No Heat and It Is -10 Below Zero in Wisconsin
Vehicle has no heat from air ducts. The fan inside the car wasn’t working but now it does. Fan speed control did not work on any setting. The day it was dropped off the fan work on all speeds and heat was good.
This job sounded like it would be a straightforward diagnose with an inoperative HVAC blower motor but became an intermittent electrical problem. Anytime a problem becomes intermittent, it makes the diagnosis more difficult. It can also lead to inconclusive results and excessive labor time. My testing approach is very different between the two scenarios. I start with pulling up a wire diagram regardless of the failure type, making sure I select the correct HVAC system (auto/manual). This car has a manual HVAC temp control system but does use a blower motor module to control fan speed instead of a resistor block. That adds a little complexity to this system along with more failure points. When I started testing this cars blower motor, it was fully operational. If it wasn’t operational, I would have started testing the power and ground at the blower motor located under the dash. I would have worked my way back through the circuit as needed and into the control side if no problem in my initial testing. Most blower motor circuits are two parts (control/feed) which come together at the fan relay.
Since the blower motor was working, I already know my power supply and ground must be present. With an intermittent problem like this car had, I started at the most common failure point that can produce these symptoms, which is the blower motor itself. My first assumption is a bad spot on the motor itself, usually a bad commutator that is making poor or no contact with the brushes. An open or shorted circuit in the motor winding is another possibility but can be harder to spot when the motor is working. I will use a low amp probe to test the electrical motor while it is running. This will allow me to see the amperage flowing through the motor and any bad spots will show up as low or high spots. I will connect the low amp probe to my digital oscilloscope and view the waveform pattern. The low amp probe can be hooked around either the power feed or ground to the blower motor. The pattern will appear the same on either side of the motor, just make sure your low amp probe is facing the correct direction. The pattern should be fairly flat and even. Comparable to a calm lake with light waves. A jagged or erratic pattern indicates a problem. Low spots are opens or areas of high resistance in the motor. High spots are shorts to ground or between windings. The waveform from this blower motor shows an area of higher resistance in one portion of commutators or in the winding. If the blower motor ever stops in this bad section, it may not work or start. I am assuming that this bad spot is worst than it appears now when the problem happens, caused by bad brush to commutator contact which can change. Brushes in electric motors are made of carbon and are held lightly against the motor copper commutators with light spring pressure. This connection can vary and change from many factors including fan speed, heat build up, or even vibrations.
I decided to tear into the electrical motor and see what I could find wrong. I do this often as a way to compare my electrical waveform pattern to the actual defect. Once the motor was removed from the plastic case, there was a noticeable amount of dirt, rust, and other foreign material all over. As I dug deeper I found that the one of the brushes was well worn but not in terrible shape. You could see a visible discoloration on about a 1/3 of the commutators grouped together. I am assuming this was caused by excessive heat build up at. I was really expecting badly burnt and damaged commutators but it wasn’t the case this time.
With the new blower installed, I took another amperage waveform to confirm the repair and verify this new motor was in good shape. Notice how even and clean the pattern is. The new motor also has a much lower amperage draw on high speed than the one I removed, from 14 amps down to 5 amps.
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