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BMW and MINI DIY – No Heat – Diagnosing Heater Problems How To Diagnose

September 5, 2012

Your BMW ain’t so hot! (A.k.a. diagnosing heater problems.)

Driving your BMW or MINI for a while in the colder weather, you may notice a degradation (or complete lack) of output from your  heater system. Causes of this lack of heat can range from worn parts to failure of new electronics. Most issues can be traced to either a problem with coolant flow or with the blower.

Coolant Flow:

1) Thermostat.

Typical mechanical thermostat for early BMWs

Typical electrically controlled thermosat for later BMWs and MINIs

The engine’s thermostat controls the engine’s temperature and keeps it from becoming too cool. A thermostat that is stuck open will cause the engine to run too cool to get the coolant hot enough to produce adequate heat. In this instance, you would tend to have more heat when you’re idling and the blower is set at a lower speed. One quick way to diagnose this is to watch the temperature gauge. Does it take a long time for the gauge to start to go up after a cold start? Does the gauge read in the middle of the range, as it should, after a few miles of driving, or is it below normal? If so, the thermostat may be stuck open.

2) Water Control Valve.

Typical water (heater) control valves

Also referred to as a water valve or heater valve, this is the control valve for the flow of hot engine coolant through the heater core. More flow = more heat; less flow = less heat. On older BMWs, the water valve is operated manually via a cable; on newer BMWs and MINIs it is electronic. If the engine temperature gauge reads properly, as noted above, the water valve could be stuck closed, partially open or fully open. In this case, diagnostics are not as clear as with a faulty thermostat.

•    If you can’t turn the heat off, you should suspect a faulty valve or a malfunctioning control cable or  electronic control unit.

•    If you have low or no heat, the valve may be faulty, the heater core may be clogged, or the control cable or electronic control unit may be faulty. A mechanically controlled valve is relatively easy to inspect for operation of the control cable. (Refer to the Bentley manual for your BMW or MINI.) Electronic valves, however, cannot just be visually inspected. Typically, if all other functions are working properly (e.g. blower speeds, A/C, air direction control, etc.) the control unit will not be at fault. But one common symptom of a faulty electronic valve is that there is heat when idling, but as engine speed (and, hence, vehicle speed) go up, the heat goes away. A relatively quick way to decide if you have a valve problem or a heater core problem is to  bypass the valve. Remove the input and output hoses from the valve and use a plastic connector nipple or elbow to connect the hoses together. This will mimic a fully open valve. If you now have full heat, either the valve is faulty or the control for the valve is faulty. If all other heat and A/C functions are working properly (just no heat), then it is likely that the valve is faulty. If bypassing the valve makes no difference in your lack of heat, you are likely dealing with a clogged heater core.

3) Heater Core.

Heater core

This is the actual heat transfer unit. Hot engine coolant flows through the core. The passages for the coolant inside the core are relatively small and can become clogged with age, especially if the coolant has not been maintained. You can effectively test the coolant flow through the core by disconnecting the inlet and outlet hoses from the water valve, in the engine compartment, at the firewall. Apply a garden hose to the inlet hose and start with a moderate water flow. The same amount of water entering the inlet hose should flow from the outlet hose. Slowly increase the flow. If the outlet flow is noticeably reduced, you likely have a partially clogged heater core.

Blower problems:

Blower motor assembly

If your BMW is experiencing problems with the flow of air from the blower, this could either be a faulty blower fan or a problem with the control. Symptoms can range from no air flow at all, to just certain speeds working, to erratic blower operation.

1) No air flow. If there is no air flow and you cannot hear the blower running, it is likely that the blower motor is faulty. However, be sure to check the fuses. There is also the possibility that the blower controls are faulty. Access the blower (using the appropriate Bentley repair manual), which is typically not difficult, and disconnect the two wires from the motor. Use a voltmeter to test for voltage between the two wires, when the blower switch is on high (with the ignition switch turned on). If voltage is present, the motor is faulty. If voltage is not present, there is a problem with the blower controls.

2) Blower only works on certain speeds or only on high. This is likely a faulty blower resistor pack or “final stage unit” (the electronic version of the older blower resistor pack). These units are typically not too difficult to replace and are addressed in the applicable Bentley repair manual.

Blower resistor

3) Erratic blower operation. This would apply to electronically controlled BMWs. The blower may go on and off, or vary its speed, with no input from the control unit. This is typically a failing final stage unit. This is very common on the later model BMWs (3 series 92 on with automatic heat/AC, 5 series 97 on, 7 series 95 on, X5, Z4, etc.). BMW has redesigned these units and we offer the new versions (below) for replacement. Again, replacement is covered in the Bentley repair manuals.

Early style final stage unit - commonly called "the sword"

Late style final stage unit

If you try all of these steps and are still having difficulty identifying which parts have failed, please call and speak with one of our phone reps at 800-535-2002. They’ll help you select the right part(s) to make your BMW or MINI “hot” again.

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