PLEASE Help- Wiring 2 relays in parallel

Discussion in 'The Projects Forum' started by jwilli, Jul 12, 2009.

  1. jwilli

    Thread Starter Active Member

    Apr 21, 2009
    I am trying to wire 2 standard 12v 40 amp automotive-type SPDT relays in parallel to increase the amperage. I know it's not the best option but it's a quick fix I need to do for something.


    1 relay works fine but when the second relay is connected, neither relay works. I can connect all the wires to the second relay but anytime both of the coil connections are hooked up (85 and 86), thats when both relays do nothing.

    I've done this in the past with no problem with a different brand (but same type) of relay.

    Does anyone know what may be causing this? I am in desperate need for some quick assistance on this.

    ANY help is much appreciated!!!
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    If terminals 85 are tied together, and then terminals 86, the relay coils are in parallel. Only some restriction of current can prevent the relays from operating.
  3. jwilli

    Thread Starter Active Member

    Apr 21, 2009
    I simply took two relays with wiring harnesses like these:


    ...and connected all of the same colors to each other like this:


    What could be restricting the current?
  4. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    Possibly the control lines coming to 85 & 86 dont have enough amperage to latch two relays. Confirm if your line provides enough current for double relay drive. If it doesnt, there is a workthrough other than this, but you will lose a pole of one relay. Drive the second relay using the first one's single pole and use the remaining 3 poles of which 2 can be tied together. Do you really need double poles?? What aplication is it for? you have numerous options. you could also get one more relay and wire it in parallel with the second relay. So you will get what you need except that you need a relay more.

    If there is enough current on the line, either the relay has a problem, or the reduced impedance of the parallel relays causes a problem. If its the second, wire a small valued resistance in series of sufficient wattage.
  5. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    Couple of things. Do the relays work when you try one at a time. I've found that some of these types of relays have a supressing diode in one direction, others in the other direction, or a burnt diode.

    Check your wire positions, your harnesses may not be wired to the same terminals.

    We've had these relay harnesses and relays come in control circuits for diesel engines, and I've found they aren't the best quality.
  6. mik3

    Senior Member

    Feb 4, 2008
    Check for a blown fuse.