My 2004 XK8 85K on acceleration the a/c fan slows only to blow hard again on normal throttle. I read in what search I was able to do to look for vacuum leaks in control lines but i don't know enough that that helps. Any input?

 

The ac will cut out on WOT, but the fan speed should not change.

 

It may be normal for the fan to slow down when airflow over the condenser is adequate to condense the refrigerant. Most cars use a pressure sensor to determine fan speed requirements.

 

A/C fan can be Cabin Blower or Radiator Fan Motor???

Which one one??

bob

 

Cabin blower.

 

I was wondering how he would be able to know what the cooling fans were doing while driving.
Maybe a voltage drop to the blower?

 

I POSTED THIS WITH A MISLEADING TITLE SO THIS IS A BIT OF A REPEAT. Hope no-one minds.

My 2004 XK8 85K, on acceleration, and when the a/c is running, the cabin air blower slows, just like the fan speed had been dialed down, only to blow hard again on normal throttle.

Anyone ever had this or got any ideas of why this is?

 

Just thinking out loud.. maybe because of the venturi effect? The cabin intake is near the firewall below the hood so the amount of air inbound is decreased at a higher speed because of low pressure? Causing the blower to work harder? (I know stupid.. probably not)

ORRrrr.. the computer limits the pressure from the compressor and lowers the blower for some reason?

I don't know.. that's a good question! :/

 

Actually Matthew, that was good out of the box thinking, I almost said isn't it Boyles law but you're probably closer than I .

Kidding aside, this one is a stumper. Here's a question: Is the AC on Auto or on manual (you set the fan speed)? I'd bet the house it is on Auto but let's see. Does the same thing happen if AC temperature is on "LOW" setting? I wonder if the speed appears to change because for some reason the air flow is diverted to another port. That's what made me think of auto and low.

Auto gives it permission to choose the vent; Low restricts system logic to some extent I'd think (logically you asked for coldest possible). You'd really have me scratching my head if you said it is on manual fan, full blast.

I don't imagine recirculate could/would have anything to do with it.

John

 

In my case, temp is manual set to LOW and fan is on maximum. Weather is hot - about 95F. So as you say, manual fan and full blast. Also top up.

 

Here is info from my Climate Control Student Guide back in the early 2000s.

"Blowers
The blower assemblies each contain a motor, fan, servo and fresh air / recirculation flap. They connect to either side
of the air conditioning / heater unit with ducts. Each blower incorporates a power transistor assembly to regulate air
flow output volume. The A/CCM varies the voltage applied to the base of the power transistor to vary the blower
motor speed and thus the air flow output volume. During normal operation, power is supplied to the blower motors
via the blower motor relay. The motor ground is completed by the power transistor circuit to provide varying blower
motor speed. When maximum blower output is required, the A/CCM activates the high speed relay, which
switches the blower motor ground circuit directly to ground and the motor runs at maximum speed.
In AUTO mode, blower motor variable speed is controlled automatically by the A/CCM. In manual control, 11 speed
steps are available. Through the 1997 MY, to compensate for the effect of ram air on the air intakes, the A/CCM
adjusts blower speed at vehicle speed above 25 mph (40 km/h). Blower speed compensation is canceled during the
following conditions:
• Maximum blower speed required
• Fresh / recirc flaps positioned to recirculation
• DEFROST selected
• AUTO – full cooling required (fresh / recirc flaps positioned to recirculation)
In the heating mode, blower operation is canceled by the A/CCM until the engine coolant temperature reaches 30 °C
(86 °F) to prevent cold air distribution. The blowers operate at all times in the cooling and defrost modes.
The A/CCM does not monitor the blower motor circuits for diagnostics, however; the A/CCM receives a feedback
signal from the power transistor circuit that indicates blower motor speed. This signal is used to adjust blower
speed. If a fault occurs in the blower speed feedback circuit, the A/CCM uses a default speed that depends on the
speed selected when the fault occurred. If the equivalent speed was above 7/8 bar segments, the A/CCM drives the
blower motors at maximum speed; if the equivalent speed was below 7/8 bar segments, the blower motors are
switched off.
The electrical load placed on the vehicle charging system by high speed blower operation is compensated for by the
ECM (engine control module)."

 

If you want to experiment, Try setting the RECIRC and see if you get the same blower reaction.

bob