My Honda Brio EMT 2015 model is having AC problems. The AC mechanic says the problem is in the ECM. Please enlighten me whether the BCM controls the AC or ECM does.

  • Unexplained abbreviations? You could at least try to communicate better than this. Do we assume BCM is the Body Control Module and ECM is the Engine Control Module? – zipzit Apr 6 '16 at 21:51
  • followup? What was wrong with the car? – zipzit Apr 13 '16 at 5:06

I don't know about this car specifically but I've had a whole lot of time working as an air conditioning (AC) engineer at a Detroit based Original Equipment Manufacturer (OEM)

The body control module (BCM) provides an interface for relays in the system, generally for things inside the passenger compartment of the vehicle. The Engine Control Module (ECM) controls how the engine is controlled, including air, fuel and spark.

First off, its not clear to me on what you mean by "AC problems"

The heating, ventilation and air conditioning (HVAC) system has a few systems that control things.

You have ventilation mode control (which controls which vents the hot or cold air comes from.) Generally in hot weather most folks want cold air to come out to them from panel vents so the air is pointed at their face. Your car uses either cables and cams or small electric motors to control these doors. The power for the electric motors is routed thru the Body Control Module. Additionally there is a recirculation door... this shuts off outside air to the interior of the car. In hot weather you want this door to be shut, other wise it should remain open. This door is also controlled by cable or motor. If its stuck open to outside air, it will adversely affect air conditioning cooling in hot weather.

You have a fan / blower control. This allows you to change the air speed of the cooling air. This is a pretty large current draw, and this is definitely controlled via the Body Control Module. You didn't say if you have automatic / infinitely variable or multi-stop fan control. If automatic or infinitely variable your car will be using some sort of Pulsed Width Module (PWM) control circuit. If its a three or four discrete step fan they could be using a resistor block WITH thermal fuse. (When that fuse goes the HVAC fan will NOT spin, you have to replace the resistor block.) That resistor block should be mounted somewhere on the HVAC air duct area inside the car behind the instrument panel.

We've talked about modes (where the air comes from), about fan speed, now lets talk about refrigeration stuff.

The air conditioning refrigerant gets pumped by an engine driven compressor, cooled off from hot compressed gas to a liquid in front of the radiator in the condenser, forced thru a control valve system, expanded from liquid to gas in the Evaporator core (where it gets cold) and then cycled to the compressor again. The compressor is a very large load on the engine. In fact the engine control computer is very much a part of that system. The compressor is controlled with a magnetic clutch / pulley system. You can turn it on / off with an electrical relay. But... because the load is so high, the computer bumps up the engine speed a bit BEFORE engaging the compressor. If they didn't do that the car would die when at idle. With that in mind, can you see why the circuitry is routed thru the Engine control module?

Additionally, the engine control module also controls the engine cooling fan, some thing very critical to the refrigerant system. It bumps up the fan speed when the A/C is running. In fact, if the car sees a near overheat condition, then it will shut off the air conditioning so there is no damage to the engine.

So, with that said... you've not provided any real details on what's not working with your air conditioning system... We don't know if it's a mode issue, a blower fan issue, or a refrigerant related cooling issue. There could be any one of fifty defects (I've probably seen most of them...) that could be wrong. I wouldn't assume it was an expensive control module. It's possible, but frankly I doubt it.

More likely:

  • a burnt thermal fuse on the blower.
  • a crossed or frayed wire or pinout (Pin not seated within an electrical connector)
  • a sending unit issue related to the engine cooling system.
  • a refrigerant leak.
  • etc...
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