Printer Friendly Forum Posts - relay to turn off power

I apologize in advance if this is a basic question. I tried a quick search but couldn't find an exact answer/diagram that I thought matched my situation exactly. I just finished installing my rear view backup camera in my Nissan Titan. The location is on the rear bumper by the license plates. The camera has night vision capabilities.

The problem I have is that at night, the license plate illumination lights blind the rear view camera. I would like to be able to wire up a relay so that when I hit Reverse, the relay would cut off power to the license plate lights as to no longer blind the rear view camera.

From the basics of what I've read, I'm thinking of wiring up the BOSCH relay as so:

30 = Positive wire originally headed to license plate light (now source of + signal for license plate lights)
87a = Positive wire headed to actual license plate light
85 = Positive wire tapped into reverse signal wire
86 = Ground

Similarly based on the starter kill diagram:

posted_image

Is this the correct way of cutting off power via a relay? I have set up basic relays before to turn on accessories such as fog lights but have never used a relay before to turn off an accessory.

That diagram is exaclty what you need.

Thanks. Are the connections I posted above correct then for my setup? Based on the diagram, it appears that I have the ground and signal reversed as the diagram has 85 as the negative ground.

Should my connections be like this instead then:
30 = Positive wire originally headed to license plate light (now source of + signal for license plate lights)
87a = Positive wire headed to actual license plate light
86 = Positive wire tapped into reverse signal wire
85 = Ground

as long as you do not put the diode in the circuit it will not make a difference. If the diode is used then it will matter. If using the diode the band of the diode has to have the positive wire connected to it.

Not planning on installing any diodes. Thanks for the clarification. I'll give this a shot. :D

Had to bump this back to the top, because I think this will do what I want it to..

FIRST THE STORY: I've got a relay that turns on an electric fan, it has a thermostatic controler that activates the relay and of coarse then the fan runs from direct power from the battery. Now, the controller has another wire that allows you to "bypass" the thermostat, and turn on the fan manually..BUT, when I wired this up using a simple on/off switch, it blew fuse after fuse, and now it has burned up the switch. I cannot find a short, It would work for a day or two, then the fuse would blow. Now the switch no longer works, but the thermostatic control does still work...

SO here's the solution I need help with.. I want to wire a relay to turn off the thermostatic controller, and run the fan, from a switch on the dash.. This is so I can run the fan manually to cool the A/C, when the radiator is not hot enough to turn on the fan. I've already been asked, why don't I just wire the fan to the A/C so that it runs when the A/C is on?... Answer: I take long trips and don't want the fan running as I'm traveling down the highway at 70 mph, with the A/C on for 3-4 hours... Please help.. I just can't see how to wire up the relay to cut something off, and turn something on at the same time...

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

The relay on the controller, does it switch power or ground to the fan? The fan will be supplied either power or ground, and get the other from the relay. Since you said it runs on power straight from the battery, I am assuming that the relay provides ground for the fan. This will be a piece of cake, I just need to know what you have to be able to draw you a picture. This can be done with one additional relay and a switch, we won't even burn up a switch.

Thank you so much for your help..

Actually, I'm pretty sure the controller relay supplies the (+) to the fan.. Seems like I remember running a ground wire from the fan and connecting it to the ground from the controller, and then connecting all the ground wires to the fenderwell, under one screw. I've been running the problem through my head for 3 weeks now, and I'm starting to think that the problem might have been the spike from the relay.. it has no diode, and perhaps that could have been blowing my fuses? perhaps that could have toasted my switch?.. I dunno.. I barely have relays figgured out, I sure as hell don't know much about diodes.

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

I am sure that the lack of a diode did not take out your switch. The only thing the diode does is prevent the spike that could cause a noise in your audio system. It will also not cause a fuse to blow. You had something connected wrong. When you check and can confirm that the fan is grounded and the controller provides it with 12 volts, I will be happy to draw you a diagram.

Ok, I'm confused.. you say that the "lack of a diode took out the switch" but the fuse blowing was not caused by it? do I have that right? This is the problem. It all worked fine for about a month before the 1st fuse blew.. the second after about 5 days, and the 3rd and 4th fuses about the same.. THEN, the switch burned up, and it hasn't blown a fuse since. I dunno..It's like the switch was shorted, but I just couldn't find any evidence of that...anyway, I did look, and the (-) wire from the fan, absolutley goes straight to ground. so It's a sure thing the controller/relay provides (+) to the fan.

Thanks again for your help, I always find folks on here willing to help.. posted_image

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

I edited it after you read it I happened to read it again and caught the error. I am sure that the lack of the diode Did not take it out.

Ok, thats kinda what I thought you meant, but you know on the internet, you can't read facial expressions or hear the tone of voice... Anyway, I can't see where I have something hooked up wrong.. I started to think something was wrong with that part of the controller.. It works fine "thermostatically".... just the stupid manual by-pass thats built in.. So really what I'm doing is trying to make my own manual by-pass.

Thanks again.

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

You need to cut the wire between the controller and the fan. I still do not know what wire it is, ground or power. Cut the wire and connect the fan wire to terminal 30 of the new relay. connect 87A to the wire that comes from the controller. Terminal 87 needs to be provided with a high current capable connection to either power or ground. Whichever the controller supplies to the fan is what you need to connect to 87. 85 and 86 are the coil of the relay, power to one and ground to the other will energize the relay and turn the fan on. with no power on the coil the controller is connected to the fan.

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

So this is what I drew from your discription.. What do you think?.. I left the fuses out, but I thought I'd put one in the (+) to the 87 terminal...a 40 amp, because thats what it has before the controller.. Is this good to go?.. THANKS AGAIN!!

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

All looks good. You just need to put a 3 amp fuse feeding the switch.

Are you sure that diagram is right? How does your thermostatic controller switch the relay without a connection to the coil? It's just going to the normally closed connection to the fan power?

kaptan wrote:

FIRST THE STORY: I've got a relay that turns on an electric fan, it has a thermostatic controler that activates the relay and of coarse then the fan runs from direct power from the battery. Now, the controller has another wire that allows you to "bypass" the thermostat, and turn on the fan manually..BUT, when I wired this up using a simple on/off switch, it blew fuse after fuse, and now it has burned up the switch. I cannot find a short, It would work for a day or two, then the fuse would blow. Now the switch no longer works, but the thermostatic control does still work...
SO here's the solution I need help with.. I want to wire a relay to turn off the thermostatic controller, and run the fan, from a switch on the dash.. This is so I can run the fan manually to cool the A/C, when the radiator is not hot enough to turn on the fan. I've already been asked, why don't I just wire the fan to the A/C so that it runs when the A/C is on?... Answer: I take long trips and don't want the fan running as I'm traveling down the highway at 70 mph, with the A/C on for 3-4 hours... Please help.. I just can't see how to wire up the relay to cut something off, and turn something on at the same time...

I must be missing something here- just have to wire the switch to the same control line as the thermostatic controller, diode isolating if necessary. It'd be better if the relay was changed to use ground switching, if the thermostatic control can be used that way too.
Then the relay coil can be connected to a switched ignition source, and it won't run the battery down if the manual override is accidentally left on.

The thermo controller has a relay built into it. He wants to be able to manually turn the fan on also. Switch off and only the controller can turn fan on. Switch in the on position, the fan is on.

Great to hear my diagram will work.. About the fuse---- Your saying put a 3 amp fuse between the hot source, (ign) and the switch?... or between the switch and the relay?....

Oh and thanks dualsport, as two pieces of iron can sharpen one another, so one forum poster can clarify another... I think I can wire this up sometime next week.. I'll post up results..

Thanks again to a fellow idiot, from Louisiana

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

Any time you connect a wire to a power source, you really need to fuse it at the point of connection. The fuse goes before the switch.

Oh- so the thermocontroller has the relay built in; knew I was missing something- posted_image

So it just closes an internal relay contact from the battery connection to the output to the fan- I don't know what it looks like, but if it looks like you can open it up, I'd see if you could run a wire in to the coil of the internal relay and use that for switch control. I'd guess it's pretty simple to figure out, but it depends on how easy it is to open up and if you're willing to tinker with it at all. If it's all plastic welded together then just add the extra relay.

You could also use the thermo relay output to just switch the external relay coil instead of running it to the 87A contact, because the NO contacts generally handle more current than the NC ones, and it sounds like the fan draws a good amount. Lighter gauge wiring for the thermo relay could then used, but your setup probably already has the heavy wire in place, so I guess that doesn't matter now.

Yeah, the thermo controller has a built in relay.. as for just adding a wire to the controllers relay, well, thats how this whole mess started.. The thermo controller has a seperate wire to by-pass the thermostat, and just close the relay and make the fan run manually.. Unfortunatley, that wire was blowing fuses at an alarming rate.. it seemed to work good for about 2 weeks, but then it would blow a fuse every 3-4 trips.. THEN, it burned up the switch.. my good freind the idiot above, assured me that a diode on the relay would NOT prevent the switch from burning up, so I don't know what it was.. That being the case, I endeavor to supply my own relay, and have it cut off the thermo controller, and turn on the fan manually from a switch, so as not to blow the fuse going to the thermo controller, and hopefully, not burn up the switch. Hence the drawing above... Hopes this makes everything clear, I think this mess is just about cleaned up, and I'll post up the results soon.. just gotta get a new switch, and fire up the soldering iron.

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

Sounds like either there's an intermittent short on that wire for the thermostat bypass or it's not actually controlling the relay, but instead a direct power output to the fan. If it were just controlling a relay inside, there's no reason it'd be stressing out your switch and burning it up.
If you have a meter, what kind of resistance do you measure on that line to ground with everything hooked up? It should be over 100 ohms if it's a relay control.

OK, just to let everyone know, IT WORKS!! Thanks so much to my new friend "the idiot" for his help on this.. I still don't know what the problem was for the original controller by-pass, but it's not really important now.. I've got some cleaning up to do, and put some wires back into some loom, and secure a few zip ties, but that's mostly cosmetic.. Thanks again! The Kaptan,

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Sony Mp3 HU, Dig/Aud 300Wts
(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!