Lose power when AC/DF is turned on.


pdxracer

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1993 Ranger XLT 2.3 ... 281k Miles

Last year or so, when the AC or Defrost is on, I lose significant power to engine when idling at stop or downshifting from 3->2 and 2->1 at stop. I recharged AC recently but same issue. You see a big drop in voltage and the engine will stall, or come close to stalling out completely. Shop tested alternator (5 years old) and says its fine.

If I am driving, at any speed, there is no issue. Its only when idling or downshifting that I experience this big voltage drop. (you see the needle drop to zero)

What is next step to investigate/remedy this situation?
 


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Denisefwd93

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When air conditioning starts up there is a heavy load on the compressor which translates to horsepower robbing from the engine, once it's cool, refrigerant isn't "saturated"from the excessive heat inside of the cab. Unless you grossly overcharged, it there's probably not much you can do about it. Easiest way to tell if you are overcharged is to check the compressor refrigerant lines, if the compressor is cold you're over charged, if the refrigerant line is cold coming to the compressor you're probably okay,. You may also check the radiator fan clutch, and listen to Unca Gump, shrimp under the hood while driving....
 
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Uncle Gump

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Sounds like the IAC isn't responding to the load created by the A/C compressor engaging.
 

Denisefwd93

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pdxracer

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Sounds like the IAC isn't responding to the load created by the A/C compressor engaging.
I did change the IAC last October, but didnt make a difference.

Planning on taking to a shop to have them diagnose it likely this week.
 

RonD

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Assuming Factory AC

The engine computer is connected to the AC "on/off" circuit
When AC is on the computer will set idle RPMs 100 higher than "normal", so if engine idles at 750-800 then with AC on it should be 850-900
The computer controls idle RPMs using the IAC(idle air control) Valve on the upper intake manifold, by the throttle plate

So either the computer is not "getting the message" of AC on, or IAC Valve is not responding to computer

First, the IAC Valve
On cold start your idle RPM should be around 1,100 or higher, Choke Mode
As engine/coolant warms up the idle RPMs should go down gradually until full warm up idle of about 750, assuming Manual Transmission from "downshifting" comment
Automatics have/need higher "in gear" idle

If this is what happens, high idle cold/lower idle warm, then IAC Valve is working
If thats not happening then you need to clean the IAC Valve, or replace it
You can test the 2 wires at IAC Valve to make sure computer is varying the voltage as engine warms up, should be 12volts before engine starts, then drop to about 11volts after startup, then down to 8v when warmed up


AC signal to computer
When you select AC on(or defrost), 12volts is sent to TWO pins on the computer, pin 10 and pin 43
Should be a Purple wire on 43
and a dark green/orange stripe wire on 10


The reason the voltage drops is because the alternator can not generate voltage below about 600 RPM it actually sucks up voltage when engine RPMs get that low, and spark and fuel shut off so engine dies

Your only issue is idle RPMs with AC on
 
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