60Driver

Does anyone else's engine start working alot harder at idle when you turn on your windshield defogger? It bogs mine down more than the A/C Compressor! Think this is an indication of an alternator working too hard?

PorscheDude1

Check your grounds.

VWaddict

Emmmmm...

That's because it DOES engage the AC compressor, then IN ADDITION it sets the fan to full speed, turns on the AC evaporator fan, and tells the stepper motors in the distribution box to move as quickly as they can to direct all the air to the windshield.

So in addition to just switching on the A/C compressor, it also adds a static load of about 40 amps (which -in this instance- equals a dynamic load of up to 50 Amps peak) of load on the electrical system.

-So if there's a 951 out there which DOESN'T get more heavily loaded when you press the defrost switch, then THAT car has a problem, not yours!

Keith

60Driver

Wow, all that for some air on the windshield. Thanks for the info. I thought it was sort of odd...

theedge

How much does your engine bog down? My car has never had a major issue with defrost/etc/etc....

CarbonRevo

When I turn my A/C on after driving for about an hour and let the car idle, oil pressure drops to 1 bar (from 2-2.25). I have even had no oil pressure on a few occourences. What is MY problem?

bearone

when i use the defroster i can feel a drop in power/rpm.

87951
95SL320dd

95ONE

Motors worn.

eniac

Mine doesn't....anymore. When my stock alternator was starting to go bad(slowly over 3 years), pressing the defrost switch added a huge load to my engine. I got to the point that I would only use it while driving and shut it off at lights. Now however, with the new Nissan alternator, the idle doesn't change a bit when the switch is on and all the lights stay bright.

VWaddict

It's not just to VENTILATE the windshield... it's to force-feed DEHUMIDIFIED air onto the windshield.

The air is dried by switching on the A/C, then -because the chilling effect of cold air on the windshield would encourage the formation of condensation on the glass- the heat is set to full-on. -Try it before the engine is warmed up and you'll notice that it's ICE-COLD (assuming that your A/C is in good shape of course!)

In addition, something else which I forgot: -The 'recirculate' button is disabled... -try it and you'll see.

Basically in ANY vehicle, (Living in Florida, with the steamy heat when the daily summer afternoon monsoon hits, you get to learn this!) the fastest way to defrost the screen is:

AC: -On, Temperature: -Maximum heat, Recirculate: Off, Blower speed: Fastest... oh, and all other vents off apart from the screen (of course!).

The defrost button in the 951 just does all of these things for you, and allows you to 'go back to where you were' simply by releasing the 'defrost' button. -In my VW, I have to twist a few dials and remember how I had everything!

Oil pressure shouldn't be significantly affected by switching on the A/C. -Oil pressure READING can indeed be significantly affected, because the electrical load can cause significant gauge-reading drops if any part of the circuit (wires, ground connections etc.) are high-resistance. -Look at your battery voltmeter display.. -does that move significantly too?

It's still producing the SAME load, but when an Alternator goes bad, it becomes much less efficient at converting mechanical load into electrical energy, so the mechanical load produced by the alternator trying to address the additional electrical load can be much greater.

There are two sources of increased engine mechanical load when you press the defrost button: One is from the A/C compressor and the other is from the alternator. -You can estimate what proportion comes from each by first engaging the A/C and seeing how much load is added by that, and THEN seeing how much the load apparently increases when the defrost button is engaged. -If you press the defrost button when the AC is OFF however, you're suddenly adding BOTH loads at the same time.

Keith

eniac

The load is less because I now have a more efficient alternator. It's better then a good new stock Bosch unit and obviously way better than a worn out 20 year old one. So as you stated, the more efficient alternator is creating less of a mechanical load while suppling enough amps for the electrical load. When an alternator has to work harder, it creates a GREATER mechanical load and thus an overall greater load on the engine....but you already knew that.

All I stated was the OVERALL load on my engine is MUCH less now that I have a better alternator and barely noticable.