Adding Hot at Start Lead

Toadclub · Feb 9, 2016

Couldn't find this info through search, the terms are too generic. I'm adding a set of heated seats to my 98 B4000 and I want to neatly install the switch circuit for the heat. The position controls and heater power sources are routed through an accessory fuse block that I put in, but the heat controls should be only on at ignition so the battery isnt drained. It's a 7.5A fuse.

Where can I tie in this circuit? I'd like to keep it neat and not mess with the original fuse block. Also, any ideas on clean routing for passenger side routing? I have everything coming through the firewall on the driver side but I'm debating routing through the engine bay and into the passenger side, what have you guys used?

Here's some rough shots in progress

RonD · Feb 9, 2016

You will want to use a couple of 30amp 12volt relays, one for each seat heater.

The seat power can then come from battery.
The relay power can come from any 12volt key on source, relays draw about .5amp, so won't effect any circuit it is added to.

Best place for relays would be next to added fuse block, run wires inside to each seat.
Get key on power from coil pack or fuel injectors, or you could lift fuse box and tap those wires from underside.
Just need one wire and daisy chain it to both relay's coils.

Run 2 small wires into the cab for relay coil's Ground, on/off switch for relays, ground switches in cab.

Toadclub · Feb 10, 2016

Thanks Ron. Yeah, I have a 10ga wire with a 50A circuit breaker feeding the fuse block in the picture, and each seat circuit is going to have it's own 30A mini fuse. I was planning on using common ground for everything, 12ga, do you think I should isolate the relay grounds? Is the coil pack mounted to the firewall?

The reason I don't want ugly mystery wires is I've been pulling out lots of PO installed wiring thats either deadended or just loose in the engine bay and have to make sure they're not a broken factory wire.

RonD · Feb 10, 2016

Yes, on using relay grounds as the control, that's what Ford and most auto makers do as well.
Less of a chance of fuse blowing shorts with Ground wires.
You make the 12volt wires as short as possible for relays, then use Ground to turn on or off.
Fuel Pump relay is this way
Fuel injectors
Even coil packs
All operated by grounding

And it is cheaper, lol, which is why auto makers like it

Gavzach · Feb 11, 2016

what seats are you using?

Sent from my XT1585 using Tapatalk

Toadclub · Feb 11, 2016

I found a pair of leather seats from a 2002 Explorer Limited, converted the memory to direct power. The heater circuit on both seats is separate from the control circuit.

I'm putting a basic write up together once I can sort out a couple of details, so people with really simple questions can find some info easily.

Adding Hot at Start Lead

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