andres008

It started 4 days ago. I describe the symptoms: When i turn the lights (low beam) on, the fuse # 40 blows. After that i do not have low beams nor high beams. Parking lights are working and the foglamps are woking. When I replace fuse # 40 the high beams ( flash-to-pass) are working. Switching to parking lights...everything is ok, switching to low beams...fuse # 40 blows instantly.
What could cause the problem ? What is the solution?

By the manual fuse # 40 specification is:
DRL relays, DRL foglamps, DRL
headlamp relay coil, Headlamp switch
(headlamps), Multifunction switch
(flash-to-pass), BSM (autolamp
headlamps relay), High beam relay coil,
Fuse 25 (LH low beam), Fuse 23 (RH low beam)

donhall

If there are no obvious signs of moisture (corrosion) in the fuse panel, especially fuse #40, then the next step would be: key in off position. disconnect the headlight switch and measure the voltage between the headlamp switch C205a pin 8, circuit 196 (DB/OG), harness side and ground to determine if the voltage is greater than 10 volts. Since you are blowing a 20 amp fuse, that is a reliable indication that the voltage is in excess of 20 amps. Depending on your voltage reading, there is a diagnostic path to follow to solve the problem (see factory service manual).

OC2009

We are having the same problem with our Expedition. We keep replacing the fuse, it lasts for anywhere from 1 minute to 1 week.
We took to a vehicle electrician at pep boys, but he couldn't figure it out.

Did you figure out what the problem was?

Thanks

wwwabbit

Eh? The voltage is not amps. Two VERY different things, although with ohms law they can work in relationship with each other.

Using a multi-meter in resistance mode would be your best start. Take the "-" lead and find a good ground to attach it to. Pull the fuse, in "DC Voltage mode" then find the "hot" side of the fuse, that is the side with 12v on it. Put the meter in resistance mode and stick your probe in the other side. Now, pop the hood and disconnect both your headlights. Now with the switch off, take a look at your resistance reading, you should have a very high resistance. Now turn the switch on to the parking lights, take note and adjust the scale as needed to see a value, there should be a lower resistance reading (but not a "short" to ground ("0" resistance)) now switch the headlight switch to turn on the headlights, if the short is in the wiring, you will see the resistance go to "0" to indicate a short. If it shows no or little change in resistance then start to connect your headlights, it may be one of them that is shorted internally. If it shows a short with the headlights disconnected trace your wiring, something has the wire for your headlights grounded (touching the metal body)

If when the headlights are on, and the bulbs are connected there is no short noted, take a look for a corroded connection. Poor connections can cause voltage drops, which with no change in resistance will increase the amperage. However, normally a poor connection will also cause high resistance and you end up with low voltage and low current.