M1Cruiser

Is it possible to turn off A/C on defrost? When I tried it, the A/C light blinks a few times and then stays on. Why do I want to know? A friend said the defrost setting tells the ECM to richen the A/F so you get a little more HP out of a stock tune. Does that sound right?

BlackZ06

The only thing the PCM "knows" is whether the HVAC controller is requesting the A/C on or off .... the PCM has NO CLUE whether you are in "defrost" mode or "cool my *****" mode .... your "friend" is a little confused.

To quote from your Owner's Manual (which you might try reading ....)

"The air conditioning cannot be turned off in DEFROST and DEFOG, as it helps to remove moisture from the vehicle."

aquaholic

there is no way to turn off the ac when in the defrost mode. its using the ac to dehumidify the inside of the car. its like that on every gm and toyota made. havn't you ever noticed when the windshield and side windows fog up and you just manually turn on the ac that they instantly defog. even if the position adjustment is on FLOOR.

M1Cruiser

I'm looking for a way around what is stated in the manual.

BlackZ06

OK ... you're possibly dumber than your "friend" ....

I already explained to you .... the PCM has NO CLUE what setting the HVAC unit is in. It doesn't "richen" the mix for DEFROST because it doesn't know you're in DEFROST.

Running the car with the HVAC in the OFF mode would be the same as running the car in DEFROST with the A/C off ... which you can't do. The decision to run A/C in DEFROST is programmed into the HVAC controller .... let me know when you find a way to reprogram the HVAC controller ..... nobody has ever tried it because there is NO BENEFIT to it.

CamaroAJ

unplug the low pressure switch and then the A/C won't work at all if thats what you want. and no your friend is a moron. the only thing the HVAC tells the PCM is A/C is requested that way it will bump the RPM up a little bit when the compressure tuens on.

BlackZ06

OK .... let me explain why what your "friend", and you, claims makes sense is

The A/C compressor is controlled by the PCM .... it is NOT controlled by the HVAC computer. The HVAC computer can "request" that the A/C compressor be turned on ..... but ONLY the PCM decides whether or not to actually engage the clutch.

An example of the PCM deciding "NO" would be if the engine is overheating ..... the HVAC may be asking for A/C .... but the PCM will not engage the compressor because it is trying to lessen engine load in an overheat situation.

When the PCM decides to engage the A/C compressor, as CamaroAJ says, the PCM will "throttle up" for that instant that the compressor comes on-line to minimize that "hesitation" you feel in most cars when you first turn on the A/C.

The PCM does not run the car "richer" in any particular HVAC mode .... the only thing the PCM knows is if the HVAC wants the A/C compressor ON or OFF .......

Screwing with HVAC modes has as much effect on engine "tuning" as flushing your toilet has on the space shuttle engines.

dgrant3830

I agree, The PCM controls the compressor. The HVAC controller makes the request to the PCM as stated above. In the old days, it wasn't that way, but now it is.

M1Cruiser

Maybe I'm confused but I thought the idea behind these forums was to share information and help one another. Seems like some of the members need to read the forum owners manual before responding to posts.

BlackZ06

We are "sharing" information .... I'm giving you facts .... what are you contributing ?????

lucky131969

I disagree. I gained 25 RWHP by unplugging the A/C pressure switch and installing a performance hood seal.

Mike Mercury

EXACTLY. And the info that's being shared is that the OP's friend is pulling "facts" out of his azz.

the first mistake the OP made was "A friend said ...". And he came here because he suspected his friend was wrong.

There are no second-set of fuel tables that are inserted when the defroster is enabled. The fuel tables are already optimized (during closed-loop operation) for perfection for all driving (except for WOT).

The WOT tables are static and can only be changed via extermal programming.

There is fuel enrichment during a cold start - to help a cold engine run better. But as soon as the engines operating temp is reached... the fuel enrichment is stopped; and it should be because it would be too rich a mixture and would hamper engine performance after that.
You can't fool the PCM into a "cold start enrichment cycle" by turning on the heater or defroster. It simply doesn't use the HVAC settings to detect if the engine is being started cold.

M1Cruiser

Thanks Mike. That's what I was looking for - a good explanation of what the computer does in each mode. I haven't fooled around with car mods in 20 years and obviously I'm on a learning curve with the computer stuff.

bestvettever

Hi M1, I once did a defeat to the intended mode of defrost operation by splicing a on/off switch into one of the wires that go to the compressor that activate the clutch, and ran the wire inside the cockpit and mounted the switch in an inconspicuous place. It's a bit unorthodox, but it will work to achieve the task you are wanting to do. I'm not claiming any major decrease in 0 to 60, or quarter times for this, because that is not my expertise, just showing a way to do it. Also I may be mistaken, but I think I remember reading in the manual that under full throttle acceleration, and/ or above a certain RPM the signal is sent to the compressor to cycle off.
Good Luck

bighank

Seriously,

Not using your AC for MONTHS can cause compressor problems.
That is one reason why most auto manufacturers run the AC in defrost or defog mode. If the temp is below about 35 degrees the compressor WILL NOT OPERATE. Besides a low refrigerant pressure sensor there is also a low temperature sensor to prevent running the beast when its freezing out.

If you are racing the car why in the world would you want to put the HVAC in deforst? BIGHANK

DPG

i can see where he is coming from. in defrost mode, the a/c works, but on the drag strip you don't really want to be dripping water from your a/c onto the track. This would be the only reason that i can think of for having the a/c with the defroster. I have had my window fog up a little at the track, but then you cant really turn the defroster cause the a/c kicks in

SDPC

If you want to run the defroster without a/c. . .put the pedal to the floor. The a/c shuts off at WOT

DPG

hmmmm.