This is a good write up, wish I had seen it beforehand. A buddy just purchased a '99 Tahoe and wanted better illumination. It had been a while since I did this on my own truck so I pieced together how we did it.
Time Estimate:
20 minutes
Materials: (about $12)
1. 1 Relay (with plug-in harness, optional). Relay has 4/5 prongs, with #85, #86, #87 and #30 terminals. These are very common relays for cars. I used a spare SPDT relay & harness that has a fifth terminal (87a) that is not used for this project
2. 3-4 feet 16 gauge wire
3. Flexible Wire Conduit
4. Solderless wire connecters
5. Ring terminals for power and ground
6. Shrink wrap
7. 1 inline fuse
Step 1
Disconnect the battery
Step 2
Look for the wiring bundle inside flexible conduit on the driver’s side fender well near the ABS system. Open the conduit and find the brown and lime green colored wires.
Step 3
T- Connect the vehicle’s brown wire to terminal 87 of the relay
Step 4
T- Connect the vehicle’s lime green wire to terminal 86 of the relay
Step 5
Connect a ground wire to the fender well (there are several existing bolts that you can leverage for ground) and connect it to terminal 85 of the relay
Step 6
Connect an in-line fuse / power wire to terminal 30 of the relay. Use your choice of power source (the underhood relay / fuse block is 12 inches away from where we spliced into the headlight wires on the driver’s side wheel well towards the firewall - we chose to tap the battery power supply to the fuse block)
Tape / shrink wrap all connections and hook the battery back up. You now have 4-high!
Other Stuff:
WIRES FOR SPDT RELAY HARNESS (optional- these were the wire colors on the harness that corresponded to the terminals on the relay- you can just match your wires to the terminal numbers on the relay using female solderless connectors):
30 BLUE fused power source
86 BLACK high beam wire- lime green in OBS trucks
85 WHITE ground to vehicle body
87 YELLOW low beam wire- brown in OBS trucks
87a RED (BLANK for this project)
SPDT Relay: (Single Pole Double Throw Relay) an electromagnetic switch, consisting of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87).
When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity.
For this application, the high beam circuit is activating the relay’s coil and closing the 30 – 87 circuit in the relay sending power to the low beam lamps. Once the high beams are turned off the coil is de-energized and the low beam circuit operates normally.