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Truck is a 2003 f250 7.3l truck with auto headlights.
I have the 3 power blocks under the hood. One is 4wd hi/lo and lo/hi relays. One is trailer tow stuff and with a/c and 3rd is DRL relays? But I have auto headlights?
Its a Canadian truck.
From my searching MFS could be culprit but wouldn't be the problem if I had power at rd/yel wire and rd/bl wire with lts on. This checked out ok
No power at headlight pigtail for low beam. Does have power for high beam
Low beam Fuses 46 & 47 have no power at all on either side of fuse. Fuse itself is ok.
Fuse 7 is ok
Help? Can the headlight switch be the issue? If so how can I test. How can I test continuity between MFS and low beam fuses? Not sure what pins on harness are headlight power on the back of the power distribution block. Can it be something else?
I'm not to familiar with the later models, but I do know that in pretty much any circuit the overload protection is the first link in the chain. It's there to protect the wiring from overheating and catching fire, so they tend to be as close to the power source as feasible. The fact that you're getting no power to either side of the fuse on either light has me asking why, so that's where I would start my troubleshooting.
What about fuse #14 under the dash?
Do the DRL work?
It looks like power to #46/47 comes from Pin #30 of the DRL #2 relay. The MFS pin #9 (red/black) feeds pin 87a on DRL#2 12v when the headlights are on low beam. The DRL circuit feeds DRL #2 pin 87 constantly which should power the low beams while the MFS doesn't.
What about fuse #14 under the dash?
Do the DRL work?
It looks like power to #46/47 comes from Pin #30 of the DRL #2 relay. The MFS pin #9 (red/black) feeds pin 87a on DRL#2 12v when the headlights are on low beam. The DRL circuit feeds DRL #2 pin 87 constantly which should power the low beams while the MFS doesn't.
Fuse 14 is good.
My drl haven't worked in years actually. Everything else worked so I just didn't bother with it.
I'll have to Check the power at the relay. Red/black wire from the MFS must be the trigger wire for the relay.
Now dumb question of the day. Which of those 2 relay boxes is the trailer tow and which is drl? One closest to the fender or one closest to the engine?
The info you provided is exactly what I am looking for thank you!
I'll have to Check the power at the relay. Red/black wire from the MFS must be the trigger wire for the relay.
No, the rd/blk is the headlight power, the drl relay is a double post relay that switches between normally open and normally closed to choose which power to sent to the headlights (fuse 14 thru the drl resistor or the rd/blk from the MFS).
The drl relay trigger wire comes from the drl module, it uses a series of relays and I didn't look into how they work together.
If your drl is inop and you don't care, (I dislike drl and disable it on all my vehicles) I believe you could remove the drl relay and jumper 30-87a, which should disable the drl and hard wire the MFS to the headlights. I'll double check the schematic...
I'm not sure which relay box is which.
You said you have power at the rd/blk coming out of the MFS, does it switch with the headlights switch on-off or the hi-low selector?
No, the rd/blk is the headlight power, the drl relay is a double post relay that switches between normally open and normally closed to choose which power to sent to the headlights (fuse 14 thru the drl resistor or the rd/blk from the MFS).
The drl relay trigger wire comes from the drl module, it uses a series of relays and I didn't look into how they work together.
If your drl is inop and you don't care, (I dislike drl and disable it on all my vehicles) I believe you could remove the drl relay and jumper 30-87a, which should disable the drl and hard wire the MFS to the headlights. I'll double check the schematic...
I'm not sure which relay box is which.
You said you have power at the rd/blk coming out of the MFS, does it switch with the headlights switch on-off or the hi-low selector?
Funny you ask. With headlights off, the red/black coming out of the MFS was still showing like 9v. With low beams on, 12v. I think high beams was also showing 12v.
I wasn't overly worried about the 9v thing since that should be below the threshold to activate the relay 🤷♂️
I was a little wrong about how the DRL's are triggered, but my suggestion of bypassing the DRL relay (30-87a) is still valid.
Schematic shows the DRL and headlight relays are in 'Relay box 2' at the left rear of the engine compartment, for whatever thats worth.
I'll try to upload a schematic for you here, but I've had bad luck maintaining resolution uploading to FTE in the past.
I don't think it likes my .pdf, I'll try a screenshot as well.
So I jumpered 30/87a and low beams work as they should. With headlights off I'm seeing 10v at 87a and with headlights on I'm seeing 12v at 87a. But low beams are off when they should be and on when they should be while jumper wire is in place.
Id prefer to fix this properly then run the jumper.
Pin 85 has constant 12v.
Pin 86 is a good ground.
30 to headlights
87 is drl with resistor. (resistor probably bad? Hence no drl?) I have power at fuse 14, but no power at pin 87
87a seems to function correctly with MFS.
Other than a bad relay, which doesn't seem to be the case, I've now tried 3 or 4 relays. Why would my low beams not function with a relay in place
So I jumpered 30/87a and low beams work as they should. With headlights off I'm seeing 10v at 87a and with headlights on I'm seeing 12v at 87a. But low beams are off when they should be and on when they should be while jumper wire is in place.
Id prefer to fix this properly then run the jumper.
Pin 85 has constant 12v.
Pin 86 is a good ground.
30 to headlights
87 is drl with resistor. (resistor probably bad? Hence no drl?) I have power at fuse 14, but no power at pin 87
87a seems to function correctly with MFS.
Other than a bad relay, which doesn't seem to be the case, I've now tried 3 or 4 relays. Why would my low beams not function with a relay in place
So you now want to also regain the DRL functions as well?
If so your gonna need to fix the open circuit or failed component between Fuse #14 and terminal 87 on DRL Relay #2. I agree from looking at the schematic that it's likely the resistor or connections to it.
I assume this is all with the key on?
Are you using the Auto function, or just using the headlight switch in the manual mode? Everything below is assuming the headlight switch is in the On' position, not 'Auto'...
If you have 12v at 85 the DRL relay #1 is closed and attempting to power the DRLs. Check for voltage at pin #1 of DRL Relay #1. For troubleshooting I think you can also remove Relay #1 and see if the headlights function normally sans DRLs. If DRL Relay #1 has failed (in the normally closed position) I think it would cause your low beam issues as well.
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