Fuse location for relayed headlight harness

urq urq at pacbell.net
Thu Oct 30 23:24:37 PDT 2008

Oftentimes the fuse is located in the individual circuit because the
operating current flowing in the supply branch is so high that even a short
circuit in the branch circuit would not be enough to blow the fuse.  In fact
the larger circuit may be protected by a fusible link.  When I build a relay
box for headlights I put a fuse of the same rating as the factory put in the
line to each individual bulb, and then I use an in-line fuse for the feed
line from the battery.  I've found a blade type in line holder rated at 40A
which I install near the tap point (usually at the starter/alternator for
headlight relays) and a small blade fuse panel in the box for each filament.
You can also incorporate the factory bulb out warning module in the box and
route the sense wire back in to the relay box.  

Steve B
San José, CA (USA)

-----Original Message-----

If the purpose of the fuse is to protect the maximum amount of wiring
harness (and other parts of the car) possible from any short circuits due to
mechanical wiring faults, it makes sense to locate the fuse as close as
possible to the power source. If the fuse were right at the designed ground
point of the circuit it wouldn't blow if a powered wire upstream were to
contact another ground - the current would go through the short until
something melted. You want the fuse to blow instead of other wiring melting

> I'm getting ready to fab up a relayed headlight harness and now I
> am wondering where the fuse should really go. I've made them
> before and always put the fuses between the relay and the power
> source. Why do we do that rather than put the fuse between the
> relay and the headlamp? Input appreciated and TIA

quattro mailing list
quattro at audifans.com
Watch this space for ads :)

More information about the quattro mailing list