Electrical Gremlin

1992 Mustang LX Convertible:

I'm experiencing a bizarre behavior, but perhaps someone could add light or schematic to my issues.

1 - Press the Brake pedal, headlights -and- Brake lights come on

2 - Dimmer switch : must turn on dome lights in order for instrument cluster to illuminate
 
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1992 Mustang LX Convertible:

I'm experiencing a bizarre behavior, but perhaps someone could add light or schematic to my issues.

1 - Press the Brake pedal, headlights -and- Brake lights come on

2 - Dimmer switch : must turn on dome lights in order for instrument cluster to illuminate
1 - Did you buy this car or build it yourself?
If you built it yourself, what was the last thing you did?

2 - The dimmer switch may have gone bad. There is a variable resistor built into the dimmer, and that is what varies the voltage to the instrument lights. Check the fuse and then it comes back to question #1.
If the fuse is OK, there is a possibility that one of the instruments (one of the aftermarket ones) was wired incorrectly caused the variable resistor to burn out. After the defect in the wiring was corrected, the instrument lights work, but do not dim because the variable resistor is bad.

Diagram courtesy of Tmoss & Stang&2birds

Fuse box layout
MustangFuseBox.gif


Diagram from Ford EVTM
92-93-mustang-instrument-illumination-wiring-jpg.92689





More diagrams from www.veryuseful.com
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg
 

Attachments

  • 92-93 Mustang Instrument illumination wiring.JPG
    92-93 Mustang Instrument illumination wiring.JPG
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Yes sir, extensive rewiring and changes everywhere... and I follow your thinking.

For many months, I ran without issues, which gives me confidence in the Instrument Cluster (IC) not being root cause. More recently however, I replaced the wideband O2 gauge and "intended" to address a flickering in the stereo power. Next, I installed an aftermarket brake light blinker. So, I could be dirty on any of these 3 changes. In hindsight, I should have tested more thoroughly after each change. Overconfident, aging fool that I am. After the symptoms began, I also found the #4 fuse burned-out.

So, I began the process of Backing out my changes. Started with the Rear brake light blinker. Then I rewired center console gauges, stereo receiver, 12v lighter receptacle, etc. I removed the center console to better organize and simplify the configuration and have it look Pro. I'm confident this is fine now as I only have 3 connectors which are currently detached; Power, key-on, lamps on. All grounds come to a common post.

I'm keen to your sentence:
If the fuse is OK, there is a possibility that one of the instruments (one of the aftermarket ones) was wired incorrectly caused the variable resistor to burn out. After the defect in the wiring was corrected, the instrument lights work, but do not dim because the variable resistor is bad.

I suspect this summarizes my issue. In fact, I ordered a new dimmer earlier today and it arrives tomorrow. When I get time to install it, I'll continue to leave the center console detached for a short time to ensure that extended use, with and without dash lights, are problem free. Then I can reattach console, get similar usage in AGAIN. And finally, if I still have the interest, I'll reattach the rear blinker taillights.

It's been many months since I've requested help, but as usual, you're like a Timex... "You take a lickin and keep on Tickin'"

Thanks again !

-D