hey the dog names a summer are soon upon us and with that comes the AC complaints that we all look forward to every year leading the charge my AC isn't blowing cold you get the vehicle inside you've used some basic checks you quickly discover that the AC compressor clutch isn't engaging a lot of reasons it can cause that isn't there how to narrow that field down real quick is the topic of this edition of the trainer oK you've quickly determined that the compressor clutch is not engaging when the AC is commanded on a lot of reasons that can be right on an obd2 vehicle that's in 1996 and later the AC compressor clutch is controlled by a relay which in turn is controlled by the engine control module and as with any other circuit controlled by the ECM certain parameters have to be met before the ECM will give the green light and turn the circuit on what the factors that affect that well what about low refrigerant that's certainly a possibility or mechanical issue with the compressor clutch itself that's another possibility or how about an electrical circuit issue and either the switch side of the relay circuit or the solenoid side of the relay circuit all of these are possibilities we can narrow them down very quickly though going right to the relay and doing some basic tests but before we do that we need to understand exactly what components are involved in the AC system on the vehicle we're working on four that we need to review the wiring diagram there are lot of things that can cause the AC compressor clutch not to engage we're going to start off by checking a few logical portions in the circuit before we do that we kind of have to end up by exactly what's involved in the circuit so we're going to take a look at the schematic first this is the schematic for the Ranger that we're using in today's example so the first thing any schematic is to identify the load component that's doing the work again this case is the AC presser clutch which is controlled by the clutch feel coil and you can see that right here you're the center of this diagram now they're a couple of wires coming off of the clutch field coil so we're going to find out where it's getting power where the controls are in other words we want to follow this path all the way back to the battery so let's just pick one we'll start here and we'll see that this wire is eventually leading to ground so that's one half of the path isn't it because once we get to you know the rest of the path is taken up from the body ground back to the battery that ground table so we have that side completed for the flush now let's take a look at the other side and follow this wire back to the AC clutch control relay well that's not unusual the circuits right and many things are controlled by relay Oh following this through we can see that this is actually passing through the switch portion of the relay continuing to follow that path back up to the AC clutch system fuse fuse right here in the engine compartment fuse relay box which we have identified on our component locator so we know where that is and notice here hot at all times means that no matter where the key position is there is value voltage present at this fuse now if we want to follow a path all the way back to the battery you can do that we pull up the power distribution diagram and follow that path back to the battery to diffuse all the way back for positive post of the battery so we have that side of the circuit complete but what we want to need to understand here is that any time we have a relay control like we do here there's actually two circuits and we have to worry about something's got to close the switch and the real end of it and that's going to be able to sell on the white side of the relay so this now becomes another circuit we need to trace know where that's going what is causing this solenoid to receive power and ground we still have not identified what's going to turn this relay on but we have identified to control the switch in turn on the relay for the clutch coil itself so I'm going to back up here's the solenoid fortunately relay now we're going to check the wiring that will start here and we can see that that's going to the powertrain control module the ECM well that makes sense most over YouTube cars and relay ability to guard controls the AC compressor clutch the ECM does that because it's considered emissions why is it considered emissions turn off and supposed to is putting an extra load on the car it's going to increase the stuff coming out tailpipes their decreased fuel economy so it's a big deal it's going to be controlled by the ECM so let's go check to the other side so we know that's so is that our ground not sure that yep take a quick look here and see the other side goes red usually good indication there's a power on the wire and sure enough is coming from the museum power relay so I think there's the power coming in your solenoid so this is going to be some kind of a ground control so let's follow that see what we got in order for that clutch control to to turn on ECM it's looking for some signals ended yeah see an AC demand signal that's coming from the AC control assembly another way the drivers got it turn the switch on right tell me you see them that yeah I want the AC on so that's what that does that's the signal coming in if that's not working that's going to keep the clutch coil for coming on isn't it so that's back to be a possible problem now here we have the AC clutch cycling switch here we have a cycling pressure switch here and pressure cutoff switch here well that's our low site pressure and our high pressure switches aren't they and keep following that path that's going all the way back to ground so what we have to have here is a request from the control head telling ECM that we want the AC on the ACM is going to close the switch internally plus control switch internal and route the power from this relay through the solenoid to ground through the to pressure cutoff switches so there's a lot going on here just to get this switch to close what's going to be easiest way to find out where the circuit problem lies is it on the solenoid side is it on the switch side of the relay or is it actually in the component itself maybe or a cannibal problem that's preventing this from working unless we're going to find out next using you activate okay here's the process in a nutshell I'm going to go in access the circuit for the AC compressor clutch a coil directly at the relay now this couple of ways you can do that some of you may use two bypassing the relay with a piece of wire or some other nonsense or swapping relays does that really get you where you want to get to very quickly now I don't I don't think so I'm going to show you a little better way but there are a few ways that you can do it one we can access the fuse that feeds to relay to do some current testing there I'm going to use this though this is called that you activate and this bypasses or takes the place of the relay in the circuit and allows me to do some checks on both sides the switch side that feeds the power to the compressor clutch coil and the control side the solenoid side that the ECM is turning on and off and once I get some indications of what's doing what to who I can narrow down and where exactly or what the cause is of that compressor clutch not engaging okay there are several adapters that come with the tool to allow you to made it to nearly any relay that you run into so we're just going to install the the proper connector in place of the factory relay and then I'm going to install the other end on the back of the box now this is staggered so you can't get it on backwards and then once we're in place where we're ready to start now the first thing I'm going to do is find out on the switch side of the relay which lead here which one of those is the battery power now you recall from the schematic battery power is hot at all times so all I do is move this down it lights up red that tells me that the red pin is the leak coming from the fuse and I do indeed have power to the relay if it a turn green is just a matter of which one is connected green guns it doesn't necessarily mean that ground it's just going to tell me which one is hot so now that I have this in place I can actually make the move to on and I can physically see in here that the clutch coil is engaging clutch is indeed working so that tells me right now that the switch circuit path is intact and functional relay could be now questionable as to whether this is the part that's at fault or not so to ignore the - to verify that though you need to do one more thing okay so the next thing I need to do is verify that the cell annoyed side of the relay circuit is working the way supposed to and that's going to let me know that the ECM is seeing all the parameters it wants to see it's decided to make the decision to go ahead and turn the AC compressor on it completes that path to ground you know for the relay circuit on this solenoid side and everything's fine and dandy there if not and then we start looking at pressure switches refrigerant charge and electrical circuit issues on that side of the circuit now to make that happen I've got to get the car set up to where it's going to turn the AC on that means in this case engine set running and the control set to max AC so let me go ahead and get that done okay with the engine running the a/c on you can see here that the little green LED light is lit this is the side of the tester that actually takes the place of the solenoid side of the relay just we force a bypass to relay with this tool I'm not going to actually do a voltage drop measurement on that side of the circuit if I wanted to by using these two points as the points on my load remember voltage drop testing might be as close to utilities that you're possible with here's my load right here on that circuit I could check either side power and ground and make sure that I don't have any excessive drop or grows and issues if this wasn't lit I could do the same thing in my having power to the circuit like I'm supposed to is there a problem on the ground side voltage drop do a part of the circuit and maybe I don't know about yet I can just think of it as a breakout box I can use that right here to find it where the problem is now that I know that I have this lick which means the solenoid side of the circuit is okay and we already checked the switch side of the circuit that was okay we did successfully engage the clutch there's really only two things left that could be causing this problem what do you think they are I just like that segue pretty cool huh well there was the question we've already seen that the little green diode lit here on the relay tester the you activate tester that let me know that the ECM wasn't the completing the circuit power was being fed to the solenoid side of the relay circuit ground was being completed that side the circuits okay the ECM is seeing everything that it wants to see it's happy so the pressure cut out switches the low side by side switches they're not going to be an issue refrigerant charge is certainly not an issue we know that's not the cause of the pressure not engaging because the ECM saw wouldn't wanted to see so we also did the switch side which means that I can bypass this I can engage the compressor clutch anytime I want but we don't know what the real underlying causes do we well you should have a pretty fair narrow down by now the only thing that's left is the relay remember though when you're replacing the relay make sure you use the one the proper type many of these have a diode in the circuit and if you fail to put that in there you could lead to some problems by burning up some spots on the ECM may not cause a failure right away but it will lead to a failure down the road now what if the clutch wasn't engaging when I hit the switch because that still means something could it be a mechanical issue or a circuit issue well we can find that out too I'm just going to take a low amp current clamp and I'm going to put that right there on that test loop and then I'm going to engage that switch and measure the current flow now of course we know this one's working we can hear it engaging but if I didn't get that click I didn't visually see that compressor working if I got current flow anywhere I would say three to five amps then I would know the clutch windings the electrical portion of the coil circuit the clutch compressor clutch coil circuit is working the way it's supposed to it's got to be a mechanical issue of mechanical fault with that clutch and I've seen where that happened more than once so before I start trying to dig that one that's buried at the bottom the engine compartment where you can't see it just on a guess I want to know for sure that we just confirmed for sure now you can do a lot of the same type of test with just an amp clamp and a digital multimeter if there's a fuse powering both the scylla need and the switch side of the relay with the current relay the stock relay in place just bypass that fuse putting a little jumper little fuse buddy hook your low amp clamp there and then engage the AC system by saying the controls to max AC is starting the engine when you get no current flow you know you have an electrical problem and it's going to be somewhere on the solenoid side of the circuit because obviously the solenoid is not engaging now could be wiring it could be a pressure sensor it could be a lack of refrigerant that's causing the ECM to see something it didn't like and not turn the solenoid on if however that you see some current say half an amp three tenths of an amp very small amount less telling you that the ECM is happy and it did indeed turn the solenoid portion of the circuit on now the problems is on the switch side the circuit going through the - the compressor clutch coil itself and as we just demonstrated if you see three to five amps but the collector is not engaging well the circuits okay it's a mechanical problem with the clutch so either method will work just fine and you can even apply what you learned here on any relay control circuit I don't know like fuel pumps that's going to do it for this edition of the trainer see you next month you you