1966 Mustang Gauge Calibration Adjustments - Ford Mustang Forum
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post #1 of 35 (permalink) Old 07-01-2009 Thread Starter
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1966 Mustang Gauge Calibration Adjustments

I am in process of calibrating my gauges and have forgot the oputput range of the senors. Can anyone tell me what the range of the sensors (oil, temp, and fuel) are in ohms. The range in ohms being what it takes to drive the gauge from zero to 100 percent of scale. All three gauges require the same current from 0 to 100 percent, therefore the range (in ohms) of the sensors are the same. The numbers, 70 ohms for zero and 10 ohms for full scale, are what I seem to remember, but I need to be sure.

The gauges are out of a 1966 Mustang and I am using the constant voltage regulator with an average output of 5 vdc.

Thanks in advance for any help someone may provide.

john

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post #2 of 35 (permalink) Old 07-02-2009
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I don't know the answer to your question and I believe your assumptions are off a bit but the 10 to 70 ohms is the resistance of the fuel sender - the fuel gauge works on a bi-metalic strip that heats up to move the needle - it is the only gauge that uses that mode. I don't think the gauges are adjustable so I wish you the best of luck in "calibrating" them.


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post #3 of 35 (permalink) Old 07-02-2009
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copied below is a response I got to same question on one of the forums.



"They are calibrated to read empty or zero at 70 Ohms of resistance. The full or maximum reading will be at 10 Ohms. All of the senders are calibrated for this 70-10 Ohm range. The fuel sender at 1/2 tank should read 40 Ohms.

~Ohms >> Reading
150 >> No movement - same as if the wire were broken or disconnected
73 >> Off scale 1/2 needle width below E
68 >> E
50 >> 1/8
26 >> 1/2
15 >> 7/8
12 >> F
10 >> Off scale a needle's width above F "
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post #4 of 35 (permalink) Old 07-02-2009
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Quote:
Originally Posted by joef21 View Post
copied below is a response I got to same question on one of the forums.



"They are calibrated to read empty or zero at 70 Ohms of resistance. The full or maximum reading will be at 10 Ohms. All of the senders are calibrated for this 70-10 Ohm range. The fuel sender at 1/2 tank should read 40 Ohms.

~Ohms >> Reading
150 >> No movement - same as if the wire were broken or disconnected
73 >> Off scale 1/2 needle width below E
68 >> E
50 >> 1/8
26 >> 1/2
15 >> 7/8
12 >> F
10 >> Off scale a needle's width above F "
Yep - That is for the fuel sender - not for temp or oil pressure - they will be different.

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post #5 of 35 (permalink) Old 07-02-2009
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Those numbers originally came from me so I know they are accurate. It took me a week adding one gallon at a time to come up with those numbers. Actually, they are/were accurate for MY fuel gauge which then quit working a few weeks after I took the measurements.

The fuel, oil and temp gauges are IDENTICAL except for the painted dials. I.e., within the range of their accuracy the same resistance gives the same proportional reading. 10 Ohms is full scale on all, 73 Ohms is the zero end of the scale. The needle position vs resistance is very non-linear and unfortunately, some replacement senders are built with a linear response which screws up the gauge readings at anything other than the two ends of the scale. The mid-point on all the gauges should be with a sender resistance of around 26 Ohms. (All dependent upon my VOM calibration which is compliments of the Sears Bureau of Standards.)
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post #6 of 35 (permalink) Old 07-02-2009
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I bought some aftermarket electric gauges for water temp, oil temp, oil pressure and volts with their own senders. I mounted them under the dash. My idea was to have a back up independent indication of what the dash was showing and this way if both are showing a problem, it is a real problem.
If the calibration does not work for you on the gauges, you can change the 5vdc IVR to this 1-10 volt DC adjustable regulator for 15 dollars:
Adjustable step down switching voltage regulator

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post #7 of 35 (permalink) Old 07-02-2009 Thread Starter
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Thanks to all for your input - I traced my problem to my test setup. Apparently I introduced additional resistance when I set up my test equipment and when I thought I had 70 and 10 ohms I actually had something else - I started over and re-connected my test equipment and everything worked as it should. I have calibrated a lot of these over the last few years, but the last one I did was September of 08, then when I ran into this problem the first thing I thought was that I had the wrong resistance range and that is when I posted for a conformation. (I don't keep good records.)

FYI - the CVR is also adjustable but requires something other then a digital or analog voltmeter - also in the three and half plus years of calibrating these I have found a few bad CVRs, but I have yet to find a bad gauge (other then amp gauges). The gauge repeatablity is fairly good at 0 and 100%, other then that they are good for "indication" only, indicating there is gas in the tank, and the oil pressure and temperture readings are where they normally are. In reality they should be known and addressed as "indicators" and not "gauges". One thing to keep in mind is if one of your gauges is indicating a problem, 99.9% of the time the problem is the sensor and if all gauges other (then the amp) indicate a problem, the problem is the CVR.

thanks again for the replies
john
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post #8 of 35 (permalink) Old 07-02-2009
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calibrate the gauges?

Did you actually calibrate the gauges?
I was not aware they had a zero or span adjustment for the gauge movement. If they can be zeroed (10 ohms) and spanned (70 ohms) where are the adjustments made?
Thanks

Good Luck and Be Safe
Ron

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post #9 of 35 (permalink) Old 07-03-2009
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Quote:
Originally Posted by jonsee View Post
One thing to keep in mind is if one of your gauges is indicating a problem, 99.9% of the time the problem is the sensor and if all gauges other (then the amp) indicate a problem, the problem is the CVR.
I agree.

By calibration I think John means measuring each gauge to see what various readings would mean. I don't know that there is any way to change the gauge, its just a way of knowing what it means whatever it reads. The oil pressure guages are the worst. The standard 55 psi can be anywhere form 1/4 to 3/4 full scale movement of the needle just by changing senders.

If you are handy with electronics it is sometimes possible to add resistance in series with the senders to adjust the readings you will get on the gauge. This only works if the gauge is indicating higher than you might like. I will do that for my temp gauge soon since it indicates too high for my liking and by adding 5 Ohms in series with it the needle will run at about half scale where God and Henry Ford intended. If your gauge is reading low then your only option is to change the sender which might help or make your problem worse. In general, new temp senders read high so the resistor addition will usually work with them.
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There are two adjustment on the back of the guages. You can see them thru two small holes on the back of the guage.
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I never had one out to look for such. Do you know what they do? The shop manual makes no mention of any such adjustments that I can find.
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One of them will zero the guage. I suspect the other will set the high end, but I have not tried to adjst one to test this out.
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post #13 of 35 (permalink) Old 07-05-2009 Thread Starter
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Yes there are two adjustments on the back of the three gauges, oil, tem, and fuel - the adjustemt 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 deacrease in resistance, increases current flow through the gauge increasing the reading.

hope that provides some light

john
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Thanks John. I had never taken them completely out of their housing to look for such adjustments. Sounds like those must be trim pots in series (for zero) and parallel (for span) to the meter movement. Interesting. I'll have to play with those before I stuff this set back in the car.

Mine are all Ford original but do you know if the repro units sold today have those adjustments?
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How do you adjust them? And for that matter, how do you test them? I am willing to bet my fuel gauge is a bit optimistic. Although, it doesn't help that the shop that had my car put in a 12 gallon tank instead of a 16 gallon tank.


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