Blowing the 10 amp heater fuse anytime A/C or defrost selected

Hello,

Below you will find the diagrams for the AC circuit.
The AC button goes through the ECU comes back through the 10A fuse then through the MG CLT Relay then goes right to the AC clutch. Be sure the relay you are checking is the MG CLT and not the heater relay. Location in diagram.

If the relay is good then you have a short somewhere in the line from the button to the ECU or more likely from the ECU to the clutch. Test both circuits to ground and make sure you have no reading.

Here is a walk through on circuit testing:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

As for the cooling fans not coming on, that would have nothing to do with the AC so I would check both relays and the cooling fan switch.

Rich

First lets verify which fuse is blowing. I suspect it's fuse # 37 since that is just for the heater system. Fuse # 14 is also a ten-amp fuse, but there's a lot of other stuff on that circuit.

For these first three diagrams, I have the first two turned around. The second one is the first diagram for this system. The last two diagrams are just the first two with red lines added to show fuse # 37, (red arrows), feeds five circuits. The first circuit is the coil for the heater fan relay. That coil is not likely to be shorted. Same with the coil for the compressor clutch relay. You had the compressor clutch relay unplugged already, so we can ignore that circuit for now. That was the third circuit.

The last two places are fed through the joint connector. At first it looks like two different fuses are feeding that circuit, but if you look closer, you'll see there's actually two different circuits there. My green arrow is pointing to the very hard-to-see dot that signifies that is a connection. From there, current goes to the controller, then, on the last diagram, it feeds the air inlet control servo motor. From there I can't tell what takes place, but it suggests there's electronics in that assembly, so that is a good suspect.

I'd start by unplugging that motor. If that doesn't stop the fuse from blowing, you're better off using my trick to avoid having to buy a lot of fuses. In place of the blowing fuse, plug in a pair of generic spade terminals, then use a pair of clip leads to connect them to a 12-volt light bulb. For this type of circuit, a 3057 brake light bulb will work well, and it has a base that makes it easy to connect the clip leads.

Now, when you power up the circuit and the short is present, the bulb will be full brightness and will get hot, so be careful where you lay it. It will limit current flow to a safe one-amp. When you unplug something, move a wiring harness around, or do anything that makes the short go away, the bulb will get dim or go out.

If you unplug the controller, and that makes the light get dim, there is something on the other side, (the output side), that is shorted, but at least this will narrow it down for now.

Please let us know what you find. We are interested to see what it is.

If it appears the short is after the relay, you can use the light bulb trick to bypass the relay instead of the fuse. I'm not familiar with the type of relay used in that model, so I added a drawing of the common styles. If you have the 1"-cube relay, use terminals 30 and 87. If you have the skinnier relay, use the two terminals with the arrows pointing to them. Some other relays will have a drawing on the side that shows the two contacts. On some relays, the terminals for the contacts are wider than those for the coil.

Plug the terminals into the relay socket, attach the bulb, then stick in a new fuse. The bulb will again be full brightness when the short is occurring.

Sorry that they didn't show up. We're all having multiple computer problems lately. I'll try it again.

Thank goodness. I just went into our online service manual web site we use to try to find the relays used in your vehicle, and that has all changed too within the last few days. I'll no longer be able to paste diagrams. They removed the manufacturer's diagrams too. I hope this isn't permanent because it's going to put me out of business, so to speak. I can still help you with any diagrams I've posted already. I have those saved on my computer.

If I can jump in here the servo motors all use this fuse. I would unplug the blend door actuators to see which one doesn't pop the fuse then replace that one. Here are the complete air conditioner wiring diagrams for the front and rear air. Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.

Remember, the light bulb trick is perfect to bypass the compressor clutch relay. The only problem is the clutch coil draws quite a bit of current under normal circumstances, and in relation, a brake light bulb doesn't draw so much, so most of the 12 volts will be dropped across the test bulb. That means the bulb will already be fairly bright when the short is not in the circuit, and it can't get much brighter when you make the short occur. The difference in brightness can be hard to see when you're not working near the bulb.

The solution for that is to use a bulb that allows more current to pass. A 9004 or 9007 head lamp bulb will work better than a 3057 brake light bulb. Brake light bulbs allow one amp of current to flow. Typical head lamp bulbs will allow five amps to flow with the low-beam filament and around six amps with the high-beam. That's if you use the ground / common terminal and one of the other terminals. If you use the low and high terminals, it will allow around 2 1/2 amps to flow.

Good find.

Glad it is fixed.

Thanks for using 2CarPros.

Roy

The problem with that solution is you don't know what caused the short or whether it's going to affect other wires in that harness. Two common causes are the harness is draped over the sharp edge of a metal bracket and this is the first wire to rub through the insulation, and water snuck in and caused corrosion, either at a soldered splice or where someone previously sliced the insulation to take a voltage reading.

I never allowed my students to cut or poke a wire to take a reading, and running a new wire without knowing what was wrong with the original one was also not an acceptable repair on a customer's car. I've also been involved with wire harnesses laying on the inner fender, sliding back and forth as the engine rocks, and they rub through the paint and the wires' insulation. By the time the first wire shorted, in this case for the back-up lights, (which were the clue), and the radiator cooling fan were on the same circuit, and a half dozen adjacent wires were almost rubbed through too. The complaint was intermittent overheating due to a dead radiator fan. Our AC system specialist found the burned fuse link wire, replaced it, and the fan motor which he logically assumed was tight or shorted. The van came back at least four more times over a few weeks for the same fuse link wire. It was just by chance and luck I noticed he had no back-up lights when he went to run the van back outside. The new fuse wire had already burned open. Putting it in reverse caused the engine to rock, the harness to slide, and the short to occur. If that cause hadn't been found and he just ran a new wire, who knows which wire would have rubbed through next, and would it have left the owner sitting on the side of the road, ... On a Saturday night, ... After midnight, ... In the middle of winter, ... Too far to walk home?

A similar problem occurred on a truck after someone installed an aftermarket dual exhaust system. They did a real nice job, but they didn't see the wire harness that was being tugged up by the pipe where it went over the rear axle. That harness melted through after a few weeks. The problem was fuses would blow intermittently when the brake light or the left signal were turned on. In this case we had the luxury of knowing the recent history, so we knew where to look first. If we didn't have that knowledge, someone might have cut the wire for the left rear brake / signal, and ran a new one. The wires for the right brake / signal, tail lights, and ABS speed sensor were in that harness too. One by one each of them would have developed the same intermittent short.

One advantage to your AC compressor clutch circuit is there's no other wires spliced into it, so that eliminates the likelihood of a corroded splice. And the need to follow multiple wires. Other than what I've mentioned, that wire you eliminated has to be shorted to something. This diagram indicates there's no ground wire in the harness for the clutch coil. The feed and ground wires touching each other is another possible cause eliminated.

One thing to be aware of is, as with an ignition coil, when voltage is switched off to any coil of wire, it creates its own voltage spike. Even little relay coils do that. To prevent those spikes from feeding back and possibly damaging other components, they're shorted out by diodes placed backward in the circuit. During normal operation, it's like those diodes aren't even there. They're "forward-biased", meaning turned on, and act like a wire when those spikes occur. That shorts them out making them harmless. It is customary to use a diode in the compressor clutch's circuit for the same reason. Some manufacturers put the diode right across the two terminals in the coil's plug. Some splice them into the wire harness, but to do that you have to also have the ground wire running through there. If there is some other ground wire I didn't see, and if they did put the diode in there, it is eliminated now by cutting out that wire. I have a suspicion that won't cause a problem, but the manufacturers had some reason for wanting to include them in the circuit.

What's more confusing is in your first post you did a real nice job of describing everything you did already, and that included removing the clutch relay, and unplugging the clutch, and the fuse still blew. If the wire to the clutch has a short, it would have been isolated / removed when the relay was removed. I know you've been at this quite a while, and sometimes you gotta just throw in the towel and run the new wire. I hope the system keeps working, but it sure would be nice to know what really happened. If it gives you more trouble, we'll be here for you.