by Jacques Gordon
When you’re trying to find out why an electrical device doesn’t work when the switch is turned on, do you start at the device itself and trace the circuit backwards through the system till you get to the power source, or do you start at the battery and work your way out towards the device? We’ve all had success with each approach, and we’ve all lost time chasing through the whole circuit only to find the problem all the way at the other end. But as cars with newer technology come out of warranty and start showing up in your bay, you’ll start seeing more ‘compressor failures’ that are actually part of an operating strategy. That is; when the compressor clutch or variable displacement valve doesn’t work, it may be the result of a command decision from the PCM and not a malfunction in the HVAC system.
Load- or heat-shedding strategies that prevent the compressor from running are not new, but the number of parameters that can enable that strategy is growing. Of course it varies by make and model, but here are some of the items to examine if the compressor is not commanded to run.
Most techs know the Powertrain Control Module (PCM) won’t engage the compressor if ambient temperature is below about 40˚F, and that the PCM will disengage the compressor when coolant temperature rises above a set limit. However, an increasing number of Ford engines have a cylinder head temperature sensor, either in addition to or in place of the coolant temperature sensor. Compressor operation strategy depends on that sensor’s reading too.
Every model will turn the compressor off at wide-open-throttle to provide more acceleration power, but compressor operation strategy on newer cars depends on high and low engine rpm limits too. Compressor engagement may also be delayed for several seconds after start-up. On cars with a power steering pressure switch, the compressor might be momentarily disengage at idle when that switch is activated, just to prevent engine stalling.
Most techs know that MIL illumination associated with catalyst-damaging misfire will prevent compressor engagement, but the PCM may also shut down the A/C if it thinks the charging system isn’t working properly.
Every vehicle has pressure cut-off switches or a pressure sensor in the refrigerant system. Consider this: if the car is cold-soaked at 40˚F, static pressure in the system will be about 34 psi. On some models, this is 5 psi below the low pressure cut-off limit, so even though evaporator temperature is above the minimum limit, the compressor still won’t engage when the defroster is turned on until engine heat raises the temperature/static pressure of the refrigerant.
Toyota has recently introduced a new concept in computer controls. On models with a relative humidity sensor, the control unit reads interior humidity and windshield temperature and then calculates the risk of windshield fogging if the compressor is disabled to reduce fuel consumption. What’s new here is the “neural network control system,” a type of control strategy that considers the system’s own actions as well as input sensors. A neural network is a building block of artificial intelligence, and it’s capable of much more subtle decisions than traditional ‘if-than feedback’ software used in conventional machine learning.
Next time the PCM does not command compressor activity, you may need to access and interpret OEM fault codes to find out why.