How I Test Relays Using the Car Battery
08-02-2010, 01:55 PM
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How I Test Relays Using the Car Battery
For a few of my recent mods, I've been testing relays lately to make sure they work properly before wiring them into the car. So I thought I'd make this guide, with some pictures, showing how I tested them. As long as you're very careful, the way I did this works fine.
Disclaimer: this job is very sensitive, so doing this is at your own risk. I'm not responsible for anything that happens with you. Being careful is on you. If you're not going to follow the safety precautions, then don't use this guide. Also, this guide only applies to typical 12V car relays.
Okay, I tested SPDT relays. The point is to make sure terminals 30 and 87a have continuity when the coil is NOT powered, and terminals 30 and 87 have continuity when the coil IS powered. The coil terminals are 85 and 86 on the relay. Knowing how to power the coil is your job. Note that suppressed relays must be connected the right way. I'm assuming you have this part down.
So the first step is to test continuity between terminals 30 and 87a without hooking the relay up to power. You don't need to do anything in the car or power the relay at all for this first part. Just touch the DMM probes to 30 and 87a (with the selector switch set to continuity/beep). You should get conitnuity here because the coil is not powered. If no continuity, then something is wrong with the relay.
You should also get no continuity between 30 and 87 when the coil is not powered. If you do, then something is wrong with the relay.
The next step is where you need battery power. Basically, you need to make sure 30 switches over from touching 87a to touching 87 when the coil gets power.
Now this is something that requires extreme care because you're hooking test leads up to a powerful car battery. You have to be careful not to cause a battery short, and you have to fuse the + test lead in case you do happen to cause one. I used a 1A fuse... small enough to not be a problem if it shorts. It'd actually be safer to use even a 1/2A or 1/4A fuse.
First, a pair of good test leads would be optimal, but I'm not going to buy them just to test 4 relays real quick. So I made my own leads out of some alligator clips, a fuse holder, and about 4 feet of 18 gauge wire (2 feet for each lead). I would still recommend you buy good leads with a fuse holder.
Ground lead: its just a plain wire with a clip on each end
Positive lead: I insulated the part of the alligator clip my finger touches while connecting it. Not that it matters, but its a little safer to not touch the +. I used a fuse holder in line as close to the battery as possible.
Safety: While testing, it is very important to make sure each clip only touches the one relay terminal that it is supposed to touch and no others. Also, stagger the directions that the clips point (like the pic below) to lessen the chances of them touching accidentally. Remember the two leads cannot touch each other or any other relay terminal when connected to the battery. The + lead cannot touch the chassis either.
What I said about staggering the ways the alligator clips point... they can accidentally touch more easily if you do it INCORRECTLY like this pic below:
Now find a secure spot to place the relay so you can reach the fusebox. It should be in a place where it won't fall, move, roll over, etc. Place a cloth under as well so the relay is not resting on the metal frame. Don't let any car metal near the relay or alligator clips.
Connect the ground lead to the battery ground. You can use the frame somewhere else too... as long as its a ground.
Finally, connect the + lead to the battery terminal in the fusebox. This should be the LAST connection you make.
Once the clip touches the battery terminal, the relay coil gets power, and the switch inside moves. You should hear a click, and then you'll test continuity: you should get continuity between 30 and 87, and you should get no continuity between 30 and 87a. If this isn't the case, then something is wrong with the relay.
Overall scetch of the connections
Now you're done testing the relay.
The only rule when taking everything apart is the first thing to disconnect is the + battery connection from the fusebox terminal.
Also, if you're testing an SPST relay, then its the same procedure except you won't have terminal 87a, so you'll only measure continuity between 30 and 87 while connected to the battery.
Disclaimer: this job is very sensitive, so doing this is at your own risk. I'm not responsible for anything that happens with you. Being careful is on you. If you're not going to follow the safety precautions, then don't use this guide. Also, this guide only applies to typical 12V car relays.
Okay, I tested SPDT relays. The point is to make sure terminals 30 and 87a have continuity when the coil is NOT powered, and terminals 30 and 87 have continuity when the coil IS powered. The coil terminals are 85 and 86 on the relay. Knowing how to power the coil is your job. Note that suppressed relays must be connected the right way. I'm assuming you have this part down.
So the first step is to test continuity between terminals 30 and 87a without hooking the relay up to power. You don't need to do anything in the car or power the relay at all for this first part. Just touch the DMM probes to 30 and 87a (with the selector switch set to continuity/beep). You should get conitnuity here because the coil is not powered. If no continuity, then something is wrong with the relay.
You should also get no continuity between 30 and 87 when the coil is not powered. If you do, then something is wrong with the relay.
The next step is where you need battery power. Basically, you need to make sure 30 switches over from touching 87a to touching 87 when the coil gets power.
Now this is something that requires extreme care because you're hooking test leads up to a powerful car battery. You have to be careful not to cause a battery short, and you have to fuse the + test lead in case you do happen to cause one. I used a 1A fuse... small enough to not be a problem if it shorts. It'd actually be safer to use even a 1/2A or 1/4A fuse.
First, a pair of good test leads would be optimal, but I'm not going to buy them just to test 4 relays real quick. So I made my own leads out of some alligator clips, a fuse holder, and about 4 feet of 18 gauge wire (2 feet for each lead). I would still recommend you buy good leads with a fuse holder.
Ground lead: its just a plain wire with a clip on each end
Positive lead: I insulated the part of the alligator clip my finger touches while connecting it. Not that it matters, but its a little safer to not touch the +. I used a fuse holder in line as close to the battery as possible.
Safety: While testing, it is very important to make sure each clip only touches the one relay terminal that it is supposed to touch and no others. Also, stagger the directions that the clips point (like the pic below) to lessen the chances of them touching accidentally. Remember the two leads cannot touch each other or any other relay terminal when connected to the battery. The + lead cannot touch the chassis either.
What I said about staggering the ways the alligator clips point... they can accidentally touch more easily if you do it INCORRECTLY like this pic below:
Now find a secure spot to place the relay so you can reach the fusebox. It should be in a place where it won't fall, move, roll over, etc. Place a cloth under as well so the relay is not resting on the metal frame. Don't let any car metal near the relay or alligator clips.
Connect the ground lead to the battery ground. You can use the frame somewhere else too... as long as its a ground.
Finally, connect the + lead to the battery terminal in the fusebox. This should be the LAST connection you make.
Once the clip touches the battery terminal, the relay coil gets power, and the switch inside moves. You should hear a click, and then you'll test continuity: you should get continuity between 30 and 87, and you should get no continuity between 30 and 87a. If this isn't the case, then something is wrong with the relay.
Overall scetch of the connections
Now you're done testing the relay.
The only rule when taking everything apart is the first thing to disconnect is the + battery connection from the fusebox terminal.
Also, if you're testing an SPST relay, then its the same procedure except you won't have terminal 87a, so you'll only measure continuity between 30 and 87 while connected to the battery.
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