Low Beam Headlight Circuit Fault

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lightman02

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Does anyone have the lighting circuit diagram for an 2002 LS V8?? I was getting a bunch of bulb out messages on the message center when driving at night. However all the lights were working. I was able to read the control module with my friends scan tool and it said "low beam headlight circuit fault" So I am thinking I have a problem in my lighting circuit causing all these wierd errors when the lights are on. No problems during the day though.
 
I would check to make sure all the fuses or relays are good.
 
The gen II manual lists two troubleshooting charts for DTC B1794, one to use if the headlights don't work, and the other to use if the headlights won't go off. Neither would seem to fit with your description of the problem. If your headlights always function correctly, then I would suspect the FEM. Otherwise, it should be something in the wiring between the FEM and the headlights. It doesn't appear that it could be a fuse issue. There should be no relays (external to the FEM) involved unless you have factory HIDs.
 
joegr said:
The gen II manual lists two troubleshooting charts for DTC B1794, one to use if the headlights don't work, and the other to use if the headlights won't go off. Neither would seem to fit with your description of the problem. If your headlights always function correctly, then I would suspect the FEM. Otherwise, it should be something in the wiring between the FEM and the headlights. It doesn't appear that it could be a fuse issue. There should be no relays (external to the FEM) involved unless you have factory HIDs.
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Sorry to break off the topic, but factory HIDs require extra relays?
 
M4rk said:
Sorry to break off the topic, but factory HIDs require extra relays?
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They don't seem to require them - some members have added factory HID assemblies to their non-HID LSes without problem. However, the factory HID option does add a relay or two relays (sorry, I don't have my wiring diagrams handy) for the low beam HIDs.
 
joegr said:
The gen II manual lists two troubleshooting charts for DTC B1794, one to use if the headlights don't work, and the other to use if the headlights won't go off. Neither would seem to fit with your description of the problem. If your headlights always function correctly, then I would suspect the FEM. Otherwise, it should be something in the wiring between the FEM and the headlights. It doesn't appear that it could be a fuse issue. There should be no relays (external to the FEM) involved unless you have factory HIDs.
Click to expand...

Well a scan of the FEM is where I got the fault for headlight low-beam circuit fault. I thought there was a relay that the FEM controls for the lights themselves?? Also is there a grounding point I should be looking for that deals more with the headlight circuit??? I was getting high voltage of 16.5 when I had an alternator issue which is now resolved, I hope I didn't fry my FEM cause I only get the alerts sometimes and sometimes I don't get them at all which is strange.
 
lightman02 said:
Well a scan of the FEM is where I got the fault for headlight low-beam circuit fault. I thought there was a relay that the FEM controls for the lights themselves?? Also is there a grounding point I should be looking for that deals more with the headlight circuit??? I was getting high voltage of 16.5 when I had an alternator issue which is now resolved, I hope I didn't fry my FEM cause I only get the alerts sometimes and sometimes I don't get them at all which is strange.
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If the gen I is like the gen II, and you don't have HIDs, then the only relays involved are internal to the FEM.
 
joegr said:
If the gen I is like the gen II, and you don't have HIDs, then the only relays involved are internal to the FEM.
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I'm very surprised to see this because every car I have seen that uses a computer to interact with the headlamps will use a relay in the fuse box. The FEM does have an internal switch such as a transistor type device to control I would think for an external relay coil, that relay is usually what carries the actual headlight load and not the computer itself. I could be wrong I just have never seen it like this on a modern car. If that is the case, that sucks.
 
lightman02 said:
I'm very surprised to see this because every car I have seen that uses a computer to interact with the headlamps will use a relay in the fuse box. The FEM does have an internal switch such as a transistor type device to control I would think for an external relay coil, that relay is usually what carries the actual headlight load and not the computer itself. I could be wrong I just have never seen it like this on a modern car.
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Sorry, I don't have a link to gen I data to post, but here's what the gen II service manual has on it. I do have the gen II wiring diagrams in print form. They show headlight relays only for the HID option. Ford has been somewhat consistent (on the cars I have dealt with) about just having non-replaceable (unless you have a soldering iron) headlight relays in the FEM or LCM (Lighting Control Module) or GEM, depending on what car you are talking about. In the early 2000s, the LCMs on Crown Vics and Grand Marquis are well known for internal relay failure and PCB soldering issues.

( I should point out that power for the headlights as well as most of the rest of the car is switched by relays in the junction box in the trunk. These relays turn on when you open a door or do most anything else, and turn off 20 minutes after you last disturb the car (if the key is off). If one of these were at fault, you should have far more problems than headlights.)

http://deneau.info/ls/s6x~us~en~file=s6xh1003.htm~gen~ref.htm
 
joegr said:
Sorry, I don't have a link to gen I data to post, but here's what the gen II service manual has on it. I do have the gen II wiring diagrams in print form. They show headlight relays only for the HID option. Ford has been somewhat consistent (on the cars I have dealt with) about just having non-replaceable (unless you have a soldering iron) headlight relays in the FEM or LCM (Lighting Control Module) or GEM, depending on what car you are talking about. In the early 2000s, the LCMs on Crown Vics and Grand Marquis are well known for internal relay failure and PCB soldering issues.

( I should point out that power for the headlights as well as most of the rest of the car is switched by relays in the junction box in the trunk. These relays turn on when you open a door or do most anything else, and turn off 20 minutes after you last disturb the car (if the key is off). If one of these were at fault, you should have far more problems than headlights.)

http://deneau.info/ls/s6x~us~en~file=s6xh1003.htm~gen~ref.htm
Click to expand...

Actually you are right I was able to find a block diagram off all least likely places, Autozone!! It seems the gen2 is like the gen1 in that respect and I do not have HID's so no relays. If you could send me the diagrams for the gen2 I would appreciate it as it looks probably the same as gen 1.
 
lightman02 said:
Actually you are right I was able to find a block diagram off all least likely places, Autozone!! It seems the gen2 is like the gen1 in that respect and I do not have HID's so no relays. If you could send me the diagrams for the gen2 I would appreciate it as it looks probably the same as gen 1.
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Okay, I have them, but they are too big to post here. PM me your e-mail address, and I will e-mail them to you.
 
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