INTRODUCTION:
There has been quite a bit of cussing and discussing the Instrument Cluster Voltage Regulator. One FTE'er recently asked me how to put a volt meter on them to determine if one is working correctly. In an effort to answer that question I started doing a bit of testing and thought I'd share what I've learned. But, along the way I realized that I was having ICVR problems on Dad's truck when the gauges kept indicating that the engine was overheating when I was pretty sure it wasn't. To prove that it wasn't overheating I installed a mechanical temp gauge and learned that the engine was fine but that the gauges were frequently reading very high due to a malfunctioning ICVR. So, what began as an exercise to answer a question (that turned out to be difficult to answer by the way) has morphed into finding a way to solve the persistent problem many of us face with gauges that read incorrectly.

First, a little bit of theory. The ICVR is a voltage chopper that does its work based on a heating coil and a bi-metallic strip with contacts. The coil goes about heating the bi-metallic strip, which then bends due to the different coefficients of expansion of the two metals, and opens the contacts. Then the strip cools and closes, and the process starts all over.

So, that puts out a "square wave" of almost battery voltage at the top (although there is an 8 - 9 ohm resistor in the wiring harness that brings the voltage down some) and zero volts at the bottom. I've attached a shot of two of the waveforms from an oscilloscope, and the vertical scale is 2 volts/division and the horizontal is .1 second. That means the top of the wave, before the upward spike, is about 13 volts and the length of the wave is about .7 seconds.

What does that mean? First, there is no chance you are going to be able to use a run-of-the-mill digital volt meter to measure that because on the DC scale you will get a confusing display of varying numbers depending on how often the sampling is taken. And, that's probably true of the AC scale, but some DVM's might show something somewhat consistent but no where near meaning anything as they are expecting a sine wave and this is nothing near that.

Second, there is no way I can think of to say what the real DC voltage is. Yes, I know the factory manual says it is 5.5 volts, but they didn't tell us over what period it was measured or even how it was measured. I don't think we can reproduce that.

However, it is possible that this waveform gives the same readings on the gauges that a 5.5 volt DC source would give. That will take further testing, which I hope to do some day, but not today. I tried to get there by placing a 1000 microfarad capacitor across the output, but that didn't get it very close to DC, so it would take far more capacitance to accomplish that, which I don't have easily available.

Anyway, I welcome thoughts, suggestions, questions, or whatever.

 

I am probably confused a little, so bear with me..

The 5.5 volts is to power the gauges I am assuming.

The resistance from the sending units,etc. controls the gauge.

If 5 volts DC + or - is needed, why couldn't you build or acquire a 5 volt regulator.....It would depend on how many AMPS the cluster draws off the 5 volt.

Just questions or curiosity...Trav..

 

Yes, that can be done. In fact, Stangrcr1 has done it. But, his gauges are reading low, so apparently it needs to be bit more than 5.0 volts. In fact, it may be that Ford was right that it needs to be 5.5 volts. However, what I plan to do at some point is to document the various resistors the factory manual says are required to calibrate/test the gauges, collect said resistors, and find a set of gauges that work as they are supposed to work. Then I'll use a variable power supply to determine what DC voltage it takes to replicate those readings.

At that point I could build solid state ICVR's that would give the readings the gauges are supposed to give, assuming the sending units are correct. Or, I could just buy one off of ebay for $24.50, but so far all I've seen on there for our trucks are units fixed at 5.0 volts.

 

I don't like the idea of buying something that ain't gonna work any better than what we have ( no matter the price).

Like you, get the varible supply and see what works right....
Then, build something with enough current to work and stay correct..

 

The guy on ebay that is selling the units says he discontinued the variable voltage units because it "confused people". I'm betting they adjusted it and then had wrong readings. I think it would be far better to use precision resistors to set the voltage rather than using a potentiometer to allow adjustment. Just figure out what voltage it takes and nail it down so it will never change.

We'll see if I can get to this at some point. But, there seem to be so many problems with the factory gauges that it might be worth it if we could get them to be accurate.

 

Getting the correct voltage set to the gauges is the key..

Most voltage regulators will work from 9 to 20 volts on the input side...

Even if you had a low voltage system say 10 volts, you would still be regulated at 5 + or - to the gauges...

Same thing on the other side(running 14 to 16 volts) to the input of the regulator....

At least, that way all the gauges would be consistent all the time(as long as your sending units work right)...I hope I'm right in that assumption..Trav...

 

Yes, that's the way it works. The various regulators take a wide range of input voltages and output a constant voltage - up to a certain current limit, which is usually about 1.0 amps. I still need to ascertain how much current is required to run the gauges in order to know how HD the regulator needs to be. However, Stangrcr1 is running a 1 amp unit that seems to be hanging in there, so hopefully that's all that is needed.

Also, having seen the huge EMF spike when the ICVR's contacts open, probably caused by the several heating units acting like coils, I realize that Ford wasn't too worried about noise in the wiring. But, I would add capacitance to provide a decent DC output, which might help some of the radios. In any event, a steady and consistent voltage should go a long way toward getting good readings on the gauges.

 

Sounds good...I haven't messed with hardly any electronics since the mid 70's...Didn't always be a truck driver..

Radio Shack carries (or did) the 1 amp regulators..But I'm sure they are available other places with higher amp capabilities..
And the resistors,diodes,caps,etc...maybe a small heat sink.

I see a NEW AND IMPROVED voltage regulator in the future..

 

Tomboy (Tom Gunby) sent me this link to a guy making a regulator: 1968 FORD FAIRLANE INSTRUMENT CLUSTER VOLTAGE REGULATOR - YouTube Is that what you are thinking it'll look like?

Kinda fun to watch him mess up his heat shrink tubing. Why, oh why would he put it on first when the other end of the wire is open? And why try to shove it over a recently-soldered joint? And, he touches his soldering pen to the tubing to shrink it? Gross!

But, the more important question is why use an 8 volt regulator?

 

And, for future reference, here's a link provided by klricks to a simplified diagram of the gauge system: Fuel Tank Selector/Gauges.

 

Something like I had in mind....but...I want the correct voltage(5,5.5,6, or whatever)...

I believe I would hook it up and check out the output voltage BEFORE installing it in the vehicle..Do it on the bench,check it out,then put the finishing touches on it...

However, to each his own..
At least it is a decent way to do it...Hope it has durability...

And for the little bit of money involved, you could make your on modifications to get the voltage you need....Something to consider...

And,what is wrong with using a lighter or heat gun for heat shrink..(not soldering tip)?

Gotta go, I'm tired.....Trav...

 

The gauges Ford used until 1989 were basically "left over" from the 6 volt days. The originals were built by King-Seeley and used two bi-metal strips in the gauges. Ford started using a regulator to allow the old 6 volt system to work with 12 volts. Chrysler used a similar system in their vehicles. GM (Delco) used a balanced coil system as did 6 volt Chryslers. The interesting feature of them was as long as the gas gauge was swinging you had fuel, when it stopped swinging, look for a gas station fast!

The newer Fords use a damped balanced coil system, but the oil pressure gauge is not a gauge, it is a fancy idiot light.

 

There is a bit more going on with the IVR. This is how I think it works:
The output is a square wave or sorts with a varying duty cycle. When the senders are all at their lowest ohm values, then the wave from the IVR will have a shorter on time and longer off time. (More current flows which heats up the bi-metal strip faster throwing the switch off faster).

When the senders are at their highest ohm value then the wave from the IVR has longer on time and shorter off time.
(Note the on/off cycle could be reversed as I don't know if the IVR is normally open or normally closed)?

The IVR also compensates for varying input voltages. 12V on battery and 13.8 or so with engine running. Notice that the gauges do not fluctuate as the engine is started or shut off.

The IVR is limiting the voltage AND current by opening and closing.
A common 3 terminal voltage regulator holds voltage constant and allows current to vary up to 1A or so.
So there is a bit different dynamics going on between the IVR compared to a regulator chip.

You would need to look at the waveform with senders at their different full scale values in order to see what is going on. Then an electronic version could be developed.

Also note that the 3 senders (resistance) are in parallel. I know the fuel sender is 10 ohms full and 73 ohms empty but don't know the ranges of the other 2 senders in the circuit. The gauges and the IVR coil have some resistance as well.

 

If you look in the instrument portion of the shop manual, there is a gauge tester that is a simple resistor box with three settings low, mid, high which correspond to the gauge positions. There is also a way to test all three gauges using a fuel tank sender which would imply that the resistance values are very close if not the same.

One of the things I have observed over the years with my Fords is on a long highway run they will slowly read higher, particularly the oil pressure as that is the most noticeable.

On my Shelby, I had a mechanical gauge in addition to the electric one. The engine held 65psi on the highway which was 2/3 of the way on the dash gauge. On a run to VIR, about the point you needed to stop for a rest break, it would be closer to 3/4 scale, but the mechanical one still showed 65psi.

 

Well, that's interesting: The factory shop manual shows the IVR/ICVR differently than the diagram in the link above. The shop manual shows the heater to be between the input and ground, and the diagram above shows it between the output and ground. And, that makes a big difference. If between the input and ground it will always be on, but since the voltage at the input will drop significantly when the points close and the load comes on, the heating will be less then. But, between output and ground means the heater is only on when the points are closed. I think the latter is highly likely to be the way it is wired, and I think the fact that output voltage spikes in my scope shot proves it as that was the only coil that was in the circuit at the time and there was obviously a coil being discharged given the voltage spike.

Anyway, I agree there is more going on than what I described. But, I don't think that the ICVR is limiting, or regulating, the voltage or the current. What it is actually doing is to regulate the watt-hours, meaning the heat which is current x voltage x time, that is produced in the heating elements of the gauges. Basically the "regulation" is done in two ways:

1. First, the 8 - 9 ohm resistor in the wiring harness reduces the voltage to the gauges, and therefore the current through the gauges, by dropping the input voltage delivered to the ICVR, which delivers it to the gauges. If we assume that all the senders are like the fuel sender and are 10 ohms when at the top of the range, and using 12 ohms as the resistance of the gauges themselves, we have the equivalent of 7.33 ohms because the three are in parallel. (In reality the ICVR's heater is in parallel with them but its resistance is only 64 ohms so doesn't really change the overall resistance appreciably.) For convenience's sake let's assume the wiring harness resistance is 8.66 ohms, so the total resistance is 16 ohms. If battery voltage is 12.8 then the voltage at the ICVR would be 5.9 volts, and that would also be the voltage at the top end of each gauge - when the ICVR is closed. Similarly, if the truck is running and the battery voltage is 14.4 then the voltage at the gauges is 6.6 volts.
   
   And, by the way, if all the senders give 73 ohms at the other end of the scale you have three 85 ohm resistors (the 73 ohm senders plus the 12 ohm gauges) and one 64 ohm resistor in parallel, and that gives 19.6 ohms by my calc's. That plus the 8.66 ohms of the resistor gives 28.26 ohms, and the voltages at the gauges would be 8.9 volts when the engine was off and 10.0 when running.
   
2. But, the ICVR isn't closed full time so there isn't any voltage applied to the gauges and, therefore, there is no current flow and no heating when the points are open. So the ICVR doesn't change the voltage one iota, it just changes the time the voltage is applied and that manages the heat.

In any event, I am confident that a fixed-voltage can be found that produces the same readings on the gauges, regardless of battery voltage and load. We'll just be making things more simple by taking "time" out of the equation for wattage.

As for the tester in the shop manual, we aren't told the resistances it contains. However, we are told what resistors it takes to make some of the gauges read certain values, so I think we can figure out how to test the gauges from that. I hope to take the time to figure that out tonight while I watch StL clinch their berth in the World Series.