Fog light install: 2 relays, 2 triggers

Thread Starter

rfranzel

Joined Jul 15, 2018
8
Greetings,

I'm new to All About Circuits, and hope someone here can help me. I am a newbie to these things, and am installing fog lights on my Toyota Prius. I would like them to be on when the low beam and parking lights are on, but not the high beams. I've identified two signal wires, one has voltage with the parking lights, low beam and high beam (Trigger 1). The other has voltage with the lights off, DRL, parking lights, and low beam on, but 0 volts when the high beams are on. (I'm actually putting a switch between Trigger 1 and the relay, but for simplicity, I'm leaving that out.)

I am not getting voltage out of Pin 87 on the second relay, and I'm not sure why. I thought I'd start with sharing a block diagram of my wiring and see if any of you can identify my logic error. Thanks for your help!

 

dl324

Joined Mar 30, 2015
9,122
Welcome to AAC!

For those of us who don't work with automotive relays enough to remember pinouts:


Start by measuring on pin 86 of the top relay (and make sure pin 87 of the bottom relay is giving you 12V).
 

Reloadron

Joined Jan 15, 2015
4,584
Looking at your circuit, if the circuit is wired as drawn, it should work. Both relays need to be energized for the fog lamps to come on. Both relays should have 12 VDC on pin 86. Make sure both pin 85 have a good ground path. Then start at the bottom with pin 30 does it have 12 volts? Yes, then pin 87 should have 12 volts, does it? Yes, then pin 30 of the upper relay should have 12 volts, does it? Yes, then pin 87 should have 12 volts to the fog lights, the fog lights should be grounded and work with 12 volts applied to them. Again, make sure all of your grounds are good reliable grounds.

Ron
 

Thread Starter

rfranzel

Joined Jul 15, 2018
8
Good Morning, and thanks for your replies.

I had disconnected everything last night, and wanted to start afresh this morning to ensure all grounds are solid.

I connect pin 86 of the bottom relay to the trigger 1 wire. When I switch the lights on to apply voltage to the pin 86, I hear the relay click. I checked for continuity across pins 30 and 87, and it is good. Without the trigger voltage there is no continuity. I am conclude that the bottom relay is functioning as I want.

I verified that I have 12 volts at the trigger 2 wire. When I connect that to pin 86 of the top relay I hear a relay under the hood click, and the voltage drops to 4 volts. That's not enough, apparently to trip my relay, but more importantly, that's doing something unexpected to the car's electronics that I don't understand.

Any idea why that's happening?

Thanks again.

Robin
 

Reloadron

Joined Jan 15, 2015
4,584
I verified that I have 12 volts at the trigger 2 wire. When I connect that to pin 86 of the top relay I hear a relay under the hood click, and the voltage drops to 4 volts. That's not enough, apparently to trip my relay, but more importantly, that's doing something unexpected to the car's electronics that I don't understand.

Any idea why that's happening?
Apparently your source voltage for that second relay is not supplying enough current. I am not sure where that second relay voltage is sourced from? One option would be to get 12 volts from the high beam itself so when you go to high beam the second relay closes. Then just move your fog lamps from 87 to 87a (just swap the Normally Open #87 to the Normally Closed 87a. Not knowing where that second relay's coil voltage is actually coming from all I can do is guess. Another possibility is that there is a faulty connection in the path for that second relay coil voltage which could cause the voltage drop you see from 12 volts to 4 volts.

Ron
 

dl324

Joined Mar 30, 2015
9,122
I verified that I have 12 volts at the trigger 2 wire. When I connect that to pin 86 of the top relay I hear a relay under the hood click, and the voltage drops to 4 volts. That's not enough, apparently to trip my relay, but more importantly, that's doing something unexpected to the car's electronics that I don't understand.
What are you using to drive the coil for the top relay? Looks like coil resistance is 68 ohms (75 ohms for the coil in parallel with 680 ohms for the surge suppressor).
 

Thread Starter

rfranzel

Joined Jul 15, 2018
8
What are you using to drive the coil for the top relay? Looks like coil resistance is 68 ohms (75 ohms for the coil in parallel with 680 ohms for the surge suppressor).
I may be missing the question, but the trigger wire is from connector F17, pin 1 on my 2018 Prius 4 (non-touring). The documentation refers to this pin as "Headlight Dim Circuit" (just "DIM" on the wiring diagram). Testing shows about 12 volts when the lights are off, DRL, Parking Lights and Low Beam are on; and 0 volts when the high beams are on.

My objective is to have the fog lights go off when the high beams go on.

- Robin
 

dl324

Joined Mar 30, 2015
9,122
I may be missing the question, but the trigger wire is from connector F17, pin 1 on my 2018 Prius 4 (non-touring). The documentation refers to this pin as "Headlight Dim Circuit" (just "DIM" on the wiring diagram). Testing shows about 12 volts when the lights are off, DRL, Parking Lights and Low Beam are on; and 0 volts when the high beams are on.
It seems that you're expecting that signal to drive an additional 175mA load and it's not capable of doing that. As evidenced by the voltage dropping to 4V.
 

Thread Starter

rfranzel

Joined Jul 15, 2018
8
Looks like coil resistance is 68 ohms (75 ohms for the coil in parallel with 680 ohms for the surge suppressor)
It seems that you're expecting that signal to drive an additional 175mA load and it's not capable of doing that. As evidenced by the voltage dropping to 4V.
Dennis, you are probably right that that is exactly what I was expecting. ;) I conclude that I either need to find another high beam signal, or just manually ensure that the fog lights are switched off when I have on the high beams. Or, are there other automotive relays that use a higher resistance coil to activate the internal switch?

Seriously, I am very interested in how you knew the coil resistance is 68/75 ohms. (I assume the relay has a built in surge suppressor.) I understand the 175mA is 12v/68ohm.

I am not sure how to interpret the specs that came with the relays, and apply it to the draws you indicated. For example:
Initial contact resistance (at 6VDC 1A): 50m ohm Max
Initial insulation resistance: 100 M ohm Min. (DC500V)​
 

dl324

Joined Mar 30, 2015
9,122
Dennis, you are probably right that that is exactly what I was expecting. ;) I conclude that I either need to find another high beam signal, or just manually ensure that the fog lights are switched off when I have on the high beams. Or, are there other automotive relays that use a higher resistance coil to activate the internal switch?
My assumption is that automotive relays with the footprint you're using have similar specs so they're interchangeable. That prevents problems caused by plugging in a plug compatible relay that has different drive requirements.

You can decrease the load on the signal by using a Sziklai pair if the relay coil will operate at around 11V.
https://en.wikipedia.org/wiki/Sziklai_pair

You might be able to get away with just buffering the signal with an NPN emitter follower, but the load on the control signal will be higher.

BTW, you're lucky you didn't damage something by putting an extra 175mA load on that signal.
Seriously, I am very interested in how you knew the coil resistance is 68/75 ohms. (I assume the relay has a built in surge suppressor.) I understand the 175mA is 12v/68ohm.
I just Googled for automotive relay specs and used the first PDF I saw.

This is from a different datasheet that shows a 76 ohm coil resistance.
upload_2018-7-16_9-13-32.png
I am not sure how to interpret the specs that came with the relays, and apply it to the draws you indicated. For example:
Initial contact resistance (at 6VDC 1A): 50m ohm Max
Initial insulation resistance: 100 M ohm Min. (DC500V)​
The contact information doesn't have anything to do with the coil spec. Since you're using this in a vehicle, I'd look for specs at 12V.
 

Reloadron

Joined Jan 15, 2015
4,584
Seriously, I am very interested in how you knew the coil resistance is 68/75 ohms. (I assume the relay has a built in surge suppressor.) I understand the 175mA is 12v/68ohm.
General use automotive relays, depending on manufacturer, all have about the same coil; resistance give or take a little. They also have the same pinout as dl324 posted in post #2. I have one laying here and the coil resistance is about 80 Ohms so at 12 VDC the coil current works out to be 12 volts / 80 ohms = 136.36 mA or we can say right around 136 mA. In reality as the coil voltage increases to around 13.6 volts that current will also increase. These are just generic automotive relays and designed with commonality making it easier to swap a relay out.

The better relays will have a manufacturer's data sheet. Here is one such example.

I will only add that in an automotive system that while we say 12 volts in reality the running voltage is normally 13.6 volts give or take.

<EDIT> I see while I was typing at my usual snail's pace dl324 answered your questions. :) </EDIT>

Ron
 

Thread Starter

rfranzel

Joined Jul 15, 2018
8
Dennis/Ron,

Thanks so much for guidance and support. You've both been a great help, and I can see an end in sight.... at least for the fog lights. We'll see what problems I run into connecting up the running lights and turn signals! I'll start a new thread if I do run into problems. This car is only about 6 weeks old, so I'm questioning my sanity taking this on!

I am breathing a sigh of relief that I did not do any damage (at least that I know of yet) putting the relay on that signal wire.

Best Regards,

Robin
 

dl324

Joined Mar 30, 2015
9,122
This car is only about 6 weeks old, so I'm questioning my sanity taking this on!
IMO, better cars include fog lights as a standard option and functionality that turns fog lights off when high beams are on.

The oldest vehicle I have that didn't come with fog lights is a 1990 F150. My 1990 BMW 5 series has them; so did my 1980 3 series...
 

Thread Starter

rfranzel

Joined Jul 15, 2018
8
IMO, better cars include fog lights as a standard option and functionality that turns fog lights off when high beams are on.
Not sure that "better cars" describes it. Just trim levels. My 2000 VW Passat had them, as did my 2003 Toyota Camry; and they both worked per DOT requirements. Toyota makes a Prius Four [and Three] Touring. The Touring trim comes with larger wheels and fog lights, but no moon roof. I didn't care about the wheels, wanted the moon roof, and figured (rightfully, I hope) that I could add the combination fog lights/driving lights/turn signals.
 

dl324

Joined Mar 30, 2015
9,122
Not sure that "better cars" describes it. Just trim levels.
If that's the case, the wiring for fog lights and control should already be installed.

I bought a used Explorer that didn't have a garage door opener. The wiring (power) for the opener was already installed, just not used, so all I had to do was buy a used console and swap them.
 

jaredwolff

Joined Jul 1, 2017
58
Did something similar to what you have in your schematic @rfranzel for my Jeep LED install. Though, I did run a separate fused power source from the battery to avoid overloading any of the control signals in the wire loom. (It's a Jeep, highly doubt it but you never know)

If you're running new wiring, I do recommend some multi-conductor wire with a nice jacket over it. (Something like this) That way it will blend in with your already existing wiring and will provide some extra protection from the elements and wear and tear over time. Also, definitely adjust the wire gauge for the current you expect to be drawn by the lights.

Good luck and be careful. ;)
 

Reloadron

Joined Jan 15, 2015
4,584
IMO, better cars include fog lights as a standard option and functionality that turns fog lights off when high beams are on.

The oldest vehicle I have that didn't come with fog lights is a 1990 F150. My 1990 BMW 5 series has them; so did my 1980 3 series...
Interesting is both our trucks have fog lights. What's interesting is I have never used them. Driving for decades less any fog lights I just never remember they are there if I want them. :)

Ron
 

dl324

Joined Mar 30, 2015
9,122
Interesting is both our trucks have fog lights. What's interesting is I have never used them. Driving for decades less any fog lights I just never remember they are there if I want them.
They make a big difference if you're driving at night and need extra light to see things on the side of the road (like deer or elk). But headlights are getting better. There's a huge difference between the headlights on a 2013 Explorer and a 2003.

I use them on our private road when it's dark. Speed limit is 15MPH, but it helps to see the deer before they can dart in front of you.
 

Reloadron

Joined Jan 15, 2015
4,584
I use them on our private road when it's dark. Speed limit is 15MPH, but it helps to see the deer before they can dart in front of you.
The deer are my worst nightmare on country roads at night when I am on the bike. My side spots are my best friends. :) I want all the illumination I can get.

Ron
 
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