Cabin blower issue

[brucemc777]

HAPPY EASTER EVERYONE!!!

THEN:
We are once again working on my daughter's 2009 Rogue S AWD. My oldest daughter sold this car to one of her younger sisters and i recall that the oldest had to replace the cabin air blower at some point in the past. At the time I instructed her to get an aftermarket one from RockAuto. Perhaps a year or so later she claimed it did not blow very well. I do not know if this poor performance was existing prior to replacement for it could have been the reason to get a new one.

Fast forward to recently - a couple years later with the younger daughter: Late last year the idiot compressor seized, stopping the car from starting and costing Dad (me) a bunch in towing and diagnosis. Counting my blessings, it was the pulley bearings and not the compressor grenading. As it was cold at the time i just stuck in a new compressor for the sake of the pulley (with the prior one frozen the car would not start as the frozen pulley put too much resistance on the serpentine belt to allow the engine to crank).

As it was finally getting hot outside, a few weeks ago i vacuumed the system and charged it back up.

OK! Now that we have all the back-story...

NOW:
Presently when running the a/c in normal mode (not max) and the fan is set to it's highest setting (the range from low to high does blow variable as the dial is turned so i presume no "resistor" issues, and dang it, i can't remember what the name of the item modern cars use instead of a resistor assembly!) we get variable blowing. In the blower highest speed setting we get some airflow (i use the word "some" as i don't know how to quantify it - we get a breeze...).

If i do either of the following two actions i get a significantly more powerful breeze as if a second stage engages, and one can easily hear the power of the airflow from the blower significantly increase:

• Turn the temperature control all the way to it's warmest setting

• Set the A/C control to "Max A/C"

I believe this means that something is wrong as i am not able to achieve that degree of flow when using the A/C normally (set to in the "blue" temperature range with the fan set to high and NOT on MAX A/C).

As i have no understanding on how the controls interface with the blower motor and vent system beyond the rudimentary, i don't know what to go after, or even if this behavior is possibly intentional.

Would you smart people kindly give this old guy a clue please?

Thank you VERY much!!!

-Bruce

P.S. In case it helps, VIN JN8AS58V69W435485

[VStar650CL]

If you look at the system diagram and flow on page VTL-3 here...
https://www.nicoclub.com/service-manual ... %2FVTL.pdf
...you can see that the A/C evap and heater core are in series with one another. That's so the evap can dehumidify air when using defrost. Basically air flows through the evap at all times, but only flows through the heater core when heat is selected. On max heat 100% of the air flows through both. On max A/C, 0% flows through the core. The only difference on max A/C is that it automatically closes the inlet door and puts the system on recirculate, so if there's any sort of clog in your cowl limiting the outside airflow, that can account for better flow on max A/C. There shouldn't be any difference on max heat, and since you get full flow on max A/C when the core is closed off, that can't be an obstruction issue. I don't see any logical explanation for that.

[brucemc777]

Many thanks!

I apologize for my late response; i never saw any notification that someone was out there helping me. I do appreciate it very much.

-Bruce

[brucemc777]

Studying the diagram, and IF our impressions of a change in air flow are accurate, this would lead me to believe i best now verify the problem (i do believe it exists, but nothing wrong with trying to reverify and further define it i suppose) and if it exists, the issue would have to be electrical. Perhaps i need to then begin by sampling voltage at the blower motor to see if there is a difference when i believe there should not be (high fan s/b high fan regardless).

[VStar650CL]

That's easy, you can see voltage drop across the motor by measuring from the blower fuse to the Red wire at the VBC (blower resistor), or voltage drop across the VBC by measuring from Red to ground. Those two drops will always equal the voltage at the fuse when added together. When the speed signal to the VBC is equal (Blue wire if you want to measure it), a decrease in 12V->Red or increase in Red->ground means the motor is doing more work, an increase in 12V->Red or decrease in Red->ground means it's doing less work. Along with the audible noise from the blower, that can tell you a lot about airflow. Counterintuitively, blower speed will increase and the work done by the motor will decrease when there's an obstruction. That's because the cage is pushing less air, the same principle as when you block a vacuum cleaner hose and the motor screams. It's screaming because it has no air to push, and with no work to do, the motor accelerates to top speed. It sounds like it's doing more work but it isn't, voltage across the motor actually increases.

[brucemc777]

Because i absolutely love to show my ignorance, are blower motor amplifier and that which is commonly referred to as a blower motor resistor the same thing?

Does Air Conditioner amplifier go by any legacy name that i might recognize?

[VStar650CL]

Because i absolutely love to show my ignorance, are blower motor amplifier and that which is commonly referred to as a blower motor resistor the same thing?

Does Air Conditioner amplifier go by any legacy name that i might recognize?

The common name in the trade is Variable Blower Control, or VBC. Nissan calls it an "amplifier", but that's a misnomer because it doesn't really amplify anything. It's more like using an op-amp in "voltage follower" mode (google that if interested, I won't go into it here). It can also be called a "resistor, but it operates differently from an old school resistor which actually dissipates (as heat) some of the power feeding the blower. VBC's instead work on the ground side of the circuit (in between the blower and ground) and use a FET tran-sistor. The HVAC controller sends a Pulse Width Modulated (PWM) signal to the FET's gate, basically turning it on and off thousands of times per second. The more time it spends "on" and the less time it spends "off", the faster the blower runs. Your voltmeter will "average" the on and off times, so what you'll see when measuring in between the blower and VBC will look like a voltage which rises and falls with blower speed. However, when you put the blower at a fixed speed and then measure while you change the flow controls, that allows you to evaluate the change in power load on the motor caused by the airflow. So if there's an obstruction in any particular path the air is taking, the blower voltage will show it up.

[VStar650CL]

PS - If you find the voltage on the Blue wire changes when it does this thing, it means something in the software is causing the speed change. I'll be surprised if that's true, but on the other hand, it won't be the first time I've been surprised by screwy software on Nissans. Far from it.

[brucemc777]

Thank you!

I remember that i read that these were no longer resistors, but i couldn't recall the proper term. To see the word "amplifier" seemed very strange. And i agree, from my limited DIY-because-i-am-the-dad experience, i am not entirely thrilled on how Nissan builds cars, but i can say at least it isn't part of the KIA family. They do great visual design work but when it comes to materials, parts and engineering i have told my daughters don't ever buy one of those things again!

Now, with that in mind... My wife destroyed my fully loaded black 2008 Ford Explorer some three or four years ago. I loved that car. I think she is allergic to watching engine temp gages as it isn't the first time. Since i don't really need a car i've just been thinking about it. Here is the part you'll laugh at given my complaining about goofy maintenance - i'm thinking about a 1998 - 2005 Jaguar XK8 (preferably hard top, but they cost a bit more).