[bigbird]

Quick question, a 1997 5SPD 240SX (ka24de obviously) , Completely stock.

Problem: Car operates normally, car starts normally, EXCEPT - when the AC is left ON, and the car is started it will start, but stall in a matter of seconds.

Why does the AC being ON cause IDLE to go down and engine to die

Sidenote : beyond that, the AC works fine and the car runs fine with the ac on, just the starting is affected.

 

[osama]

Its the alternator, its gonna die
They are both on the same belt.

Another way to check is at night, when you turn on the AC, does it dim the lights?

 

[kenshi]

The A/C compressor puts a load on the engine that the ECU is supposed to compensate for at idle. Unless the ECU is not getting a signal for the A/C switch, I would guess the compressor might not be turning properly. If the system has been recharged and they didn’t replace the lubricant correctly, it can cause problems.

 

[osama]

a 97 is fuel injected

 

[osama]

The A/C compressor puts a load on the engine that the ECU is supposed to compensate for at idle. Unless the ECU is not getting a signal for the A/C switch, I would guess the compressor might not be turning properly. If the system has been recharged and they didn’t replace the lubricant correctly, it can cause problems.

how does that cause the car to stall at startup? I'll put cash on the alternator showing early symptoms of death.

 

[MelanieMonroe]

No it's not the alternator. Most likely it has an electronic control that increases the idol speed when the AC is switched on. And for some reason Pouya's car does not recognize the AC being on from a cold start. So, when you park turn off the AC, and just turn the AC on AFTER you start the car.

 

[kenshi]

how does that cause the car to stall at startup? I'll put cash on the alternator showing early symptoms of death.

You're probably right.

 

[osama]

aah, ok
should look into it though, crazy things started happening to me after the pulley and the alternators on altima/240sx are weak and we chew them up easily.

 

[Biohamhock]

maybe you should check over at freshalloy

 

[MelanieMonroe]

How is it the alternator and what does it sharing the same belt have to do with the idol speed??? NOTHING. If the alternator was freezing up the belt would squeel and/or snap. It's NOT the alternator.

 

[bigbird]

Well as of now it does not seem like a serious problem, I didn't notice that the lights were dimming or loss of power. To me it seemed that a relay wasn't working somewhere and it was more of an electric/tronic problem, rather than something hardcore and mechanical. Oh well.

 

[kenshi]

You'll see dimming anyway if the idle speed goes down.

 

[MelanieMonroe]

Yeah that's what I'm saying, its not a big deal. Tell big P to just make sure he turns the AC on AFTER he starts the car.

 

[osama]

Here is something I found on the subject:

Have a dealer check the engine's base idle. If this isn't correct, or the idle adjustment passage is partially blocked, you will get what you describe. The ECU controls the engine rpm from base idle (usually about 600RPM) and above.

The reason it goes away then the AC is on is that the engine idle
speed is purposely increased when the AC is on (or when there is high hydraulic line pressure in your power steering).

Also have them check the TPS sensor.

 

[osama]

How is it the alternator and what does it sharing the same belt have to do with the idol speed??? NOTHING. If the alternator was freezing up the belt would squeel and/or snap. It's NOT the alternator.

ok, I had similar symptoms cause of my pulley, I guess its the ECU thing.

 

[kenshi]

Yeah that's what I'm saying, its not a big deal. Tell big P to just make sure he turns the AC on AFTER he starts the car.

The interesting thing about this is that the compressor shouldn't be engaged when the car is being started. It would be an unnecessary load on the starter. It's set to kick on about 3 seconds afterwards.

 

[MelanieMonroe]

Unless its an aftermarket AC then the ECU has nothing to do with the problem.

 

[osama]

ok I'm stumped, I'll ask the 240sx gurus and tell you what they said.

 

[MelanieMonroe]

The interesting thing about this is that the compressor isn't engaged when the car is being started. It would be an unnecessary load on the starter. It's set to kick on about 3 seconds afterwards.

Well if it was designed to kick in a few seconds after it started then the ECU would tell the car to boost the idol at that time or when he switched it on manualy. Maybe the compressor is running when the car is started and therefore the ECU is not notified that the AC is on. Hence my reasoning to wait until the car is started to turn it on.

Whatever "devise" is in the relay compressor is not working, its that simple.

 

[kenshi]

Well if it was designed to kick in a few seconds after it started then the ECU would tell the car to boost the idol at that time or when he switched it on manualy. Maybe the compressor is running when the car is started and therefore the ECU is not notified that the AC is on. Hence my reasoning to wait until the car is started to turn it on.

Whatever "devise" is in the relay compressor is not working, its that simple.

Then just eyeball the compressor at startup to see when the clutch engages. Also look at when the compressor is engaged at idle. If the ECU isn't getting a signal from the A/C relay, I think the car should stall every time the compressor engages.