LOST JEEPS :: View topic - Using Backup Lights to trip relay for rear facing work light

I have built a circuit to use the backup light feed to trip an auto relay to light up my rear facing rack mounted work light. I tapped into the White/Grey backup light wire just below the plastic sill at the bottom of the tailgate. Problem is the voltage on the backup light wire will not trip the 12V relay. When I measured it I found 8.9V. Going back through the Service Manual I found that the power for the backup lights comes directly out of the 545RFE Transmission Range Sensor components buried inside the transmission (on top of the valve body). So I'm stuck I guess trying to find a relay that will trigger at 8 Volts. The power for the work light comes from a dedicated fused feed and is certainly 12V. Anyone else run into this problem or have a suggestion?

Thanks for that thread, unfortunately the wiring loom and connector pinout for the 2007KJ CRD is very different. The reverse wire power comes from the Transmission Range Sensor (inbedded inside the transmission). The active wire is White/Grey. I am just going to give up on reverse activation and put in a driver controlled switch to activate the relay (from a known 12V source). The other switch will still be in the rear cargo area. Thanks for your help with this.

This guy used a diode in the circuit, I'm not sure it is necessary, but maybe thats your issue?

kapalczynski wrote:

This guy used a diode in the circuit, I'm not sure it is necessary, but maybe thats your issue?

A diode is needed to quench the back emf kick from the relay coil.

Kapalczynski
(You probably know this but) a relay has a contact side (switch) and a coil side (electromagnet). You put current through (voltage to) the coil side and it magnetically picks up the switch.

The coil (being an inductor, and more) gets charged up. The problem is when the coil releases, it releases the energy back into your controlling circuit and worst still it is a negative voltage. The diode shorts the back emf (electromotive force) before it gets kicked back into your truck.

This back kick can be quite a punch too. Gassers use this phenomenon to produce a spark (think greater than 10K volts). The little relay may kick back 20 volts or more.

So for us electro-idiots are you saying the diode has to bridge relay pins 30-87 or 85-86 and which way does the arrow point?
:banghead:

Squeeto wrote:

k, so i need a diode or its bad for the electrical system it sounds like...but why then do starter relays not have this??? They have been around since time itself...lol and if you've ever seen an old Ford starter relay there's no diodes, etc. Is this because there is no amperage hardly at all and low voltage or what?

I think I get what you're saying, but is it really a concern or not???

Still :dizzy:

- Mark

Makes sense... Thx. Will go to radio shack soon.

kapalczynski wrote:

Makes sense... Thx. Will go to radio shack soon.

Know that I haven't looked at the wiring diagram for our trucks (05,06 or 07). It could still just be run off a switch (doubt it though since something is stealing ~2 volts) but for a $2 part, why take the chance.

Author:	Glend [ Sat May 15, 2010 5:47 am ]
Post subject:	Using Backup Lights to trip relay for rear facing work light
I have built a circuit to use the backup light feed to trip an auto relay to light up my rear facing rack mounted work light. I tapped into the White/Grey backup light wire just below the plastic sill at the bottom of the tailgate. Problem is the voltage on the backup light wire will not trip the 12V relay. When I measured it I found 8.9V. Going back through the Service Manual I found that the power for the backup lights comes directly out of the 545RFE Transmission Range Sensor components buried inside the transmission (on top of the valve body). So I'm stuck I guess trying to find a relay that will trigger at 8 Volts. The power for the work light comes from a dedicated fused feed and is certainly 12V. Anyone else run into this problem or have a suggestion?

Author:	Squeeto [ Sat May 15, 2010 10:33 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Maybe the first question is - should it be 12 volts to the backup lights. The battery for my truck is on the bench under trickle charge and I have no coolant. I am waiting for a part. Maybe someone on the forum could check this for you.

Author:	Squeeto [ Sat May 15, 2010 11:42 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Just trying to flush out a volunteer for you. I am busy this morning but if you get no help, I will check it this afternoon.

Author:	Squeeto [ Sat May 15, 2010 4:26 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Mine, with one bulb out, sits at 10.8 volts. I was surprised to see this actually. Either our Jeeps have a really poor drive circuit (electronic) for this or a small resistance is added in to measure current for bulb out conditions. I am not going to cut into the harness as you did, so measure the voltage with no relay and no bulbs, then add one bulb and measure, two and measure and finally add in the relay. If it is something like 12.4v - 10.2v - 9v - 8.9v you are probably ok to install a lower voltage relay.

Author:	kapalczynski [ Sun May 16, 2010 3:42 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Glend wrote: I have built a circuit to use the backup light feed to trip an auto relay to light up my rear facing rack mounted work light. I tapped into the White/Grey backup light wire just below the plastic sill at the bottom of the tailgate. Problem is the voltage on the backup light wire will not trip the 12V relay. When I measured it I found 8.9V. Going back through the Service Manual I found that the power for the backup lights comes directly out of the 545RFE Transmission Range Sensor components buried inside the transmission (on top of the valve body). So I'm stuck I guess trying to find a relay that will trigger at 8 Volts. The power for the work light comes from a dedicated fused feed and is certainly 12V. Anyone else run into this problem or have a suggestion? Interesting. My back up light mod uses a relay that turns on 2 rear facing fog lights. I hid the relay behind the drivers rear foglight. I tapped into the reverse light circuit to power relay. Worked for me...I will have to measure voltage for you and see what I run...also grab a part number for the relay if it has one. I have been using this setup for about a year now and has worked every time I put it into reverse. Here is thread where others have done this mod. This guy used a diode in the circuit, I'm not sure it is necessary, but maybe thats your issue? Hope this helps: http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=59&t=39402

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Author:	Glend [ Sun May 16, 2010 3:57 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Thanks for that thread, unfortunately the wiring loom and connector pinout for the 2007KJ CRD is very different. The reverse wire power comes from the Transmission Range Sensor (inbedded inside the transmission). The active wire is White/Grey. I am just going to give up on reverse activation and put in a driver controlled switch to activate the relay (from a known 12V source). The other switch will still be in the rear cargo area. Thanks for your help with this.

Author:	kapalczynski [ Sun May 16, 2010 4:12 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Glend wrote: Thanks for that thread, unfortunately the wiring loom and connector pinout for the 2007KJ CRD is very different. The reverse wire power comes from the Transmission Range Sensor (inbedded inside the transmission). The active wire is White/Grey. I am just going to give up on reverse activation and put in a driver controlled switch to activate the relay (from a known 12V source). The other switch will still be in the rear cargo area. Thanks for your help with this. Ahh, that makes sense now. I didn't read you were driving an 07...my fault. I think I am just used to not having any 07 and up CRD's here in the US. Lucky Aussie! (did I spell that right?) I'll take a CRD Rubicon wrangler if you can get one for me...lol Good luck on teh light project! - Mark

Author:	Glend [ Sun May 16, 2010 7:22 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Mark, The Rubi Wrangler CRD is not available here. What we can get is petrol Rubi's or CRDs equipped with the Off Road Group (which is one rear locker only - I think). Most are buying the CRD JK with the Off Road Group, and adding the front locker later on if at all).

Author:	Squeeto [ Sun May 16, 2010 10:48 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
kapalczynski wrote: This guy used a diode in the circuit, I'm not sure it is necessary, but maybe thats your issue? A diode is needed to quench the back emf kick from the relay coil.

Author:	Glend [ Sun May 16, 2010 9:11 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Now as to the Work Light dramas, I think I have it sorted now. My research told me that the Bosch relays have a lower 'pull in voltage', as low as 8 volts. So I picked up a Bosch relay. They are more expensive but the Narva's just won't work. Anyway I connected ithe Bosch up and the relay tripped and light works! Very happy that is resolved because I was thinking of just going back to a second switch on the dash if it had not worked. I still don't have a diode in the circuit but I will pick one up later today. Thanks for your help with that problem.

Author:	kapalczynski [ Sun May 16, 2010 11:19 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Squeeto wrote: kapalczynski wrote: This guy used a diode in the circuit, I'm not sure it is necessary, but maybe thats your issue? A diode is needed to quench the back emf kick from the relay coil. Ok, I don't speak that language...or rather I am not fluent in it. lol Heres what I know, a diode only alows electricity to flow one direction. Thats it...I dont know anything else...LOL I currently do not have a diode in my setup, whay is it necessary? What is EMF (Electromagnetic Field?)? What happens if I don;t run one, does it fry something??? Should I be concerned? I'm great at mechanical and understand most electrical, but EMF threw me.

Author:	Squeeto [ Sun May 16, 2010 11:35 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Kapalczynski (You probably know this but) a relay has a contact side (switch) and a coil side (electromagnet). You put current through (voltage to) the coil side and it magnetically picks up the switch. The coil (being an inductor, and more) gets charged up. The problem is when the coil releases, it releases the energy back into your controlling circuit and worst still it is a negative voltage. The diode shorts the back emf (electromotive force) before it gets kicked back into your truck. This back kick can be quite a punch too. Gassers use this phenomenon to produce a spark (think greater than 10K volts). The little relay may kick back 20 volts or more.

Author:	Glend [ Sun May 16, 2010 11:40 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
So for us electro-idiots are you saying the diode has to bridge relay pins 30-87 or 85-86 and which way does the arrow point?

Author:	kapalczynski [ Sun May 16, 2010 11:44 pm ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Squeeto wrote: Kapalczynski (You probably know this but) a relay has a contact side (switch) and a coil side (electromagnet). You put current through (voltage to) the coil side and it magnetically picks up the switch. The coil (being an inductor, and more) gets charged up. The problem is when the coil releases, it releases the energy back into your controlling circuit and worst still it is a negative voltage. The diode shorts the back emf (electromotive force) before it gets kicked back into your truck. This back kick can be quite a punch too. Gassers use this phenomenon to produce a spark (think greater than 10K volts). The little relay may kick back 20 volts or more. k, so i need a diode or its bad for the electrical system it sounds like...but why then do starter relays not have this??? They have been around since time itself...lol and if you've ever seen an old Ford starter relay there's no diodes, etc. Is this because there is no amperage hardly at all and low voltage or what? I think I get what you're saying, but is it really a concern or not??? Still - Mark

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Author:	Squeeto [ Mon May 17, 2010 12:19 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Glend wrote: So for us electro-idiots are you saying the diode has to bridge relay pins 30-87 or 85-86 and which way does the arrow point? I am not sure about pin-out numbers since no relay is the same but you should be able to get this info off the Bosch site, box it came in or on the relay itself. Usually relays show a symbol of the switches on one side and a squiggle or a circle on the other. The diode connects directly (in parallel) to the coil side. There is a 'band' painted near one leg of the diode. This leg goes to your positive voltage signal. If you get the diode backwards, you will make the 'magic' smoke.

Author:	Squeeto [ Mon May 17, 2010 12:27 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
kapalczynski wrote: Squeeto wrote: Kapalczynski (You probably know this but) a relay has a contact side (switch) and a coil side (electromagnet). You put current through (voltage to) the coil side and it magnetically picks up the switch. The coil (being an inductor, and more) gets charged up. The problem is when the coil releases, it releases the energy back into your controlling circuit and worst still it is a negative voltage. The diode shorts the back emf (electromotive force) before it gets kicked back into your truck. This back kick can be quite a punch too. Gassers use this phenomenon to produce a spark (think greater than 10K volts). The little relay may kick back 20 volts or more. k, so i need a diode or its bad for the electrical system it sounds like...but why then do starter relays not have this??? They have been around since time itself...lol and if you've ever seen an old Ford starter relay there's no diodes, etc. Is this because there is no amperage hardly at all and low voltage or what? I think I get what you're saying, but is it really a concern or not??? Still - Mark Not really needed. The kick back goes back to the key switch. Electronic devices are far more delicate. Integrated circuit installers work off a static mat and wear grounding wrist straps to not damage components from simple static electricity you might cause from scratching your head (well not really but wearing wool might do it).

Author:	kapalczynski [ Mon May 17, 2010 12:33 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Makes sense... Thx. Will go to radio shack soon. Hey, check ur mail!!! Sent you a link to what I was working on on my charger... - Mark

Author:	Squeeto [ Mon May 17, 2010 12:53 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
kapalczynski wrote: Makes sense... Thx. Will go to radio shack soon. Know that I haven't looked at the wiring diagram for our trucks (05,06 or 07). It could still just be run off a switch (doubt it though since something is stealing ~2 volts) but for a $2 part, why take the chance.

Author:	kapalczynski [ Mon May 17, 2010 1:02 am ]
Post subject:	Re: Using Backup Lights to trip relay for rear facing work l
Squeeto wrote: kapalczynski wrote: Makes sense... Thx. Will go to radio shack soon. Know that I haven't looked at the wiring diagram for our trucks (05,06 or 07). It could still just be run off a switch (doubt it though since something is stealing ~2 volts) but for a $2 part, why take the chance. Yeah, for sure.