First Speeds Repairs

There was a time when the heating and ventilation system on most cars was simple. A three-position switch, a couple of levers and maybe some separate vents that would allow outside air to enter the passenger compartment. Times have changed and today’s car buying public wants to dial in a desired temperature and turn over regulation to a processor (See Photo 1). Many cars have multiple-zone HVAC systems where different temperatures can be dialed in for certain areas of the car interior.

BMW has used the same basic system for the last 25 years or more for regulation of interior temperature, window defrosting and ventilation. The focus of this article will be the 3 and 5 Series cars of the last 15 years, but the information on repair and diagnosis is applicable to all models. Like a jigsaw puzzle, there are a lot of pieces to this system, but when they are all put together, the picture on how it all works is pretty clear.

COMPONENT PARTS
Beyond the obvious parts of the HVAC system, the radiator and heater core, blower motor, A/C evaporator and condenser, there are other components that help regulate the interior temperature regardless of the outside conditions. On all of these models, there is an auxiliary coolant pump, a switching valve assembly and various temperature sensors, flap actuators and a control head and processor (See Photo 2). Later models will have at least one cabin or pollen filter, and some will have two or more.

Not so obvious parts are the engine water pump (See Photo 3), engine thermostat, fan speed control or, as BMW calls it, “final stage” and the on-board diagnostics system. They all contribute information to allow the processor to regulate heat or A/C operation to control temperature throughout the car’s interior.

Before doing any diagnosis of the climate control system, the integrity and operation of the car’s cooling system must first be reviewed and any faults must be repaired (See Photo 4). Because these cars have an auxiliary coolant pump for the heater system, some coolant will flow through the system whenever the key is on and the heater system is in a “heat demand” setting. Since water pump failures are common, lack of heater output at higher engine speeds is often a clue to a cooling system problem that must be resolved before the climate control system is suspected.

THE BIG PICTURE
In this article, I’m going to list some of the more common symptoms related to the climate control system and point out some things to look for to resolve them.

One of the first places to go to make sure everything is working properly is the owner’s manual to get a picture of how the system is supposed to work. Because a number of factors go into the processors’ adjustments to achieve a certain temperature, as with most climate control systems, temperature control is not instantaneous; so always give the system a chance to adjust before moving to the next test.

1. 1. No climate control display. This is common on 3 Series cars. Usually after a few tests for power supply from the fuses and relays involved with the climate control system, replacement of the control head and processor is the only fix. Both new and remanufactured units are available. Often a good, hard slap on the instrument panel will cause the display to light up, unless that is what the car owner has been doing for the past six months and it no longer works.

1. 1. Temperature regulation is poor. There can be a number of problems related to the system not working, but temperature regulation is way off the mark. The first two items that come to mind are the interior and ambient temperature sensors. The ambient temperature sensor is usually located in one of the under-bumper air ducts to reduce the possibility of being influenced by radiator or condenser heat. Unfortunately, the plastic pieces under the front of the car are vulnerable to attack from curbs and things, so they are often damaged or missing.

One reason the climate control system won’t work properly is because the ambient temp sensor left with the piece it was connected to. The interior temp sensor is subject to a different type of attack, from dirt and dust being drawn into the control panel by a small aspirator fan (See Photo 5) that is attached to the control panel. Often, where no other component failures are found, just cleaning the inlet of the panel, or removing the faceplate and cleaning the sensor, will restore operation (See Photo 6).

1. 1. Blower does not operate. There can be a number of reasons why the blower fan won’t blow. The obvious first steps are to check all of the applicable fuses; as there are always several fuses related to the climate control system. If all of the fuses are OK, look at the control panel and verify that the control buttons change the requested fan speed. If the graphic indicates that the control module is generating the request; go to the next step.