Thread: D4 A/C Compressor issue

maybe a glitch with gap, i ran again and it seems more accurate, also note the change in ac pressure with fan rpm (engine rpm not changed, just viscous change)

Screenshot_20230102-150048_IIDControl.jpg

I logged a lot of mine today. The PWM viscous fan commanded was weird. Alway down to zero when stopped, and immediately ramped up between 10-30% when moving.

My GAP wouldn’t show me fan RPM - does that sometimes and doesn’t show me all the applicable logging options.

At idle, not moving, Evap temp was in the 5-8deg C, but while moving was in the 0-5 deg range.

When I was back in the garage, popped the bonnet and there was a decent flow from the fan at what it says is 0% PWM.

It definitely wasn’t roaring.

Originally Posted by DiscoJeffster

I logged a lot of mine today. The PWM viscous fan commanded was weird. Alway down to zero when stopped, and immediately ramped up between 10-30% when moving.

My GAP wouldn’t show me fan RPM - does that sometimes and doesn’t show me all the applicable logging options.

At idle, not moving, Evap temp was in the 5-8deg C, but while moving was in the 0-5 deg range.

When I was back in the garage, popped the bonnet and there was a decent flow from the fan at what it says is 0% PWM.

It definitely wasn’t roaring.

Ha ha, I got what seems to be an accurate fan speed, but n/a for control? Maybe we could combine our data

I always thought they operated like a normal viscous fan, and the ECM control was only to heat it up more if required, therefore making it faster, so yours having no pwm makes sense because it's already hot enough.

Please note, I have nothing factual to back that up

For additional airflow through the radiator matrix, particularly when the vehicle is stationary, there is an engine driven electro-
viscous fan unit fitted to the rear of the radiator. The fan is used for engine cooling and for Air Conditioning (A/C) system cooling.
This unit functions as a normal viscous fan, but with electronic control over the level engagement of the clutch. The Engine
Control Module (ECM), which determines the required fan speed, controls the level of clutch engagement. The ECM determines
engagement based on the coolant, charge air, ambient and transmission oil temperatures and the A/C pressure. The fan is
mounted using a left hand thread.
The viscous fan unit is electronically controlled by the ECM to optimise fan speed for all operating conditions.

Regulation of the coolant temperature is achieved via engagement of the electro-viscous fan assembly. This is controlled by a
Pulse Width Modulated (PWM) signal with a duty cycle of between 0 and 100%, provided by the ECM and derived from inputs
based on:
Coolant temperature
Ambient air temperature
Engine inlet air temperature
Air Conditioning (AC) system pressure
AC switch operation
Transmission oil temperature
Fan speed control is variable; however, because the fan is driven directly from the engine, the maximum fan speed available is
tied to engine speed. At high engine speeds the fan is progressively disengaged to protect the clutch unit. This system provides
very high levels of fan power, up to 5 kilo Watts (kW), with enhanced noise and fuel economy benefits compared to mechanically
controlled viscous fans.

NOTE: If the electrical connections to the viscous fan are disconnected the fan will 'idle' and overheating may result. The ECM stores the appropriate fault codes in this case.

lack of power idles it, so doesnt seem to be activating the slip, more the grip

generic google result:

Viscous drives control how fast an engine fan spins with remarkable accuracy. Oil is released from a reservoir into grooves that allow for increased or decreased friction depending how much cooling is needed.

The valve arm opens to push silicone oil in to the working chamber. As the silicone oil flows between the input and output clutch plates, it creates shear forces. This gradual buildup forces the two plates to spin at similar speeds, thus allowing a smooth change in the fan speed.

Since the fan slows when when high power is not needed, your vehicle gains horsepower and torque for better engine performance and mileage. When cooling is needed, the engine control unit (ECU) triggers the clutch, which puts the fan into faster rotation again.

I’m guessing it goes to zero at idle to avoid making too much noise and ruining the lovely LR ambience

Originally Posted by DiscoJeffster

I’m guessing it goes to zero at idle to avoid making too much noise and ruining the lovely LR ambience

Yes, unless needed for the AC example, you will hear it if it ramps up, its sounds just like an electronic thermo fan you have on cars with sideways motors.

I looked to see if there was a way to trigger it in GAP but no, only monitor.

Screenshot_20230107-081431_IIDControl.jpg

I made another capture this time showing the pressure spiking and then dropping in response to the fan speed

Something is not correct. With spikes like that you are risking losing another compressor. There is a lot more to diagnosing an a/c problem than looking at a snapshot showing a graph. What temperature is the suction line, how much superheat is dissipating across the condenser?
I feel that you may still have an ongoing problem and are risking your new compressor.