Credit for all the work goes to Dick Kreutzer (DickK93 on TT)
Those who have attempted to install LED headlights in the Isuzu chassis have discovered that the Isuzu headlight circuit isn’t like the standard American system. Since LED’s are polarity sensitive it is not just a matter of changing the bulbs. It will require the addition of at least two relays and rewiring the hi beam indicator.
Another issue is protecting the power source to the headlights. American cars for years had a thermal circuit breaker built into the headlight switch to protect against overloads. I’m not sure if it was by design or just good luck but I had a chance to test the one in my 53 Studebaker one night. The headlights went out but almost immediately flashed back on for an instant several times before I got stopped. Fortunately, I was familiar with the road and it was mostly straight.
Had it been a fuse, things likely would have turned out for the worse. The Isuzu circuit divides the lights to left and right so that a short will only take out the lights on one side. The circuit I have drawn has separate breakers for low and high beam so switching to the other beam would return the lights. It would require 4 relays to divide the system to left and right and still have hi and lo beam but relays are cheap and the road gets real narrow when the lights go out.
The circuit is also useable to reduce the voltage drop for regular bulbs or H4 conversions. There are pre-wired harnesses available but they will require some changes to use in the Isuzu. They seem to use one fuse to protect the lights so that for me is unacceptable but could be changed to circuit breakers.
Another consideration is that the low beam bulbs in the Isuzu have the common in the center position while most LED’s seem to have the ground on one side which may involve changing the position of the wire in the bulb socket. I will also attach the original Isuzu diagram for comparison.
Here are the same diagrams in pdf format: