Both Low Beams not working
Thread Starter
Driver School Candidate
Joined: Nov 2020
Posts: 23
Likes: 0
From: Arizona
Do you know how a mechanical relay works?
In short a low energy circuit gets a signal, this signal drives an electromagnet, the electromagnet closes a switch connecting the battery +12V to your headlights in this case.
Pin #1 is part of the electromagnet circuit. When off, it floats. When triggered by the body computer, it goes to ground making a circuit. Place your test lamp ground to Batt Neg. Stick the probe in pin#1 and turn the lights on. Do NOT use a 55w lamp. Just a low voltage/low current test lamp.
Google schematics for relays. It will help. Many systems use high level logic (+12v) to trigger subsystems. Others like this one use low level logic -12v = ground to complete the circuit and trigger the relay.
In short a low energy circuit gets a signal, this signal drives an electromagnet, the electromagnet closes a switch connecting the battery +12V to your headlights in this case.
Pin #1 is part of the electromagnet circuit. When off, it floats. When triggered by the body computer, it goes to ground making a circuit. Place your test lamp ground to Batt Neg. Stick the probe in pin#1 and turn the lights on. Do NOT use a 55w lamp. Just a low voltage/low current test lamp.
Google schematics for relays. It will help. Many systems use high level logic (+12v) to trigger subsystems. Others like this one use low level logic -12v = ground to complete the circuit and trigger the relay.
Driver
Joined: Jan 2019
Posts: 83
Likes: 19
From: nil
Troubleshoot electrical issue is always time consuming, and at time frustrating. it is probably best you take it to your mechanic and have him/her troubleshoot it.
The body ecu that you should check next is inside the fuse panel on the passenger side by the glove compartment. I only know that pin #8 is the pin that energize the relay, but I have no idea where that pin is on the body ecu harness. It should be the red/white (or white/red?) wire. If you want to tackle this yourself, try google as much as you can or go get a subscription at alldatadiy.com and download all the relevant wiring diagrams and troubleshoot guide.
By the way, the body ecu might be working fine, it could be just contact issue due to corrosion, or loose connector on the back of the HEAD LP relay. You can first try remove the fuse box and check the relay socket connection, make sure there is good contact on all the pins.
The body ecu that you should check next is inside the fuse panel on the passenger side by the glove compartment. I only know that pin #8 is the pin that energize the relay, but I have no idea where that pin is on the body ecu harness. It should be the red/white (or white/red?) wire. If you want to tackle this yourself, try google as much as you can or go get a subscription at alldatadiy.com and download all the relevant wiring diagrams and troubleshoot guide.
By the way, the body ecu might be working fine, it could be just contact issue due to corrosion, or loose connector on the back of the HEAD LP relay. You can first try remove the fuse box and check the relay socket connection, make sure there is good contact on all the pins.
Thread Starter
Driver School Candidate
Joined: Nov 2020
Posts: 23
Likes: 0
From: Arizona
Troubleshoot electrical issue is always time consuming, and at time frustrating. it is probably best you take it to your mechanic and have him/her troubleshoot it.
The body ecu that you should check next is inside the fuse panel on the passenger side by the glove compartment. I only know that pin #8 is the pin that energize the relay, but I have no idea where that pin is on the body ecu harness. It should be the red/white (or white/red?) wire. If you want to tackle this yourself, try google as much as you can or go get a subscription at alldatadiy.com and download all the relevant wiring diagrams and troubleshoot guide.
By the way, the body ecu might be working fine, it could be just contact issue due to corrosion, or loose connector on the back of the HEAD LP relay. You can first try remove the fuse box and check the relay socket connection, make sure there is good contact on all the pins.
The body ecu that you should check next is inside the fuse panel on the passenger side by the glove compartment. I only know that pin #8 is the pin that energize the relay, but I have no idea where that pin is on the body ecu harness. It should be the red/white (or white/red?) wire. If you want to tackle this yourself, try google as much as you can or go get a subscription at alldatadiy.com and download all the relevant wiring diagrams and troubleshoot guide.
By the way, the body ecu might be working fine, it could be just contact issue due to corrosion, or loose connector on the back of the HEAD LP relay. You can first try remove the fuse box and check the relay socket connection, make sure there is good contact on all the pins.
5th Gear
Joined: May 2023
Posts: 5
Likes: 0
From: CA
My low beams don’t work on my 2007 is350
This issue has been beyond frustrating. I tried checking all of my fuses that related to the headlights but they’re all intact and fine. I changed the headlamp relay and they still don’t work. But my high beams still work. Does anyone have a clear answer on how to fix this?
Racer
Joined: Feb 2017
Posts: 1,266
Likes: 331
From: Ontario
This issue has been beyond frustrating. I tried checking all of my fuses that related to the headlights but they’re all intact and fine. I changed the headlamp relay and they still don’t work. But my high beams still work. Does anyone have a clear answer on how to fix this?
5th Gear
Joined: May 2023
Posts: 5
Likes: 0
From: CA
Thread Starter
Driver School Candidate
Joined: Nov 2020
Posts: 23
Likes: 0
From: Arizona
So I wasnt able to exactly pinpoint my issue, but im aiming towards maybe a fault in my BCM/MPX. My fix was wiring a relay inline with the park lights to power my low beams. Basically when my park lights go on, so do my low beams. My stalk and indicator were working perfectly fine. Had no power going to the pass side engine fusebox to energize the relay. I believe my issue stemmed from an aftermarket HID kit the PO installed and it wasnt really quality (Drilled holes through the plastic weather cover over the low beam and used weather seal to seal it up while having the ballast and relay basically hanging outside the headight not supported or strapped in on anything). Issue happened right after I went to a touchless car wash and im guessing the high pressure water or soap shorted out the wiring from the ballast low beam wiring all the way to the BCM. Hope this helps. I do plan on properly fixing this issue but im not sure if the BCM is trim related (LUX pkg, sport pkg, x pkg) since other trims have other modules like electric telescopic adjustment, rear shade, afs etc. If all trims share the same BCM ill go ahead and replace mine to see if it will fix my issue.
Last edited by jusss2is; May 29, 2023 at 10:02 PM.
5th Gear
Joined: May 2023
Posts: 5
Likes: 0
From: CA
So I wasnt able to exactly pinpoint my issue, but im aiming towards maybe a fault in my BCM/MPX. My fix was wiring a relay inline with the park lights to power my low beams. Basically when my park lights go on, so do my low beams. My stalk and indicator were working perfectly fine. Had no power going to the pass side engine fusebox to energize the relay. I believe my issue stemmed from an aftermarket HID kit the PO installed and it wasnt really quality (Drilled holes through the plastic weather cover over the low beam and used weather seal to seal it up while having the ballast and relay basically hanging outside the headight not supported or strapped in on anything). Issue happened right after I went to a touchless car wash and im guessing the high pressure water or soap shorted out the wiring from the ballast low beam wiring all the way to the BCM. Hope this helps. I do plan on properly fixing this issue but im not sure if the BCM is trim related (LUX pkg, sport pkg, x pkg) since other trims have other modules like electric telescopic adjustment, rear shade, afs etc. If all trims share the same BCM ill go ahead and replace mine to see if it will fix my issue.
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Thread Starter
Driver School Candidate
Joined: Nov 2020
Posts: 23
Likes: 0
From: Arizona
In simpler terms, the previous owner of my car installed an aftermarket HID kit, since my car had the halogen headlights as opposed to the HID version. The way they installed it isnt ideal, though it works. Somehow the kit shorted out my stock wiring harness and messed with the BCM which is one of the body ECUs are cars have the control other peripherals, in this situation the low beam headlights. When did this happen to yours? Do you have an aftermarket HID kit?
5th Gear
Joined: May 2023
Posts: 5
Likes: 0
From: CA
In simpler terms, the previous owner of my car installed an aftermarket HID kit, since my car had the halogen headlights as opposed to the HID version. The way they installed it isnt ideal, though it works. Somehow the kit shorted out my stock wiring harness and messed with the BCM which is one of the body ECUs are cars have the control other peripherals, in this situation the low beam headlights. When did this happen to yours? Do you have an aftermarket HID kit?
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