What is a resistance wire?

[87gtVIC]

I am looking over a 1985 Mustang EVTM and there is much talk about resistance wires within the graphic warning module circuit that came equipped on some of those cars. I am looking into the idea of installing one in my car, but I would need to know what this resistance wire talk is about. Anyone have an idea?

[87gtVIC]

Found a pic of what I was talking about for reference:





Looking further into it is seems like a paint to do....but I would still like to know what a resistance wire is.

[GM_Guy]

A resistance wire acts like a ballast resistor. Drops the voltage to whatever the design calls for.

ie: On older vehicles, the coil was designed to operate at 9v. Since its a 12v system, you need a way to drop the voltage, and a resistance wire does that for a low cost with no extra bits (such as using a ballast resistor as Mopar did, two extra connectors, a place to mount the ballast resistor, a screw to retain the ballast resistor the firewall.. you get the idea). Resistance wire also tends to last the life of the vehicle. If you are old enough, you'll know about keeping a spare ballast resistor in your Mopars glove box.

For your lamp out warning, the resistance wire is necessary to detect a change in voltage drop at the point where it connects to the module. It provides a reference. If all the lamps are good (each lamp type has theoretically the same resistance, and subsequent voltage drop across it), the voltage the module detects is X volts at that junction. If a lamp burns out, the module detects Y volts and sets the warning. In short, to retrofit this, as installed in the mustang (no modifications), you need to use the same lamp and number of lamps for each function (no substituting LED's, no running 168's in place of 194's or vice versa). If the mustang only uses one brake light per side, you're screwed. =-)

Ideally, you will want a working vehicle to take measurements on at the module inputs. If the EVTM provides a range of acceptable voltages, you may be able to redesign this thing.

Alex.

[gadget73]

usually its like a 1 ohm resistance, and voltage drop is measured across it. You could probably get away with changing bulb types if you alter the value of the resistance wire to maintain the same normal-state voltage across it. Its theoretically possible to duplicate this with regular resistors, but keep in mind that the entire current load of the lamp goes through it, so you'd need somewhat large wattage resistors to handle it, and sufficient space to dissapate the heat produced.

[86VickyLX]

I am looking over a 1985 Mustang EVTM and there is much talk about resistance wires within the graphic warning module circuit that came equipped on some of those cars. I am looking into the idea of installing one in my car, but I would need to know what this resistance wire talk is about. Anyone have an idea?

Check your bulbs. :p A resistance wire is just a wire with resistance in it. Not to sound like a dick, but that's what it is. In a series circuit, the larger resistance will use more voltage, so that resistance wire won't use up most voltage when all the lights are working properly.

When there's enough voltage applied to the module, the LEDs will light. The LED would act as the actual light.

[87gtVIC]

Interesting guys. Sounds like something I would need to ponder upon for a while. Is a resistance wire something I can purchase from a store (not an actual resistor but this mythical resistance wire) or more likely have to rip them out of a parts car?


As of now the only things the graphics warning module could show me was if I was low on fuel and if my washer fluid is low (if I find an aero or equivalent washer reservoir with a low switch in it) as those two use no resistance wires.

[87gtVIC]

So just to put this in terms that I understand....in my case as per the pictures above the resistance wire as well as the wire leading to the module directly before the resistance wire and directly after the resistance wire are merely taking readings of the voltage before the resistance wire and after the resistance wire. Lets say there is normally x volts (or whatever) leading to the headlight. Suddenly a filament burns out and now there is less voltage being used on the other side of the resistance wire so the indicator light will signal on the warning module?

[GM_Guy]

When there's enough voltage applied to the module, the LEDs will light. The LED would act as the actual light.

No one said LEDs would not work, the LAMP OUT warning is what won't work.


87GT, you basically have it. Google voltage divider.

Alex.

[87gtVIC]

Interesting and confusing formulas but I think I understand the concept.

[gadget73]

Voltage = current X resistance. thats really the only formula, ohm's law. You can monkey that around to get whatever you need.

[86VickyLX]

No one said LEDs would not work, the LAMP OUT warning is what won't work.


87GT, you basically have it. Google voltage divider.

Alex.

The lamp out warning uses LEDs to indicate which bulb is out.

[gadget73]

the LEDs in the outage indicator really have nothing to do with it. Where LEDs and the lamp outage module become relevant is when you put LEDs in place of incandescent bulbs in the marker lights and stuff. They draw less current, so the module sees a lower than expected drop across the resistor, and thus triggers the lamp out indicator. If you have a car with this sort of system and put a headlight relay harness on it, you get the same results.

[johnunit]

that could be cured by a resistor after the LEDs though, right?

[87gtVIC]

If you have a car with this sort of system and put a headlight relay harness on it, you get the same results.

Solution:

[gadget73]

that could be cured by a resistor after the LEDs though, right?

yes, but it would eliminate the reduced current draw advantage of the LED. That, and you'd have to put the resistor somewhere that it could shed the heat safely. Resistors of suitable size and resistance would probably be more expensive than the light bulb you're trying to replace. It would be easier to actually just mount the stock bulb somewhere and use it for the load.