Register now to get rid of these ads!

Relay-ing your brakes lights

Discussion in 'The Hokey Ass Message Board' started by cosmo, May 2, 2013.

  1. OK, I've harped on this enough, here's the skinny:

    You'll need one Bosch-type relay and socket, easily sourced at your local spare parts emporium (junkyard, too). I used a relay that had a built-in mount, (not strictly necessary). You'll need some wire, try to match the color codes for best effect.

    Fire up your soldering iron for best results, though a good crimper will work also.

    Let us first look at the relay a minute. Head on looking at the terminal end, you will see four or five terminals. Three will be in one direction, one located 90º to the edge. Hold the relay with this terminal at the bottom. Look closely at the bottom for the number 30. This being at the bottom terminal means you have a Bosch type relay. Up and to the left, should be terminal 86. Opposite this is 85, and at the top, 90º to all others, 87 (there may be a terminal in the middle labled 87a or 87b. Ignore it). Follow these numbers. Note, not all square relays are wired like this, Ford and BMW (at least) use relays that look the same but are not wired the same, and will not interchange. However, the numbers mean the same for all.

    Now it gets a lot simpler.

    If your car is like mine (1961 Ford Falcon), the brake switch is hydraulic and mounted near (or on) the master cylinder. Pull the wires off the switch. Now, cut both wires close to the switch, leaving about 1" of wire after the connectors. From the harness, determine which wire is hot with a test light. This wire is connected to the relay #30 terminal AND to the #86 terminal. The other wire (which goes to the brake lights, usually through the turn signal switch) gets connected to the relay #87 terminal. From the brake light switch, one lead gets grounded. For life :) . The other lead goes to the relay number 85 terminal.

    You are done with the wiring. Probably take you about 5 minutes or so. Mount the relay, plug in the connectors, and do a bulb check. The relay should click when you press the brake pedal, and, of course, the brake lights should go 'on'.

    The photos illustrate the sub-harness that goes from the car's harness (in my case) to the switch. You will be making something similar. The other photo illustrates the install. Note the ground lead is connected to the relay mounting screw, for simplicity. Ignore the rusty master, that's the subject of another tech article.


    Attached Files:

  2. A Rodder
    Joined: Jul 13, 2008
    Posts: 2,477

    A Rodder


    I am uncertain I have ran across a situation where the brakelights overload a stock stoplight switch.

    Is there a benefit I am missing?

    Great detail on how to do it though.
  3. stealthcruiser
    Joined: Dec 24, 2002
    Posts: 3,729


    I like it Cosmo!

    back in '05-'06, someone posted a link on here, of a PDF file, of every relay, ( automotive), known to Man, and how to wire them for the different applications, desired effects, etc.

    I printed off a copy, and have it at home .......somewhere.

    It may still be searchable, were I to be "search savvy".......................
  4. mj40's
    Joined: Dec 11, 2008
    Posts: 3,269


    I'm glad you are concerned of the rust corrosion on the master cylinder and brake lines.
    Register now to get rid of these ads!

  5. Roger Walling
    Joined: Sep 26, 2010
    Posts: 1,142

    Roger Walling

    Again, WHY?

    I was going to install relays on my 24 volt motorhome so as to operate 12V lights on a trailer that I use on different cars, But why go through all the trouble on a car?
  6. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,096


    I have had a circuit breaker kick out when pulling a trailer that had two pair of brake lights (two big lights on each side). It may have been the suspect wiring on it as much as the lights though.

    This one spooks me a bit as I've done a lot of wiring repair on brake/turn light circuits that had shorted out wires in the past that had fried wires because they had too big of a fuse in the fuse box and this would create a situation where there may be no fuse in the line.
  7. Why?? Because hydraulic switches fail due to the arcing that is a component of make-and-break contacts. It takes awhile, true, but it happens.

    Recently (earlier this week) Hyundai issued a recall of brake light switches for, guess what?? Arcing and failing. I took a bunch apart, out of curiosity: all of them were arced and the contacts had failed. A relay would have prevented this. And on THAT note, Nissan have a 'kit' to 'fix' a problem with their own brake light switches in certain Altimas. What is in the kit, you ask?? A couple of relays and the wiring necessary to put them in circuit with the brake switch.

    So, to further the "why" - because it is a damn good idea, that's why!!

    Oh, yeah, its downright easy, and should take most folk under 30 minutes to do.

    Last edited: May 3, 2013
  8. Look again, this simply puts a relay into the line that is already there, and already fused. All it does is ease the load on the switch.

  9. bct
    Joined: Apr 4, 2005
    Posts: 3,068


    nice work there a bosch style 6 volt relay?

    i replace my Echlin hydraulic switch every couple years as the contacts usually arc closed.
  10. oj
    Joined: Jul 27, 2008
    Posts: 6,200


    You are doing this because hyundai had a problem? don't forget the turn signals, they are part of the brake light circuit.
  11. tommy
    Joined: Mar 3, 2001
    Posts: 14,758

    Member Emeritus

    Just something else that can and therefore will fail leaving you with no brake lights and you will probably never know. IMHO
  12. atch
    Joined: Sep 3, 2002
    Posts: 4,297


    so true. i carry a spare hydraulic brake light switch in clarence's glove compartment. i have no idea how many i've replaced. these things fail at the most inopportune times. looks like i need to consider a relay addition...
  13. mustang6147
    Joined: Feb 26, 2010
    Posts: 1,847

    from Kent, Ohio

    I didn't think this was legal? Here's why.... Remember back in the early 70's crooks switched the brake lights to get people to rear end them. Thus going to the hospital with whiplash and collecting $$ I thought a law was passed regarding Brake light switching at that point. Meaning Hard wired only....

    I am sure every state is different. Some are no fault states ect....

    I am a big fan of relays, I use them when I build my drag cars... But thought they weren't legal for the street....
  14. VoodooTwin
    Joined: Jul 13, 2011
    Posts: 3,455

    from Noo Yawk

    I don't like it. It adds complication to what should be a very simple wiring design with few points of failure. If you're concerned about the pressure switch life, you can change to a mechanical type switch, which is a better way to go anyway.
  15. pinkynoegg
    Joined: Dec 11, 2011
    Posts: 1,135


    I never have any luck with relays. If I put one of these on my car the relay would fail much sooner than the brake switch would. as stated above it just complicates things. its a good thought though
  16. BootleggerMatt
    Joined: Aug 17, 2011
    Posts: 258


    I don't personally see a benefit with doing this. Even if the switch failed there is a fuse in the system. I don't know what happened to hyundia but you can't compare the nightmare cheapness of todays cars with the wiring in a correctly put together hotrod. Good info on a ground switching relay though.
  17. I could see this being really beneficial to the guys using hydraulic pressure switches.
  18. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,808


    Just jumping on the pile, but this makes little to no sense to me. Guys seem to be going relay crazy putting them in places where there is little to no benefit and some substantial downside risks. The OEM systems worked fine for decades without relays.

    Oh, and just an OT anecdote: I'm a snowmobiler and those relays have such a high failure rate that many snowmobilers carry spare relays to avoid being stuck out on a cold trail at night with a dead engine or no headlights, or both.

    Arcing occurs inside relays, too. That's why they fail.
    Last edited: May 7, 2013
  19. Mr T body
    Joined: Nov 2, 2005
    Posts: 2,077

    Mr T body
    Alliance Vendor
    from SoCal

    The Hyundai issue is complicated by the inter-relation of other systems on the brake light switch. Functions related to the switch are:
    Turn signals
    Brake interlock
    Cruise control
    Brake lights

    The concern with the switch failures is the switch itself, not excessive current flow requiring a relay.
  20. rfraze
    Joined: May 23, 2012
    Posts: 1,995


    Keep It Simple! Make sure it works and KEEP YOUR FOOT OFF THE BRAKE!
    If it quits working, replace the switch with one of GOOD quality.

    Use relays as........................................REMOTE SWITCHES.
    Last edited: May 7, 2013
  21. mustang6147
    Joined: Feb 26, 2010
    Posts: 1,847

    from Kent, Ohio

    You mean a Hyundai had problems? Really?
  22. nickleone
    Joined: Jun 14, 2007
    Posts: 319


    Yeah, back in the fifties when a new car had a component failure and the driver was killed there were no complaints about faulty products. The driver was dead.
  23. Cantstop
    Joined: Jul 11, 2005
    Posts: 236


    Two 1156 or 1157 bulbs can draw anywhere from 10 to 15 amps. The problem is the quality of the switches in production now, they cant handle the current draw. A good quality relay can handle the load much easier, and takes the load on the switch to less than an amp. Any switch made usually wears out from the electrical load placed on it not from the physical action of turning off and on.
  24. hillbilly4008
    Joined: Feb 13, 2009
    Posts: 2,870

    from Rome NY

    I still don't get it, you explained it, but I still don't understand the need for it. You said hydraulic switches can fail, but this doesn't prevent that from happening. Does it just put less power to the switch to make it last longer?

    Relays scare me.
  25. Yutan Flash
    Joined: Aug 6, 2008
    Posts: 663

    Yutan Flash
    from Gretna, NE

    If you were running 6 taillight bulbs a'la '64 T-bird I could understand it. In fact, Ford had to add a relay in production for '64 and conduct a recall campaign to retrofit relays in cars without it for the taillight circuit as the load from all 1157 bulbs, with a turn signal also turned on, would blow fuses. In '65 they ended up going to a mechanical brake light switch and the sequential system which had (drum roll, please) relays (cymbal crash).
  26. Weasel
    Joined: Dec 30, 2007
    Posts: 6,708


    What is the typical load from 1157 type LED bulbs just out of curiosity?

  27. ........ Voltage - Amps. Watts Candela Lumen Hours

    High Element 12.8 ----2.10 26.8 32 402 1200
    Low Element 14.0 ----0.59 8.3 3 38 5000

    Awe crap that formatting is screwed up
  28. 42merc
    Joined: Dec 19, 2010
    Posts: 621


    Why ? Especially necessary when using Bright Bulbs, or Halogen bulbs for stop & turn lights on the older cars with smaller tail lights. The higher amperage (wattage) of these bulbs will kill the standard hydraulic switch in short order.
  29. hillbilly4008
    Joined: Feb 13, 2009
    Posts: 2,870

    from Rome NY

    Thanks, thats all I wanted to know.
  30. 28TUDOR
    Joined: Jan 25, 2007
    Posts: 419


    Thanks Cosmo, good info.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.