Headlights + Horn Issues – Dim/Yellow Lights, Cutting Off While Driving (2006 Mustang GT)

[metrolink]

I've been chasing headlight and horn issues on my 2006 Mustang GT. The passenger side headlight is rarely even on, just a very faint dim glow. The driver's side works for a moment in park, but as soon as I shift into drive, it shuts off after a few minutes. When it does light, it's yellowish and dim instead of bright white. It isn't as dim as the passenger side.

The only time both headlights worked properly was right after I tightened the positive battery terminal. They were bright and white for a day or two, then went back to the same problem. This was the only time it actually ever worked:

The horn doesn't work from the steering wheel either, but it does sound when locking the car or when the alarm goes off. That makes me think the horn itself is fine, and the issue is in the wiring or control path.

I've checked all the fuses and cleaned the grounds near the radiator and under the glove box (G100 + G201). Fog lamps work perfectly.

When I opened the passenger compartment SJB under the glove box, I noticed corrosion and found that Pin 39 on connector C2280B for the horn switch control is broken off and stuck in the connector. I tried cleaning the pins with 90% isopropyl alcohol and an acid brush, but no change to the headlights.


Could the pin be repaired or does the whole SJB need to be replaced? If replacement is the only option, what's the best place to source one?

Any advice is appreciated.

 

[metrolink]

Going to try and snag a pic and video of the headlights at night when i get the chance to share.*

 

[metrolink]

UPDATE: The First image is on the low beam position, and the second image is with the front fog lamps on.

After a couple of minutes, the headlights go out as seen below.


Originally, I thought it was the bulbs, so I replaced them with H13 halogen bulbs, but no change. Been driving with just the fog lamps.

 

[DieHarder]

Do you have a VOM (Volt/Ohm meter)? If not, get a decent one (Fluke/similar; you can find them on feebay) and check DC volts across the battery with the engine running. What do you have? Second, troubleshoot the circuit using the wiring diagram. Check fuses/relays and wiring for the headlights. Check the output of the SJB. Flex the wiring for the headlights. Do symptoms change?

Probably difficult to change that one pin for the horn but possible if they're individually separate. You'd have to check with a dealer.

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang 2005 Mustang Headlamps. autolamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Wiring Diagrams Headlamps. Autolamps

iihs.net

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang 2005 Mustang Headlamps. autolamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Wiring Diagrams Headlamps. Autolamps

iihs.net

 

[metrolink]

Do you have a VOM (Volt/Ohm meter)? If not, get a decent one (Fluke/similar; you can find them on feebay) and check DC volts across the battery with the engine running. What do you have? Second, troubleshoot the circuit using the wiring diagram. Check fuses/relays and wiring for the headlights. Check the output of the SJB. Flex the wiring for the headlights. Do symptoms change?

Probably difficult to change that one pin for the horn but possible if they're individually separate. You'd have to check with a dealer.

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang 2005 Mustang Headlamps. autolamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Wiring Diagrams Headlamps. Autolamps

iihs.net

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang 2005 Mustang Headlamps. autolamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Wiring Diagrams Headlamps. Autolamps

iihs.net

I checked with the key ON (engine off). Battery ~12.4V.
Also found that the headlight(s) only comes on fully when I hold the red tab halfway — all the way in or out doesn't work.

Passenger headlight connector readings:

• Low beam (brown/pink): ~7.2V (lights off/red tab is not positioned) → jumps to ~10.6V when I move the red lock tab halfway (lights on).
  
• High beam (green): ~7.0V with stalk forward (high beams ON) — bulb doesn't illuminate, low beam just dims. (left image below)
  • When I let the stalk go back to low beam, voltage jumps to ~11.6V. (right image below)

I also checked temps with an infrared gun. The passenger side wiring/connector was ~99°F and hot to the touch, while the driver's side was only in the low 80s.


Looks like resistance/heat buildup right at the connector, so I'm planning to replace the pigtail. I'll retest with the car running to confirm voltage. Haven't had much time because of work but just wanted to give a quick update on this.


Anything I should double-check before I splice in a new connector?

 

[DieHarder]

I checked with the key ON (engine off). Battery ~12.4V.
Also found that the headlight(s) only comes on fully when I hold the red tab halfway — all the way in or out doesn't work.
View attachment 110634
Passenger headlight connector readings:

• Low beam (brown/pink): ~7.2V (lights off/red tab is not positioned) → jumps to ~10.6V when I move the red lock tab halfway (lights on).
  View attachment 110638View attachment 110639
• High beam (green): ~7.0V with stalk forward (high beams ON) — bulb doesn't illuminate, low beam just dims. (left image below)
  • When I let the stalk go back to low beam, voltage jumps to ~11.6V. (right image below)

View attachment 110641View attachment 110644

I also checked temps with an infrared gun. The passenger side wiring/connector was ~99°F and hot to the touch, while the driver's side was only in the low 80s.
View attachment 110635View attachment 110636

Looks like resistance/heat buildup right at the connector, so I'm planning to replace the pigtail. I'll retest with the car running to confirm voltage. Haven't had much time because of work but just wanted to give a quick update on this.


Anything I should double-check before I splice in a new connector?

Anything I should double-check before I splice in a new connector?

Review attached for troubleshooting procedures to narrow down the problem (though the connector is highly suspect at this point).

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang Exterior Lighting Headlamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Workshop Manuals Electrical Lighting Exterior Lighting DIAGNOSIS AND TESTING Headlamps

iihs.net

Then, before you replace the pigtail strip the tape covering the wires back about a foot. Inspect the harness for burned wires and shorts between the wires. (I found a short in the headlight bulb wiring for my son's car that was also intermittent). I highly recommend soldering wires vs crimping if you have the means.

Using Ohms (key off) check for shorts between pins 1,3 at the connector C1041). Flex the wires and connector. If you read ohms (or you get a tone for continuity) there's a short between the hi beam/low beam wiring. Hopefully, the issue is within the connector itself (most likely) and/or between the connector and BEC vs the SJB (low current board).

After you replace the connector recheck voltage between pin 2 (ground) and pins 1 (low beam) & 3 (high beam) for both C1021/C1041. When measuring voltage each side of C1021 & C1041 should be the same (~ 12v). The headlight bulb itself is not needed for these tests.

 

[metrolink]

Anything I should double-check before I splice in a new connector?

Review attached for troubleshooting procedures to narrow down the problem (though the connector is highly suspect at this point).

Ford Workshop Service & Repair Manuals - fordrepair.info - 2005 Mustang Exterior Lighting Headlamps

Ford Workshop Service and Repair Manuals, 2005 Mustang Workshop Manuals Electrical Lighting Exterior Lighting DIAGNOSIS AND TESTING Headlamps

iihs.net

Then, before you replace the pigtail strip the tape covering the wires back about a foot. Inspect the harness for burned wires and shorts between the wires. (I found a short in the headlight bulb wiring for my son's car that was also intermittent). I highly recommend soldering wires vs crimping if you have the means.

Using Ohms (key off) check for shorts between pins 1,3 at the connector C1041). Flex the wires and connector. If you read ohms (or you get a tone for continuity) there's a short between the hi beam/low beam wiring. Hopefully, the issue is within the connector itself (most likely) and/or between the connector and BEC vs the SJB (low current board).

After you replace the connector recheck voltage between pin 2 (ground) and pins 1 (low beam) & 3 (high beam) for both C1021/C1041. When measuring voltage each side of C1021 & C1041 should be the same (~ 12v). The headlight bulb itself is not needed for these tests.

You were right — the connector was the problem. When I stripped the loom back, I found the passenger and drive side headlight wires had been spliced into the wrong positions.

I also found exposed copper causing resistance/heat.


After correcting the wiring to the proper pin order per the diagram and re-splicing it cleanly, everything works — low beams are solid and the high beams finally function as they should.

Appreciate the detailed steps they helped me a lot.

 

[DieHarder]

Amazing. Previous owner (or some hack in the past) couldn't even compare and follow the wiring order from the driver's side connector. Glad you found it.