I'll try to describe this problem as best as I can. It has to do with moving 2 and adding one gauge. The two gauges that were moved had been installed and working flawlessly for 2 years. A boost gauge was added. When the lights were wired back into the dimmer switch everything went down hill. The voltage in the blue/red stripe wire that powers the dash lights is collapsing/bleeding down. When you initially turn on the lights it's 12V but goes down to .1 (without moving the dimmer switch) BUT the dash lights do NOT dim. If you cycle the dimmer switch to dim then back to bright the voltage returns to 12V but quickly bleeds down again.
Also, the power wire for the EGT and fuel pressure gauge, which used to come on at key on now only powers when the lights are turned on. Neither of these wires was moved from where they were originally tapped.

None of the gauge lights is working correctly but the gauges seem to function correctly.

 

Tim,

It sounds like the addition of the new gauge has overloaded the circuit. Each incadescent bulb in the gauge is in essence a resistor which in turn increases the load on the circuit. The increased load on the circuit is to much for either the wires or the dimmer switch or both to handle. The dimmer switch must dissipate the same load as the total load on the circuit, so as you keep adding loads to the circuit the dimmer switch must dissipate greater loads.

Basically you may have created a "voltage or current divider circuit" by adding the aftermarket gauges to the factory wiring. Here is a little reading on voltage dividing circuits and resistance effects on a circuit.
www.allaboutcircuits.com/vol_1/chpt_6/1.html
www.allaboutcircuits.com/vol_1/chpt_6/3.html
http://en.wikipedia.org/wiki/Voltage_divider

You may have permanently damaged the dimmer/headlight switch or it may have an overload protection circuit in it ( i doubt this but without checking the factory wiring manuals i cannot verify).
Also understand that the factory gauges & lighting pass through the on board computer and many devices are interconnected and have common grounds & feed back circuits.

1) I would disconect the new gauge and see if this solves the problem. If removing the new gauge solves the problem then I would run a seperate power source from the battery to a separate dimmer switch to control the lighting on all the aftermarket gauges
2) I would get the power for the gauges from a new separate circuit and use a key switched wire for the control trigger on a relay supplying this new circuit.

With all the electronics & computers on modern vehicles it is always a bad idea just to tap into the factory wiring to add accessories.
Best practice is to always add new fused & relay controlled circuit originating from the battery. Furhermore any time you tap into a factory wire to use it, even for a trigger wire, always diode isolate the new tapped trigger wire from the factory circuit to prevent feed back when the relay (switch) opens and closes.

For more information on 12 volt wiring etc go to "the12volt DOT com" and join.
Car Alarm, Car Stereo, Mobile Video, and Cruise Control Info for Installers

Hope this helps

 

Wow.....long time no see. Welcome back. It's more than a pleasure to have you and your expertise here again.

I did fail to mention that the gauges were removed from the circuit when the collapsing voltage was occuring. I'm just puzzled that the dash lights weren't affected and didn't dim when the voltage drop occured but that they responded correctly when the dimmer switch was cycled and voltage returned to the wire, only to drop again.

Maybe DChamberlain can provide the wiring circuit for the head light switch.

The truck involved is an 04 virtually identical to my 04 with virtually identical gauging and they're wired the same way but obviously something has changed. I checked the dimmer circuit in my truck but maybe I need to watch it for an extended period to see if the voltage drops also, which I didn't do. The only other thing that changed is that the gauges are no longer mounted to the metal frame of the truck, they're mounted in an overhead gauge pod and I wonder if the ground is insufficient for the circuit. That still doesn't account for the collapsing voltage when they're removed from the dimmer.

 

Not sure exactlt the situation but it almost sounds the way you describe the voltage like you are measuring across a capacitor. Keep in mind all a multimeter actually measures is potential difference. so you have to know what each wire of the multimeter is attached to. When you connect across a capacitor you will measure the stored charge of 12v that will decrease as the cap discharges through the multimeter. it will look exactly like you describe like it is collapsing.

 

Tim,

It sounds like the dimmer/headlight switch has been damaged and/or one of the wires has been overheated and melted to an adjacent one thus causing high resistance. I would opt for a bad dimmer/headlight switch. I will have to dig around and see if i still have an 04 wiring diagram. Understand that as the resistance on the circuit increases the dimmer switch (potentiometer) must dissipate a higher load. The higher load over time will degrade the dimmer switch to the point it will not operate properly.

As far as grounding the gauges, they should have a separate ground wire and not be relying on their case for ground, so the new mounting position should have no effect.

As I said previously tapping into factory wiring without the use of diodes & relays is a bad idea do to the potential harm from back-feed and increased load on the circuit. All of these effects can be small and take time to slowly degrade the circuit. Far to many times I have seen this effect with poorly installed stereos & alarms. The circuits today pass through the on-board computer and are monitored for many things. The trucks automatically leave the interior lights on for an extended time so you can see to exit and turn on with key-less entry for a timed event.
I will look around and see if I can locate a wiring diagram.

 

I'm making notes as you write and I'm at work on a (thank God) busy Friday. Everything has been duly noted and I'll pass it along. I don't have the truck in my possession at this point. It's 3 hours away.

 

Okay Tim,

Found my wiring diagrams, when the main light switch is turned on to the park lamp, headlight or auto (note light sensor must close as well) position or the dimmer switch is turned fully on (interior lights illuminated) power flows through the Main Light Switch "Microprocessor". Once the Microprocessor receives power it supplies power to the Interior Illumination Circuit (LB/RD wire, #19) and also a reference voltage to the Instrument Cluster Microprocessor(VSM) OG/LG #402.

The power from the Main Light Switch Microprocessor circuit #19 LB/RD supplies voltage to both the non-dimmable & dimmable interior illumination.

One of two things has happened, the Main Light Switch Microprocessor is temporarily malfunctioning do to the overload condition or more than likely the excess load has finally permanently damaged the Main Light Switch Microprocessor and your friend will need to replace the entire Main Light Switch assembly.

If it was my truck I would run a new separate fused circuit with its own independent dimmer switch to the aftermarket gauges. Tapping into factory microprocessor controlled circuits is a recipe for disaster.

Hope this helps.

 

Glad to hear work is busy today, work is slow around here today.

 

Haven't seen you in a long time. Welcome back blackhat. Hope you stay for a while.