relay advice for newby

I have basic automotive electrical background, but not in my 2014 Dodge Journey. I am installing a trailer brake controller that needs a brake signal feed from (normally) the brake switch. I cannot do this due to the Can bus.

I have located a wire from the BCM that is normally .20 volts and goes to 13.38 when the brake pedal is depressed. I want to use a relay to activate the signal to my controller. In my reading here I have learned that because of the Can bus it is desirable to have a diode protected relay that in my simple terms prevents a voltage spike back into the Can bus. Correct?

I have found this: https://www.delcity.net/store/Relays-with-Diode/p_73578 during a brief search. Although it does not say exactly, is this the part I can use to accomplish what I described above? Also, is it true that the low .20 volts will not energize the relay coil unexpectedly?

Thanks in advance for reading and any advice.

Dave

I prefer adding my own diode to 'normal' unprotected relays, preferably using 1N4007 else 1N4004 diodes.

But make sure your 13V signal can handle the relay coil (eg, up to 250mA).

There might be alternatives like the brake bulb (if not combined with flasher) or the brake switch itself.

.20 volts will not energize a relay. I too recommend catching it at a bulb.

Bulb and use a 1N4004 diode, band towards bulb.
BTW it takes around 7.5 V+ to trigger the average automobile relay.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I read da180 as saying he has >12V to trigger the relay. IE - the relay is off when the BCM is only 0.2V.

He did ask " Also, is it true that the low .20 volts will not energize the relay coil unexpectedly"?
So I answered.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Aha - you're talking about later detail...
I missed that!
My apologies.

But if it did (hold to pull in), a series diode would fix that.

WOW, thank you all for the quick responses.

Re: connect at the bulb, I have two 1157 bulbs in my popup camper tail lights. Since the lights illuminate when the turn signals are on I would imagine there is no way to connect at the bulb for a brake signal. BTW, I wired the car for this before I ran into this problem, ha ha.

Based on your responses, since the relay coil won't energize with only .2v then I would be able to use a single fused 12v source to the relay (#30) and #87 to my brake controller. The wire I found with the 13.3v to #85 on the relay and #86 to ground?

Again, I can use the relay I linked to originally? It is an 85 NEGATIVE as opposed to a 85 POSITIVE. And...does 85 negative mean the diode is preventing a current spike from returning to the Can bus and instead, it is dissipating it when the coil de-energizes?

I quote the description from the website: "This relay has an 85 negative polarity and allows a low current flow circuit to control a high current flow circuit. They are remote control electronic switches that are normally controlled by another switch, computer or control module. The internal diode within this relay protects sensitive equipment against voltage spikes."

Thank you all for your patience.

A relay coil is a non-polar coil. However because some relays have inbuilt spike quenching diodes, the convention is that 86 is +ve, 85 is -ve.


You can combine flashers with other functions like stop or reverse, but it requires either repositioning of the flasher can (the flasher switch controls relays that connect the can etc) else time delay circuits - eg, exledusa's TPC v2 or v3.

So...following convention, I attach the 13.3v to #86 and ground #85.

I then would purchase the "85 negative" relay with the diode because the 85 terminal goes to ground.

Because I already had to get a tail light converter for my four wire, I really cannot complicate this more with the exledusa setup.

85 NEG is the proper convention, you'll still need another diode to prevent feedback, the relay built in diode is a quenching relay to prevent the 200 volt spike flashing back to your circuitry when the relay is switched off.
Light bulb via 1N4004, band away from bulb to 86.
Ground to 85, power fused 10amps to 87,
output to extra bulbs from 30.
For indicators you will need extra circuitry and relays (2).

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

There are no -ve or +ve relays. (Excluding solid state etc...)

I suggest you buy all relays as SPDT (aka 5 pin) assuming they are the same price as SPST (4 pin), and add your own 1N4004 or 1N4007 diodes if required.

I think you guys are trying to confuse me. My objective here is to supply one 12v brake signal to my Tekonsha brake controller which triggers my blue wire and subsequently, the electric trailer brakes.

It is only in the last 18 hours that I ever even saw a picture of a diode, much less soldered one. I cannot attach diodes to bulbs and screw up a very expensive tail light converter/wiring setup I had to buy to overcome the Can bus and my LED tail lights and still have trailer lights.

I understand how relays function and I know now what a diode does. I need to know if the 85 negative relay I referenced in the OP will suffice in providing adequate protection from the anticipated current spike.

Yes but VOLTAGE spike not current that's what the built in diode is for.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

In that case buy the relays with inbuilt spike quenching diodes. Just make sure 86 is always the more +ve compared to 85.

Whether you ground 85 or supply +12V to 86 does not matter.   


But again, ensure that ~13V BCM output can supply the extra current for the relay coil. (If using the BCM output and not brake lights etc.)

No stick to ISO conventions, 30 = power, 31 = ground, 56 = lighting etc. etc. When you're doing dozens per week like me, keep everything the same.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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Yes but VOLTAGE spike ...

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Correct, I meant voltage spike.

In my original post I identified a place that sells them online. Does my local Radio Shack sell something like this or do I have to do the online?

Yes but without the diodes.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Thank you all for your help and for generously sharing your vast knowledge. You cleared up many issues for me and I wish I could contribute more to the forum.

I will be going forward with this and I'll try to remember to advise of the outcome. Maybe we'll talk again when I put in my backup camera and monitor.