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740 electrical gremlins after installing LED head lights


Active member
Sep 5, 2005
I have installed LED lights in the headlights of my 780. They work fine.
But after doing that i sometimes (well, 50% of the time actually) have the warning light for a broken bulb light up on the dash.
It will come on with the city/park lights, the low beams and high beams.
All the time NO bulbs are blown. No fuses are blown.

At the same time i run the LED headlights i see the voltage meter drop well below 12V.
Yet, when i measure the battery is still see 14.2V so i conclude that the alternator is charging.
When i switch the LED headlights off the voltage meter goes back up to about 12.5V,

Anybody any idea what is going on here?
The bulb out relay isn't sensitive enough to work properly with the lower current of the led bulbs. It's triggering because it senses the lower current in the circuits.

As per the dash gauge. it may have something to do with the lighting circuit on the cluster. Like a weak ground for the dash lights so the circuit is finding ground through a gauge.
I inspected the ground of the dash gauge cluster. Could not find anything wrong.
Would it be possible to trick the 'bulb failure' light into switching on later? For instance by adding a dummy load like a ceramic resistor in parallel?

brain wave: i realised i have also installed a tow hitch on the 780. This means i also included a 7pin socket to connect trailer lights. The wiring works. I have tested it.
Offcourse i spliced the wires to the socket into the existing wiring for the real light clusters.
Could that be the cause for the 'bulb failure' light to come on? Even when nothing is connected to that trailer hitch socket?
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It will come on with the city/park lights, the low beams and high beams.
Comes on even without the headlights themselves being on?
That rules out the headlights as being the *only* culprit.
I would carefully check the condition and contact surfaces of ALL the other bulbs.

The relay itself is fairly simple, when you look at it.
There's a single reed switch in the middle, surrounded by pairs of wire coils.

Each pair will conduct current for opposing bulbs or circuits; left and right.
There's a pair for headlights, a pair for taillights, a pair for signals....

When a circuit is active (lights turned on), current will flow through the coils on the way to the bulbs, creating a small magnetic field around each coil. The wire coils are wound in a way that the magnetic field from one coil of each pair will cancel the field of the other coil, so nothing happens.

If a bulb burns out, or is a different wattage, or has a bad connection that reduces the current flow through that side, the magnetic fields become unbalanced, and the reed switch contacts are pulled together, turning on the warning light.

Assuming your new headlights are drawing the same current on each side, it shouldn't matter if they draw more or less than before.

great explanation. Thank you very much!
This might just help me track down the problem. I have already found 2 bulbs in the rear light clusters that had a black haze inside the glass so i replaced those.
I also discovered that both license plate lights were not working and badly corroded. I haven't been able to get new bulbs so i hope when i get new ones in there that it will fix the issue.