tachometer mystery?

So I was driving to school one day, and was sitting at a red light when I thought to myself, what would happen to my RPM’s if I turned my air conditioner on while I’m just sitting here. I hypothesized that they would go up because i figured the air conditioner would provide a greater load on the engine so it would need to spin faster to keep up. An amazing thing (to me) happened, they went down. I tested it out a few times, and sure enough if I turned the air conditioning on all the way the needle would pop down and if i turned it it off, it would pop back up. Why is it like this?

What kind of car? I never really notice a difference with mine. Anyhow, an AC compressor places load on the engine, causing it to slow down. Just the same as if you there was friction in the drivetrain of your robot. It is a load, and would cause it to slow down. I am surprised you notice a difference though because for a big engine (like a car engine) and AC compressor shouldn’t be too much of a load.

Your ECM/PCM will probably open the throttle a little to compensate for the load if the RPM drops past a certain point, but if it’s within the idle speed limits it will probably just rev a little lower.

In some smaller cars with 4-cylinders and low horsepower, you can notice the difference in acceleration when the A/C is on or off :cool:

It doesn’t matter the age of the car or the size of the engine, the AC will slow your acceleration. Some cars just have enough excess power that it isn’t very noticeable. (The AC load robs the car of a smaller percentage of power to the wheels)

Wetzel

if you have an automatic, you can even see it drop more when you have the brakes on with the tranny in drive rather than neutral,

that was my case, I have a 99 dodge intrepid (automatic) and I was sitting still w/ brakes on and in drive.

Now what causes this:

On a car(gasoline engine anyway, i dont know much about deisels) the RPMs are adjusted by two different means depending upon whether u have Fuel Injection or Carbeuration:

FI- You adjust via the Go pedal how much air is allowed into the engine through a butterfly valve, the computer then determines how much fuel needs to be injected into the cylinders based on the pressure and amount of O2 in the air.

Carb- You adjust it via the go pedal but this time the butterfly valve and air are used to create a ventury(i think that is spelled right and this method is at least for all aoutomobile carbs that i have seen) which sucks the gasoline allong with the air into the cylinders.

When you add load to the engine it does not say to anything(at least not in most cars some fancy ones do this)“hey im underload gimme more powa’!!” instead the explosion in the chanber does not have enough force to continue at the same speed and the RPMs drop. More gas and Air = more Force explosion = faster RPMs.

Car Man :cool: ,
Cuog

I dont agree entirely

I think every car on the US market has been fuel injected for some time now. The engine controller (computer) holds the engine at the idle RPM when you have your foot off the gas. It compensates for air temp, engine temp, air pressure, the load on the alternator, and keeps the engine at its proper idle speed.

the on/off button on the air conditioner is just another input to the engine controller.

also, on most cars, when you accelerate with the pedal all the way to the floor, the engine controller turns the A/C off, because you obviously want full power to the drive train (or you wouldnt be mashing the gas pedal).

My Saturn Vue has a electic gas pedal. There is no cable to the intake butterfly, its controlled by the engine computer. (and my Vue is by no means an expensive vehicle: $14,500 new).

Also, since I’m assuming the wheather was warm, your power production is probably down anyway, due to knock thresholds and density.

The air conditioner provides an additional load which slows down the engine. Many engines have an idle solenoid that will engage when the A/C is on. This solenoid will attempt increase the speed of the engine. Depending on how it is adjusted, it may or may not cause the engine to speed up.

Ken’s explanation is correct. Both the Honda’s and the Dodge van I own have throttle positioners that are controlled by the computer. All of the various sensors on the engine are looking at idle speed, all of the environmentals (i.e. outside temp, engine temp, etc.) transmission selector, etc. When at idle, in gear or out of gear, lean or rich, cold or hot, the computer adjusts the idle setting based on the RPM sensor and whatever the software guys programmed in. The service manuals for the above cars specify that engine RPM when warm should not change more that 50 RPM as the lights are turned on, the A/C cycles, the brakes are applied (brake lights on) or the fan is set to high. There is some delay from one manufacturer to the next but they should all recover within a few seconds. If not, there is a good chance that the throttle positioner is loose or worn, the throttle plate is dirty, the mass air sensor is dirty or not up to temp, the injectors are dirty, the fuel pressure is not within spec, or you have a manifold leak. (Assuming you have kept to the maintenance schedule and have good plugs and clean filters) Most of these problems are minor and they result in a small loss in economy. There are a number of shops that are able to clean intake parts and I know of one shop that has equipment for cleaning the intake, injectors and fuel rail for a very reasonable price.If your RPM drops 200 or more (quite common with post '96 Dodge big V6 engines with dirty throttle parts) you might have a problem with engine stall in the future.

I noticed that mine likes to bounce. When there is a load, the tach drops. When the control system tries to recover, it overshoots. Then it realizes it went too high and comes back down. The A/C does this when it turns on. If I turn the bass on the radio up, it’ll follow the music.

On a side note, I did notice that there’s a certain temperature that my engine doesn’t like. The problem is that it’s a little above ambient for this time of year. When the engine’s at this temperature (at start up), it revs really low. Every so often, it’s so low that it feels like it’s getting ready to stall. It’s fine when I give it some gas, but it doesn’t like to idle properly. Once the engine warms up, it’s fine though.

Phil,
The first one is easy, the music is too loud. It might be an engine response or it might just be accoustic coupling.
The second one is a prime suspect for a temp sensor gone bad. It probably works above a certain temp and is way off below that temp.

I was thinking that the music thing might be from extra power consumption. The electrical system is connected to the alternator which is connected to the engine.

As for the temp sensor, it just doesn’t work at that temperature. When the engine is really cold or already warm, it’s fine.

It’s possible that you are drawing too much current through the dash power wiring. As the current goes up the voltage drop on the wire goes up. (just like the wiring to speed controllers and motors.)

I had the same problem with a normally aspirated engine years ago. The carb just didn’t like wet weather between 32 and 45 when it was cold. Seemed that ice would form across the venturis as the air was accelerated.

I dont agree entirely

I think every car on the US market has been fuel injected for some time now. The engine controller (computer) holds the engine at the idle RPM when you have your foot off the gas. It compensates for air temp, engine temp, air pressure, the load on the alternator, and keeps the engine at its proper idle speed.

the on/off button on the air conditioner is just another input to the engine controller.

also, on most cars, when you accelerate with the pedal all the way to the floor, the engine controller turns the A/C off, because you obviously want full power to the drive train (or you wouldnt be mashing the gas pedal).

My Saturn Vue has a electic gas pedal. There is no cable to the intake butterfly, its controlled by the engine computer. (and my Vue is by no means an expensive vehicle: $14,500 new).

I adressed that in the fancier post that i originally posted but i had accidentally closed the browser and didnt feel like typing it again. Yes some newer cars infact the number is increasing do more “thinking” for throttle input but many cars(my experience with new cars runs till 2000 on Jeeps so try not to hold that against me) but what i posted earlier is mainly exident in less advanced fuel injection and carbueration not so much on todays more electronically controlled machines

slightly off topic but kinda on. my truck when i rev it in neutral and then as i let go of the throttle, the speedometer whips up about 5mph and then settles down again. this is when i have my foot on the brakes and the truck hasnt moved a cm. how can it? the transmission is not in gear… why is this?

instrumentation error

or

your engine is warping the time/space continuum!

yea. figured as much.

Flux capacitor is obviously uncalibrated…