What Is The Max Amp Draw The OEM Front And Rear Fog Light Circuits Will Support?

[Steve_In_29]

As the title states, I'm interested in what amp load the OEM circuits will support. From what I gather there are no fuses for these circuits?

Wanting to know when it's safer to just use them to activate an add on relay to provide power to aftermarket lights

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[JediMcMuffin]

The wires for the aux switches run to fuses.
The switches in the cabin control relays.
The pre-run wires are all 16AWG, and are a bit of a choose your own adventure, but max amp draw will depend on what length you're interacting with.

Some AI slop to pile on:

How that maps to the Bronco (this is the “don’t fry stuff” layer)

Engine bay taps (B1/B2/C)

• Usually short runs (3–6 ft effective)
• Realistic usable current:
  • ~5–10A clean
  • 15A+ if you accept voltage sag

Translation:

• Your 15A AUX2 is fine here
• Even 30A AUX1 won’t be fully usable unless the load is very close

Windshield / roof (Circuit D)

• Longer path, more connectors
• Treat like 10–15 ft equivalent

Safe range:

• 2–5A ideal
• 6–8A max if you don’t care about drop

Rear cargo (Circuit E)

• Longest run in the vehicle
• Think 15–20 ft equivalent

Reality:

• 1–3A ideal
• ~5A practical ceiling

This is why people get confused when their fridge “works but acts weird”


The uncomfortable truth Ford doesn’t spell out

• The system is fuse-limited, not wire-optimized
• Just because:
  • AUX1 = 30A
    does NOT mean:
  • You can pull 30A at the rear of the truck without consequences

This is basically:
“We gave you power everywhere”
but quietly:
“We did not guarantee voltage integrity”

 

[Steve_In_29]

Thanks for the info however it is NOT what I asked.

My question was specifically about the OEM fog light circuits. Both the (US spec) front and (Euro spec) rear fog lights.

Which are controlled by the headlight switch.

 

[CalvinT]

As the title states, I'm interested in what amp load the OEM circuits will support. From what I gather there are no fuses for these circuits?

Wanting to know when it's safer to just use them to activate an add on relay to provide power to aftermarket lights

Delcity sells a number of different relays that will switch 30A, the coils draw less than 150ma. This should be no problem for the fog light circuits.

It seems you're correct, there are no fuses for the OEM fog lights. But that shouldn't be a problem as long as you don't make any mistakes in hooking up the relays. If one of your added wires shorts to ground you're SOL.

Ford uses low power FETs to control power to those three circuits. I'm guessing they designed the system so that one of the FETs will blow before a wire melts. That means at best case you'll have to replace your BCM, at worse case you'll be replacing your Bronco.

What you propose doing is feasable. It boils down to your confidence in your wiring skills and risk tolerance.

By the way, DO NOT use wire they sell in Auto Zone, O'Reilly's, etc. Buy wire that meets modern automotive specications. The wires I mentioned don't. They don't meet temperature or abrasion standards.

 

[Rumbloki]

The DD SS3 pulls 3A (38.5W), and works in that spot.
You could use a meter to check the draw on the factory light. Won't tell you the max available, but may raise the bar over the SS3.

 

[CalvinT]

You could use a meter to check the draw on the factory light.

Only two ways to measure current. Cut the wire and insert an amp meter or find a meter that uses a clamp on probe for DC. Measuring the resistance of the Fog lights won't work because they're LEDs.

 

[JediMcMuffin]

Sorry I misunderstood. I'm running Baja SAE fogs off the front wires no issue, but couldn't tell you what the limits are.

 

[Steve_In_29]

Sorry I misunderstood. I'm running Baja SAE fogs off the front wires no issue, but couldn't tell you what the limits are.

What is the wattage of a pair of your lights?

 

[Rumbloki]

Only two ways to measure current. Cut the wire and insert an amp meter or find a meter that uses a clamp on probe for DC. Measuring the resistance of the Fog lights won't work because they're LEDs.

You can use scrap wire to plug into the female connector.

 

[Steve_In_29]

You can use scrap wire to plug into the female connector.

There has to be a load on the circuit to measure the amp draw. Volts are pushed but amps are pulled.

And the amp measurement will only tell you the load at that moment, not the max load the circuit will handle.

 

[Q1svt]

Wanting to know when it's safer to just use them to activate an add on relay to provide power to aftermarket lights

… normally the go to is Ford Pro Upfitter documentation
https://www.fordpro.com/en-us/upfit/publications/

But as posts here, just to much unknown. I chose to be safe and added relays I trust…. (@CalvinT ).

There is a nice space inter driver fender. I’m adding eight, one for each aux, plus two

 

[Ducati1098]

Ford does not give a spec anywhere. This is all it says in the workshop manual:

Field Effect Transistor (FET) Protection

The BCM utilizes a Field Effect Transistor (FET) protective circuit strategy for many of its outputs, for example, lamp output circuits. Output loads (current level) are monitored for excessive current (typically short circuits) and are shut down (turns off the voltage or ground provided by the module) when a fault event is detected.

A FET is a type of transistor that the control module software uses to control and monitor current flow on module outputs. The FET protection strategy prevents module damage in the event of excessive current flow.

Output loads (current level) are monitored for excessive current draw (typically short circuits). When a fault event is detected the FET turns off and a short circuit DTC sets. The module resets the FET protection and allows the circuit to function when the fault is corrected or the ignition state is cycled off and then back on.

When the excessive circuit load occurs often enough, the module shuts down the output until a repair procedure is carried out. Each FET protected circuit has 3 predefined levels of short circuit tolerance based on a module lifetime level of fault events based upon the durability of the FET. If the total tolerance level is determined to be 600 fault events, the 3 predefined levels would be 200, 400 and 600 fault events.

When each level is reached, the DTC associated with the short circuit sets along with DTC U1000:00. These Diagnostic Trouble Codes (DTCs) are cleared using the module on-demand self-test, then the Clear DTC operation on the scan tool (if the on-demand test shows the fault corrected). The module never resets the fault event counter to zero and continues to advance the fault event counter as short circuit fault events occur.

If the number of short circuit fault events reach the third level, then DTCs U1000:00 and U3000:49 set along with the associated short circuit DTC . DTC U3000:49 cannot be cleared and the module must be replaced after the repair.

All that being said, I think a safe/conservative limit would be ~4, maybe 5A per side.

 

[Rumbloki]

There has to be a load on the circuit to measure the amp draw. Volts are pushed but amps are pulled.

And the amp measurement will only tell you the load at that moment, not the max load the circuit will handle.

Your meter goes between the two connectors. Use a wire between the connectors for the other leg. The factory fog is the load.
As I said earlier, " Won't tell you the max available, but may raise the bar over the SS3. "

 

[dgorsett]

Ford does not give a spec anywhere. This is all it says in the workshop manual:

Field Effect Transistor (FET) Protection

The BCM utilizes a Field Effect Transistor (FET) protective circuit strategy for many of its outputs, for example, lamp output circuits. Output loads (current level) are monitored for excessive current (typically short circuits) and are shut down (turns off the voltage or ground provided by the module) when a fault event is detected.

A FET is a type of transistor that the control module software uses to control and monitor current flow on module outputs. The FET protection strategy prevents module damage in the event of excessive current flow.

Output loads (current level) are monitored for excessive current draw (typically short circuits). When a fault event is detected the FET turns off and a short circuit DTC sets. The module resets the FET protection and allows the circuit to function when the fault is corrected or the ignition state is cycled off and then back on.

When the excessive circuit load occurs often enough, the module shuts down the output until a repair procedure is carried out. Each FET protected circuit has 3 predefined levels of short circuit tolerance based on a module lifetime level of fault events based upon the durability of the FET. If the total tolerance level is determined to be 600 fault events, the 3 predefined levels would be 200, 400 and 600 fault events.

When each level is reached, the DTC associated with the short circuit sets along with DTC U1000:00. These Diagnostic Trouble Codes (DTCs) are cleared using the module on-demand self-test, then the Clear DTC operation on the scan tool (if the on-demand test shows the fault corrected). The module never resets the fault event counter to zero and continues to advance the fault event counter as short circuit fault events occur.

If the number of short circuit fault events reach the third level, then DTCs U1000:00 and U3000:49 set along with the associated short circuit DTC . DTC U3000:49 cannot be cleared and the module must be replaced after the repair.

All that being said, I think a safe/conservative limit would be ~4, maybe 5A per side.

Oh my, This made my head hurt, I just spliced my $35 fog lights in to the factory wiring and went with it, I guess I should check it out a bit more....

EDIT: Just checked 42w so 3.5 a...OK

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