Tech Tip: Uncovering Hidden Culprits Speeds A/C Diagnostic Process

Tech Tip: Uncovering Hidden Culprits Speeds A/C Diagnostic Process

Many of our customers consider air conditioning to be a necessary, rather than an optional, vehicle amenity. For this reason, it has to work on demand. Why shouldn’t your shop garner these A/C service and repair opportunities? To aid your efforts, this article is going to look at some of the most common A/C problems we encounter on the Honda vehicle lineup.

By Bob Dowie
Contributing Editor

It still surprises me that the customer who contemplates having a CV boot or timing belt replaced does not hesitate to spend the money to have the air conditioning repaired.

Many of our customers consider air conditioning to be a necessary, rather than an optional, vehicle amenity. For this reason, it has to work on demand. Why shouldn’t your shop garner these A/C service and repair opportunities? To aid your efforts, this article is going to look at some of the most common A/C problems we encounter on the Honda vehicle lineup.

While this article focuses on Honda models, much of the information here applies to other nameplates, as well.

While you’re pulling your customer’s car into the bay, you can start the diagnostic process. Operate the heater and A/C controls. Does everything move smoothly? Pay close attention to rotary-style heater control knobs. They can crack along the shank that slides onto the heater control shaft. If in doubt, pull it off and take a look. This issue has tricked more than one good technician. Of course, the same problem will cause a “no heat complaint” in the fall.

While you were operating the A/C controls, what else were you looking for? Did the indicator light by the A/C switch light up? Did you hear the compressor clutch close? Did it stay engaged? Did the idle step up in anticipation of the compressor dragging it down, or did the cooling fans start to run to increase the airflow over the condenser? Don’t forget to check blower motor operation, as the ground circuit for the A/C system is provided by the fan switch. Don’t forget that if the fan switch is off, the A/C is also off.

With the reliability that Honda enjoys, it’s a safe bet that you got some indication that the A/C system was willing to work for you but needs some service to cool your customers. But, what if you didn’t get any signs that the system was trying to work? Either way, I would continue your diagnosis by checking the system pressures.

Honda, like all other carmakers, protects the compressor with a low-pressure switch that opens the power lead to the compressor when the system pressure is low. If the pressures are very low, take the time to visually inspect the system for obvious leaks. Look for broken lines, loose fittings or damage to the lines or condenser from collision damage. At the same time, look for the telltale stains of compressor oil.

In the last few years, we’ve all become accustomed to using the various dye kits that are available for leak detection. Although these products work well, they require installing the dye and charging the system to locate a leak. We can use the compressor oil in the same way. If you detect an area that is oily and dirty on the condenser or a fitting, it’s a safe bet that refrigerant is leaking along with the oil. Confirm the leak with a leak detector and you can make the repair without having to add refrigerant, only to remove it so the system can be opened.

We’ve found that the most common leakage problem on Hondas stem from the condenser. Usually, they leak in the corners, but with their exposure to road debris, it pays to give them a thorough inspection. In the older Hondas, we’ve seen some sight glasses and pressure switches that would leak. Again, look for the oil stain. Make sure that the pressure relief valve or plug did not blow off. Usually the customer will report that something seemed to blow out.

If you find evidence of excessive pressure, be sure to check cooling fan operation. Honda uses a couple of different methods to turn on the condenser fan. Some systems simply turn on the radiator and condenser fan when the compressor clutch is on, while others use a high-pressure switch that turns on the fan when the high side reaches a certain pressure. The method used isn’t as important as being sure that the fans run and the high-side pressures are not extreme.

A word of caution on selling Honda A/C condensers. It’s a good idea to make sure you’re able to remove the lines from the condenser before quoting the job and ordering the parts. With the longevity that Hondas enjoy, the aluminum fittings may have been together a long time, resulting in a real challenge to get them apart. You can spend a lot of time fighting with a fitting to get it apart, only to find the threads are unusable. The lines aren’t expensive and most dealers have the common ones in stock. But a little planning can avoid a disappointed customer. This is also the time to convert any R-12 systems to R-134a, as well as install a fresh receiver drier.

What if the system was full and the compressor clutch still was not closing? Check the connector that goes to the compressor. Is battery voltage present? If so, probe the wire close to the clutch. No current here would indicate a problem with the thermal protection device in the compressor.

On some of the Civic models, there have been reports of problems with the clutches themselves. Some don’t work at all, while others work intermittently – they’ll engage the first time they get current, but may not re-engage as the system cycles. The problem seems to be affected by high ambient temperatures. Some technicians report good success with setting the clearance tighter between the magnet and the driven plate, while others report a bad ground between the compressor and the engine. Simple voltage drop checks will let you know if the ground is good.

At the same time, you might as well check the power lead for drops. If the voltage checks were good, I would recommend replacing the clutch and magnet unit.

It’s a tough job to remove the compressor, and the only way you’ll know whether closing up the clearance is going to work is to remove the compressor, tighten the clearance, and reinstall and charge the system. It’s too big a gamble for me to take. Sell the clutch unit and be sure of the repair.

The other side of the coin is a system that seems to have enough at rest pressure (40+ psi) to allow current to be sent to the compressor, but you find none. When you tested the controls, what did you find? No reaction to the switches would send me to the fuse box with a two-wire test lamp in hand to check all the fuses.

I find it easier to just check all the fuses rather than try to figure out which fuse works what. I like to use the two-wire light since even with the key on/engine off, some fuses will not have power. Since the two-wire light also shows a ground circuit, all the fuses can be tested.

If you find a blown fuse that gets the A/C working or one that blows as soon as it’s installed, make note of the number and you have a place to start when you get out the wiring diagram. No bad fuses would send me to my repair information source and wiring diagrams. We work on too many years and models of Hondas to try to cover them. The trick is to have good information on hand. The circuits are nothing tricky, but it’s hard to test whether the compressor relay is getting its ground, if you can’t find the relay on the car.

Suppose when you tested the controls, the indicator light illuminated and the idle stepped up, but the compressor clutch received no current. Since this is the same situation you had when the system was low on refrigerant, check that the pressure switches are getting the current you would expect according to the wiring diagram. On different models, these switches operate and look different. Some will be simple two-wire switches, while others may be dual switches that look at both high and low pressure. Other models may have more than one switch. Again, go to your service information source to locate and check these switches.

The last thing to check would be the A/C thermostat. Its job is to prevent evaporator freeze-up by cycling the compressor to control temperature. Mounted in the evaporator housing, these sensors are usually not a problem.

Another situation is that the A/C appears to be operating properly – clutch engaged, fans running and the blower motor works on all speeds – but you’re not getting cold air out of the vents. Sometimes this can be the most frustrating problem of the bunch. Don’t assume the vents are open and set to A/C operation. I’ve repaired many “I don’t get any cold air out of the center vents” complaints on older Accords by reminding the client to flip the lever back to the A/C position. But not all of the problems are that simple. Since you checked the operation of the controls earlier and didn’t think anything was amiss, you will have to dig a little deeper. Chances are something is not happening when you are telling it to.

The first place I would look is at the heater valve under the hood. Located in the inlet heater hose, its job is to shut off the flow of hot water to the heater core. Make sure that the lever on the valve is moving when the temperature control dial or lever is moved. It’s not unusual for the valves to stick in one position after being left in the heat setting for eight months. The customer may think that just a little extra effort might be all it will take to move it. Of course, something will move, but it’s usually not the valve. The cable may slip from its bracket, allowing the outer cable to move when the control is operated. Or, the inner cable bends, either at the valve or control unit.

Now is a good time to double-check the heater knob. Either way, the heater valve ‘thinks’ you are asking for warm air, and the A/C system gets the blame. In addition to controlling the water valve, the heater dial or lever also controls the operation of the blend or air mix door. Though it’s not as easy to check as the heater valve, it’s certainly not difficult to diagnose.

The blend door, as the name implies, directs the airflow that enters the car. If the door is not in the proper position, the air will not pass through the evaporator on its way to the vents. The most difficult part of the diagnosis is getting a look at the lever. Be sure there isn’t a problem as described when you checked the heater valve.

Although some cars exhibit the problem of the blend door sticking in the housing, Hondas don’t seem to have that problem. To be sure, you should be able to hear the door move as the lever is operated. Problems with the blend door are not very common. Most of the ones we’ve seen are a result of vandalism (a radio being stolen) or a head unit installed by less-than-skilled people.

The last thing I’ll touch upon is the seized compressor. Although it is not nearly as common today as it was in the past, a locked-up compressor is not unlikely. It’s probably the easiest problem to diagnose. The heat damage is usually quite obvious, not to mention the noise the belt will make. The problem here is not replacing the locked-up compressor but rather how to sell the job and be sure you can provide a quality, profitable repair.

The problem is that when the compressor locked up, all of the pieces that resulted from the failed compressor entered the system and became lodged everywhere. What else should you replace with the compressor? At the very least, recommend a new receiver dryer – the filter of the system. It’s a safe bet that the expansion valve will be plugged. While you’re replacing the valve, be sure to flush the evaporator. It will be out anyway when you replace the valve.Don’t forget about the condenser. In the best case, the above-mentioned pieces will be in there, waiting to make the return trip to your rebuilt compressor. In the worse case, the condenser is plugged solid. Either way, the safe thing is to replace it, flushing all the lines as you go.

The last thing you’ll need is to install an inline filter in the suction line at the compressor to catch any debris you missed during the clean-out process. I know at least one rebuilder who has made many of these steps mandatory to keep the warranty in place. Needless to say, we’re talking a lot of money here. Be sure the customer understands what he or she is getting into. There is no way to cut corners on this job. Don’t let the customer talk you into trying it.

I hope you feel better prepared to handle the Hondas – or any other marque – that finds its way to your door during the first hot days of this spring. There is no reason not to welcome this work.

Like everything else on a Honda, the A/C system is well-designed and reliable – generally. Problems can occur, and you need to take a measured approach to diagnosing and solving the problem. Remember: A cool customer is a happy customer!

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