The factory wiring is the real demon in the scheme of things when it comes to poor headlight performance. Whether converting to H4 bulbs or not you need to upgrade the electrical path to the headlights to get the maximum performance from them.
The workings of the headlight circuit are as follows: The battery feeds power to the ignition switch. From the ignition switch the circuit goes to the headlight switch which then feeds the fuse box as well as the high and low beam connections on the headlight relay. The headlight relay then connects the power to the appropriate set of headlights, which are grounded to the chassis thus completing the circuit. Electrical losses from corrosion inside the ignition and headlight switches (and the old headlight relay too) leads to lower voltages to the headlights and decreased output. To remedy this the circuit needs to be rewired as follows:
See this page for the wiring schematic.
1) Find a convenient place to add 20 amp relays under the dash, one for low beams and the other for high beams. It is convenient to use the Hella relays with the built in fuse holder to simplify the wiring.
2) Disconnect the low beam wire from the headlamp relay and connect it to terminal 87 on the the new low beam relay. Add a new wire from the headlamp relay to terminal 86 on the new low beam relay. These relays also have a mounting tab with screw hole to fasten to the body.
3) Disconnect the high beam wire from the headlamp relay and connect it to terminal 87 on the the new high beam relay. Add a new wire from the headlamp relay to terminal 86 on the new high beam relay.
4) Run new, 12 guage or better, power and ground cables from the battery to the 20 amp relays. Connect the power cable to terminal 30 on the new low beam relay and daisy chain to terminal 30 on the new high beam relay. Run the ground cable to terminal 85 on the new low beam relay and daisy chain to terminal 85 on the new high beam relay.
Now the old headlight switch and relay are only used to activate the new 20A high or low beam relays instead of to the lights directly. This insures that the power goes directly from the battery through the corresponding 20A relay out to the lights thus significantly reducing the potential for losses in the circuit. The end result is more voltage to the headlights producing brighter output on the road.
Installing the H4 headlights is straight a forward matter. Just remember that if you are going to use higher wattage bulbs the wiring will need to be upgraded to handle the extra load without suffering performance losses and/or damage to the wiring harness and vehicle. I will add some pictures of this procedure soon.
©1997-2003, Thomas M. Ainlay