Every car I’ve owned with factory air has exhibited this phenomenon. I’m hoping you guys have some theories, opinions and/or ideas that’ll explain the source of it. I know you’ve got WAGS, but post those, too, and maybe we’ll get to the bottom of this.
Here’s the poop. When sitting motionless, with my foot on the brake pedal, and the air-conditioning compressor cycles (on or off, I’m not sure), I can feel the pedal sink a little bit. Like I said, this has occurred in every vehicle I’ve owned, automatics and sticks, V-8’s and four-bangers, imports and domestics, new, used and very used. Any ideas on the cause of this?
Perhaps it has something to do with the vacuum controls on the air conditioning affecting other controls such as the brakes… this doesn’t happen on my car.
This is because either a vacuum or an electric solenoid actually moves the throttle open a bit, to supply extra power to the engine to run the A/C. I can watch mine do it when it’s idling with the hood open.
Even though the A/C and brakes are directly connected, the energy that powers them both is the engine.
So when the A/C engages, it takes a little energy from the engine, slowing it down. The power brakes then don’t have as much power going to their pressure system, which requires a deeper press of the brake pedal.
Oops! I thought you meant the gas pedal was depressing - in my old carburetted Subaru, the gas pedal would actually depress when AC solenoid kicked in. Sorry about the misunderstanding.
I am so poor I cannot afford AC but here’s my guess: the car depresses the accelerator a bit to give a bit more power to the engine so it does not stall under the new load. The engine revs up a bit and sends more power to the brake servo.
Everyone knows that the air conditioner was invented by Willis Carrier. However, the automobile air conditioner was invented by the Rosenberg brothers back in 1926.
They took their prototype and installed it in a Ford. They then went to Henry Ford with the car and tried to sell him the invention. At first he didn’t want to buy it, but after sitting in the air conditioned car on the hottest day of the summer, he heartily agreed.
The Rosenbergs, however, attached a condition to the deal… the invention had to be named after them. Henry Ford, being the great antisemite that he was, was in no way going to install a “Rosenberg” in every car.
They discussed the matter back and forth, until finally they agreed that the first names of the brothers would be displayed in every car with this device.
And that’s why today, on every car air conditioner, we find the names HI, MAX and NORM.
The AC draws energy from the engine, slowing it down, thus decreasing the available power for the brakes.
The AC causes an increase in engine speed thereby increasing the availble power for the brakes.
And I have to say “Hi,” to Max and Norm.
I gotta say I like the first (AWB’s) better. A decrease in power available to the brake assist seems to mean the manually applied pressure must increase to maintain the same braking pressure. To generate the increase in the manually applied pressure, the pedal obviously must travel further. Okay. I’ll buy that, pending any other silly jokes that may make more sense. Shivering, indeed; I do usually try to achieve meat locker temps in the car.
Warning - Info on Auto A/C you probably don’t care about…
I always found it strange that auto A/C’s frequently have more cooling capacity and use more energy than home A/C’s. The heat load in a car is huge because it has poor to no insulation, huge window areas that let lots of sun in, and when moving is exposed to great volumes of air. Typical home A/C’s are about 3 tons, which with a 10 SEER come to about 5 horsepower. Auto A/C’s, counting belt losses et al, easily pull much more horsepower off of a car’s engine. A car with a 100 horsepower engine can easily experience a 10% drop in fuel efficiency with the A/C on, so you can attribute about 10 horsepower to the A/C. The fan energy is much less in the car than in a home, 'cause in a car you cool a much lower air volume, but the descrepancy on energy related codes always blew me away. I don’t know of any laws related to car A/C efficiency, but residential and commercial codes on minimum efficiencies, windows, insulation, et cetera abound.
Just a random thought on the weird workings of well intended government agencies…
I’ll shut up and go back to a pointless analysis of pumper drums for a mid sized low temp ammonia system now.
Useless, Don? I think not. Your post is a beautiful segue (and gives me an excuse to post this) to a Cecil Adams’ column from 1986. Gas Mileage & Air Conditioning Well done, sir.
Unclebeer, AWB’s theory implies the engine slows down when in my experience the opposite is true. What happens in your car? does the engine rev down when the AC kicks in? or does it rev up? That should answer it.
had a car w/ factory a/c w/o power brakes - it didn’t exhibit this behavior.
the a/c clutch puts a strain on the engine some cars compensate for this some don’t but either way the idle speed changes, vacuum changes and the powerboster assist changes