Gary, I had thought of that, but not looking for a band-aid. After much research and discussion with some electronics geek friends, I am going to make an adjustable regulator good for up to 5 amps based on the LM338T chip. The circuit will only require two more resistors than my first experiment above. One a fixed resistor and the other adjustable.

And with the cheap price of parts, I got enough to build a few of them. The parts are being shipped so it will be a few days before they get here.

 

Don - You might want to consider one more resistor, and put it in parallel with the pot. Makes the pot a lot easier to adjust. Figure what resistance you need for the top resistor in the voltage divider, and then double that and you have the resistor and pot value since in parallel that's what you'll get.

 

Gary, here is the schematic that my geek friends went to as soon as I told them what I wanted to do.

While this regulator is set up for up to 25v output with a 28v input, and since we only have 14v to work with, it will end up being a 1.2v-11v regulator. I suppose we could increase the value of R1 to increase minimum output voltage.... I plan on playing with the resistor sizes to get it down to a 4v-7v range.

They also recommended increasing the cap sizes to 1uf on the input and 10uf on the output.



On edit: I am still unsure of just what voltage is actually required here.... Do the gages need 5v? 5.2? maybe a bit more? Do I use a spare IVR as a reference and then set the regulator to where the gages read the same?

 

That's the classic regulator alright. A couple of thoughts: First, the automotive environment is one of the most harsh around since the starter motor sends large spikes through the system when it comes in and drops out. Hopefully that regulator is happy working in that environment, but if not you might need a zener diode across the input to limit the voltage it sees.

Second, I wouldn't worry about the size of the output cap. Since the gauges are used to seeing a square wave anything would be better. But, it really doesn't matter since they are thermal and, therefore, very slow to change.

On the required voltage, I'm thinking there are two approaches. First, we could get scientific (yes, Chris, another 8 page micro-analysis) and test several IVR's with differing loads to find an average. Or, probably better yet, set the voltage to give the correct reading on the only gauge that has some calibration - the fuel gauge. I think setting the voltage to show a full tank, when the tank truly is full, would be the best approach. Who cares what the voltage really is, just get the reading to be correct.

 

You can't really use a spare IVR as it's adjustable.

There are slight variances between each individual trucks. The resistor wire before the IVR may have slightly different values due to the manufacturing process. Why there is a range between 8 and 9 ohms on the resistor wire.

The gauges may need 5.2V, but the IVR may need to be adjusted differently to each specific truck in order for them to get that precise voltage. The reason why the IVR were adjustable in the first place, and why the gauges do not have precise set values written on them.

 

Not sure if that would be nessecery, as the instrument panel is open in the start position of the Ignition switch. Most circuits are open during the start mode to protect components from the spikes you mention, and to free up amperage needed for the starter motor.

Don't think it would hurt either, mind...

 

Good point, and one I hadn't thought of.

By the way, my '81 FSM shows the in-line resistor to be 8.5 ohms, FWIW. That's in both the Fuel Indicating System writeup as well as in the Oil Pressure Gauge & Temp Indicator sections.

And, here are the calibration steps for each gauge:

• Fuel: Ford used a Rotunda Instrument Gauge System Tester Model 21-0015 to simulate the fuel sender. Apparently it had two positions: Full & Empty. The guage was to be tested in both positions and the center of the needle had to be "within the "F" or "E" white band of the graphics". I take that to mean somewhere within the E or the F, but I may be wrong. If the pointer was wrong at Empty you were supposed to replace the gauge. If outside the Full you were to replace the IVR and re-test. If still outside the range replace the guage and re-install the original IVR.
• Oil: Either use the Rotunda tester or a 22 ohm and a 73 ohm resistor for testing. With the 22 ohm resistor the gauge should read "mid-scale", and with the 73 ohm it should read at the "L".
• Temp: Use Rotunda tester or a 10 ohm and 73 ohm resistor. With the 10 ohm the "side of the gauge pointer closest to the "H" graduation should not be more than one pointer width away from either side of the "H" graduation. With the 73 ohm resistor "the side of the gauge pointer closest to the "C" graduation should not be more than one pointer width away from either side of the "C" graduation.

In other words, at no point did it say "adjust the IVR". And, by the way, I found in the drawings that the voltage is supposed to be "5.0", but I don't think that is a usable figure since measuring it would take something like a 'scope and shops didn't have that. Anyway, I'm thinking that for an individual there are only two ways to adjust the new IVR's voltage:

1. Adjust it for Full on the fuel gauge and assume that the other guages are going to follow.
2. Adjust the IVR as in #1, but then use the prescribed resistors and test the Temp and Oil gauges to ensure they are reasonably accurate.

 

You are trying to steer this thread to be solely about your new circuit, and that's ok. My comments were aimed at your comments about the original circuit being a faulty design.

About your new circuit. Do you realize what we are talking about here? These gauge setups are just for relative values or positions day to day, they really give you no useful information. All they say is "cold", "hot", "normal" or some other relative term, no real numbers. So your problem of the gauges reading "low". That is compared to what they used to read, but is that a problem? If you feel your truck is running "normally" with no problems, why not call your lower readings good? As long as the fuel gauge goes to full when you fill the tank(I am sure it would, they have a lot of lee-way).

 

Yes, however that is with testing the gauges using the rotunda tester on a bench right? The resistor wire is still in the truck. After you get all the gauges reading correct on the bench, you still may need a slight adjustment when installed back into the truck, due to the possible resistor wire difference as I understand it.

My Ford diagrams state a difference between 8.0-9.0 ohms, is nominal on the resistor, which 8.5 ohm is the mid way between average.

Again I think that's the average, as you say, a lot of shops wouldn't have state of the art test equipment (at the time) for something like this, so this would be close enough. Which is Franklin2's point really.


Personally I'd adjust it to read empty, on an empty tank, if at all possible. That way the gauge will not read 1/8th or something and run out of fuel. It's better to be accurate at the empty end, than at the full end.

I do think this is a worth while endevour because the old IVRs tend to make the gauges do the hula when they wear out. This new one would eliminate that problem, but it would need to be adjustable for the slight variances that are seen in the field between the different trucks.

 

No, those tests are in the truck because it says to disconnect the fuel sender and connect the Rotunda there. So, the in-line resistor is in the circuit.

Actually, we'd have to pick the writer's brain to find out what was meant by "5 volts". Is it with an analog or digital meter, which will certainly read differently. Is it using the DC scale or the AC scale - which wouldn't be accurate because it assumes a duty-cycle. Is it...... That's the old news. It is what it is and we could measure it with today's equipment and document that if needed, but few if any will be adjusting their IVR so that's a moot point.

And, I think I agree with you about showing Empty with an empty tank. That's probably the most critical, as you point out. Good thinking.

Dave - In post 21 Don showed the readings and said that the temp and pressure were reading one full letter lower, and the fuel gauge showed 3/4 when it had been reading "just under full". So, I don't think he's satisfied with that and I know I wouldn't be as I'd at least want the fuel to use the whole range.

 

I do like the look of the volt meter. Is this the correct part number?

Auto Meter Sport-Comp 3592 - Voltmeter | O'Reilly Auto Parts
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It seems kind of pricey for my taste as my local junk yard only charges $2 for a volt meter. I guess I am not used to NEW prices.
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I was going to try the DVM look but could not get a nighttime DIM mode to work with my $6 DVM. I also did not like the look of mixing analog and digital meters.
Since my 3G ALT swap the AMP meter is a dead gauge and your volt meter has a very nice look.
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I think I would make the new VR adjustable so while cruising down the freeway I could dial the voltage up and raise the gas tank reading. At least it would FEEL like I am getting better gas mileage.
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Humor aside, these gauges are “feel good” gauges that give you a rough idea of what is happening as you head down the road. I don’t see the point of making the VR rock solid accurate while the rest of the system is still old school.
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Here is a photo of the DVM in place of the AMP meter. You can also install this DVM in place of the Emissions Light or in the 4x4 location if you have 2WD.
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Have fun with your project.
Jim

 

Dave, As I started this thread, yes, I am steering it to my original intent of replacing the IVR with something more robust, stable, and adjustable. The original circuit I used is not faulty and works fine. The output is low and not adjustable, but otherwise exactly what I am after. I am going to top off the tank tomorrow and see if it still gets to full.

And I understand about the gages being relative. I just hated it when turning off the truck, going in to a minimart and getting a drink, then starting the truck and the gages read different. Hell, they would change driving down the road. Guess it is a pet peeve of mine. One thing I learned from racing is to look at the gages as much as possible, cause problems usually happen fast. And when they start moving off what you expect them to be at, there is a problem. So when these gages start floating, I think there is a problem.

I want the temp to come up and then sit. I then notice which letter the gage is sitting on(It used to sit on R). Next time I look at that gage, if it is off that letter, then something is wrong(It now sits between N and O). Oil pressure I expect to change some, but not at steady state cruise. And when the fuel gage starts going up cruising on level ground.....

This thread is about experimenting and finding a good circuit that is easily built with off the shelf parts, feeds the required voltage with plenty of power, stable, adjustable(new requirement after the first test), and hopefully for cheap.

Gary, exactly. I want the gages back where they were, but stable, and for the reasons above.

The gages are very stable now, just lower than what they were. No float, no wander, just lower than before. I think the "new" adjustable circuit above will do what I want.

As a side note, the new circuit will not be much bigger than the original IVR, but I cannot use the original case mostly due to the larger heatsink. I will use the plastic cover with the snap connections though.

 

Jim,

This is the meter I used. I had it left over from a racecar I had, so...

You may see my gage panel with all Autometer someday.....

I like gages and look at them a lot. And I did the 3G swap.

I also have an LCD meter like yours but it is not lit, so no good at night. And in keeping with the Old School gages with pointers....

 

Well, that makes the difference then. As long as that circut is in line, it should take up the variance.

The only time I've ever seen it documented to adjust it is if it's a new IVR and all gauges are low across the board. IE: like Stangrcr1's first attempt.

I've had the unfortunate experience of that happening to me with a 1973 F-100. Their instruments are relatively the same in function.

 

Reminds me of Dad's '72 Mustang Grande. Borrowed it when one of ours was down and drove it home thinking "that gauge is basically on empty, so I'll have to fill up tomorrow". It didn't start the next morning. Later he told me that it runs out when the needle touches the empty line.

Personally, I wouldn't want the gauge to read that way as other vehicles don't work that way and no one will expect it. Like I didn't.