuses?

[samahi72]

2004 Impala, Variable Speed blower.

Why are there so many fuses and or relays for the fan(s)?
When I look at the two fuse boxes under the hood, the upper box has a fuse that says blower motor. The lower box has 3 large 1 inch square fuses or relays. One says Fan Cont #3, The second Fan Cont #2 and the Third says Fan Cont #1. Then under them there are two small 10 amp fuses that says Fan Cont #2&3 and the last one says Fan Cont #1.

Why are there so many fan controls fuses / Relays? Are some of them for the engine cooling fan?

[DeltaP]

Most of them are. Relays and fuses are different and serve different purposes. There's relays that control each engine cooling fans on/off functions and there's fuses that protect each fans circuit. That way you dont lose all of them at once. The blower motor is for the a/c blower fan.

[J-Ri]

Originally Posted by samahi72

2004 Impala, Variable Speed blower.

Why are there so many fuses and or relays for the fan(s)?
When I look at the two fuse boxes under the hood, the upper box has a fuse that says blower motor. The lower box has 3 large 1 inch square fuses or relays. One says Fan Cont #3, The second Fan Cont #2 and the Third says Fan Cont #1. Then under them there are two small 10 amp fuses that says Fan Cont #2&3 and the last one says Fan Cont #1.

Why are there so many fan controls fuses / Relays? Are some of them for the engine cooling fan?

A bit more clarification on the cooling fan fuses and relays-
Many dual-fan GM cars use 3 relays to control the 2 fans. Two of them are "on/off switches", but the third is on/on.

In low speed mode, "fan cont 1" supplies power to fan 1, then the cooling fan relay #3 routes the power into the second fan, which always has ground on the negative side. Fan relay 2 is off in this mode. This provides half the voltage to each fan, which results in low speed operation (which is quiet). What if one fan stops working and the circuit cannot be completed? The low speed mode doesn't work. But if the engine temperature gets high enough, the high speed mode kicks in. High speed mode will come on any time the engine coolant temperature is too high, not only if low speed mode fails.

In high speed mode, fan relays 1 and 2 are on. Instead of routing power through the second fan, fan relay 3 switches fan 1 to ground. The result is both motors running at full speed, because they are both given the full system voltage.

Aside from expensive transistor controls for the fans (which I can't say I've ever seen in a production car), this is the best way to do it. Many other vehicles that have a high and low speed have two relays. The high speed puts power directly to the fan, while the low speed goes through a resistor that drops the voltage. That wastes power in low speed mode (turned into heat by the resistor), but the worst part is that the ones I've seen turn the low speed off when the high speed comes on. If the high speed fuse blows or there is a problem anywhere in the control system, the fan doesn't run, which leads to overheating when stopped or driving at low speed. Some also have dual-winding motors with the same relay setup (minus the resistor), but the fan still stops working if there is a problem with the high seed circuit when high speed is commanded on.

The 3-relay system can have any one component fail and it will still work, in most cases it functions well enough that there are no cooling problems.

This is a quick drawing, the control circuits for the relays are not included, and it's possible that I switched the numbers around, but it works well to convey the basic idea.