My original post: "I bench tested the heater blower and found it blows better wired one way. Positive to brown and neg to yellow ( a 75 ) . When I reverse the polarity the air flow is weak, + to yell and - to brown. The difference is very noticeable

Then I installed the heater fan motor with the intent of getting the stronger air flow: 12v green from relay board to fan motor brown, and brown ground from relay board to fan motor yellow. Blows fuse every time

Next I reversed the wires: green to yell, and brown to brown. Works, no fussiness blowing. The air flow is weak....

Is there a way to get the change the fans rotation so I get the stronger air flow like when I bench tested it ? "

Jim,
DC motors turn a certain way when they are hooked up properly, IE: suck in one way and blow out the other. reverse the power and then they will reverse the air flow direction. Not sure why the fuse would blow one way and not the other.
Tom

The only thing I can think of is that when the fan motor is bolted into it's holder, the motor shorts out when the wires are reversed ?

I remember a thread several months ago that discussed this very subject.


I disconnected the wiring plug and removed the blower motor in Dianne's car for some reason or other a couple of years ago, and when I reinstalled it, I mistakenly reconnected the wiring reversed polarity (aka: 'backwards'.)

We took off for late afternoon drive, and later that evening when the outside temperature got cooler, I pulled back on the heater lever for some comfortable HEAT. The engine started missing and I felt a loss of power. Since the blower motor wiring was the only thing I had messed with, I assumed that was where the problem was.

I reconnected the wiring correct polarity and everything went back to normal.


As far as I remember, brown wires should always be connected to the ground side on any [Porsche/VW] electrical component ... not sure why your motor is rotating backwards

Mark,

That's the same thing. Why can I reverse polarity on the bench, but can't in the car ? Tomorrow, i'm going to unbolt the motor and see if I can reverse polarity when it's not attached to the chassis

The unit is grounded when mounted in the car, not when it's on the bench.

OK. If I insulate the mounting bracket with a rubber pad, should I be able to reverse the polarity and make the fan run the other direction--the direction that puts out a good blow ?

If the motor turns faster in one direction, I suspect the brushes (coals) inside are toast. Whatever you do, its working on life support right now.
Take the motor apart and replace them.

Looking at the inlet side of the fan it is designed to spin counterclockwise given the incline of the blades.

I had a similar problem a couple of years ago with blowing the fuse.....found out that one the connecter pins from the harness....was not setting completely down and when I pulled the heater lever...which I think just grounds out the relay...andwould short against the lead next to it.

Might want to make sure the connector pins aren't touching each other first Jim.

It's not turning faster; when wired in reverse, the fan actually sucks air from the engine bay. I can put my hand over the fans open center and feel the pull.. When wired correctly ( no fuse issues ), there is no positive pressure going out through the hose.

Jim,

Your assessment of operation is correct ... squirrel-cage fan and designed to pull in through the open side and blow out through the 'nozzle'.

When I removed the blower motor in my '75, I found a rubber pad glued to the mounting saddle, but nothing surrounding the circumference of the motor body.

So, with a pad on the saddle alone, the body of the blower motor may still have a mechanically-grounded electrical connection with the saddle through the clamp (due to chipped-away paint on the saddle), and if connected reverse polarity, would blow the fuse when activated.

I guess you could try wrapping the entire body of the motor with a a thin insulating material, then clamp it to the saddle

I still can't understand why the motor would be rotating backwards though

I can reverse the polarity and run the motor either direction when the fan motor is removed from the mounting bracket.

Getting a rubber insulation pad around the motor will not work unless I use a larger clamp.

Fan goes CW when wired like original, and CCW with reversed polarity.

I'm making progress, thanks for the suggestions.

It works better in one direction because the slow way you're pushing air, and the 'fast' way you're doing nothing. It's like saying that your car has less power when going uphill, so you're only going to drive downhill because it works better.

If you reversed the wires, you would also have to reverse the hose fittings to get air blowing in the direction you want.
Tom

When I reverse the wires there is definate pull on the open side of the fan motor and a significant flow at the output.

Should the fan turn clockwise with original wiring scheme--looking from the open side it's a late fan motor.

The fan itself has C-shaped blades. Those are meant to grab the air like little cups. If you turn it backwards, the air doesn't get scooped up. There will still be some air flow when nothing is hooked up and there is to resistance to flow, but once you install it in the car and have LOOOOONG air flow path with lots of resistance that fan won't do anything.

Brown is ground, yellow is +12v.