I want to add a new circuit to my fuse box - PeachParts Mercedes-Benz Forum

Admiral Ahani · #1 03-19-2012, 09:24 PM

My '82 300TD has an unused fuse holder, and I'm adding power seats to the car. I was about to run a power wire from the battery to somewhere under the center console, when I thought about the possibility of using the fuse box.

But I don't know exactly what is involved in making that happen. Has anyone here done it before? I'm sure other MBs would be similar in principle to my w123. TIA.

vstech · #2 03-19-2012, 11:12 PM

two screws hold the fusebox to the firewall, just loosen them and pull the box out, and attach the wire to the output of the fuse panel empty spot. you should also attach a power supply wire to the other side of the fuse.

tangofox007 · #3 03-19-2012, 11:46 PM

Quote:

Originally Posted by Admiral Ahani

I was about to run a power wire from the battery to somewhere under the center console, when I thought about the possibility of using the fuse box.

You might investigate the possibility of wiring into the window circuit(s) at the center console.

Admiral Ahani · #4 03-20-2012, 04:28 AM

vstech, Where would my power supply wire come from? I know I could install my own wire and tap/splice a nearby hot wire, but then I might overload the spliced wire. I could run my own wire from the battery to the fuse but the whole point is to avoid running a wire from the battery. I don't suppose the factory ran any extra (i.e. unused) power wires in the harness?

I'm just worried about current carrying capacity (ampacity). The seats probably draw 5A per motor, and if someone hits the memory button and it moves 4 or 5 motors at once, it could draw 25A, which would be the limit of the 12 gauge wires - if the wire I spliced into has its own load currently drawing current, it would melt. I really don't want to have to tell all of my passengers to move one motor at a time. It would be embarrassing.

Where do all of the fusebox's power wires originate? I.e., where is the place where all the power wires in the fuse box originally connect to the fat battery + cable?

tangofox, I thought about that, or even the rear window heater, because what are the chances someone will be moving the seat at the same time as opening a window, but I've been trained to prepare for every contingency so I can't help but think my first passenger after the swap will get in, move 3 motors on the seat simultaneously, while opening the window.

If all else fails, maybe I'll make a neat harness of my own, carrying a few power wires from the battery to the fusebox, and friction tape it to make it look inconspicuous. There are two free fuse slots (5 and d), and if I ever need more, I can add an aux fuse panel somewhere under the cluster. Or maybe I'm getting ahead of myself?

WhipperSnapper · #5 03-20-2012, 09:02 AM

Is there not already one of the fuse locations in the box designated for power seats? If so, there might very well already be stubbed out wiring in the right location. That's the way it works on many cars. Uninstalled options still have the wiring connectors in place.

Whip

Phillytwotank · #6 03-20-2012, 09:08 AM

Quote:

Originally Posted by WhipperSnapper

Is there not already one of the fuse locations in the box designated for power seats? If so, there might very well already be stubbed out wiring in the right location. That's the way it works on many cars. Uninstalled options still have the wiring connectors in place.

Whip

Yes. On the lid of the fuse box you see a chart of whats what in the fuse circuits. You will see equipment listed that you may or maynot have in you wagon.

Attach the wiring for the power seats to one of the existing circuits that has some "head room" to accommodate the increased load from the power seats.

Admiral Ahani · #7 03-21-2012, 04:30 AM

The w123 was never offered with power seats (at least not in the USA market) so there's not supposed to be any wiring or fuse slot for it. It's a custom installation of w126 seats into a w123. After messing with the wires on the seats and switches, all it boils down to is getting a power wire to each seat, and providing each seat with a good ground. The ground was easy.

If what I've read is right, then I'm looking at a possible 25A draw from each seat. I know that's not going to happen often, and probably not ever both seats at the same time, but I'd like to definitely give the seats their own 30A circuit to avoid maybe overloading some other circuit.

The only optional (powered) equipment my car does not have is the power sunroof (the w123 wagons had manual sunroofs). However, I checked, and the power sunroof fuse slot is not powered - there are no wires going to or from it.

I think what I'm going to do is run 4 new power wires from the battery, along the firewall, through one of the grommets on the driver side, to the area behind the fuse panel. 2 of them will be used for the seats, one each, run from here to the seats. I'll use an inline fuse holder near the seat, under the carpet. The other 2 wires will be hooked up to the fuse panel for future projects I have in mind (which draw less current

).

Thanks all for the ideas!

Renntag · #8 03-21-2012, 09:46 AM

Quote:

Originally Posted by Admiral Ahani

I'll use an inline fuse holder near the seat, under the carpet.

I would caution running direct battery power that far. If there was ever a short prior to the fuse, you could have a fire in the passenger compartment. I would install a weather proof fuse holder on the engine bay side of the firewall to be safe. If you really want to be slick with your install, you could get a fuse holder work box kind of thing and bolt it to the car in the brake booster/fuse box area. (Rather than have an inline fuse holder hanging out). I personally wouldnt mind the weather proof holder hanging out if the wiring was all wrapped and secured nicely.

Best of luck. Nice project.

Question, will you be sitting any higher with the electric seats? I dont care about the power one way or the other, but the seats in my W126 are much more comfortable than those in my W123.

PackerEdgerton · #9 03-21-2012, 11:15 AM

Tie into the battery and be sure to put the fuse as close to the battery as possible. Do NOT run unfused wires unless you like the possibility of electrical fire.

PE

charmalu · #10 03-21-2012, 11:16 AM

When make an installation of additional wires, it often looks kind of tacky seeing red, blue, green etc... colored wires running various places around the engine bay.

When I have added or changes a wire size etc... like adding the Al-129-X 115 amp alt, or the Electric fan and up grading the Fog lights to 100watt. I purchased some Heat Shrink tubing in 4 ft lengths to slip over the wires. then it isn`t so noticable and looks all factory. also run the wire through the MB Zip ties in the wire bundles and add an additional black Zip tie where needed.

As far as the fuses go. I have seen in 126, 124 and maybe 201`s, in the box the fuses and relays are in, there is a small 2 fuse plug in, has 3 or 4 pins where it pushes in the plug receptical like the Relays do.

On your 123 under the black box on the drivers fender where the 2 relays are, is space for 2 or 3 more plugs. knock out a couple of them at PNP along with 8 inches of pig tail and add these. then your fuses are under cover, and another factory looking install.
Once in a while these fuses will have a small black box lid that snapps on to protect them from the elements. they look about the size of a relay.

Charlie

Admiral Ahani · #11 03-22-2012, 03:47 AM

Thanks guys, great ideas!

OK, I will put the fuses closer to the battery (in the engine bay), and not next to the seats. I like the idea of using the black box on the driver inner fender. If I can find some of those fuse-holding inserts, I'll use those. If not, I'll probably buy some inconspicuous looking small black fuse box and mount it in this area.

I always wrap new wires in 3M friction tape, which looks exactly like a factory-wrapped wire harness.

Renntag, check out my thread on the w126 seat conversion. I'll update it with pics soon. Put W126 seats into my W123 - first impressions