There seem to be many members who don't know what a relay is, or don't know how to wire one in. this guide is here to help solve that issue...

what is a relay?
-a relay is a device that allows you to control a high-current electrical load with a low-current electrical 'signal'. they are usually electro-magnetic, but are also available in solid-state forms. they can be used with a switch (to allow control of a high-current load with a small switch) or they can be hooked up to a switched power source in the car like the ignition or accessory power circuits (to allow power to be switched on/off automatically with the ignition key).

Why do I need a relay?

-when hooking anything up to a car's factory wiring, it's important to remember that factory wires are designed to carry the load of only the factory installed components. they are not 'general use' power circuits like the power outlets in your house. for example, the ignition (IGN) circuit is designed to power the car's ignition system and nothing else. hooking up a high-current device to this circuit can create a fire hazard. by using a relay, you can use the IGN circuit to control a high-current device without directly powering it from the IGN circuit itself.

Is a relay hard to hook up?

-no... most relays require only 4 wires.

Where can I buy a relay, and how much do they cost?

-you can buy standard automotive relays at radio shack, autozone, walmart, or at almost any electronics store or automotive store. a typical SPST relay will cost about $5(US).

I bought a relay, but I don't know how to hook it up. all I see are a few weird numbers and strange symbols, what do they mean?

-a standard bosch-style relay will have 4 or 5 numbered leads (30, 85, 86, 87, and sometimes 87a). why they picked those numbers, I have no clue; but I can tell what they hook up to.

-30 = constant [positive (+)] power (usually wired directly to car battery)
-85 = coil ground (wired to the negative (-) battery terminal or any grounded metal panel in the car)
-86 = coil power (wired to the control source. could be a switch, or it could be the car's IGN or ACC circuit.)
-87 = switched [positive (+)] power output. (when the relay coil is powered, lead/pin 87 is connected to lead/pin 30)
-87a = [on 5 lead/pin relays only] this lead/pin is connected to lead/pin 30 when the coil is NOT powered.

here is the bottom view of a standard 4-lead Bosch-style automotive SPST relay


here is what happens inside the 4-lead/pin SPST relay. on the left, the coil is NOT powered. on the right, the coil IS powered. notice the switch changes positions when the coil is powered. when the coil is powered, pins 30 and 87 are connected. when the coil is NOT powered, then pin 30 is not connected to anything, therefore it is in the 'off' position.


here is the bottom view of a standard 5-lead Bosch-style automotive SPDT relay. you'll notice that the only difference is the addition of the 87a lead/pin.


here is what happens inside the 5-lead/pin SPDT relay. you'll notice that instead of an 'off' position, there is an electrical contact (87a). on the left the coil is NOT powered, and pins 30 and 87a are connected. on the right the coil IS powered, and pins 30 and 87 are connected.


here is a typical setup to control power to a carPC via the car's ACC/IGN circuits. this is generally use for laptop installs or 'no-write' setups (not good for normal carPC because abruptly cutting power without proper shutdown can cause data corruption);


I want to use the relay to turn on/off with the car. how do I hook that up?

-connect lead/pin 86 to the car's IGN, or ACC circuit. these circuits can be tapped into in the wiring harness that goes to your car's key switch. the ACC circuit can also be tapped into at the fuse box or in the stock radio harness

What is the difference between the ignition (IGN) and the accessory (ACC) circuit?
-they are both powered when they key is in the 'RUN' position, and they are both not-powered when they key is in the 'OFF' position.

-the 'ACC' circuit is powered when the key is in the 'ACC' position, but is not powered when the key is in the 'START' position. when the key is turned, power to the relay will turn on as the key passes the 'ACC' and 'RUN' positions, then turn off in the 'START' position, then turn back on as the key is released (springs back to the 'RUN' position. this is not an issue if you're using a manual on/off switch, or have a delayed on startup controller.

-the IGN circuit is powered when the key is in the 'START' position, but is not powered when the key is in the 'ACC' position. this circuit avoids the issue stated above, but requires that the key is left in the 'RU' position if you want to use the carputer with the engine off. on some cars, this can burn out the ignition coil.

I bought a 5-pin relay, can I still use it as a simple on/off switch instead of a changeover switch?

-yes. simply leave pin 87a disconnected.

My relay has more than 5-leads/pins?
if your relay has more than 5-leads/pins then is is most likely a DPDT, MPDT, or MPMP relay (M=multiple). it will work the same way, it simply has two or more separate switched inputs/outputs inside it. regardless of the number of switched contacts, it will still use a single coil, so it will still be controlled by a single power source.

Are there others types of relays other than the standard 'bosch style automotive relay'?
yes, just like regular switches, there are several different types, sizes, and styles of relays. some have multiple switched contacts, some are circuit-board mountable, some have high-current capacity, some have low-current capacity, some use higher or lower voltage coil power (generally you would only use a 12v coil in a car).

What is a 'valet switch'?

-the 'valet switch' is a standard SPST switch wired between the coil power source and pin 86. it allows you to leave the relay turned off, so nobody (including a 'valet' driver) can turn your carPC on. how effective this will be depends on how well you hide the switch (it should be accessible, but hidden from plain sight).

does a relay take the place of a fuse?

-no! a relay provides no protection from overload or short-circuits.

do I need to use a fuse if I use a relay?

yes! you must still fuse your power wires!


thanks in advance

 

i always get the " NITROUS?... your gonna blow that motor up!!" im bout tired of hearing that ****. lol. great write up, and im ready for that 200 shot now.

 

dam right man! i finally figured out my going rich problem, which was the bottle tilt, the freaking custom mounting plate i purchased with the kit came with built in 15* tilt, i reconfigured the tilt to about 40* and i pulled an 11.176@124.2 with the 100w on jul 9th. the 200 is up soon, i just want to richen up my fuel curve to 12.0 afr, but only the last third of the run in 4th gear, i'm too close to 13 in that area. and then i'm gonna let rip.

 

hell yeah, hell of a run on my BIRTHDAY!!! lol. congrats and let us know how she handles the 200 hundo.

 

jetblast-

so is that a total of 18* that you are running? cuz that seems to me like you didnt take any out then, i didnt take any out to run the 100 shot. not sure what to do for the 150 or the 200 but ill figure her out, i thought the 04 goat was around 18-19 stock

 

I have a stock bottom end 05 ls2 gto. getting ready to install L92 heads & cam in the next couple of weeks. I already have a LS3 intake w/fogger system & plate from Nitrous Outlet. Plan is 100 shot out the hole while bringing on another 200 going down the track.

 

My buddy who is insane btw has an 04 GTO Trex cam only. M6 with LS7 clutch, not even sure what nitrous kit and no wideband at all. Has ran 12.4 @ 111 mph n/a. He has been spraying a 100 shot with a cold bottle, keep in mind no tuning software around 28-29 degrees of timing and who knows about AFR no wideband. On a 100 shot with a cold bottle he ran 11.4 @ 121 mph. I let him borrow my bottle heater and he said he got the bottle real hot and it ran a 10.98 @ 130 mph lol. He did put some 109 trick fuel in it I think, also has TR-6 plugs. It is a wet kit but who knows what jets he has in it. Pretty crazy if you ask me must be about 200 shot with 28-29 degrees of timing. This is all at 2200' elevation and I do not recommend what he does at all. Stock fuel pump and injectors.

 

update: so far i have sprayed a total of 28 passes with 100w no progression set to kick in from 4k to 6950 rpm plus 2 passes with the 150w this season. all pasess weere made with vp103. i pulled a 11.176@125.89mph on the 100w and street wheels and hoosier dr and catless mids , timing was at 15deg , 2deg less than with the cats, i did use vp109 to see how the afr changed if any from the vp103. i plan on using vp109 for the 200. for comparison i pulled a 10.998@125.49mph on the 150w and 18deg of timing aug. 03, so i'm pretty confident with 3 or 4 more out the mph would have gone up. i hit a deer in nov 08. 10k bucks. to fix. one last note: it seems with the nano operating at max efficiency with the new tilt angle, i can no longer lean the motor out more than 11.5 or i get kr between 5.6k and 6.9krpm but it seems to not mind 12.5 in the lower rpm section of the run. considering the eng. has hit a few very lean runs in 08 and can still put down good #s today is very impressive keep the timing low and you will be rewarded. stay tuned for the 200!!

 

what about a stock bottom L92 6.2with cam and headers, l76intake??? I was lookin for 150wet. But I hear it wont be safe or it wont last long