relays general information

Before I posted this I searched on the keyword "relays" and even though this is a very basic topic, I didn't quite find what I was looking for, at least not in a way I understood as applicable to the following circumstance. so...here goes...

I've done a fair amount of vehicle wiring (usually successful) but not too many alarms per se as complex as the Carvox I'm currently installing..

One thing that still gets me when reading the alarm install instructions for aftermarket alarms is about relays...when to and when not to use them.

Ok, so I know that you can't just arbitrarily connect an alarm output wire to a circuit that requires more current than that device/circuit/wire can deliver (ie a 500ma wire to a starter), but in many instances, modern vehicles have relays already in place for most every circuit.

In most circumstances that I encounter, all that appears to be needed is a low current pulse to trigger an existing relay.

For example, in my Caravan, the parking lights are controlled by relays. As far as I can determine, (and my lack of understanding becomes clear here....) all that is needed is a small current to trigger the existing relays coil to close the main circuit and thus power up the desired circuit. So, how does one determine how much current is needed for the small input to the existing relays coil? (Assuming my assumption is not flawed).

(I know, assumptions are not a good thing when doing wiring, thus the questions before I cut and connect :-)

And, is it safe to make the assumption that because the vehicle already uses a relay for the circuit you are attempting to power up, that one can simply tap into the circuit before the relay and simply take advantage of the existing relay without the need for a "relay before the relay"?

I am currently in the process of connecting my vehicles horn to my aftermarket alarm also. I understand that the horn is a high current draw device. So I would not try to connect the alarm directly to it. However, since the vehicle already HAS a horn relay, and since I'm going to tap into the wire at the steering wheel that provides a ground which I assume triggers the horn relay, why would I still need another relay?

Is it necessary to determine how much current is being used by the factory wiring scheme to power a relays coil or can it be assumed that it's low current (ie 200-500mA?)

Thanks for your time.

My general (conservative) rule of thumb  has always been if you don't know how much current a device needs use a relay.

This is just like any other part of wiring a car up though - verify before you make any connection.  Once you have used a digital volt meter to verify you have the right wire set the meter to it's highest current setting and apply the proper signal to activate the signal.  If it is higher then ~200mA use a relay.  If you don't get a valid reading on your meter, but the function works (ie the horn is honking) then gradually turn down the settings on your current meter until you are in the correct range for a valid reading to be captured.

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Kevin Pierson

Hello KPlerson,

Sound advice.

One reason I personally try to avoid installing relays unless absolutely necessary is because every relay adds 2 additional wires to the spider web. And, of course you have to find a place to secure the additional relay. I guess, at least IMHO, avoiding the addition of components unless absolutely necessary is part of good planning? It just keeps the process as simple as possible and clean as possible.

I recently purchased a MIN/MAX VOM so it is much easier now for me to take those types of measurements as you suggest.

thx so much for your reply

Good planning would involve wiring everything up on a bench before you put it in the vehicle, so the "spider web" should be minimized!  Also, using "stackable" relays really makes it easy to keep things clean.  You can buy stackable harnesses at several places.

I agree 100% with you though, if there is no reason for the relay, don't bother with it.  But, ONLY if you are 150% positive you don't need it - don't make assumptions about current draw.  Put that new meter to work!

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Kevin Pierson

I do basic preps on almost every alarm in stock...And do full preps when installing the unit...Put the brain in a vice and start taping from brain to tip of wire...This makes everything a little easier/cleaner...And on most installs...you cant even differentiate between factory and aftermarket wiring...Tape...Tape...And more tape...One friend actually used double sided tape...So he could "dirty" up the tape...Basically making it even more diffucult to tell the difference...(this is over-kill) Ive tried to explain the process of prepping but keep having to delete it because its kinda hard to follow...I include any by-pass im using in the full prep also...

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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

But basically you would gather up all wires going to the ignition harness and tape them from the brain to connection...Now lets say you have the system ground ,parking lights,siren wires...Again you would start at the brain and start taping up...These wires are obviously going to diiferent locations...But thats where you would just leave out the wire...Then continue taping up...So about a quarter up from taping you would seperate the parking light wire...Continue taping...then let go of the ground wire and keep taping up...Usually i would twist the door lock wires with my drill...This is going to differentiate between the unlock wire and the - door trigger(same color...usually green)...Code used 2 white wires for there parking light input and output...And cant tell you homany times i would have to cut the tape...Prepping is just so natural now...And really decreases total install time...!

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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

Check out this thread for info on prepping the system (with pics)

https://www.the12volt.com/installbay/forum_posts.asp?tid=115358&KW=KPierson

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Kevin Pierson