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Per the troubleshooting guide, here are my results:
Checked vehicle battery voltage at 24.8 V
Checked battery cables and terminals for good condition.
Disconnected wire from ignition switch to gauge. Voltmeter indicates 24.8 V with ignition switch ON and grounded. Also 24.8 V when grounded to case of gauge.
With jumper wire connected to sending unit side of gauge, ignition on and grounded to vehicle, fuel gauge indicates empty. When removed from ground - fuel gauge indicates 1/4. (should read full).
Replaced sending unit wire at gauge and removed sending unit wire at fuel tank. Anticipated 24 V at wire when grounded to vehicle and ignition on. Voltmeter indicated 4.0 V
Testing sending unit:
Ground from fuel tank to vehicle ground reads 0.4 ohm
Sending unit reads 33.1 ohms with tank more than 3/4 full
I believe the results indicate the fuel gauge is bad and the sending unit is good. Please evaluate this conclusion based on the data above.

Per the troubleshooting guide, here are my results:
(Step 4e[2]) Checked vehicle battery voltage at 24.8 V
(Step 5b) Checked battery cables and terminals for good condition.
(Step 5c) Disconnected wire from ignition switch to gauge.
(Step 5d) Voltmeter indicates 24.8 V with ignition switch ON and grounded. (Step 5e) Also 24.8 V when grounded to case of gauge.
(Step 5i) With jumper wire connected to sending unit side of gauge,
(Step 5j) ignition on and
(Step5k) grounded to vehicle, fuel gauge indicates empty.
(Step 5l) When removed from ground - fuel gauge indicates 1/4. (should read full).
[/b]NoteIg gauge does not conform to reading oulined in k & l above, REPLACE gage IAW Manual. If gage readings conform with this test proceed with steps m thru r below.[/b] (At this point you were to change the gage.)
(Steps m, n, o, p & q were in here) Replaced sending unit wire at gauge and removed sending unit wire at fuel tank. Anticipated 24 V at wire when grounded to vehicle and ignition on. Voltmeter indicated 4.0 V
( Troubleshooting guide says to just ground the wire above and gauge should read empty. Said nothing about voltage reading.
(Step 6) Testing sending unit:
(Step 6b missing)
(Step 6c[3]{a})Ground from fuel tank to vehicle ground reads 0.4 ohm
(This reading was suppose to be 0.0 so your indication here is a poor ground from tank to body.)
(Step 6c[3]{b}) Sending unit reads 33.1 ohms with tank more than 3/4 full
I believe the results indicate the fuel gauge is bad and the sending unit is good.
(Step 6c[3]{c}), (Step 6c[3]{d}) & (Step 6c[3]{e}) skipped. {d} was not needed & {e} is unknown without first performing {c})

Please evaluate this conclusion based on the data above.

My short answer is: Change gage - YES) Sender still needs test (Step 6c[3]{c}) before you can have a firm answer about sender.

In addition you need to remove all traces of grounding resistance between the tank and the tub. This means removing a lot of paint and corrosion or simply add a 14 gauge ground wire between a paint free stud or bolt and hole on the tank to a paint free stud or bolt and hole in the tub.

Here is a conclusion to the thread for future readers.
The gauge was bad based on the results from the troubleshooting guide.
I found a used fuel gauge that I believe is correct for the M38 based on the Ord 9 page 151. The working gauge is Ord# 7728852 and AC-1517561.
Please correct me if I'm wrong about the correctness of this gauge.

I also added a dedicated ground for the instrument panel and fuel tank sending unit per recommendations.
Thanks Wes!

I found a used fuel gauge that I believe is correct for the M38 based on the Ord 9 page 151. The working gauge is Ord# 7728852 and AC-1517561.
Please correct me if I'm wrong about the correctness of this gauge.

That gauge has the internal resistor and is correct for M38's after MC17855.

Reviving an old thread.

Evaluating two 24 volt fuel gauges. The gauges are AC-1517561 and S.W. 505 J 8376374. Gauge ground confirmed with dedicated ground from gauge to dash.

Senders: Evaluated 2 NOS sending units. Both provided proper ohms (0 empty, 15 at 1/2 full and 30 at full tank).

Gauges: Followed troubleshooting guide (see earlier posts). To summarize, disconnected wire from ignition switch to gauge. Voltmeter indicates 24.8 V with ignition switch ON and grounded. Also 24.8 V when grounded to case of gauge. With jumper wire connected to sending unit side of gauge, ignition on and grounded to vehicle, fuel gauge indicates empty. Both gauges peg to full when sending unit ground removed. I thought these tests indicated the gauges were good.

However, with sender out of tank and grounded, neither gauge reads full with sending unit float in the full fuel tank position (30 ohms). Both gauges read just over 1/2 full.

Are both gauges bad?

When asking for a confirmation of your troubleshooting you should provide an itemized list of results as shown in the TB 9-2300-228=20. Also tell us which model jeep you have.

IE:

5 d: Voltage to gage = 24.8

5 e: Voltage to gage = 24.8

5 i thru k: Neg side of gage to veh. grd. = Empty

5 l: Grd jumper removed gage = full

5 m thru q: Grounding sender lead at tank =

6 a thru c (3) (a): Sender reads how many ohms when connected to fuel tank and veh grd.? -

6 a (3) (b): Ohmeter reads ? ______ when tank is at what level? _______. Note in your post you say you have 0 @ E, 15 @ 1/2 and 30 @ full. The TB only offers somewhere between 0 and 40 ohms. Where did you learn that o, 15 and 30 were proper ohm readings?

Perhaps you can answer the blank readings and tell us the part number of your sender?

Are you aware that the Stewart Warner gage is only used on the M170?

1952 M38
Fuel gauges tested: 7728852 AC 1517561 and 8376374 S.W. 505-J
NOS sending units tested: 11640674 and 1200923-2
All parts obtained from reputable MV parts suppliers

Data from Troubleshooting for Military Standard Instrument Cluster Gauges This information provided by Bob Westerman www.cj3a.info and www.willysmjeeps.com Trouble shooting guide
5. Gages and Related Wiring
a. Check vehicle battery voltage. Charge or replace battery if voltage is not as prescribed in pertinent technical manual – 24.8 volts.
b. Check battery cables and terminals for good condition. Repair or replace battery cables and terminals, as required. Be sure that they are properly secured. - all removed and cleaned
c. Disconnect wire from ignition switch to gage being tested at gage, and connect wire to positive lead of voltmeter. - done
d. With ignition switch ON, ground negative lead of voltmeter (B, fig 1) to vehicle ground, voltmeter should indicate battery voltage. Note. If lower or zero reading is indicated, check circuit for loose or broken connections. Repair as required. – 24.8 volts
e. Remove voltmeters negative lead from vehicle ground and connect to case of gage (C, fig. 1), voltmeter should indicate battery voltage. Note. This check is important. If case of gage is not properly grounded, gage will not operate. - 24.8 volts
f. Turn ignition switch OFF. - done
g. Disconnect voltmeter positive lead from ignition switch-to-gage wire and connect wire to gage. - done
h. Disconnect wire from gage to sending unit at gage. - done
i. Connect suitable jumper wire to sending unit side of gage. - done
j. Turn ignition switch ON. - done
k. With jumper wire grounded to vehicle (D, fig. 1), the gages should indicate the following:
Temperature gage Maximum temperature
Oil pressure gage Zero pressure
Fuel gage Empty YES
l. Remove jumper wire from ground (E, fig. 1), the gages should indicate the following:
Temperature gage - Zero temperature
Oil pressure gage - Maximum pressure
Fuel gage - Full YES
Note: If gage does not conform to reading outlined in k and l above, replace gage in accordance with pertinent technical manual. If gage readings conform with this test, proceed with steps m through r below.
m. Turn ignition switch OFF. - done
n. Remove jumper wire from gage and connect wire from sending unit to gage. - done
o. Disconnect wire from sending unit to gage at sending unit. - done
p. Turn ignition ON. - done
q. Ground wire removed from sending unit (F, fig. 1), the gage should indicate the following: Temperature gage - Maximum temperature
Oil pressure gage - Zero pressure
Fuel gage - Empty - YES
If gage does not conform to reading outlined above, repair or replace wire from gage to sending unit.
r. Turn ignition switch OFF. - done
Connect wire from gage to sending unit. Note: Electrical gages indicate an approximate reading, for accurate readings, it is recommended using master gages connected to vehicle
Chart of resistance checks done on operational components. All resistance checks are done with the
component out of the circuit.
Fuel Level Sending Unit - resistance to yield corresponding gauge
readings (per Westerman Troubleshooting for Military Standard Instrument Cluster Gauges)
30 ohms = Full
15 ohms = Half
0 ohms = Empty

Both gauges read from zero to slightly over 1/2 full when tested with sending unit out of tank and moving float thru its full range.

Part number of your sender is????

And did you ohm check the fuel tank to vehicle ground?

When you tested the sender out of the tank using battery voltage and getting your 1/2 full reading with full float up where were each of the tested gauges housing's grounded? And where was the sender grounded?

Part number of your sender: NOS sending units tested: 11640674 and 1200923-2 See photos below.

And did you ohm check the fuel tank to vehicle ground? Yes. 0.1 ohm. Also have a dedicated ground wire from sender body to frame.

When you tested the sender out of the tank using battery voltage and getting your 1/2 full reading with full float up where were each of the tested gauges housing's grounded? - Yes. Dedicated ground from each gauge in instrument cluster to body tub. Confirmed tub grounded to frame and frame to under hood battery negative.

And where was the sender grounded? Grounded to frame and also tested with ground to under hood battery negative terminal.

There's your problem!

Wrong senders.

1200923-2 is a universal hi ohm (240-30) marine sender.

11640674 is an unknown unless you spend a great deal of time trying to research the number.

The sending unit that matches your correct gauge (7728852, AC 1517561) is AC 1517375 or ORD# 7524909.

The sending unit that matches you M170 fuel gauge (8376374, SW 505-J) is any one of these three AC GG8799, AL EO1199F or SW 507N. Keep in mind that the M170 gas tank is a different shape, and holds a different capacity of fuel compared to the M38A1 and M38 tanks.

Thanks Wes! Appreciate your help.

I have now tested 3 NOS senders from reputable MV parts suppliers.

Where in the world does one find a correct 1952 M38 sending unit AC 1517375 or ORD# 7524909 that matches gauge 7728852, AC 1517561?

I don't understand how a sending unit that provides the proper ohms per the Westerman guide (0 ohms empty, 15 ohms 1/2 full and 30 ohms full) does not yield correct readings on the gauge.

When I got this M38 it had the original sending unit (with cogged wheels) but it was corroded and did not function.

Any suggestions for locating the correct sender?

Have you called John at Midwest Military?
If anybody on this planet has one, he does.

I don't understand how a sending unit that provides the proper ohms per the Westerman guide (0 ohms empty, 15 ohms 1/2 full and 30 ohms full) does not yield correct readings on the gauge.

I am not sure where Bob got those readings but you should note that, that OHM range is not what the TB 9-2300-228-20 uses. It only offers a max OHM reading of 40 for the correct sender.

Is it possible you are using external resistors with your fuel gauge? Early M38's used a 95 Ohm external resistor mounted on the gauge and later M38's used a newer gauge with an internal resistor.

This is the early cluster with the three 95 Ohm resistors on the back of the gauges.


This is the late cluster without the external 95 Ohm resistors:


Also the transision from external resistor mounted on early gauge PN 1517270 to internal resistor in the later gauge 1517561 had one small problem causing confusion. Both gauges had the ORD # 7728852

Rond2, John at MWM is my go-to guy. He has been very helpful and provided 2 of the senders.

The other sender (0-30 ohm) came from AJP and was listed as NOS M38 sender.

All have Douglas connecters, but unlike the original, the sender float goes up and to the right (with Douglas connector facing forward). The original sender float went up and to the left.

Wes, both gauges tested do not have the external resistor pictured.

So I need a 0-40 ohm sender, correct?

It has been so long that I don't even remember what "operational component" sending unit I tested that yielded the 30 ohm result. Strange that other M series sending units tested by JeepdaddyRC also are 30 ohm, while TB 9-2300-228-20 states they should be 40 ohms. TB 9-2300-228-20 is dated 8 July 1960 and superseded TB ORD 659 dated 29 October 1956. Seems like TB ORD 659 should be more accurate for M38s because of the 1956 date. Perhaps the sending unit resistance value changed over the years? I do not have a copy of TB ORD 659, so I just ordered one from Portrayal Press.