Alarm Dome Light Output

Installing a Viper 5901 in my Pontiac Firebird and I have a question regarding the dome light output. My dome lights have three wires, a ground, a constant 12V, and a white wire, which when grounded will illuminate the dome light.


The dome light wire in the Viper 5901 is a negative output but it specifically states not to use it without a relay. I'm just a little confused as to why I can't just use this wire, which will be grounded on unlock, and connect it directly to the white wire as opposed to running it through a relay.

Also for what its worth the white wire seemed to read ~.25 volts when the dome light was off and went to 0 volts when a door opened which turned the dome light on. Not sure what the .25 volts signifies?

If you're not yet familiar with relays, go to the top of this page where it says Relays and click there.....definitely worth learning if you're planning on installing more alarms.

Also, the Viper's install guide probably shows how to configure the relay to connect to the domelight.

Regardless, your question is a valid one.....it does seem weird, doesn't it?

Anyway, the output from your Viper (or most any other car alarm) is a low-current output that is very weak. It probably says something like 200mA.... 2/10 of an amp. This probably wouldn't even be enough to drive a single overhead light in an old economy car.....let alone the multiple bulbs in multiple locations that GM usually uses.

If you hook the dome supervision output directly from the alarm to the lighting circuit in the car, it might work once if you're lucky....but will almost surely burn out the alarm's output.

However, a standard relay's coil only draws around 1/10 of an amp...the Viper's output can comfortably drive at least one relay, and usually two or three.

So, look at the relay's pins:

85 and 86......when these receive power and ground at the same time, the relay is "engergized" or "on." When not, the relay is "at rest" or "off."

Then you have three pins going up and down the middle: 87, 87a, 30.

At rest, 87a and 30 are connected together (inside the relay of course), and 87 has no connection to anything.

When the relay is energized, 87a is disconnected, and now 30 and 87a are connected together.

As you may be imagining by now, you can use these three middle pins in lots of creative ways, activating (or deactivating) things as desired.

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IN YOUR CAR:

86: BLACK/ white domelight output from alarm
85: constant power
87: ground
87a: nothing at all gets connected here
30: white domelight wire in car
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HOW IT WORKS:

When everything as it rest, the relay performs no function, and the car stays just like it was from the factory.

When the Viper system puts out a ground on 86, the relay's coil energizes (because you've already connected positive to 85).

This causes 87 and 30 to be now jumped together inside the relay, which applies chassis ground to the car's domelight wire.

As you can see, the only item with the heavy workload in this case is the car's chassis ground.............the alarm only does a tiny bit of "work" to tell the relay to turn on........and then the relay only does a tiny bit of "work" to jump 87 and 30 together.

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POSSIBLE SHORTCUT:

I've done this on my cars, including many GMs, but never on an F-body like yours.

The car's BCM already has some sort of a relay built in. The relay gets activated by the car's two doorpin switches, by the trunk pinswitch, and by the dimmer wheel on the dashboard.

It's very, very likely that these items are low-current, and don't draw more than the 200mA your Viper alarm puts out.

If you can pull out the headlight switch, try to find the wire behind the dimmer that turns on the domelight.....it should test as a ground only when the switch is moved to "on."

If you can't find it there, go to either of the door pinswitches.

You can probably (but it's hard to promise) connect the Viper's BLACK/ white directly to the car's domelight-switch or doorpin wire without using any relay at all.

Yes, driver's door pin. Now 1 question? If you unlock the car, but don't open the doors does the dome light come on? If so you don't need to worry about dome supervision, it's already there!
Secondly to moderators:- Could Chris's post be made a sticky? I think it's a brilliant answer.

Thanks Chris for that detailed description. I am fairly familiar with how relays work and I understand how using the negative output, along with wiring the relay to ground the wire directly when the relay is activated.

I guess what I was confused on is why the alarm didn't automatically have an internal relay that went to ground instead of outputting a 200mA negative output.

Actually as I type this I think I'm starting to see why that is. Is it because not all cars have a wire that is grounded to turn on the dome lights so this negative output can be used to drive a relay for positive or ground output?

Also thanks howie I never even thought to try unlocking the car from the inside to see if the domelights would turn on or not. I'll have to test that before I wire up a relay.

Oh and one last thing Chris regarding the BCM, my fbody is a 95 which doesn't have a BCM (started in 96).

However that part is correct which is what the installer did here:

https://www.bfranker.badz28.com/fbody/alarm96up2.htm

Excellent, was just about to post a question asking about the electronics central control unit and whether I could wire the 200ma wire up to the door pin. Seems that this should be the case. Thanks Chris.