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How Gauges work

Mach1 Driver

Well-Known Member
I've been having discussions on two other forums (some with Midlife) and thought I'd share what I've learned.

I wrote:
If it were me, I would get rid of the original gauge constant voltage regulator, and replace it with a modern electronic one. The old ones try to provide around 5v by pulsing 12v on about 42% of the time and off about 58% of the time. Since the gauges are dampened it doesn't affect the readings. The new electronic regulators give a rock solid 5v. Edit, for reasons mentioned below*, I have revised my opinion about electronic IVRs.
Just for some background information; Ford intentionally developed gauges that take time to react- they call this dampening. This is so the gauge doesn't bounce all over when you take a curve or do a jump like the General Lee. All the gauges are reportedly the same except for the scale. Even though the oil sender measures pressure and the water sender measures temperature, and the fuel measures fluid level, all three sensors are just variable resistors that read from 78 ohms on Low to 10 ohms on Hi. The gauges have high and low adjustments, and an internet search turned up this information. What he is saying below is that the top adjusts the low end of the scale and the bottom adjusts the high end of the scale. Within some (unknown) "range" they are all adjustable.
2007 jonsee on allfordmustangs said:
Yes there are two adjustments on the back of the three gauges, oil, temp, and fuel - the adjustment on the top is to adjust the zero at 70 -73 ohms and the bottom adjustment is to adjust the 100% reading (better known as the span) at 10 ohms - with 5 -6 volt supply, an increase in resistance decreases current flow through the gauge (better known as indicator) which decreases the reading and a decrease in resistance, increases current flow through the gauge increasing the reading.

And then this morning there was more, I wrote:
I just saw this great picture courtesy of Jharcinske on VMF. Ford developed a gauge that doesn't use the typical D'Arsonal meter movement. It is a bi-metal device with a heater to make it move. That's why these things have so much dampening- its inherent in the design, and probably costs much less than a typical meter movement. A little more explanation is probably in order here: a bi-metal is two sheets of dissimilar metal formed together in layers like a sandwich. They use materials that have greatly different expansion rates. When heat is applied they bend, because one side expands faster than the other. The bending is very predictable and repeatable.

In the same thread (about gauges) I got a response from TheRktmn who owns a gauge repair shop. I asked him about the the two adjustments and he responds:

They set the high and low points for the gauge. You will need a face on there to do it, but with a 73 ohm resistor for low and a 10 ohm resistor for high and a 5VDC power supply you can set the gauge back to factory specs.
Use a small screwdriver in through that hole to engage the teeth and move the arm. You will see the pointer move.
Once you change one you need to check the other since it will change some too.

* Edit: it has come to my attention that the electronic IVRs do not properly mimic the operation of the old electro-mechanical ones. The electronic versions put out a nice constant 5v, which is good most of the time. It is my "theory" that the old electro-mechanical versions put out 12v for a short period of time at start-up, probably less than 5 seconds, before they start pulsing to give an average 5v. This allows the instruments to quickly reach their operating range. People with electronic IVRs report delays of 30-40 seconds before the instruments are up to their operating ranges. That isn't a problem with most of the instruments, but many people with expensive engines want to know they have oil pressure before 40 seconds. I suspect that if the manufacturers knew it was problem they would work to correct it.


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Jeez, it's been way too long since I've had the pleasure of reading such an informative and technically interesting thread. Thanks for taking the time!
Since the IVR is part of the gauge system, this would be a good place for a description of it. The stock IVR (instrument voltage regulator) is also a bi-metal with a heater, similar to the gauges. The contacts are normally closed and open when hot. Then it cools and constantly repeats. The contacts are on the right rear in the picture. The IVR is also in series with a 10 ohm resistor wire. The combination of the resistor and pulse gives an "average" of 5v. The gauges don't care about the pulsing because they are so heavily dampened it doesn't affect them. When the car is started the IVR doesn't pulse, it outputs 12v for a short period of time until it warms up. That makes the gauges come quickly to their operating range (about 5 seconds). Aftermarket electronic IVRs only output 5v, so the gauges can take much longer (some report 30-40 seconds) to come up into range. It can be unnerving to wait that long for oil pressure to rise after starting the engine.
Power from the ignition switch goes through violet 10 ohm resistor wire #30 and connects to the left rear terminal in the picture. That goes up to the cycling contact on the right rear, and flows down and around the corner through the bi-metal to the right front terminal. The heater wire is welded to the bottom side of the bi-metal on the right near the switching contact. It wraps around the bi-metal and goes out the rear where it connects to the metal case, which is grounded when attached. So, as the bi-metal heats it bends and lifts, opening the contact and removing power from the heater. It cools and closes, and constantly repeats. It is around 12v when closed and 0v when open. You can see the screw under the switching contact, used to adjust the on/off time of the device. Over a period of time, in series with the 10 ohm resistor, this pulsing output averages 5v. This is a 1960s version of a voltage regulator, and it works quite well.


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The use of a 10 ohm resistor wire is only applicable from 1969 onwards; earlier years used direct ACC power.
The use of a 10 ohm resistor wire is only applicable from 1969 onwards; earlier years used direct ACC power.
Thanks for that insight Mid, as you can see to the left, I have a 69 and it's wire diagram is where I get my information. Its hard to know why they made the change, and what if anything changed in the IVR. One good thing is that if one of the gauge posts is shorted out (as people so often do when they install them) then it limits the amps through the IVR to only 1.2.
The actual IVR design changed from 69 on to use battery snaps; the previous versions used spade leads. That change may well be due to the introduction of circuit cards in 1969. As mentioned elsewhere, the resistor wire slows the response of the gauges tremendously. When I construct hard-wired dash clusters for 69 on up, I had to test the gauges for about one second or less or else the gauge would peg very high. Even then, it took 30 seconds or so for it to return back to normal. I then added a 10 ohm resistor wire between my power source and the IVR, and things slowed down to what one would expect when installed in the car: nice, gradual movement off of zero when grounding the sending unit line.