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Discussion Starter #1 (Edited)
Hello again

While connecting the rear speakers to the stereo behind the dash, i felt the main harness was hot to the touch. Pulled out the dash and cut open the harness, and a violet wire was getting hotter than everything else, measures 40c (everything else 20c). Traced one end to the main gauge cluster, specifically pin #11 (Power to Gauges).
It heats up when in RUN and ACC. Pretty much everything is hooked up electrically, except the alternator, coolant and oil senders (which cant add as no engine currently).
Would the lack of any of those components cause a large current draw, or could something else be at fault? I rebuild the cluster back in February and everything went fine, and i don't believe i got anything wrong.
Don't know how long this issue has been around, haven't noticed it until a couple of days ago.

Any help is greatly appreciated
Thanks
 

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The violet wire 30A is a resistance wire that feeds the "instrument voltage regulator". This is the first year they used a resistor in front of the regulator, and when the key is on it feeds power to the regulator and it does heat in the process of dropping the voltage. It is a 10 ohm resistor in the form of a wire, and the cheapest way to perform the function. I have no idea if that is too much heat- but too much heat would only occur if there was a short in one of the gauges. If all three work normally (oil, gas, water), all move upscale but don't peg, then I wouldn't worry about it. What often happens is that people will take the gauges out and when they replace them one of the posts shorts out to the instrument panel. The 10 ohm resistor actually prevents the IVR from getting destroyed in that condition by limiting the amps to 1.2. You could have that condition, or just normal heating. You can see the circuit in the right middle section of the schematic below.
 

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Ohms Law to the rescue...

12.65 Volts (fully charged battery)
10 Ohms resistance
----------------------------------------------
16 Watts power

No doubt the source of your heat.
 

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....well it won't be any more than that, but we both know it's not quite that simple. There are too many variables- we don't know the resistance of the meters, or the senders, or the pulse rate of the IVR, but point taken.
 

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....well it won't be any more than that, but we both know it's not quite that simple. There are too many variables- we don't know the resistance of the meters, or the senders, or the pulse rate of the IVR, but point taken.
Of course it's that simple. A supply voltage and a fixed resistance. P = V^2/R. 16.00225 = 12.65^2/ 10.

We're not trying to determine anything other than wattage (power consumption) of the resistance wire only.
 

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Meter Circuit

A mind is a terrible thing to waste, but Covid boredom has driven me to it. I doubt that anyone cares, but the entire circuit has to be considered when evaluating the power dissipated by the 10 ohm resistor:

The gauge circuit has the meter in series with a sender which is a variable resistor ranging from 10 to 78 ohms. The meter isn’t a typical D’Arsonval meter movement. It is just a bi-metal strip wrapped with a heater wire. When the bi-metal gets hot it bends. The gauge pointer is attached to the bi-metal and it moves on the scale. All the meters are identical except for the scale, and all the senders are variable resistors. Even though they measure temperature, or pressure, or fuel level, they all have 10 ohms at maximum or 78 ohms at minimum. The point of this type of meter construction is that bi-metal reacts slowly (dampened), so when you go around a curve, or take a jump like the General Lee they don’t bounce around.

I’m told that this heater wire in the meter is around 14 ohms, and since it is just a resistance wire, it can be treated as a simple resistor. So it is in series with the sender. When doing calculations, this can be reduced to one 24 ohm resistor (14 +10) on hi or one 92 ohm resistor (14+78) on low. But, there are three of them in parallel. For calculations that can be reduced to a single 8 ohm resistor on hi, or a single 30.67 ohm resistor on low. I won’t show parallel resistor calculations- even I’m not that bored.

Now, these are in series with the 10 ohm resistor in front of the IVR. So when the IVR is closed the 10 ohm resistor is pulling 4.94 watts on high, or 0.97 watts on low. But, the IVR doesn’t stay closed, it pulses, so the wattage of the 10 ohm resistor goes down according to its pulse rate. The pulse rate varies with the load (sender resistances) to average 5v across the meters.

IF the left meter terminal of any of the meters in the example is touching the instrument panel, then the wattage on the 10 ohm resistor would be 16 watts for the period of time the IVR is closed.

So, you can see that there are sender resistances to consider as well as the IVR pulse time when looking at the wattage on wire 30. See the attached pdf
 

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