# Car engine dies with ac on



## noquacks (Jun 5, 2010)

People,

If a car engine (at idle, usually), dies while say, at a traffic light, idling, with ac on, and even while driving slowly (low rpm's, say, 5-10mph), could that be caused by bad battery? Cuz now, car wont start at all. 

Getting ready to check voltage on batt. I was just wondering, since a car "started", shouldnt the car keep running , even with a demanding power demand from the ac compressor, and not shut off? 

Thanns!


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## Cblock6 (Sep 16, 2012)

It could be your bearings going bad in your air compressor pulley that will do it.


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## cjm94 (Sep 25, 2011)

Could be a bad battery. Also check the alternator may not be charging at all times


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## ukrkoz (Dec 31, 2010)

well, let me put it this way.
a/c comp takes quite some power away from engine. so, if your engine is on its last leg, it will simply choke it.
next, due to point 1, very likely, your alternator is under-performing, or, belt/-s are stretched. hence, power taken away from engine, causes low charge rate, what kills battery - and engine.
with this being said, why don't you tell us few things:
how old is vehicle
what engine is it

start with recharging battery and cleaning cable and battery terminals WELL. AND SETTING THEM BACK ON BATTERY REAL SOLID, AS ALL THE WAY DOWN TO THE PLASTIC. oops, sorry for caps. 

but I bet engine is simply not up to par with the task of pulling a/c.


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## Billy_Bob (Sep 11, 2008)

If you have a modern fuel injected vehicle...
(Helps to say what year/model vehicle you have!)

Then the engine idle is controlled by the engine computer.

If say the alternator fails and the car is running off JUST the battery, the battery voltage will get lower and lower. Eventually it will go below say 10 or 9 volts. Then that computer will stop working. Thus it would not be able to detect that the A/C was on and would not be able to increase idle. Or even be able maintain normal idle for that matter!

A fully charged battery is 12.75 volts. After a car is started, the voltage should go up to around 14 volts DC. This is the alternator kicking in and charging the battery.

Measure the voltage on your car battery with a voltmeter. If below 12.75 volts with the car off, charge the battery.

Then start the car and see if the voltage goes up to 13 or 14 volts. If not, check your alternator and charging system.

Note: This is like a cell phone. If you don't charge the battery, the cell phone will stop working. Charge it and the cell phone works again. An alternator in a car charges the car's battery.


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## Mort (Nov 26, 2008)

On my mom's old Ford Taurus, it was the AC compressor freezing up. We noticed a considerable loss of power when it would click on (heat or AC, didn't matter), until it finally froze. You could either replace it, or get a shorter belt and bypass it.


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## ukrkoz (Dec 31, 2010)

Of course, engine management system feeds alternator power to engine and vital systems first, then to battery. Easy check is to start the engine - if you can - and take positive terminal off battery. If engine dies - your alternator is either caput, or belt is totally loose. Also, blue battery should be up, if running on battery.


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## D-rock (May 23, 2011)

I would say best guess an alternator going bad, possible loose/worn belts (but normally makes noise). If the a/c compressor was going out 9 times out of ten it will be making lots of noise. Bad battery normally doesn't kill engine while running unless it is internally shorting out. An alternator going out will make it stall out at slower speeds and not fully charge the battery. Newer cars will raise the idle speed to compensate for a/c compressor output, if at fault it could stall out engine, but battery should still be strong.


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## noquacks (Jun 5, 2010)

Geez, thanks people, for so much useful info! never thought of that (all the above). Right- I should say- its an alfa, 1987. FI, 4 cyl. Now, I can say, as an update, we checked comp clutch plate, cold engine (not running), tried to turn that plate by hand to see if comp is turning freely. It was sooo stiff. That has to be it, huh? Couldnt hear any unusual noises from comp though. We did, earlier (weeks ago) take psi on both high/low sides, and got 45-50 on low, but only a paltry 100-110 psi on high side. usually means comp cant compress, right? Also, I undserstand could be blockage. were getting pretty cool air though.

Anyway, we have to keep an eye on it. For now, were driving the car with no ac on, to be safe. Worst- put in new comp next April or so.

Thanks


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## noquacks (Jun 5, 2010)

Oh- engine/car in general, is in excellent shape......except never know when alternator starts to go (new from AZ about 2-3 yrs ago- lifetime)


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## Autobell (Jul 14, 2011)

*Engine dies with ac on*

Usually there is a idle compensating solenoid that kicks in when the ac comes on , it kicks the idle speed up so as to not stall the engine with the added ac load. In smaller engines the load will be pretty high and the idle needs to be checked and possibly turned up to at least the base idle or this car.


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## silver_flash (Sep 28, 2012)

I doubt it's the alternator. if it was your stereo, headlights, dash lights, and fan all start slowly start to quit. how many miles? what's the make and model of car? did you hear any noise before your car died? a bad fuel pressure regulator can kill your car with strain on the motor.


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## silver_flash (Sep 28, 2012)

my 2011 silverado compressor is hard to turn also. if that was going to cause problems you would have heard the bearings squealing a long time ago and it would have gotten louder. seems to me like a problem with the fuel system or a vacuum line.


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