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How to Test a Relay in Your Car – Here’s What You Need To Know

How to Test a Relay in Your Car

There are a number of relays in any car on the road today. It's a general term that describes a switch in your car that has an input circuit and output circuit. It allows a small amount of current to control accessories in your vehicle that rely on a large amount of current to operate. The relay prevents the need for heavier switches and more wiring to get these things to work. 

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A properly functioning relay uses an electromagnetic device to move the switch in a way that opens or closes, or in this case making or breaking an electrical circuit. Most relays are small black Square devices that are often called many relays or standard relays.


What is the Point of a Relay in an Automobile?


Automotive relays are generally used to switch a high current circuit using a low current circuit. The necessary for when an inline switch doesn't have the ability to handle the current required to switch a high-current electrical system. Using a relay gives you the advantage of having multiple circuits on a single switch. With multiple relays attached they can break multiple circuits and perform a number of functions with just a single input. In your vehicle and example of this is the door locking mechanism. You can press one button but unlock for doors at the same time.


One of the main reasons that relays are used in a vehicle is that they are time and money savers. Without relays you'd have to be using high current electrical circuits and those can cost a lot of money. Low current components like relays don't cost very much money at all. Then more relays you have, the fewer high current circuits you need in place. 


If you go to Autozone.com you'll see that something like a fuel pump relay may cost only $8. A multi-purpose relay might be as little as $13.


Signs of a Bad Relay


As you can imagine if something goes wrong with a relay in your vehicle it can play havoc with various electrical systems. Since every electrical accessory in your vehicle has the relay attached to it somewhere along the line, nearly any potential electrical system could end up failing. This ranges from your fuel pump to your door locks to your power windows to the air conditioning and more. They really do affect nearly every part of your vehicle.


It's possible when a relay goes bad that it may be a simple matter of it coming loose or being dirty. 


If the relays in your vehicle start to go bad, you may notice some of these symptoms.


Sudden Stalling


If you have a problem with an ignition relay in your car then you're going to be suffering sudden stalling. This happens because the spark is cut off and you are no longer able to maintain combustion because the relay can't keep the electrical current flowing to your spark plugs any longer.


Dimming Lights


The relay that is responsible for handling your headlights could be damaged or failing and that will result in your headlights not working at all or appearing dimmer than normal. This could be a result of a broken connection or relay overloading.


The same can happen to the lights in the interior cabin of your vehicle or even on your display. You could suffer intermittent dimming, the lights not working at all, or flickering lights as a circuit is overloaded or unable to maintain.


Windows Not Working


If your windows are only lowering and raising part way, getting stuck, or they don't operate at all when you press the button, that's a good indication that the relay that deals with your power windows has failed.


Air Conditioning Not Functioning


When you turn on your AC if you find that there's no cold air coming out, or no air coming out of all then there is the potential that the relay and control of your air conditioning system has failed.


As you can see it's hard to nail down the symptoms of a bad relay in a general sense of the word. Because relays are related to every electrical system, you'd be more successful nailing down the symptoms of a bad ignition relay, a bad starter relay, a bad fuel pump relay and so on. 


Suffice it to say that if any of the systems in your vehicle that run on electrical power are not functioning at all, or they are functioning in an intermission or reduced capacity that it's very possible that the relay connected to that device is the reason you're having a problem. And if that's the case, you may want to test a relay to see if there's anything that you can do about it.


Car Relay Repair Cost


If you're experiencing a problem with a relay and you want to get it fixed by a professional, you can take it into a mechanic to get it looked at. Relays are usually very long lasting, but it's possible they can short out or be otherwise damaged and require really placement. If you need to go to a mechanic to get your relays replaced, you're looking at a charge of about $100 to get it done. As we’ve seen, the cost of buying a new relay can be fairly cheap, but the labor is going to make all the difference in this case.


How to Test a Relay in Your Vehicle


When you suspect that something is going wrong with one of the relays in your vehicle you can take it to a mechanic to get it checked out or you can go through the process of getting the relay tested on your own. testing your own relays is not overly complicated and if you're comfortable doing any work in your vehicle, you may be able to handle this job on your own.


Step 1: You should use a test light and check the fuses in your vehicle before you proceed with checking the relays just in case the problem is with a few rather than the relay burning it. Every electrical component will have a fuse in the system as well that you can check and replace if necessary. Automotive fuses are typically very cheap and should cost you under $10.


Step 2: If you check under the hood of your car you should have something called a power distribution centre or PDC.  This provides a map for relay locations and what each one does. This will be handy for you to find the exact relay you need to address whatever problem you are experiencing with your vehicle. If you don't have a location map for all the relays in your vehicle on your PDC anywhere, you can check out your owner's manual and it should provide the same information. If that fails, the internet will definitely have what you're looking for so you can Google your make, model, and year and the relay you're looking for and you should be able to find exactly where you need to go.


Step 3: Now that you've located the specific relay that you're looking for, gently take hold of the relay that you're looking for and have someone turn the vehicle on.  All you need to do is hold the relay between your fingers, there's no need to pull on it or move it. As the engine cranks over, the relay should click in one of the ignition switch positions. If that happens, then you know this relay is working properly. If it doesn't happen, then you need to proceed.


Step 4:  Turn off your car and now you can remove the relay to inspect it at this point. Get a firm grip on it and pull forward, give you a bit of a wiggle if you need to help it come out.


Step 5: Give the relay a quick visual inspection. The terminals on the underside should be clean. If you see any signs of corrosion or damage from heat, you're going to want to replace the relay.


Step 6: You need to do an inspection of the terminal that the relay came out of. The terminals are encased in plastic, so if there's been an issue with overheating you may see some melting or evidence of burning around them. That also indicates you need to replace the relay. If you're seeing just a bit of mild corrosion, you could use an abrasive tool of some kind to scrape the corrosion clean and give it a good buff so that it is in proper working order again. 


Step 7:  The relay should show an illustration of the internal circuit that it applies to. On the primary side of a relay you're going to find terminal 86 and 85.  Power from the switch comes into the relay at pin 85 and it heads out to the ground at pin 86. That makes the complete circuit activating the electromagnet. The magnet closes the second circuit in the relay that runs from pin 87 to pin 30.


High amperage current from the battery enters the relay at pin 87. Pin 30 is where the current flows to whatever component it controls. 


Step 8:  To check any relay to make sure it's working properly the only tool you really need is a multimeter. Once you have the relay removed you can set the multimeter to measure DC voltage and check to see if there are 12 volts located at pin 85 in the fuse box where the relay plugs in. If there is no voltage there you need to check the appropriate fuse for whatever component that you are checking.  


Step 9: If you confirm that there's voltage in the 85 position then you can turn your multimeter to continuity and check just make sure that you have a ground at the 86 slot. If that is in good working order, then you can move to the 87 and check for voltage there. If you have no voltage present at the 87 point, then you may also have a blown fuse or problem with the circuit breaker.


Step 10: Return the multimeter to the continuity function and you can check the 30 terminal by placing one lead there and the other on the component positive voltage connection point. If that looks alright, then the relay is very likely the problem so you can replace it.


Step 11: If you're not sure that you performed all of the steps properly, it never hurts to do a double check just to make sure everything is working the way it is supposed to be. 



The Bottom Line


The relays in your vehicle are of the utmost importance in ensuring that all the electrical components in your vehicle are working properly. Because there are so many of them, the hardest part is often just tracking down which one actually controls the part of the vehicle that you're interested in checking out.


This is why checking your owner's manual and keeping it handy at all times in a place where you won't lose it, like the glove box of your car, is important. When you have the map of relays you’ll know just where to go and save yourself a lot of time. As we said, relays are not very expensive and the process of checking the relay is only going to take you a few minutes of your time. 


Since the cost of going to a mechanic to get this job done can be over $100, it's well worth your time and effort to do a job like this on your own. It's not particularly complicated at all, and it does give you a sense of ownership over what you're doing in your car and your vehicle maintenance.


So many of your car's components require relays for them to work properly. When something starts going wrong, this is definitely one of the first places you should look along with the fuses to get the problem fixed. Even if it isn't relay, doing this yourself eliminates it as an issue so that you can move on to whatever bigger issue may be present.

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