Some Fiat Electrical Basics: Part 4 Fuse box inputs and loads

[zachmac]

In this final segment (I think it’s the last part?) I’ll review the feeds into each fuse in a Fiat Spider fuse box. As far as the loads off of each fuse I posted in a supplement what the factory manual lists for certain years. This list does differ year to year so be sure to verify using a wiring diagram specific to your car. I am basing this on a 78 so no FI (sorry 80 and later FI guys). I know early cars also vary, particularly with regard to UNSWITCHED loads. I believe the power sources to the inputs are basically the same for all years.

The standard fuse box has ten (10) fuses. They are labeled by number for input and by letter for output. Each input and each output can and often does have more than one wire connected to it.

The inputs are numbered 1 – 10. The outputs are letter designated A-L Note that they skipped J and K. I don’t know why? I do know there are a couple of fuse holders on the car that are not part of the main fuse box so maybe they used the J and K designation for those?

I am going to address the fuses in what might at first seem a random order but it has a method to my madness in that I will address the fuses associated with UNSWITCHED, SWITCHED (CONTINUOUS LOADS) and SWITCHED (SITUATIONAL LOADS) in that order, followed by fuses that get power via a secondary switch. Refer back to Part 1 of this series if you need to refresh how each of those groups was defined.

UNSWITCHED: Fuses #9 and #10
Inputs for these two fuses are electrically connected; they act as one point electrically. Power into these two comes from a BLACK wire that doesn’t pass through the ignition switch first, hence the UNSWITCHED designation. That wire originates from the hot post of the alternator in the engine bay, which is itself connected to the hot post of the starter that is fed from the battery. On my car this wire is actually a heavy gauge BLACK/WHT (black with white trace) wire. On earlier cars it may be just BLACK.

The BLACK wire ALSO comes off this input and goes to the ignition switch #30/1 post. This is the feed for the SWITCHED (CONTINUOUS LOADS) side of the ignition switch.

The BLACK wire ALSO goes to the headlight switch. This is what allows you to turn on the parking lights even when the key is removed.

So if the battery is connected the #9 and #10 fuses should always have power.

SWITCHED (CONTINUOUS LOADS): Fuse #1
Fuse 1 provides power to all the loads that need power any time the car is running. The loads include things like the sensors, the gauges, brake lights, back up lights, etc. As the name implies these loads are not controlled by a driver controlled switch and receive power whenever the key is on. Some don’t always use power, like the backup lights or brake lights but the switches that control them are an “automated” function meaning the switch actuates as part of another function; stepping on the brake, putting the gear shifter in reverse, etc. Other loads like the tachometer and sensors use power continuously.
Power to the #1 fuse comes from a PINK wire form the #15 post on the ignition switch. Recall that that wire in turn is powered by the ignition switch via the BLACK wire continuing from the #9/10 fuse pair input to the #30/1 post on the ignition switch. For both SEPIA and Lada switches this power is only present when the ignition is in the RUN or START positions. It is not energized in OFF or LOCKED / KEY REMOVED.

SWITCHED (SITUATIONAL LOADS): Fuse #2
Fuse 2 provides power to the Wipers (and rheostat if so equipped), the Windshield Washer Pump, and the heater blower motor.

The #2 fuse receives power from a BLUE/blk wire from the #INT connector on the ignition switch. Note that in most years the wire changes to a BLUE/red wire at the ignition switch connector so that is the color coming into the fuse box. This fuse will be powered via the ignition switch the same as the #1 above IF you have a stock wiring setup. That is power in RUN or START but not in LOCKED / KEY removed.

If you have a Lada switch and have wired it per the factory wiring (BLUE/blk from the #INT ignition switch post then it will be hot in RUN, START and also in LOCKED / KEY REMOVED. BUT, it will not be powered in key in, in OFF. If you have wired it from the #15 post on the ignition switch it will behave exactly like the #1 fuse. BUT, as I discuss in part 2 of this series, I don’t recommend that configuration unless you also only use the BLUE/blk wire as a relay trigger to a separate power wire to the #2 fuse. Otherwise you are running all of the cars loads through one set of ignition switch contacts and it may not last long.

NOTE: The BLUE/blk #INT wire ALSO powers a post on the headlight switch and the in turn the high low beam switch. We’ll get to that in the next fuse set.

Fuses #3, #4, #5, and #6:
These fuses feed the headlights. #3 feeds the Left high beam and the high beam indicator, #4 the right high beam, #5 the left low beam, and #6 the right low beam.

Power to these fuses if controlled by the combination of the headlight switch and the high / low beam selector switch. Head light switch must be on. Then power is sent to either the #3/#4 high beam fuses or to the #5/#6 low beam fuses, depending on the position of the high / low beam selector switch. REMEMBER, the headlight and selector switches got their power from the BLUE/blk #INT post on the ignition switch (stock setup). So for the headlights to function the ignition has to be in RUN or START. BUT, is you have wired a Lada switch per the factory configuration that connection will also exist in LOCKED / KEY OUT so the headlights will behave as if UNSWITCHED. If you’ve wired a Lada switch with the BLUE/blk wire to the #15 post it will operate as a stock unit BUT all the headlight power now goes though the same single set of contacts in the ignition switch as every other load.

When using either a stock SEPIA or Lada switch a headlight relay kit is highly recommended! Those sets use the feed from the #3/#4 and #5/#6 fuses as trigger power for the relays only thereby greatly reducing power through the ignition switch, headlight switch and high low beam selector switch which should greatly improve the life of all three.

Fuses #7 and #8:
Last but not least are the misc. loads off of fuses #7 and #8. In general they supply power to the parking lights, side markers, dash lights, the license plate lights and various other lights like the trunk light, the illumination for the hazard switch and cigarette lighter surround, and the bulb for the fiber optics to the center console illumination.

BTW, these do something interesting with regard to left and right lights. Unlike what you might expect, left all off one fuse and right all off the other, the supplies crisscross. For example #7 powers the front left parking lamp, but the right rear tail lamp and the front right and rear left side markers. I assume Fiat did this as a safety measure so that if either fuse blows and you are by the side of the road you still have two opposite corners of the car lit up at night so other cars see you as a wide vehicle, not perhaps a motorcycle.

In any case, power to these fuses comes through the headlight switch, either from the UNSWITCHED#10 fuse supply or the BLUE/blk wire from the ignition switch.