2003 2.0L accord…rpm drops to 500 whenever aircon is switched to on.what is wrong?
Does the engine stall or nearly stall? Does it do this momentarily, then pick back up? There may be nothing wrong. The a/c compressor demands a fair amount of power from the engine to operate it, and many times the difference is noticeable to the driver. Let’s get some more details and maybe we can come to a consensus as to whether there is a problem with your car.
Most cars have some form of idle speed adjustment that comes on when the A/C is started up. On low powered cars, the A/C may slow the engine enough to stall it. Your mechanic should check this out; if it does not bother you and the engine does not stumble, it is not urgent.
The problem might be with the Idle Air Control valve. This is what controls the engine idle speed when variuos loads are imposed on the engine. So in your case, this IAC valve could be failing to go to the proper position for the load imposed on the engine when the AC is turned on. And the RPM’s drop too low.
Tester
hi mark9207.it doesnt stall but at 500rpm,engine vibration is strong.now,seems to be that not only with regards to aircon because even if its turned off,when transmission is shift into gear,drive or reverse,rpm would drop to 500.it would only go up to 800 when you shift back to PARK or turn the aircon off.you see,we manage a carshop here and sadly,our mechanics are at a loss as to what the problem is,let alone the solution…
we have two other accords here in our shop and embarassing as it may seem,our mechanics are at a loss to what the problem is,let alone the solution.we borrowed the whole throttle body with the IACV on it and even the ECU of another accord but still the same drop to 500…
okay,that one went in…hi doc.the woes of solving car problems such as these,and we have many.a town and country wont start after some fuse tests.brought it to our local chrysler dealer who said that camshaft and crankshaft sensors are defective but after replacing them,the car still wont start.they said PCM is at fault but due to dealer high price,i had to pull out the car and order a remanufactured PCM abroad.it just arrived this morning and thank heavens,the car started!
Does This Mean The Compressor/Compressor Clutch Is Not Turning Easily?
What happens if you run two of these cars with the air turned on for the same amount of time and point a non-contact thermometer at their compressors? I realize that one will be turning slower than the other ( by 500 rpm ), but wouldn’t a somewhat restricted compressor have to create more heat from the friction that slows it down?
Disregard this if it’s nuts.
CSA
It’s the whole approach to finding faults that is faulty. To begin the troubleshooting process, you need to thoroughly know what the normal and usual operation is. You can’t jump into the middle and expect to find a solution.
What normally controls idle rpm? What inputs, and outputs, are involved? How are they involved? What things, such as airflow and exhaust, are, also, involved?
The questions one should ask (to oneself), and find answers to, are continued until one arrives at a solution.
Temporary part substitution is alright if it’s to ascertain whether the circuit fault is external, or internal, to the component. At no time should the customer pay for an experimental part.
exactly what another shop opined about,the aircon system.but even if the aircon is off,no load from the compressor,shifting to drive lowers the rpm too…
You tell me/us: What inputs, and outputs, control idle rpm, and how do they do it?
It’s a continuous string of questions to arrive at a solution. What changes to the engine take place when going from Park to Drive (or, Reverse)? Does the engine load change? How does the engine computer know? What does it do? Does the engine computer have the right information? Does it have the correct controlling output? Is there the correct response, from the actuators, to the controlling inputs? And, so on.
i agree.we concentrated on ‘what normally controls idle rpm’.questions were asked,solutions tried,and still asking.substitutions were made to actually save the customer from paying for parts not needed.now,with rpm also dropping when shifted from park,isnt that sets aside airflow?
I Thought That’s What Cars Do.
CSA
but not THAT low,i believe.
oh yeah,engine rpm,throttle position speed sensors.actuators, injectors and igniters processed by the ECU to provide the desired RPMs?
Your dealer was just guessing at what the problem was, I think. Idle speed is controlled by letting more air into the engine, as mentioned below.
You need someone more knowledgeable who can actually diagose what the problem is first, before trying a fix.
CSA; you are not nuts; my wife’s Ford Granada with a 351 V8 had a similar problem way back. It idled rough and was hard to start as well. A tech at our local gas station quickly diagnosed it as the A/C compressor beginning to fail through a seized bearing. He cut the A/C drive belt as an emergency measure, and the car ran great
never ending strings of question but we’re sure we’d break it.as for now,we muse over what happened,why and how.
ah but ‘dealer’ was for the chrysler town and country i mentioned as an aside.as for the accord,SOMEONE MORE KNOWLEDGEABLE is really what i need.unless i gain it myself! 
got to log off for now guys,its past midnight here and i need to get to work in a few hours…thanks for the comments and instructions too! …dream cars