lets talk relays and wiring

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

The Car Stereo Company

aka grumpy car stereo guy
Supporting Member
Joined
Jul 8, 2012
Posts
31,891
Reaction score
22,073
Location
here, on frf
guess its time to have a thread dedicated to wiring, and wiring a relay. keep seeing people struggle with them.

a relay is basically a switch. its a switch powered by an electromagnet. there are several different types of relays, but the most common relays we use are spst (single pole, single throw. ie 4post relays) and spdt (single pole double throw. ie 5post relays). these come in usually 30-40 amps depending on manufacturer. an spdt relay (5post) can be used as an spst (4post) relay, but an spst can NOT be used as an spdt. the big difference between the relays is that the spst is just an on off switch. there is no 87a (center post) all it does is turn things on or off. lights for example. if you are using your factory switches you will notice that the amperage is not enough for a 40in lightbar, thats where a relay comes into play. depending on how you want to wire up your lights, will determine which relay you want. just on and off with an aux switch (or any other trigger) will be a spst. or like my reverse lights which come on automatically, but can also use a switch to activate when not in reverse. this uses a spdt. however most of the time a spdt relay is the same cost as an spst relay. on to the good bits now.

i unfortunately dont have an spst relay, but i will find one to take a pic, but essentially the difference is it does not have a center post. IMG_20231216_142143.jpghere is an spdt relay. IMG_20231216_142848.jpgthis is a diagram of the post layout. 85 and 86 get power and ground. so lets say you are using an aux switch in the truck, it is 12v so its a positive trigger. if you connect 85 from the aux switch, you must connect 86 to ground. sometimes you have a negative trigger so lets say you are using (not recommended by the way. for example purposes only) a high beam switch and its a negative trigger meaning a gound signal, if you connect that to 85, you must connect 86 to power. power and ground do not have a dedicated side on a relay. take a 9v battery. touch it to 85 and 86. then flip the battery around and touch it again. the relay still activates regardless. i have always wired relays a certain way just due to the installer in me and how we use relays. i alaays use 85 as source signal input (aux, or any trigger im using) then 86 as opposite so the relay itself has power and ground. i always use 30 as output to my lights compressors, etc. confusing? just think that one side needs power, the other side needs ground. thats it. now to get more confusing.....

on an spdt relay, 87a and 30 are normally closed. meaning when the relay does not have power, the circuit goes through.IMG_20231216_143954.jpgwhen the relay has power 87 and 30 are common, which means they are connected and that makes a complete circuit. also known as a normally open circuit. its disconnected until power is applied to the relayIMG_20231216_144247.jpgwhen wired this way, 87 (or 30) is usually battery power. heavier wire from the battery itself. a relay activated on roughly 200ma of current. so lets say aux 4 (gen 1) is a 5 amp circuit. there is no way i can power my 4 air compressors on 5 amps of current. each compressor is 25 amps. so i got 100amp of current i need to power from a 5amp aux switch. well with the relay being 30-40 amp capable, i can do just that. at 200ma (milliamps) i can power 5 relays at 1amp draw from my aux switch. and at 5 amps, that technically 25 relays i can power from that aux switch. however i highly recommend to not do that. maybe keep it to 20 relays. 4 amps of current. but we will discuss that later. so now that i have completely lost you, lets wire up a relay to power a light bar. this is an spst circuit, using an spdt relay. meaning we are not using the center post. or you already have a relay without a center post (spst)

pretty simple. 85 gets power from aux switch. 86 goes to ground. 87 is power from battery and 30 is out to your light. YOU DO NOT USE 87A (center post) on this setup. keep in mind 85 and 86 are interchangeable, meaning they can accept both power and ground (as explained earlier) and 87 and 30 are interchangeable. meaning you can use 30 as input from power and 87 out to the lights. it doesnt matter. some manufacturers say use 30 in from battery, and 87 out to lights. no difference in this setup. (however i have always ran relays as 30 output since the next set of diagrams for reverse lights need specific connections) but get in the habit of always wiring relays the same way. makes it much easier when wiring multiple relays togetherIMG_20231216_150033.jpgsorry for the sideways pic.


next topic. spdt relays and now to use them correctly.
 
OP
OP
The Car Stereo Company

The Car Stereo Company

aka grumpy car stereo guy
Supporting Member
Joined
Jul 8, 2012
Posts
31,891
Reaction score
22,073
Location
here, on frf
now on to an spdt relay set up. i have reverse lights that come on in reverse. they are also on a switch so i can use them anytime. this is where wiring becomes more specific on a relay. while 85 and 86 do not change, 87, 87a, and 30 are location specific. but if you wire relays like i do then it doesnt change except for 87a. lets discuss this more. as stated above 87a and 30 are a normally closed circuit again, meaning power flows through when the relay has no power. so i have reverse power going into 87a and 30 goes out to my aftermarket reverse lights. this means that when i put my truck into reverse, the aftermarket lights turn on automatically and shut off when taken out of reverse. acts just like my reverse lights. now 87 is connected to my battery. 85 is still my aux switch input and 86 is still ground (according to how i wire relays, so theres no change for me) so what happens is when i flip my aux switch, it breaks the connection between 87a and jumps to 87. this means that since 87 is connected to my battery, it is now sending power through the relay to 30 which go out to my lightsIMG_20231216_153243.jpgdoing this gives me the flexibility to use the reverse lights when setting up camp in the dark, or whatever you need aux lights for. it becomes just that. aux lighting, with the addition of being able to have them as reverse lights. keep in mind, you may need to wire up a second relay to power 87a depending on your lights. the oem circuitry may not be enough to power both. so make sure you got sufficient power going to 87a. i have a gen 1 which used halogen bulbs. i replaced mine with led which have 1/4 the current draw. i use 2 6in leds under my bumper which when adding up the current is less than 1/2amp max more than my oem halogen bulbs. in my case, i did not use a relay.

i have also done this with video feed for multiple cameras. i can have 2 different video feeds go into a single screen and toggle between the two. however you dont necessarily use these relays, but any spdt relay can do it.

hopefully this explains why you can not use an spst relay as an spdt relay. there physically isnt enough posts to do so
 
Last edited:
OP
OP
The Car Stereo Company

The Car Stereo Company

aka grumpy car stereo guy
Supporting Member
Joined
Jul 8, 2012
Posts
31,891
Reaction score
22,073
Location
here, on frf
wires and current.

there are different guages of wire. most everyone knows that the smaller the number, the larger the wire. when adding lights you have to understand current and how it flows. in the dc world we use stranded wire. this because there is a lot of current loss with dc equipment. so the stranded wire gives more surface area for current to travel on compared to a solid strand. solid strand is for ac current because there is a lot less current draw so it can travel much further. but we arent talking about home wiring so lets get back at it. when wiring up lights knowing what the amperage is, is very important. it allows us to know if we need a relay, and it tells us what wire we need. lets take a 40in led ligh bar. roughly 18-20 amps of current. current is the amount of power flowing through the wire. it will always be 12v (12-14v depending on vehicle) but its dc current. this means that power will always be on one side and ground on the other. it never changes. so we need 20 amps of current to turn this light bar on at full power. a good way to look at it, is look at the oem wiring. see the size and match it up, or go slightly bigger wire. for a front bumper mounted light bar it usually has 16ga wire. you are fine to continue to use 16ga wire as long as you are using it for the front bumper. the wire can handle the current. however if you are using that same light bar as a rear facing light, due to current loss (efficiency) you will need bigger wire in order to run it to the rear of the vehicle. THE LONGER THE DISTANCE THE MORE CURRENT LOSS THERE IS. so the light bar is 16ga wire but you are running 20ft to the back of the truck. well at that point you need bigger wire something like a 12ga or 10ga wire would work just fine. REMEMBER, GOING BIGGER ON WIRE WILL NEVER HURT YOUR SYSTEM AS LONG AS ITS PROPERLY FUSED AND SECURED. i have the viair 400c air compressor. 4 of them in the bed of my truck. i put out 11cfm and they draw 25 amps each roughly. they have 10ga wire on the ends. i technically would needed to have run 6ga wire to each of them due to the distance to the bed of my truck. i instead put a second battery in my truck that has a 0ga power connected to my main battery through a 200amp relay. the relay is wired to ignition (see above for relay wiring) so when the truck is running, both batteries charge, and when the truck is off, battery in back is isolated from the rest of the system. since my battery is about 6 inches from my compressors, i was able to use 10ga wire through relays to power all 4 air compressors. kept me from having to run so many thick wires throughtout the truck. along with that i ran several 18ga wires which are my aux wires. i specifically ran these to power relays since i have a battery in the bed, i dont need to run power cables. the relays do not need big thick wires for the relay power connections (85 and 86) they are just there to turn the relay on and off at 200ma max. my big power connection comes from the battery in the bed (87on relay) knowing what you are powering and how far you need to run it determines the guage of wire you need. always go bigger if you can. however i know cost becomes a factor. good wire isnt cheap.

IN TERMS OF WIRES AND CIRCUITS. NEVER EXCEED 70-80% OF ITS MAXIMUM.
if your aux switch is 20 amps and your lightbar is 18amps, its not ok to connect direct to the aux. use a relay. if the wire you are using to run to the back of your truck is getting hot, use bigger wire. if your fused circuit is 10 amps, dont draw 10 amps. overkill is good when it comes to wiring. if your fuse is melting, your wire is insufficient. its getting so hot, it gone to the melting point. use bigger wire. this usually happens when you are running something and its at max distance while even still using the wire it came with. happens a lot. i have seen it too many times. i have run all my lights off of 12ga wire. even my pods. everything facing forward has 12ga. i know this wire is sufficient for the length im running to the battery and ground.

remember, only you can prevent truck fires. fuse your power as close to the battery as possible and secure to something that doesnt move to prevent excess vibration.
 
Last edited:
OP
OP
The Car Stereo Company

The Car Stereo Company

aka grumpy car stereo guy
Supporting Member
Joined
Jul 8, 2012
Posts
31,891
Reaction score
22,073
Location
here, on frf
yeah, i posted this in the gen 1 section..... cause i have a gen 1. maybe i will get around to posting it in the other gen sections since it all still is relevent.

placeholder in case i forgot to say something.
 

fordfreek

Full Access Member
Joined
Dec 16, 2013
Posts
625
Reaction score
1,364
Location
L'Anse, MI
Back in the days of sealed beams and halogen lights for headlights, did you ever use the power going to the headlights to control a relay to power the headlights? Supposedly it made the headlights brighter by shortening the power circuit. If you did, did it in fact make a difference ?
 
OP
OP
The Car Stereo Company

The Car Stereo Company

aka grumpy car stereo guy
Supporting Member
Joined
Jul 8, 2012
Posts
31,891
Reaction score
22,073
Location
here, on frf
Back in the days of sealed beams and halogen lights for headlights, did you ever use the power going to the headlights to control a relay to power the headlights? Supposedly it made the headlights brighter by shortening the power circuit. If you did, did it in fact make a difference ?
a relay with bigger wiring can definitely help. i have run into a few issues on older vehicles where light output wasnt enough so we used a relay to the headlights with more adequate wiring.
 
Top