This time of year we spend plenty of time on blower problems. Everybody needs one, because frozen feet and hands are no fun. For the most part, blowers aren't rocket science. Yet there are certain fundamental mistakes that we see backyarders -- and even some seasoned technicians who should know better -- make over and over again. Fortunately, most OEM's including Nissan are starting to simplify matters with 3-phase brushless (BLDC) designs that don't need a resistor in order to vary speed. The Altimas have used this type of motor for quite some time, and diagnosis on them is simple: A varying PWM control voltage goes in, a varying speed should come out. Yet many modern systems still use traditional brush motors, including some very modern designs like the Kicks and gen2 Rogue. These all need a "resistor" to vary speed, but very few modern vehicles use an actual array of resistors with 3 or 4 speeds as was common from the '50's into the '80's. Instead, they use a VBC, or Variable Blower Control. The VBC uses a high-powered MOSFET transistor that can carry 40+ amps, but it's used as an amplifier and not an on-off switch. The HVAC Controller sends it a pulse-width modulated (PWM) signal that cycles the FET on and off thousands of times per second. PWM sounds mysterious but it isn't. The pulses come at a predetermined rate, but the percentage of each pulse that goes positive voltage (high) or 0V (low) changes with how fast the blower is supposed to run. The higher the percentage of time each pulse spends high, the longer the FET stays on and the faster the motor runs. Because it cycles so quickly, the blower motor -- and your voltmeter -- will see the "average" of the on-and-offs as an increasing or decreasing voltage. Only an oscilloscope or signal analyzer will "see" the actual pulses.

https://drive.google.com/file/d/1ZcdspU ... sp=sharing

The link above shows a simplified schematic of how this is used. The first thing to notice is that the VBC is located on the ground side of the blower motor, not the hot side. The only hot side switching is the Blower Relay, which is only there to prevent the blower running with the ignition off. The speed control is accomplished by varying the blower's connection to ground, not juice. This means if you're doing diagnosis on the system, the wire connecting the VBC to the blower will read lower voltage as the blower runs faster. The more ground the FET provides, the faster the blower speed. If the FET is switched off, the connecting wire will read battery voltage. You may know that about half the time when VBC's fail, the blower runs at high speed all the time and can't be controlled. This is because the MOS in MOSFET stands for "Metal Oxide Semiconductor", and when the FET fails the metal inside often melts. This forms a permanent short circuit to ground, and as you can see in the diagram, that will make the motor run flat-out whenever the key is on.

One common backyarder mistake is failing to realize that most Nissans have two fuses on the blower circuit, not one. The two parallel 15A fuses carry the same current as a single 30A, but using two lets Nissan cut down on the amount of copper in the fusebox. Very often when the complaint is, "It keeps blowing the fuse," the answer is that both fuses were blown but the customer only found one of them.

Now for some motor basics. The most important thing to understand is that all brush motors draw enormous current when they're starting up. The circuit in the diagram shows 30A fusing, but the motor may only draw 8A when it's running at speed. The excess rating isn't there to protect the wires, it's because that same motor can draw upwards of 25A for a short time when it's first turned on. Once it gets rolling the "surge" drops off sharply, but the circuit design needs to account for that extra current. For this reason, anything that stalls or slows the motor during startup puts huge strain on the rest of the system. This can be debris in the blower cage, motor bushings that are wearing out, eroded commutators or bad brushes that "bounce" and slow the motor, or anything else that impedes the motor from getting quickly up to speed. Diagnostically, that means anytime a VBC fails, you should be very suspicious of the motor, even if it seems to run fine when the VBC is "jumped out". Nissan VBC's are very rugged, with most having FET's rated at least 50A. They don't die easily. But constant heat will still kill them, and a draggy blower makes them heat up badly. It's an invisible problem, because the only way to "see" excessive startup current is with an oscilloscope. By the time your meter reacts the surge is already gone, but trust me, the FET can feel it. In our dealership, the simple rule is this: If the VBC fails but the blower seems good, replace the blower anyway. If the motor fails but the VBC tests good, then replacing the motor alone is acceptable.

Lastly, the notion that a plugged intake or cabin filter will kill the motor is a myth. It will make you freeze or sweat because no air is moving, but in fact the motor is under less strain when there's no air to move, not more. The principle is the same as putting your palm over the end of a vacuum cleaner hose. The reason the motor suddenly races is because it has no air for the impeller to push against. An obstruction in your blower's intake is no different, the motor will race in a partial vacuum, but that's because it's doing little or no work. So the next time someone tells you to keep your cabin filter fresh to make your blower last longer, you can smile and know better. "No," you might say, "I keep mine clean so I don't sweat."

Happy New Year, all!

Great post as usual. Happy new year to you too.
I've been fortunate to not have to deal with any blower issues so far in my life *knocks on wood*, but I'll be pointing people to this post if/when issues arise.

Happy New Year to you as well! Glad you liked it. Knowledge really is power when it comes to fixing stuff right the first time. I'm always happy to share a little, and maybe - just maybe - knock a fraction of a percent off the world's misery quotient.

All my s*** is WAAAAAAY too old for PWM. Most is also too old for cabin filters.

I also love Nissan's tendency in 90 models to use a FAR over-specced pot (or, I guess, motor) for the blower control, so that when it fails (fully open) you get Fan Speed 9/4. My Q was that way for a while and it was noisy as hell, but I couldn't make myself spend money on it. I don't mind the heat, so if car HVAC stuff fails in the summer I usually can't be arsed.

The big failure point in the Corvair is the defrost lever. It is cast, and snaps out of its mounting. Nothing better in winter than an old car with crappy old wipers and no defrost air!

MinisterofDOOM wrote: ↑

Thu Jan 07, 2021 9:43 pm

The big failure point in the Corvair is the defrost lever. It is cast, and snaps out of its mounting. Nothing better in winter than an old car with crappy old wipers and no defrost air!

Yeah, I distinctly remember chasing two different knobs endlessly around the floor of my '67 GTO after replacing the heater control, once they were removed they would never fully latch back in place. The good old days (not)!