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The Gentleman's Express: Tech-Tips from the JIOC

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Headlamp Relays

Author: Adam Bennett

In the past tech tip articles there have been headlight relay conversion suggestions but to those of you who have converted overto the new highpower quartz-halogen lights or are thinking about it, a single relay may not be enough to prevent switch “meltdown”. In my ’74 Mk III I’m using Hella H-1’s and H-4’s with 80/100 watt low-beam bulbs and 100 watt high-beam bulbs. These are the most powerful available. At night I can see a “Black Cat” over 400 yards away!

Our infamous “Lucas” factory wiring has the headlight switch draw its current off the parklight switch. BAD NEWS! Besides melting the parklight switch over a period of time, it can become a real fire hazard as the wiring can become incredibly “HOT”. So I decided to install a more efficient and safer “relay” system. Because of the high drain and load by my lights I chose to use a relay for each headlight (four total). Any heavy-duty relays with a minimum of a 20 amp rating should work. I used relays made by Hella Lights, PN 4RA-003 510-082-12v, which have a 30 amp rating and are relatively inexpensive at $5.50 each. The wiring schematic numbering is identical to the Lucas/ Bosch fan relay:

#85 .... Ground #86 .... Switch (from dash)
#87 .... Light #30/51 .. 12v source

I strongly recommend using at least 12 gauge wire for hot source and ground, with 14 gauge for all other connections.

Installation: Start first by disconnecting the battery. Leave the dash switches as they are.

At the fuse box, start by disconnecting the ‘R.H. H/ Lamp Dip Beam’ wire (from the fused side) and connect to #87 on one of the relays. Now replace it with a length of #14 ga and connect it to #86 on the same relay. This supplies the switched source to the relay yet retains the factory fuse as a breaker. Repeat these steps for the ‘L.H. H/Lamp Dip Beam’ and for the ‘R.H. and L.H. H/Lamp Main Beams’ using a relay and corresponding fuse for each.

Next run a #12 ga ground wire to a good chassis ground, (keep this wire as short as possible) and split it to all four relays. Connect this ground wire to #85 on each relay. You now need to run two (2) lengths of #12 ga wire to the alternator/battery post (direct hot). Connect one to each pair of relays i.e. “right and left dip beam” relays and “right and left main beam” relays; and split. Connect to #30/51 post on each relay. Double check all connections and bundle up all wires using electrical tape or wire ties.

Reconnect the battery and test the headlights.

The whole project took about an hour and a half. After doing the conversion, the light efficiency improved at least twice over! This was probably due to the more direct approach to wiring them, and to the new wires and connectors.

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Contents copyright 2008, 2009 - Jody F. Kerr

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