Engine fan runs in hi speed when ac or defrost is on

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Loftja
Posts: 3
Joined: Sat Mar 24, 2018 3:20 pm

Engine fan runs in hi speed when ac or defrost is on

Post by Loftja »

I have 2 2006 vibes and one of them the engine fan is much quieter when the defrost or ac is on. One vibe you can hear it coming because the engine fan is turning so fast. I’ve swapped the fans between cars and the problem stays the same. The vibe engine fan has 2 speeds. When the defrost or ac is on it’s on low speed and when the engine temp rises the fan will go to a higher speed. My problem is one vibe is on high speed whenever the ac/defrost is on. Any help would be greatly appreciated.
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joatmon
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Joined: Fri Mar 21, 2003 5:19 am
Location: SMC MD

Re: Engine fan runs in hi speed when ac or defrost is on

Post by joatmon »

First off, welcome to genvibe!
Loftja wrote:I have 2 2006 vibes and one of them the engine fan is much quieter when the defrost or ac is on. One vibe you can hear it coming because the engine fan is turning so fast. I’ve swapped the fans between cars and the problem stays the same. The vibe engine fan has 2 speeds. When the defrost or ac is on it’s on low speed and when the engine temp rises the fan will go to a higher speed. My problem is one vibe is on high speed whenever the ac/defrost is on. Any help would be greatly appreciated.
Here's a long winded description of the engine fan circuit

There are two FAN relays, next to each other in the underhood fuse block. Both of the relays get 12V on the relay coil from the ignition switched ECU-IG fuse. If that fuse were blown, neither relay would work and the engine fan would never come on, so you know that's good on your car

+12V from the battery through the 100A ALT fuse then through the 40A RD1 fuse (both in the underhood fuse block). From there, it is switched by the normally open FAN 1 relay. When the FAN 1 relay closes, it connects this +12V to one side of the engine fan motor.

The FAN 1 relay is always and only controlled by the ECU/PCM/computer, and is used to turn the engine fan on or off.

The FAN 2 relay is used to control the engine fan speed. While FAN 1 connects one side of the fan motor to +12V, the FAN 2 relay connects the other side of the fan motor either directly to ground, or through a fan resistor to ground. When it connected directly to ground, the fan motor runs at full speed. When it is connected through the fan resistor to ground, some voltage is lost through the resistor, and the engine fan runs at less than full speed.

The FAN 2 relay, normally connects to the resistor and should make the fan run at low speed. Two things can close the FAN 2 relay, which provides a direct ground. The ECU can can energize that relay directly, when it thinks it needs to. Also, the AC refrigerant pressure switch can directly ground the FAN 2 control when it senses a high enough pressure (my book says when refrigerant pressure is above 1520 KPa/220 psi)

For what its worth, the refrigerant pressure switch has a second signal that it sends a ground to the ECU when the pressure is between 198 and 3140 KPa (28 and 455 psi). This doesn't directly affect the engine fan speed, but likely is a signal the ECU uses to decide what fan speed to command.

Also, my service manual says:
The PCM commands low speed fan operation when all of the following conditions occur:
- The A/C system is operating
- the A/C system pressure is below 1226 kPa (178 psi)
- The engine coolant temperature is below 83°C (181°F)
The PCM commands high speed fan operation when either of the following conditions occur
- The ECT reaches 93°C (199°F)
- The A/C system pressure exceeds 1520 kPa (220 psi)
So, all that being said, there are a few things in there that could cause the fan to always and only run at high speed when the AC is on. There's always the possibility of some broken or shorted wire somewhere, but while it can happen, in reality it doesn't happen much, unless there's some undisclosed vehicle history. A shorted fan resistor would cause the fan to run at high speed when it should be low speed. I've looked and can't figure out where that resistor is, maybe hidden under the underhood fuse block somewhere, but it looks like the toyota part number might be 85928-12010

If you do not turn on the AC, does the fan speed behave the way it should and run at two speeds depending on engine temp?

If so, then it seems it would involve the refrigerant pressure switch. Either the switch itself is triggering at lower than spec'd pressure, or perhaps the system is overcharged, or some other condition causing the pressure to jump over 220 psi when the compressor is running. So maybe check refrigerant pressure on the high side when its cold and running.
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Loftja
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Joined: Sat Mar 24, 2018 3:20 pm

Re: Engine fan runs in hi speed when ac or defrost is on

Post by Loftja »

Thanks for the reply it’s going to be helpful. The fan resistor is mounted on the left front frame rail and that’s where I’ll start.
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