Notices
General Tech Ask model specific questions in the appropriate category below. All other general questions within.

Headlight brown out...short to ground?

Old 05-16-2015, 10:57 AM
  #1  
03Mach1AirForce
Thread Starter
 
Join Date: May 2013
Location: GU
Posts: 8
Default Headlight brown out...short to ground?

I have an '03 Mach 1, bone stock. The passenger side headlight is browned out. I've seen this on other cars, but it's never happened to me before. From the searches I've done, it appears I've got a short to ground somewhere.

Details:
The headlight bulb is new (replaced ~2 months ago during headlight restore) and both filaments emit light.
The headlight connector (female side) is in good shape with no apparent damage.
Fuses (30A maxi fuse in power distro box, and 10A fuse in passenger compartment) are good with clean contacts.
Following the wiring harness as best I can, I find no damage or frayed wires.

Here's the interesting part: Disconnecting the headlight ground (just beside the hood prop pivot) does not disable the browned out headlight. It does disable the running light, but upon close inspection, the running light bulb is still faintly lit, similar to the barely lit headlight bulb.

This tells me those light bulbs are finding a path to ground somewhere other than (and I guess before) the proper ground, but I have no idea where.

I am not that familiar with electrical systems, and only barely know how to use my analog multimeter. According to these how-to articles, I tested the battery (ignition off) and got 12v. I back-probed the headlight connector and got 11v. But when I test across the fuses (on the test contacts on their tops) I get nothing when the circuit is energized, so I'm certain I'm doing something wrong.

That's about how far I get. The proper headlight ground works, because it disables the running lamp when disconnected. The brown out is only affecting the passenger side light cluster. I'm assuming there's a short to ground somewhere but I don't know where and I don't know for sure.

Where do I go from here?

Thanks,

J
03Mach1AirForce is offline  
Old 05-17-2015, 01:41 PM
  #2  
03Mach1AirForce
Thread Starter
 
Join Date: May 2013
Location: GU
Posts: 8
Default

Any ideas? I realize it's only been a day. I'm going to head to the parts store and see if anybody there can give me some pointers.
03Mach1AirForce is offline  
Old 05-18-2015, 04:15 PM
  #3  
jwog666
5th Gear Member
 
jwog666's Avatar
 
Join Date: Mar 2008
Location: wisconsin
Posts: 2,260
Default

without being able to physically check your powers and grounds, getting to the bottom of this will be tough. i suggest taking it in to be looked at
jwog666 is offline  
Old 05-19-2015, 06:58 AM
  #4  
88 orangepeel notch
6th Gear Member
 
88 orangepeel notch's Avatar
 
Join Date: Mar 2010
Location: Wisconsin
Posts: 5,062
Default

Not a bad idea ^^^

Unless your willing to tear down to get to the complete wiring harness, which will be some work, then trace wires till your blue in the face. That's about the time you start thinking you should have taken it in to start with. Electrical gremlins really suck.

I'd still dig into the socket/bulb area a little more yet, wiggling wires and connections. Swapping bulbs from other side, as that area takes most of the abuse from the elements.

Good luck
88 orangepeel notch is offline  
Old 05-20-2015, 04:13 AM
  #5  
petrock
4th Gear Member
 
Join Date: Nov 2006
Location: CA
Posts: 1,670
Default

Originally Posted by 03Mach1AirForce View Post
I have an '03 Mach 1, bone stock. The passenger side headlight is browned out. I've seen this on other cars, but it's never happened to me before. From the searches I've done, it appears I've got a short to ground somewhere.
A short to ground on the power side would cause the fuse to pop. A short to ground on the ground side could cause the symptoms you describe. Most likely it is a damaged/corroded wire/connector on the ground side touching ground given the symptoms you describe.


Originally Posted by 03Mach1AirForce View Post
Here's the interesting part: Disconnecting the headlight ground (just beside the hood prop pivot) does not disable the browned out headlight. It does disable the running light, but upon close inspection, the running light bulb is still faintly lit, similar to the barely lit headlight bulb. This tells me those light bulbs are finding a path to ground somewhere other than (and I guess before) the proper ground, but I have no idea where.
Yes, you are correct that it is getting ground from more then just its normal grounding point. This can happen when a wire or connector is damaged and the bare metal in the wire/connector touches some other grounded metal. Or a connector has corrosion inside of it that links two pins together.


Originally Posted by 03Mach1AirForce View Post
I tested the battery (ignition off) and got 12v. I back-probed the headlight connector and got 11v.
You should have gotten 12 volts at the connector. That 1 volt drop is why your headlight is dim and means that you have excessive resistance somewhere on the power side, however, given your symptoms I would have expected it on the ground side. A drop of more then .5 volts on a 12 volts circuit is considered excessive. So to double check do this with the headlight on, ground lead on your meter connected to battery ground terminal, back probe the power wire at the headlight connector again. What voltage do you get? Now back probe the ground wire at the headlight connector. What voltage do you get? With the power & ground lead touching batter positive & battery negative terminals what voltage do you get?

Originally Posted by 03Mach1AirForce View Post
But when I test across the fuses (on the test contacts on their tops) I get nothing when the circuit is energized, so I'm certain I'm doing something wrong.
That is normal. A volt meter only tells you the difference in voltage between the positive and negative leads. If both sides of the fuse read the same voltage then you will get a reading of zero volts. Your reading of zero means that the fuse is good. If you had the ground lead on the meter touching the ground terminal on the battery and touched the positive meter leads to each pin on top of the fuse you should get battery voltage.


Originally Posted by 03Mach1AirForce View Post
Where do I go from here?
I’d get the wiring diagram for your year/model ride and start poking around using voltage drop testing. You can do a binary search (divide & conquer) to find the problem section of wire. The problem area of wire will be between the last place you got a good voltage reading and the last place you got a bad voltage reading. Try to avoid poking holes in wires because that just creates a vector for moisture to get into the wire and corrode it from the inside out over time. The proper method is to back-probe connectors.
petrock is offline  
Old 05-21-2015, 05:51 AM
  #6  
03Mach1AirForce
Thread Starter
 
Join Date: May 2013
Location: GU
Posts: 8
Default Headlight problem - SOLVED

You guys are awesome. Petrock, especially, thank you for the considered break down.

So after much wailing and gnashing of teeth and pulling the intake tube and battery/battery tray to physically check the harness, and getting under the car to check all the grounds I could, I finally got around to swapping the bulbs from left to right.

It's a bad bulb. I have no idea what went wrong inside the bulb, but as I said it's brand new, and the filaments aren't broken. So I went and bought a new one. Phillips doesn't warranty their headlight bulbs, and Pep Boys wouldn't replace it either. Oh well.

Turns out to have been moot anyway: I just bought replacement headlight and foglight housings from Latemodel Restoration, and they came with bulbs already installed.

This is part of an overall restoration(?) I'm doing to return the exterior of the car to original condition. 13 years of spending every single day in the sun has taken a toll. New plastic trim pieces and new weather strips are next, followed by a complete body teardown and paint job later this summer.

Thanks again for the help!

J
03Mach1AirForce is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
jwog666
Pipes, Boost & Juice
6
01-07-2018 07:16 AM
Thunderball
4.6L (1996-2004 Modular) Mustang
15
11-22-2015 11:49 PM
flash_xx
S197 Handling Section
14
10-16-2015 07:23 AM
Drastang
4.6L (1996-2004 Modular) Mustang
3
09-29-2015 03:31 PM
Mobiusone stang
2005-2014 Mustangs
7
09-07-2015 08:46 PM


Thread Tools
Search this Thread
Quick Reply: Headlight brown out...short to ground?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.