Toyota Celica Supra Forum banner

1 - 5 of 5 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1
Hey guys, been working on my 85, and noticed yesterday that when I turn on the fog lights the high beams come on. It’s got me baffled, electrical is not my strong suit. I recently replaced the bulbs in the fog lights. Upgraded to led bulbs. I can’t say for sure that this didn’t happen before the bulb change. But looking for some advice or ideas. Thanks
 

·
Premium Member
Joined
·
8,167 Posts
Check all your grounds.
 
  • Like
Reactions: JVirostko79

·
Registered
Joined
·
1,116 Posts
I had this same problem when switching to Led headlamps. Look up fog light mod. I ended up doing that to my car after doing the led upgrade, and can now turn my fog lights on or off when key is in the "on position."
 

·
Registered
Joined
·
19 Posts
Hi JVirostko79
As the previous post indicated, I am guessing you also installed LED headlamps. The issue is that the power to operate the Fog light relay passes through the high beam filaments of the headlamps. With a regular halogen incandescent bulb, the current is too small to make the high beam appear on. However, with LEDs the small trickle through the Fog lamp relay could turn on the high beams. I have attached the wiring diagram for the fog lights.

You can see that terminal 2 of the headlamps flows to the fog light switch, which when on passes power to the coil of the fog light relay. (BTW, turning on the high beams with the dimmer switch grounds the terminal 2 of the headlamps, removing power from the fog lamp switch/relay. That is why the fog lamps turn off when you switch to high beam.)

The same thing can happen in the house when you put LED bulbs in lamps controlled by dimmers/timers not meant for them. The small leakage through the dimmer or timer can allow the LED to turn on.

This does not solve your problem, but at least provides a plausible explanation. Good luck!
 

Attachments

·
Premium Member
Joined
·
8,167 Posts
SuperRed, Thank you for the very informative and educational post!!

Now I understand how and why our fogs work and what happens when high beams are activated....

Props to you Sir!!

Hi JVirostko79
As the previous post indicated, I am guessing you also installed LED headlamps. The issue is that the power to operate the Fog light relay passes through the high beam filaments of the headlamps. With a regular halogen incandescent bulb, the current is too small to make the high beam appear on. However, with LEDs the small trickle through the Fog lamp relay could turn on the high beams. I have attached the wiring diagram for the fog lights.

You can see that terminal 2 of the headlamps flows to the fog light switch, which when on passes power to the coil of the fog light relay. (BTW, turning on the high beams with the dimmer switch grounds the terminal 2 of the headlamps, removing power from the fog lamp switch/relay. That is why the fog lamps turn off when you switch to high beam.)

The same thing can happen in the house when you put LED bulbs in lamps controlled by dimmers/timers not meant for them. The small leakage through the dimmer or timer can allow the LED to turn on.

This does not solve your problem, but at least provides a plausible explanation. Good luck!
 
1 - 5 of 5 Posts
Top