AC coil burnt out

Tiny
DARYLL BRUNO
  • MEMBER
  • 2007 SATURN
  • 80,000 MILES
Okay, my AC clutch coil is out. It is showing zero ohms resistance. All other connections are okay including the AC relay. The compressor turns freely. My question is, is it gambling to get a new compressor when I will not be able to get the warranty because I am not going to do all the work like Flushing Etc. I am going to change the condenser and drier. But I do not know what caused my original coil to burn out so is it gambling to put a brand new compressor on while not knowing what made the old clutch coil burn out?
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Friday, July 21st, 2017 AT 1:57 PM

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Tiny
CARADIODOC
  • EXPERT
Before you get wrapped around the axle, lets make sure you are interpreting your test results correctly. The compressor clutch's coil must have some resistance. If you actually measured 0.0 ohms, it is shorted and you would have a blown fuse. Nothing on the vehicle can cause that other than the coil just did not want to be a coil any longer. A better suspect is a short in your headset, meaning you read the meter incorrectly. The coil is going to have very low resistance, in the order of 2 to 6 ohms. You need to put your meter on the lowest Ohms scale which is the 200 ohms scale. Your meter leads alone will have a few ohms of resistance too, so you can expect to see a total of perhaps 4 to, oh, . Maybe 12 ohms. The actual value is not important as long as you really don't have 0.0 ohms or something real high. A somewhat common mistake is to put the meter on a higher scale that can measure up to 2,000 ohms or 20,000 ohms. On those scales, you lose the accuracy after the decimal point, so an acceptable reading of 10 ohms, for example, would be rounded off to 0.0 ohms. Even that shows the coil is not open, but it is not accurate enough to say it is not shorted.

If all this wondrous information does not make sense, a better approach would be to connect a pair of small jumper wires to the coil's connector, and the other ends to the battery. If you see and hear the clutch snap into engagement, you are done. The coil is good and working, and you can move on. We never measure the resistance because that wastes the time our customers are paying for. Applying twelve volts to the coil tells us everything we need to know, assuming all our other tests have not revealed the cause of the problem yet. Even applying twelve volts is something best left to students learning how the systems work. Compressor clutch coils have such an extremely low failure rate, that is the last thing we suspect or look for.

Be aware there is usually a diode in the circuit to short out the voltage spikes generated when current to the coil is stopped. A diode is a one-way valve for electrical current flow. It is placed in the circuit backward so it's like it is not even there during normal operation. Each time the coil cycles off, the resulting voltage spike creates a momentary reverse voltage which will cause that current to flow through the diode and be safely shorted out.

That diode is often right across the coil's terminals, but on many models it is in the wiring harness on the other side of the connector. If it is on your coil, you must observe the polarity when using the jumper wires to test the coil. If you connect the wires to the wrong battery posts, the diode will be "forward-biased" and will appear as a dead short. The really high current will likely melt and short the diode, and the wires will melt or get hot enough to smoke and burn your hands. While it takes a little extra time, a way to safely do this test is to connect a head light bulb in one of the wires. If you connect them backward to the coil, the bulb will limit current to a safe five amps, it will simply be full brightness, and nothing will be damaged. When you connect them in the correct polarity, the bulb will be slightly dimmer than normal because some of the battery's voltage will be dropped across the coil, and the rest will be dropped across the bulb. Neither will have the full twelve volts so the bulb is dim and the clutch may not make a strong enough magnetic field to engage, or it will engage weakly. Either way, that will tell you the coil is connected with the correct polarity, and it is okay now to remove the bulb from the circuit. This is just a time-consuming way to insure no smoke is let out of anything and nothing gets damaged. If there is no diode on the coil, you can connect the leads to the battery in any order and the clutch will engage.

If you do indeed find out the coil is shorted or open and must be replaced, the best alternative is to replace just that and leave the compressor alone. Most require some special tools to pull the clutch off. You should be able to find those at an auto parts store that rents or borrows tools. On many models there isn't enough room to fit the tools onto the clutch unless you unbolt the compressor, then turn it for access, but you can almost always do that without disconnecting the hoses. That eliminates the need to recover and recharge the refrigerant, and to pump the system into a vacuum before doing the recharge.

If you do replace the compressor, the disconnected hoses will let air in along with the harmful humidity in it. That moisture, if bad enough, can freeze at the control valve and stop the flow of refrigerant intermittently. The common symptom is the AC cools fine for a half hour, then blows only warm air for an hour. Once the ice melts, it starts to circulate again, and the systems cools for another few minutes. What is worse is the water combines with the refrigerant to form an acid that attacks the metal parts, mainly the condenser and the evaporator. The system needs to be pumped into a vacuum for at least a half hour. In a perfect vacuum, water boils at seventy seven degrees so it turns to a vapor and can be drawn out. That vacuum also aids in drawing in the initial refrigerant charge and getting the low-pressure cutout switch to turn on so the compressor will turn on and continue the charging process.
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Friday, July 21st, 2017 AT 11:21 PM

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