A/C issue.

HI, my name is Carlos and I recently bought the online version of your book. I have been working as an automotive air conditioning technician for five years. Days ago, I was diagnosing a 2008 Kia Sportage and the problem was that the A/C compressor clutch was not engaging. Pressures were good so I decided to check the compressor coil first and then its positive wire. With the engine off I used the PowerProbe IV to energize the coil and it was good so I turn the engine and the A/C on. I back-probed
the positive wire and there was not voltage but at the same time I sent 13.8 volts to the coil with the PowerProbe for ten seconds. The clutch engaged perfectly and the AC compressor was working fine. When I stopped sending voltage to the positive wire the clutch was still engaged and my customer had Air Conditioning again. My question is : why, after sending voltage to the wire, everything started to work fine? This signal disappeared months ago and now there it is. What could possibly happened? Thanks for your help.

Sometimes that happens, I'd like an explanation myself!
I've seen it with solenoids and motors too...

I thought about that too. Thanks for your interest and help.

So the clutch power comes and goes with the A/C switch now? :huh: Functions just like it should? Weird!

My best guess is that something got disturbed while you were testing at the clutch connector. Not saying you did anything wrong, but it may be that you disturbed a poor connection or corroded wire enough to restore operation temporarily.

Hi . yes, Right now it is working fine. Before back probing, I checked the power wire and its connector., it was ok. No corrosion signs or broken wire. Maybe the AC module of this 2008 kia sportage got reset by getting a small amount of current but I am not sure. Thanks for your interest and help.

The AC wiring diagram is attached

So there are a few possibilities here.

If you had an issue of poor contact, corrosion or a spread terminal at the compressor then you should have seen power on that wire. Unless you didn't make a good enough connection with your tester. That'll throw you for the biggest loop ever. Manually powering it can't "Reset" anything really. It's just powered by a simple relay. With the relay off then it's an open circuit and the power can't back feed anything. There is a SLIGHT possibility that maybe you have a faulty relay that wasn't fully closing. Maybe the extra power gave it an extra jolt and caused it to unstick. I would think that to be very unlikely. If it comes back then you can just replace the A/CON relay. But ya, I wouldn't imagine that to be the issue.

I would hook a scanner up to it and check for codes in the aircon module. I'm thinking most likely scenario is a faulty evaporator temperature sensor. Vehicles have gone away with the simple on/off two wire low side pressure switch and moved to a thermistor or potentiometer on the evaporator to give greater control. This system uses a two wire thermistor. I'm thinking most likely the sensor is faulty and it's reporting too low of a temperature. That tells the module to turn off the compressor to prevent evaporator freezing. I'm guessing that forcing the compressor on disturbed the thermistor and moved it to a better contact. You should have a data pid reading for the thermistor. If that thermistor is the problem then be prepared for it to be less than fun to change!

I can't see your wiring diagram but, what comes to mind is a stuck low pressure switch, located on the high side of the system. By forcing the compressor on, the pressure on the high side changed from roughly 80 psi to maybe 190 (depending on temperature). On my MX5, the low pressure safety switch opens the circuit at 47 psi, to prevent the low side to fall into vacuum. I suspect Kia has a similar system layout. When you powered the A/C clutch the sudden surge of pressure could have unstuck the pressure switch.

That's the best explanation I think we'll hear! but it doesn't answer all the questions like why was the switch below pressure to even get stuck open ?

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Thank you for your interest. What you say sounds possible but to be sincere I haven't experienced a problem with a stuck pressure switch. I have uploaded again the wiring diagram. Your opinion is valid.

Your wiring diagram follows the same logic as the one used in my Mazda MX5. I will assume both Kia and Mazda a/c system are similar in their operation.
I’m afraid you will have to wait for the malfunction to occur again (if it does) before knowing for sure what have happened.

The Triple switch on your diagram (sometimes called hi/low medium, or trinary switch) is the sensor I was referring to in my post. Low opens at 47.4 psi, high opens at 484 psi. Medium closes between 191 and 249 psi, (MX5 data) but this one is used to control the condenser fan only. At rest, terminal 1-2 should be closed and terminal 3-4 open. It’s an easy test if the malfunction reoccurs. I must admit I’ve never encounter a stuck pressure switch coming back to life on a car a/c. But, I’ve seen this happened on commercial refrigeration system, back in my days as a field technician. In commercial system, evaporator temperature is controlled by ajustable pressure switches, as opposed to household application where a temperature sensor fills this role.

Of course, other variables are to be taken into account if the triple switch is found to be operating correctly while the malfunction is present. The A/C control module and ECM (influenced by the body control module, and transmission module via CAN bus) adds many obscure variables for fuel efficiency or added safety to the A/C system. Other than mentioned in my previous post, I can think of:
- deactivate a/c at kick down (auto trans)
- increase engine idle speed when a/c activated
- deactivate a/c at WOT
- deactivate a/c at excessive engine coolant temperature
- deactivate a/c when engine cranking
- evaporator temperature sensor might be used as a protection from freeze up by deactivating a/c.

Obviously, most of these items can be ruled out in your particular situation but my point is to show that a lot is going on that worth paying attention to.

I've seen a couple where the evap temp sensor was faulty. Didn't throw a code but when you checked data it was reading like 5 degrees fahrenheit. So it would just keep the compressor for engaging because it thought the evap was iced up. They're usually a 6+ hour job to change. :whistle:

Bit of an old topic,but I see it was posted in feb. if it's cold where you are it could have very well been stuck pressure switch. As I'm sure you know, it drops significantly in winter and can stick. ill usually watch the scan tool when I flip the switch to see if it even sends the message to the clutch. If not, it may think the pressure is too low even though it isn't. Tapping on the switch usually works for me