Car Electrics: If there is a PD of +12V, shouldn't it trigger a relay

[tamrat]

Hello Guys,

I have this one questions thats been bugging me since earlier. So I was modifying my car to add DRLs that are integrated into the turn indicators. I found two wires responsible for the turn indicators at the headlight combination switch. They carried 12V at all times (when probed across body ground). But when I turn on an indicator, the respective wire's voltage drops to 0V. So I assumed that I can use a relay to keep a DRL circuit open as long as there is 12V in those wires. But when I tapped into that wire and added a relay, instead of triggering the relay, I'm just turning on the indicators. What now? How?

 

[Blue Duck]

Does the relay have a ground? 

 

[Andy™]

car electrics are weird

 

[tamrat]

Does the relay have a ground? 

Yes. I did feed it body ground (which is negative as far as I know) and the +12V from the indicator switch. In theory (or in practice really), since there is a a +12V potential difference across the terminals, current should flow through the relay solenoid and open/close the contacts.

car electrics are weird

Very.

 

[Blue Duck]

https://www.12voltplanet.co.uk/relay-guide.html

Why not wire them to the side lights?

Or 

 

[tamrat]

Yes. I did this. I'm definitely sure I didn't mess up the wiring since I also tested this with the hazard switch. The hazard switch has 4 cables. All pins are at 0V except for one which is always +12V (with a multimeter: positive probe on the pins, negative probe on body ground). This +12V line also drops to 0V when the hazard switch is activated. Tried to get power from this cable, connected it to 86. Grounded 85. Nothing. Relay doesn't click. But the hazards come on. Its like the circuit was deprived of -12V (negative: body ground). And rather than flowing from 86 to 85, its like feeding -ve power from 85 to 86.

I have attached the diagram of the circuit I built below.



Additionally here is the official EWD from Toyota.

 

[tamrat]

Why not wire them to the side lights?

I could have done that. Its just my parking lights are very very dim. Very small LEDs barely noticeable except at night. Plus I personally like the amber glow and double-purpose nature of turn indicators being used as DRLs (as seen on Toyota Tacomas).

 

[Evans Electric]

Car electrics are weird. 

 

[Andy™]

thats what i said

 

[tamrat]

Car electrics are weird. 

I guess I should just drop this and move on to other ways then.

Thats for another thread though. *the one I'm gonna create now*

 

[Evans Electric]

I know  ,  I have the same opinion .  

Take  one headlight unit on my van as an example .   Its fed via a multi-plug with 8 wires  all connected .   :C

There are  4 lamps  , Head  / Dipped / parking /  R/H Flasher.     So to a Sparks  with his 240 volt   X BS 7671 hat on   he'd go with  4 switch wires  and all the lamps returning via the bodywork of the vehicle . 

 

[Sharpend]

May I suggest you try a Toyota forum as it will mainly be petrolheads, I’m not sure that it’s legal to have indicators as DRL’s ??

 

[Andy™]

dont think they have many laws / rules like that where he's from

 

[tamrat]

May I suggest you try a Toyota forum as it will mainly be petrolheads, I’m not sure that it’s legal to have indicators as DRL’s ??

I'm talking about something like this. This is a factory setup from Toyota. The amber lights are both DRLs and also indicator bulbs. They still flash when indicators are being used, so I'm sure they are legal (not that it would matter here since DRLs aren't mandatory, I'm doing this mainly for aesthetics). And yes, I have also asked this question on other car forums. Not much info, that why I tried to come to electrics focused forums.

 

[Blue Duck]

I assume that would require an optionional extra in the form of a wiring harness / loom? 

They are like that in the US.

over here and possibly  Europe there is a requirement to switch off Tomatoe'd lanterns Sorry....DRL's when indicating which looks dumb.

 

[Sharpend]

Well if that is standard in your land then why ain’t yours got it? 

In most vehicles the wiring harness is complete for all optional extras and there are usually extra/spare plug points for all these extras, I guess you’ll just have to find which one you need. 

 

[Doc Hudson]

The only observation I would make is don't assume all car electrical controls are putting a +12v onto a load to energise it to work. There may be occasions where something is controlled in 'reverse' i.e. putting a 0V (-ve earth) potential onto a circuit with +12v already sitting on one side. I know our car washer bottle motor uses a reverse polarity to operate either the front washer jet or rear washer jet.

Doc H.

 

[tamrat]

Well if that is standard in your land then why ain’t yours got it? 

In most vehicles the wiring harness is complete for all optional extras and there are usually extra/spare plug points for all these extras, I guess you’ll just have to find which one you need. 

DRLs are not required in my country. This is purely for aesthetics. The Yaris has an optional DRL for US and Canada models I think. Going that route will be expensive since I'll have to get a lot of OEM modules/relays replaced.

The only observation I would make is don't assume all car electrical controls are putting a +12v onto a load to energise it to work. There may be occasions where something is controlled in 'reverse' i.e. putting a 0V (-ve earth) potential onto a circuit with +12v already sitting on one side. I know our car washer bottle motor uses a reverse polarity to operate either the front washer jet or rear washer jet.

Doc H.

 This is the only explanation that makes a bit of sense. So does this mean relays have polarity? Might is work if I reverse the inputs and try? There was a +12V difference in the wires and the body ground. I'm just curious as to why this potential difference isn't triggering relay.

 

[Sharpend]

I suspect the problem you are having is down to the way modern vehicles work. Back in the day you could just pick up a feed and splice into it with no problems, todays cars are far more electronic and you have to have a lot of things programmed to the ECU so that the car now knows it got it. 

I changed the throttle body on wife’s car, despite it being a genuine part I still had to take it to the dealership to marry it to the car. 

On my van I tried to tap into the rear interior light for an additional led light but it wouldn’t have it. 

I think you’d be better speaking to local Toyota dealer technician to see what’s required. 

 

[Doc Hudson]

DRLs are not required in my country. This is purely for aesthetics. The Yaris has an optional DRL for US and Canada models I think. Going that route will be expensive since I'll have to get a lot of OEM modules/relays replaced.

 This is the only explanation that makes a bit of sense. So does this mean relays have polarity? Might is work if I reverse the inputs and try? There was a +12V difference in the wires and the body ground. I'm just curious as to why this potential difference isn't triggering relay.

Relay coils are not polarity conscious, but it is easy to include a diode in series to make it only operate with a specific polarity if you wanted to.

Looking at your earlier drawing L1-42  with the boxes titled 'Turn signal switch' (inside the Main dimmer switch) which then have wires going to the 'Flasher Relay box' (in the main body ECU).   I read that drawing as saying the indicator switch is actually switching an 0v signal wire back into the flasher relay box.

Earth coming into the switch at E then going out via TR->ER  or TL->EL to put an 0v signal into the flasher unit for left or right.  The 12v supply is sitting permanently at the flasher unit at the IG terminal.  Which corresponds to your initial comment that the wires you found are at 12v with respect to the body (this is coming via the flasher unit). Then when you put the switch either Left or Right the wires drop to 0v. (this completes the circuit for the flasher unit to start flashing as it already has +12v at its IG terminal)

Basically your 12v reading is across the switch not the load, which is why it disappears when the switch is operated. I think the logic for this is something to do with needing to be able to put hazards on when the ignition is off or on (via the +B terminal), but normal indicators only when the ignition is on.   I think your relay coil is just bypassing the indicator switch to activate the flasher relay.

Doc H.