Odd headlight issue i've not struck before
Thread Starter
Le Mans Master
Joined: May 2005
Posts: 5,214
Likes: 65
From: Queensland, AUSTRALIA
Odd headlight issue i've not struck before
Hi all,
Got a 1990 6 speed coupe here that has a totally different behaviour when the headlight switch is turned on or off.
When turning to the first setting, the headlights roll around but stay off. Park lights, side markers, and fog lights work as expected on this setting.
When on the ON setting the headlights come on as normal, all else operates as it should.
Turning switch to off will roll the lights back around to off position as per normal.
Not sure why they are coming up on the park light setting, never ever seen this before in almost 3 decades!
Thanks all 🙂
Got a 1990 6 speed coupe here that has a totally different behaviour when the headlight switch is turned on or off.
When turning to the first setting, the headlights roll around but stay off. Park lights, side markers, and fog lights work as expected on this setting.
When on the ON setting the headlights come on as normal, all else operates as it should.
Turning switch to off will roll the lights back around to off position as per normal.
Not sure why they are coming up on the park light setting, never ever seen this before in almost 3 decades!
Thanks all 🙂
Melting Slicks
Joined: Nov 2016
Posts: 2,256
Likes: 3,083
From: So Cal
2023 C4 of the Year Finalist - Unmodified
Put your meter on the yellow wire at the module. Backprobe the connection, so you are monitoring the complete circuit. What is the voltage in park lights? If not really close to 0-Volts, this is what is signalling the module to open the doors.
The yellow wire is the output from the headlight switch to the dimmer switch. In park lights, this wire should be 0-volts. But if something in the park lights are backfeeding into the headlight circuit, this could raise the volteage enough to open the doors. LED headlight bulbs? Bad ground on one or both sides of the front light harness?
Try your testing with the dimmer switch in both high and low beam positions. If the condition exists in only one of the beam positions, that should help narrow down the source of the back-feed. Also try turning off the fog lights with the switch and see if the back-feed ceases.
ADD: My thought is that you have LED headlight bulbs. And that with the fog light switch ON, dimmer switch in High Beam, the fog relay coil is back-feeding the yellow wire through the dimmer switch.
Cheers.
The yellow wire is the output from the headlight switch to the dimmer switch. In park lights, this wire should be 0-volts. But if something in the park lights are backfeeding into the headlight circuit, this could raise the volteage enough to open the doors. LED headlight bulbs? Bad ground on one or both sides of the front light harness?
Try your testing with the dimmer switch in both high and low beam positions. If the condition exists in only one of the beam positions, that should help narrow down the source of the back-feed. Also try turning off the fog lights with the switch and see if the back-feed ceases.
ADD: My thought is that you have LED headlight bulbs. And that with the fog light switch ON, dimmer switch in High Beam, the fog relay coil is back-feeding the yellow wire through the dimmer switch.
Cheers.
Last edited by IHBD; Feb 3, 2025 at 01:18 PM.
Thread Starter
Le Mans Master
Joined: May 2005
Posts: 5,214
Likes: 65
From: Queensland, AUSTRALIA
Put your meter on the yellow wire at the module. Backprobe the connection, so you are monitoring the complete circuit. What is the voltage in park lights? If not really close to 0-Volts, this is what is signalling the module to open the doors.
The yellow wire is the output from the headlight switch to the dimmer switch. In park lights, this wire should be 0-volts. But if something in the park lights are backfeeding into the headlight circuit, this could raise the volteage enough to open the doors. LED headlight bulbs? Bad ground on one or both sides of the front light harness?
Try your testing with the dimmer switch in both high and low beam positions. If the condition exists in only one of the beam positions, that should help narrow down the source of the back-feed. Also try turning off the fog lights with the switch and see if the back-feed ceases.
ADD: My thought is that you have LED headlight bulbs. And that with the fog light switch ON, dimmer switch in High Beam, the fog relay coil is back-feeding the yellow wire through the dimmer switch.
Cheers.
The yellow wire is the output from the headlight switch to the dimmer switch. In park lights, this wire should be 0-volts. But if something in the park lights are backfeeding into the headlight circuit, this could raise the volteage enough to open the doors. LED headlight bulbs? Bad ground on one or both sides of the front light harness?
Try your testing with the dimmer switch in both high and low beam positions. If the condition exists in only one of the beam positions, that should help narrow down the source of the back-feed. Also try turning off the fog lights with the switch and see if the back-feed ceases.
ADD: My thought is that you have LED headlight bulbs. And that with the fog light switch ON, dimmer switch in High Beam, the fog relay coil is back-feeding the yellow wire through the dimmer switch.
Cheers.
Halogen H4 headlights, problem was there before i swaped out the sealed units though.
Halogen fog lights, they are stock ones, does the same with those on or off.
Does it with both high or low beam.
I haven't had a chance to check voltage at the yellow wire with parks on yet, i've had a couple other C4s come into the workshop that have had to take priority over my car at the moment, but i'll get to that as soon as i can.
thanks!!
