heated seats - do i need a relay?

Discussion in 'Mech Tech' started by rob.e, Dec 4, 2018.

  1. Ok, so the mx5 seats i have in the bus have heating elements in them. I've hooked up a direct 12v on them and it feels like the heater elements are still working so i'm planning to wire them in properly.

    Google comes up with some wiring diagrams showing folks wiring seats in with a relay switching to a direct feed from the battery, which i'm guessing is so that there isn't a big voltage running through the switch?

    I'm planning to have mine fed from the leisure battery. I have a feed from this direct to the front of the bus into a small fuseboard. I have a toggle switch rated at 15A. Can i just wire it up direct to this, or would i need to add a relay too?

    jim mcglynn likes this.
  2. If you add a relay, you avoid extra heavy wiring under the dash.
    It also means you can route the high current through a fuse from the battery and a relay somewhere near the seat and bring a single light duty positive feed via a e.g. 3Amp fuse and then the switch to the relay. The relay ground is used to provide negative for the relay coil.

    Hopefully your split charge setup can take the extra current when the engine is running and does not blow the fuse in the split charge battery charger wire coming from the relay and starter battery side.
  3. thanks mike - that's a good point, i hadn't considered the impact to the split charge.. what options would i have if the split charge isnt' man enough?
  4. I didn’t fit a relay when I fitted mine.

    I wouldn’t run it off the leisure Battery as it’s about 10amps each so would flatten the battery quickly

    I took a feed from the back of the vans fuse box (the switched side so they don’t get left on without the key turned) through a in-line fuse To a 20 amp switch on the dash with a light built in to a cigarette lighter mounted to the seat base so I can unplug it to swivel the seat.

    Last edited: Dec 4, 2018
    Kruger, rob.e and theBusmonkey like this.
  5. I'd run them off the main battery via a relay, and arrange things so that they only work if the ignition's on (engine running). They take a lot of current.
    rob.e likes this.
  6. .. actually just thinking about this, i could re-purpose the switch for the heated rear window. it's sat there in the dash but not connected up to anything.
    Valveandy and snotty like this.
  7. no...just thermals...if yes...you might need some 'piles cream' if the relay has a fault!
    rob.e likes this.
  8. You could! Tap a +12V supply off from the fusebox.
    rob.e likes this.
  9. Do you need an external thermostat, BTW?

  10. whats' your logic here, have some sort of automated system where they kick in automatically when the weather's cold?

    Gingerbus likes this.
  11. No, I meant the seats usually have some kind of thermostat fitted to stop them just getting hotter and hotter. The Saab seats used to have a button stat in the seat itself, or an external one that came from the dash. Not sure what the MGs have. If it’s external (dash mounted), you’ll need that as well as the other wiring, or they’ll set your bum alight :eek:
    Last edited: Dec 5, 2018
    Valveandy, Bulletooth and rob.e like this.
  12. You can always buy a temperature controller on eBay provided your has good enough for him fail not in that way chInglish for it understand mouse way.

    Fit that to control the relay and have it come on when the inside of the bus is +45 degrees C.:) Then alter the settings so it works right.
    nobayinhell likes this.
  13. Or just switch it off when hot enough
  14. On ours was wired directly to 15a switch on dash, it had a inbuilt switch that worked relay so would only work if you were sat on seat and disconnected back rest part too to save load.
    rob.e likes this.
  15. So, i finally got around to doing something with the seats last night, wired up direct to the heated rear window switch, and a plug added under each seat so they can easily be removed if needed.

    I'm guessing the wiring for the heated rear window must be pretty robust so i've gone direct off that for the moment - i'll see how that works out and add a relay if needed. Seems to work fine testing it last night, seats get warm and the switch doesn't appear to get particularly hot so i'm thinking this might be ok.
  16. Does the wiring get warm?
  17. i only ran them for a minute or so to check the heaters were getting power, and i didn't perceive any noticeable heat in the wires. i'll do a more thorough test tonight :)

    of course, now they're in i expect we'll have super hot weather from now on :D
  18. Dazza

    Dazza Eyebrow not high brow

    Made me laugh at that Mike :D:D:D:thumbsup:

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