Joined: May 30, 2014 Posts: 3447 Location: Texas Hill Country
Posted: Thu Dec 11, 2014 8:08 am Post subject: 6/24 volt gauges, again
So I've been thinking about those 6 volt gauges used in early M38's with the external resistor.
There are different numbers for the temperature gauge suggesting the Army created a new ORD number for the replacement. Not so for the fuel and oil pressure. We have discussed this ad nauseum in the past.
What is interesting to me is at least the fuel level sender is the same. There is only one ORD part number, although it describes being 24 volt.
My question is this? Does the sender care whether the unit is 6, 12, or 24 volt or is it merely a variable resistor to ground with no relation to voltage that only has to handle the different loads?
I'm asking as the resistor in the gauges is on the supply side, correct? Meaning everything downstream would be 6 volt as well, into the gauge and out to the sender. Since they used the same sender the fuel sender would be applicable to 6, 12, and 24 volts?
The other question is if that is the case, can someone that has an M38 converted to 12 volt use the M38 fuel tank and sender as is?
I understand you need to match the resistance of the sender to the gauge used otherwise the level is incorrect, but maybe you just ohm out the M38 sender and find a gauge that needs that resistance or perhaps adjust with an additional resistor in the circuit?
Just thinking aloud for those guys that have 12 volt jeeps and original fuel tanks not shot full of rust holes.
Next question is will the temp and oil pressure senders work the same way?
I realize this isn't Stewart Warner's website, but found this catalog on the Internet. Lot's of good information here if one finds a Stewart Warner gauge or sender, including dimensions, voltages and resistance in many cases:
http://www.partdeal.com/media/wysiwyg/Stewart-Warner-Catalog.pdf
Brian, I am going to assume you were never in the military and never used the military supply system. Often times the ORD number represents many different things. The fuel gauge for example is one of those anomalies. There are two AC part numbers that the Ord # 7728852 represents. 1517561 is an Internal resistor gauge. 1517270 is a gauge that has no internal resistor and uses the 95 Ohm external resistor. In the military when the stock number or ORD# represented several manufacturer's part number and they did not differ in application we ordered new parts using the ORD# or FSN/NSN. In the case of the fuel gauge and the jeep we were working on had no 95 ohm resistors we would special order by manufacturer's part number stating "No substitutes". The oil pressure gauges have the same issue; 1507680 is internal resistor and 1507491 requires the external 95 ohm resistor. The authors of the parts catalogs are usually writers not technicians. As a normal practice in the field the shops would simply order the late internal resistor gauge and chuck the 95 ohm external resistor on the early M38's. Eventually the supply system quit carrying the early gauges.
Trying to use any of the three gauges with 12 volts equals an indicating range error. This is because the input voltage of 12 volts will try to run an early gauge with internal windings for 6 volt or a later gauge with internal winding for 6 volt plus a built in 95 Ohm resistor resulting in range errors.
You would have to do the heavy math with an electrics formula guide to determine the resistor rating you would need to make either gauge indicate in the correct range with 12-14 volts applied.
Keep in mind the tiny windings for motors and magnets in these gauges are designed at the outset for a specific input voltage by either wire gauge use or input fixed resistors. The senders are all variable resistors and again their resistance value was based on the original input voltage. Changing either the input voltage or the resistor values will change the indicating range of the gauge.
So if it's an experiment you wish to embark on then get your hands on an electrical theory and formula guide and play a bit with different combinations. The simplist of which would be to use the early gauges without the internal resistors. This will leave you with only coming up with the correct resistor value for the input side that will replace the original external 95 Ohm resistors. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: May 30, 2014 Posts: 3447 Location: Texas Hill Country
Posted: Thu Dec 11, 2014 3:12 pm Post subject:
Hello Wes,
Yes, you are correct. I never had the calling, as one put it, or more simply, God bless Richard M. Nixon. In 1973-74 I was in Jakarta, Indonesia. A very short flight to Vietnam after basic.
I appreciate you again tackling this subject and your patience in helping me understand. An EE I am not, which might help a great deal.
One thing you mentioned is what I am still focusing on.
You said," The senders are all variable resistors and again their resistance value was based on the original input voltage. Changing either the input voltage or the resistor values will change the indicating range of the gauge. "
I'm making an intuitive leap here and guessing replacement of the sender if required, was not determined by whether the 6 volt or 24 volt gauge was in use. Assuming the original input voltage for the M38 was 24 volts, and the 95 ohm external resistor dropped this voltage to 6 volts to allow using the old style 6 volt gauges, this does not explain how the same sender would be serviceable over the life of these vehicles with either 24 or 6 volts vehicle voltage unless perhaps the sender was rated for 6 volts at X amps and the resistance range was the same, and they are all and always have been 6 volts, and the gauges with external or internal resistors dropped the voltage to the gauge and therefore sender to 6 volts.
One could see what the voltage is going to the sender on both 24 volt gauges with internal resistors and 6 volt gauges with external resistors.
The other thing would be to ohm out the sender at full and empty. This would give you the variable resistance range being supplied. One could then take a look at Stewart Warner's catalog and see what gauge used that resistance.
In which case the same sender theoretically could be used on 6 or 24 volts
depending on whether it had an internal or external resistor as long as you matched the output of the gauge to the sender (whatever voltage that is). And as you suggest, if one could then determine the correct resistance needed, for 12 volts as well.
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