View Full Version : Lo-beam malfunction mystery

12-13-2005, 11:15 PM
Vehicle: 1990 celica st

Problem: Lo-beams will not come on when light control stick is in lo-beam mode.

Other info:
headlights pop up fine. Hi beams come on and work fine. 12 volts was detected with voltometer on both power wires at bulb socket with bulb removed when the control stick is set to lo-beam and hi-beam (only driverside headlight tested). Bulbs have been replaced with different types of bulbs. Problem persist.

Why would i still get a reading of 12 volts at the wires end point for both settings and yet only the hi beams work? IS there some sorta control unit I don't know about? What's wrong?

12-14-2005, 01:33 AM

12-14-2005, 05:51 AM
I recall quite a while back a camry I worked on that had something like this going on. It ended up being the light switch.

12-14-2005, 05:58 AM
u mean the control stick that u use to turn the light on with?

12-14-2005, 06:21 AM

actually I mean the dimmer switch.

Also according to thw wiring schematic you whould have power to the red (lH) and red and yellow (RH) wires (at the head lights) whenever the light switch (the turn signal shaft with the twist switch on it) is turned on.

The power goes to both the high and low beams.......but only lights up one side or the other depending on which mode the dimmer switch is in. The dimmer switch is responsible for grounding the circuit. And The dimmer switch I would suspect is bad if you have already substituded new lamps and the condition is still present


12-14-2005, 07:00 AM
hmmmm..... that looks like a pain in the ass to get to. Or replace for that matter.

12-14-2005, 09:24 AM
make sure your harness is in decent shape.

i had to rewire one of my harness's because the damn thing wouldnt do a lowbeam. turned out the wire coroded away.

12-18-2005, 09:15 AM
what part of the wire?

12-18-2005, 01:57 PM
probably one of the wires under the steering wheel?

12-22-2005, 05:15 PM
This happened on my supra, I haven't quite figured it out.

But how the system works is heavily dependent on good grounds. If your grounds are not very good or have broken, the circuit will be stuck in highbeam mode. I replaced the whole stalk assembly but it didn't help at all, and replaced both sealed units.

Check all your grounds, get some new ones, heavy gauge wire. The high/low system is super dependent on those grounds.

12-26-2005, 08:56 PM
yah know... I tested the power wires at the bulb socket... but i used the engine chassis as a ground for the voltometer.... STUPID! Why didn't i check the ground?!?! arg....

12-27-2005, 03:52 AM
lol if it comes to changing the light switch, its not that hard.

12-27-2005, 10:55 PM
have you ever thought that just the low beam bulb is burnt out? it is a dula filament bulb in the housing, do you have another sealed beam somewhere that you can try?

12-28-2005, 01:32 AM
Ummmmmmmm....... I'm only going to say this one more time.

If you carefully look at the circuit you can have +12V all day long at the headlamps and they still won't work. Because the ground side is the circuit with the switch on it........... not the posative side. So what is important in testing at the headlamp connector is the ground. Becasue that is the side that has the switch on it and you WILL find that the headlight dimmer switch IS bad. And I'm confident in this fact becasue I had the exact same problem with the exact same wiring schematic and the headlight dimmer switch would not provide the ground to the circuit and was faulty. Now I feel like I'm repeating myself but maybe I just didn't explain it in enough detail.

Verify that you are getting a good ground at the headlight connector. If not.... REPLACE THE DIMMER SWITCH TO FIX. There is still a slim chance that there is a bad wire, but unlikely. One final time REPLACE DIMMER SWITCH!

12-30-2005, 07:59 PM
I quote myself...

Bulbs have been replaced with different types of bulbs. Problem persist.

so the lo beams have not burnt out.

and as for Mr Auto, thank you very much for your insite... but please be PATIENT since I'm new to electronics. It's kinda hard to tell how to get to the switch. Am I gonna have to splice the wires or is there a connector? Am i gonna have to go to Toyota for this part? Do i have to remove the steering wheel and bypass the airbag?