Technical Electrical Gurus- I've got a Question About Grounds

Here's the background:

I've got a Vintage Air Heritage unit in my truck, and the fan blows harder than I need or want, and makes too much noise (These are not uncommon complaints with VA). So I want to slow down the fan speed to less than the lowest of the 3 speeds VA provides.

I asked the tech at VA if I could just add a resistor to either the power or the ground wires for the blower motor. He said yes "In theory", but he wouldn't recommend it. I couldn't get any reason why out of him, nor could I get an alternate recommendation.

I was going to buy a big, ceramic resistor and give it a try, but became concerned about how hot it would get so I looked into PWM's and found one often used for trolling motors that had good reviews and ample power-handling capacity: https://www.amazon.com/gp/product/B07B4B2X35/ref=ppx_od_dt_b_asin_title_s04?ie=UTF8&psc=1


So I wired it into the low power wire to the fan, and it just passes the power through: No ability to decrease the rpm. The pot has no effect. In reading posts from other users there is mention that the ground on the input side needs to be isolated from the ground on the output side or this will occur, but it seems to me an a car all grounds end up tied together through the chassis and back to the battery; so how would you isolate them?

Clearly there is something I'm missing here. Could someone 'splain it to me please?

 

cut some fins off the blade

 

Instead of using the vehicle frame ground you can isolate the blower motor from the vehicle ground if it has a separate ground wire at the blower motor. If not the problem is more difficult and requires insulating the body of the blower motor from it’s mounting surface and then attaching a new wire from the body to the controller ground. This will require fiber washers under the heads of the mounting screws also and extra care to make sure nothing allows the motor to ground to the original mounting. It’s doable but a pain.


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The PWM speed controllers that we use in robotics, use two wires to the motor, just like this one (I downloaded the instruction sheet). The motors all have two wires, no ground.

I would not expect it to work on a grounded motor.

Like he said, look at the AC unit and see if you can separate the ground wire for the motor. It's likely you can, since it's in a plastic housing, eh? The thing is that you need to use only the PWM controller, you can't leave ground connected for the high speed settings.

btw, I use the temperature control on my Vintage Air AC unit, and it never blows too much air. Some folks think it has to be either full hot or full cold, and you control temp with the fan. Not quite how they are designed...

 

Why not a simple rheostat to reduce power to the unit or am I missing something?

 

A rheostat would probably work. One of the old heater controls from the 50s might do the trick. but it will get hot, and I can see not wanting to go the resistor path....

 

I think he's onto something. Many PWMs the hot side is always hot, and to regulate the power it "grounds" the low side in pulses. Fuel injectors on modern cars are often controlled this way.

So I think you want both fan wires connected the output of the PWM. Do you know anybody with an oscilloscope? That would let you see what exactly is going on.

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Just a side note...all electric golf carts are isolated grounds...quite a learning curve when your back-ground is automotive electrical...lol


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Resistors a and rheostats reduce voltage, lower voltage means higher amp draw.
The PWM controllers maintain the voltage but pulse at different rates without raising the amps.

 

I think you're making it too complicated.... What controls the blower speed now?

Ford (and others I suspect) used a simple resistor set-up. A single speed motor, and a three speed switch that switched the power lead around between the various resistances before going to the motor. Yes, the resistors get hot but Ford solved that by mounting them in the blower or heater plenum, so when the fan was on it blew cooling air across them. They used this from the '50s (if not earlier) up to the '80s, so it's a proven design.

Here's a typical resistor set.... https://www.macsautoparts.com/ford_...VBh6tBh3TpgHsEAQYAiABEgLyr_D_BwE&gclsrc=aw.ds

You should be able to find a wiring diagram in most any Ford service manual in that era.

 

Aftermarket AC units generally have 3 speed blower motors, that have three windings, and 4 wires. One common (ground), and Low, Med, High. The speed switch selects which wire gets power.

Note that the resistor 60s-80s cars sits in an air plenum so it can get cooled... harder to do with an aftermarket unit

 

Good one Steve! I always wondered why the resistor in F body Camaros and TAs were located on a phenolic board in the fan cavity on the AC box. Now I know.

 

If he can find a place to install the resistor in the plenum, that actually makes it easier. Wire it in using the existing switch, then try the high speed winding. Too fast? Try the mid-speed...

 

Adding a resistor in series does NOT increase amp draw, but reduces it.

As squirrel mentioned, the resistor would have to be mounted in the air plenum for cooling. Is there a spot to do that, this would be the easiest solution.


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Actually, it does. BUT, current draw also depends on load, in this case because there's no fixed load it won't matter. This would be a bad idea if it was a window motor...

 

He’s not going to recommend it, no manufacturer will. Doesn’t mean you can’t do it, but their lawyers don’t want the responsibility for anything that happens.

An in-line resistor would drop all fan speeds, and will be turning electricity in to heat, so plan accordingly.

My Dakota uses a “standard” three resistor in the plenum housing setup. It has burned up two resistor cards so far, and about 6” of wiring harness leading to the card shows damage from overheating.




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Electric motors draw the highest current at 0rpm, and less as speed picks up.
A resistor in series reduces the voltage the most at high current, and less as the current goes down.

Too much resistor would make the motor so weak at startup so it may be unable to start at all. PWM or a more sofisticated voltage regulation is so much better at regulating motor speed.

 

That’s because they under-sized the resistor and wiring for the size (amp draw) of the blower motor...I believe they came out with a Factory up-grade because of that issue.


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Have you tried insulating the box to see if you can cut down on the noise. Saves changing the operation of the A/C system. Remember it is just a then hollow box and the noise cancelling is nil. It worked for me on a A/C conversion. I glued Hush mat to the out side of the box.

 

I was going to say what southcross said. If it's only noise, insulate it. I have a glassbeading booth, with a dust filter set up. Last time I got a new motor for it, I splurged and got the sound insulation kit, as well, What a difference! You can hardly hear it at all! used to be very loud. A bit of foam/foil or extra dynamat is all you need.

 

Another approach is to put a real motor in the car, then you'll never hear the AC blower over the engine

 

I appreciate all the input. Not sure what direction I'm going.

The unit in question is already installed, so opening up the case may be difficult. I'll have to see if I can get the front panel off without completely. FWIW it is one of these:

https://www.summitracing.com/parts/vta-674011

Wiring diagram is:
So it looks like the motor is grounded by a wire, not through it's case, so the ground wire from the blower motor goes direct to the PWM M- terminal, and a regular chassis ground goes to the PMW B- terminal? Then the pot on the PMW should work, and the one on the VA box should not. Do I have that right?

The PWM controller is my first choice, but if it can't be used I'm thinking about mounting a big-ass resistor (similar to an ignition ballast resistor) in the engine compartment where the heat wouldn't be such a big issue . I don't mind if it slows down all 3 speeds. Any problems (besides heat) with doing it this way as an alternative?

In the meantime I will get my face down there and see if I can figure out a way to insulate it better- That was a good simple idea I hadn't thought to do.

This is all in my '59 Dodge pickup ( Dodge Magnum 5.9 engine) daily driver and I'd like it to be relatively quiet & comfortable. All this is mainly because I'd like to listen to the radio without having to blast it.

 

Those under-dash units always were noisy....

Whatever you do electrically, you'll have to do between the fan switch and the motor. Both the control module and AC compressor clutch are fed off the input power side of the fan switch and neither one will like reduced voltage. If you can determine the current draw of the motor on low speed, it's possible to calculate a resistor size to reduce speed.

 

Sounds right. It always helps to draw a new schematic, so those of us that can read schematics better than following long sentences, can check your work.

 

Insert the resistor in series with the yellow wire and it will only effect the Low speed.

Or back to the PWM controller, there are 4 connections, B+, B-, M+, M-. How did you connect it so far? I believe you should have battery voltage in at B+, chassis ground at B-, out to the motor (Yellow wire) at M+, and the motor ground to M-, and it should control speed on the Low stetting only, just what you want. Power in to B+ should come from the L terminal of Blower Speed Switch (yellow wire).

 

Are you saying that a PWM controlled blower motor draws the same current at low speed as it does on high? Not true. We went to PWM controls on the heater motors at Freightliner in the early 2000's. The current draw of the motor goes up as the speed goes up. It moves more air at the higher speeds so it does more work. However, the PWM signal doesn't operate the motor, it just tells the circuit board in the motor what speed to run at.

 

The problem I see with the pwm controller will be you are going to need to isolate the ground in addition to keeping the normal ground for the higher speeds. You can do it with a bosch style relay, but it does complicate the circuit.

 

Should just leave the other motor wires disconnected. I would connect the speed controller to the high speed winding. Then you will have full speed control with the new setup.

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That would mean turning a pot switch on the circuit board. The schematic change below would use the same control switch and only affect the low speed.
Try this...

 

You guys have been busy...
Today I did this:

And it works as expected.
My mistake before was grounding the "Motor -" to the PWM and the chassis.

So now I get a wide range of speeds using the LOW position on the VA blower speed switch for power, and adjusting pot on the PWM for speed.
Thank you guys for setting me straight

So now that I know it works, I'm going to try Trendz suggestion and see if I can then have PWM low-low along with factory medium and high settings on the VA switch. I know I've got some relays around here!...