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I'm looking for a fuel level gauge, locally, but can't find a 24 volt gauge. Can someone explain why these gauges are described in Ohms, and what the M37 gauge might be?

BTW, I'm electrical disadvantaged -- and apparently the guys at NAPA are, too, because they didn't know if there was a 24 volt gauge, or the significance of the Ohms designation.

Dave

The gauge reads via resistance from the sending unit in the tank. As the float in the tank moves, it makes contact with a resistor (usually a wound coil of wire). Where the float arm contacts the coil determines how much resistance is in the circuit.

A typical GM gauge reads 0 ohms at full (arm is not contacting the coil when in the tank as the float is too high) and 90 ohms at empty (full resistance as all the voltage in the circuit must pass through the coil).

I'm not sure what the M37 ohm settings are, but various manufacturers have used different ohm settings over the years.

Does that mean voltage doesn't factor into what kind of gauge is used? The sender on my truck reads 0 at empty (float all the way down) and 30 ohms full (float up), if I recall correctly from some of the experimenting we were doing.

Dave

Dave, the basic electrical theory uses the triangle with volts - Ohms - Amps.

Volts = pressure that pushes the electrons through the circuit.

Ohms = resistance to flow of the electrons in a circuit.

Amps = Current = The actual flow of electrons.

The three are inter-related and a change in one value will effect a change in the other two values.

The triangle is meant to show how they relate to each other mathematically and offer you several equations to resolve what happens when one value changes.

That was the long answer.

The short answer is Volts and resistance are the key identifiers for electric fuel system gauges.

As for NAPA don't waste your time there if you intend to use the original sender and keep your truck 24 volts.

You'll need to shop the military vendors for the correct electrical senders and gauges used in your M37. The Army did keep it fairly simple and parts interchange is very good between vehicles. The biggest differences are sender swing and drop and connectors Douglas vs Packard.

The Army also published A TB to help with troubleshooting the electric 24 volt gauges.

TB 9-2300-228-20 Tactical Transport and Combat Vehicles: Troubleshooting for Instrument Cluster Gages, Switches, Circuit Breakers, Sending Units and Related Wiring dated 8 July 1960.

You can download a copy here:
http://www.cj3a.info/sibling/milgauge/milgage.htm

Here's an exerpt on the fuel sender:

(3) Fuel gage sending unit.

(a) Check ground between fuel tank and vehicle with an ohmmeter, zero reading should be obtained. If the fuel tank is not grounded to the vehicle, the sending unit will not operate.

(b) The ohmmeter should indicate a reading between zero and 40 ohms, depending on the amount of fuel in the tank. If the ohmmeter indicates a reading higher than 40 ohms replace the sending unit.

(c) The ohmmeter should indicate a higher reading as fuel tank is filled and a lower reading as the fuel is withdrawn. If the sending unit does not conform to this test, replace the sending unit.

(d) To test inaccessible sending units and electrical wire from gage to sending unit, omit procedure in b and c above and proceed as follows:

1 Disconnect wire from gage to sending unit at gage.

2 Connect positive lead of ohmmeter to wire from sending unit and ohmmeter negative lead to vehicle ground.

3 Follow procedures outlined in c(1) through (3) above for sending unit being tested.

(e) Defective sending units will be replaced as prescribed in pertinent technical manuals.

Here's an exerpt of the correct resistance readings for the sender:

Fuel Level Sending Unit - resistance to yield corresponding gauge readings

30 ohms = Full

15 ohms = Half

0 ohms = Empty

It is very important that the fuel tank be grounded solidly to the chassis. It is also very important that the gauge housing be grounded to the chassis. Years of paint on original trucks and heavy coats of new paint on individual parts before assembly on restos leads to poor grounds. This means bare metal has to be allowed to mate the tank to the chassis and gauge to the instrument cluster panel and the instrument cluster panel to the dash.



So if you know two of the values you can compute the missing value. The upper item uses division when equating values with a lower item and the side bu side items use multiplication when equating those to values.

V = Volts
R = Resistance or Ohms
I = Current or Amps

CUZ

Great post and TB, Sometimes we forget ohms law and run off half cocked and do stupid things and do not use the basics we learned in school to get the job done!

hb

Hey, Cuz, great information. Thanks. I take it from your answer that there are 24 volt gauges, per se. I was just trying to get a fast way to get a gauge, and one that would match the SW oil pressure gauge I got from NAPA.

Dave

Another thing to remember is don't hook up 24 volts (or 12) directly to the sending unit (this is more for modern sending units, but its good info anyway). The sending unit usually gets it power from the gauge. There are usually 3 posts on the back of the gauge; one is usually marked BAT (Battery or source voltage), the others "S" (sender) and "G" (ground).

The gauge sends only a limited amount of current to the sender so as to not overwhelm the resistor and fry the unit. Many a sender has been pooched by hooking it directly to the power source, rather than the gauge.

Electrical stuff works on smoke. Once you let the smoke out it dosen't work again...

Electrical stuff works on smoke. Once you let the smoke out it dosen't work again... Crying or Very sad Laughing

I like that.

Most modern instrument systems use a limited voltage and 5 volts seems a very common number.

The military system is straight 24 volt.

Be very careful when trying to mix and match senders to gauges. Done without close study of the units will usually result in the quoted statement above.

Is the SW oil press gauge you got from NAPA a mechanical gauge using a tube to the engine or is it used with an electric sender?

Gauges and senders are generally only sold as a matching set with any attempt to buy a replacement single part like just a gauge you must order a gauge identical to the one that came with the sender. Or at least the same voltage and resistance (ohms) rating.

Midwest Military stocks the fuel gage for $90 and the sender for $70.
http://www.midwestmil.com/m37p8.html


The sender numbers from the IPLs are:
Chrysler # 1268987, ORD# 7526038 FSN's 6680-515-3457 & -6193

Gauge numbers from the IPLs are:
AC# 1517871, ORD# 8376374,
When above # is exhausted use: AC# 1517651, ORD# 7728852,
FSN 6680-024-2276
Also available: MS24544-2, FSN 6680-933-3600