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Voltage at A/C compressor?

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Old 07-17-2015, 07:33 PM   #1
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Default Voltage at A/C compressor?

I'm having trouble with my A/C system. Compressor won't kick in. There is plenty of refrigerant pressure in the system. Voltage at the connector at the compressor is only 9V. Am I correct in assuming there should be 12V there? I'm going to test the compressor using my variable DC bench power supply (i.e. Provide 12V and see if it will kick the compressor on), but I was just wondering if anybody knows off-hand what voltage should be present at the compressor?
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Old 07-17-2015, 10:41 PM   #2
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12 v should be there, use a test light with an actual bulb ( led's dont count) to test the circuit, connect it across the connector and if the bulb lights then you have a good circuit, the reason for using a test light with a filament bulb is that it acts like a load, and if the circuit has a high resistance the bulb wont light. and remember if the battery voltage (measured across the terminals at the battery) is low the voltage at every point in the system will be low too.
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Old 07-18-2015, 10:45 AM   #3
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You never use a test light to test voltage. Use a DMM to test voltage. 9v is low. You must have 11.5 for the a/c compressor to create the electromagnetic field needed to engage the clutch. You have a resistance somewhere in the circuit. You will need to find the pins in the harness and ohm test them. More than likely you have a short-to-voltage somewhere in that circuit.
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Old 07-18-2015, 01:32 PM   #4
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if u ohm a circuit 99% of the time you wont find the high resistance, use a test light


You may be wondering why we are concerned about voltage drop. Measuring voltage drop is an accurate means of determining the relative resistance in a circuit that is powered up. Remember we said earlier that the higher the resistance, the higher the voltage drop. We can measure resistance directly with an ohmmeter; however, to do so, the circuit cannot have power applied to it and quite often the component in question would have to be removed from the circuit to measure resistance. Many circuit resistance issues are only detectable when there is high current flowing in the circuit. A classic example is a faulty battery cable. The ohmmeter measures resistance by pushing a low current through the component being measured. A battery cable, which is made up of numerous strands of copper wire could have half those wire broken and the ohmmeter will still indicate low resistance because the low current can easily travel through the remaining wire strands. However, when the starter is cranking the engine, it will require, typically, 150 - 200 amps. Those remaining strands of wire cannot carry all of that amperage (high resistance). Measuring voltage drop while cranking the engine will show a high voltage. As a general rule, the voltage drop across a battery cable should be no more than 0.5 volts. yay science!!!!
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Old 07-18-2015, 01:50 PM   #5
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I go to ohio technical college for high performance and racing technology and am already ASE certified in electrical. And I have no need to copy and paste from another website to try to make myself sound like I know what I'm talking about.

http://rsandas.com/P1_Session_7-2.html

And no I don't fix cars professionally but I have built more cars than most people on this site and have worked on NHRA, NASCAR, and pirelli world challenge teams

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Old 07-18-2015, 01:59 PM   #6
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if you dont want to use a test light with a filament bulb to qualify a circuit for high resistance be my guest, but in my 20 years experience with electrical systems (10 as a ford senior master tech) i have seen ase certified newbie techs get burned by this exact situation many times.they all say the same thing " but the ohm meter said .4 ohms!" an ohm meter doenst case if 1 strand or 100 strands are intact in a wire, it will read the same. btw sorry i didnt spend 20 minutes typing that myself,
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Old 07-18-2015, 02:08 PM   #7
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Using a test light tells you that there is power and that's it. Most test lights will light at 6v which tells you nothing. OHM testing the wires will tell you if there is excessive resistance in that section of the wire so you can find the problem and fix it.
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Old 07-18-2015, 03:16 PM   #8
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thats why i aid use the correct kind of test light, 12v bulb automotive test light, the light output at 6v is very dim and is hard to miss. times a ticking till u get burned by testing for high resistance with an ohm meter alone.
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Old 07-18-2015, 10:14 PM   #9
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Seriously wtf is a stupid test light gonna tell you other than if there is power there or not. Unless your still using your harbor freight DMM all good DMM are high impedance so you use it to find the voltage reading with out any possible damage to the circuit. Then you ohm the circuit to find where the problem causing the low voltage reading just like every service manual will tell you to do. Time to stop surfing the net trying to get stuff you can copy and paste to make you sound like a mechanic and actually go out and learn to be one.
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Old 07-18-2015, 10:35 PM   #10
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Quote:
Originally Posted by jwog666;8497254[B
]if you dont want to use a test light with a filament bulb to qualify a circuit for high resistance be my guest[/B], but in my 20 years experience with electrical systems (10 as a ford senior master tech) i have seen ase certified newbie techs get burned by this exact situation many times.they all say the same thing " but the ohm meter said .4 ohms!" an ohm meter doenst case if 1 strand or 100 strands are intact in a wire, it will read the same. btw sorry i didnt spend 20 minutes typing that myself,
LOL I just read this. I'm guessing you have no idea how electricity or electrical circuits work. Using a filament bulb test light does provide a resistance but what happens when you raise resistance in a circuit? You raise current draw. ExR=I you know Ohms law. Raising current draw in an already bad or worse a shorted circuit could cause more damage.

OP instead of reading us talking about a stupid test light. Get your DMM out and do voltage drop test or ohm the circuit if you know how or take it to a shop where they know how to do it (not jwogs)
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Old 07-18-2015, 10:35 PM
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