Resistor Wire in Gauges
5th Gear
Joined: Feb 2019
Posts: 5
Likes: 0
From: Cincinnati
Howdy, this is my first poast. Apologies for thread necromancy, but hopefully this will be useful to some folks going forward.
I was fixing the result of a coil resistor wire mishap and stumbled on this other resistor wire that leads to the gauge cluster.
On my '78 F250, the red resistor wire for the coil has overheated and cooked the insulation of adjacent wires. It was being used to drive a duraspark II coil in addition to a continuous duty relay (aux battery isolator) off the 'I' terminal of the starter relay.That problem is fixed now, I removed the red resistor wire leading to the coil and replaced it with a regular copper wire which now triggers a relay, that sends power to an aftermarket coil and the isolator relay.
However, there is also a second, factory resistor wire inside the harness, it is black and is spliced into the wire running from 'ACC1' terminal of the ignition switch, into the number 14 pin of the instrument cluster.
Additionally, I have a '79 F250 wiring harness, and it also has both of these resistor wires.
I'm assuming that the black resistor wire is the shunt resistor that was mentioned earlier.
I was fixing the result of a coil resistor wire mishap and stumbled on this other resistor wire that leads to the gauge cluster.
On my '78 F250, the red resistor wire for the coil has overheated and cooked the insulation of adjacent wires. It was being used to drive a duraspark II coil in addition to a continuous duty relay (aux battery isolator) off the 'I' terminal of the starter relay.That problem is fixed now, I removed the red resistor wire leading to the coil and replaced it with a regular copper wire which now triggers a relay, that sends power to an aftermarket coil and the isolator relay.
However, there is also a second, factory resistor wire inside the harness, it is black and is spliced into the wire running from 'ACC1' terminal of the ignition switch, into the number 14 pin of the instrument cluster.
Additionally, I have a '79 F250 wiring harness, and it also has both of these resistor wires.
I'm assuming that the black resistor wire is the shunt resistor that was mentioned earlier.
Fleet Mechanic
Joined: Dec 2007
Posts: 1,511
Likes: 340
From: Wildwood Pa
Pin 14 is what powers the instrument voltage regulator.
I've tested IVR's with both 6 & 12 volts running in to them, they seem to be able to regulate either voltage. Since the IVR pulses to regulate, it is hard to get an exact voltage with the meters I have. I would guess it is easier on the IVR with stepped down input voltage, & that is why there is a resistance wire in that location.
I'm curious as to how far the voltage is dropped by the resistance wire, but not curious enough to pull my dash apart to find out.
I've tested IVR's with both 6 & 12 volts running in to them, they seem to be able to regulate either voltage. Since the IVR pulses to regulate, it is hard to get an exact voltage with the meters I have. I would guess it is easier on the IVR with stepped down input voltage, & that is why there is a resistance wire in that location.
I'm curious as to how far the voltage is dropped by the resistance wire, but not curious enough to pull my dash apart to find out.
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