Fuel Gauge Diagnostic Problem - SOLVED!

[houdel]

My fuel gauge has never read properly since I got it from the DRMO. Regardless of how much fuel in the tank, it always measured between 1/2 and full on the gauge, which I know is wrong since I can't afford to fill the tank and generally run between 1/4 and 1/2 tank of fuel.

I tried the diagnostics in TM 9-2320-316-20. I disconnected wire 28 from the fuel sender unit, fuel gauge read full, this is good. I grounded wire 28, fuel gauge read empty, this is also good. I measured the voltage of wire 28 at the sender unit, got 2.4 volts, this is not good, it should read battery voltage, 25-28 volts.

So I pulled the gauge panel. Measured the resistance of wire 28 from the fuel gauge to the fuel sender, got a reading of 0.3 ohms. I assume this is good, the TM did not give a specific resistance measuremnt, but 0.3 ohms is pretty close to no resistance.

Measured the voltage at Pin 27 of the fuel guage, got 25 volts,. this is good. Measured the voltage at pin 28 of the fuel gauge, got only 2.4 volts. this is not good. Not clear from the manual, but I expected 24-28 volts at pin 28. Connected a good gound wire from the fuel gauge to a good chassis ground, remeasured the voltage at pin 28, still got 2.4 volts.

OK, my guess is I have a bad fuel gauge unit. The only way I can have "batttery voltage", i.e.25-28 volts at wire 28 at the fuel guage is to have "batttery voltage", 25-28 volts at pin 28 of the fuel gauge. This ain't happening.

Before I shell out the buck$ for a new fuel guage, does anyone agree, disagree, or have any comments on my diagnostics so far?

 

[houdel]

Fuel Gauge Dagnostic Problem -HELP

My fuel gauge has never read properly since I got it from the DRMO. Regardless of how much fuel in the tank, it always measured between 1/2 and full on the gauge, which I know is wrong since I can't afford to fill the tank and generally run between 1/4 and 1/2 tank of fuel.

I tried the diagnostics in TM 9-2320-316-20. I disconnected wire 28 from the fuel sender unit, fuel gauge read full, this is good. I grounded wire 28, fuel gauge read empty, this is also good. I measured the voltage of wire 28 at the sender unit, got 2.4 volts, this is not good, it should read battery voltage, 25-28 volts.

So I pulled the gauge panel. Measured the resistance of wire 28 from the fuel gauge to the fuel sender, got a reading of 0.3 ohms. I assume this is good, the TM did not give a specific resistance measuremnt, but 0.3 ohms is pretty close to no resistance.

Measured the voltage at Pin 27 of the fuel guage, got 25 volts,. this is good. Measured the voltage at pin 28 of the fuel gauge, got only 2.4 volts. this is not good. Not clear from the manual, but I expected 24-28 volts at pin 28. Connected a good gound wire from the fuel gauge to a good chassis ground, remeasured the voltage at pin 28, still got 2.4 volts.

OK, my guess is I have a bad fuel gauge unit. The only way I can have "batttery voltage", i.e.25-28 volts at wire 28 at the fuel guage is to have "batttery voltage", 25-28 volts at pin 28 of the fuel gauge. This ain't happening.

Before I shell out the buck$ for a new fuel guage, does anyone agree, disagree, or have any comments on my diagnostics so far?

 

[cranetruck]

RE: Fuel Gauge Dagnostic Problem -HELP

Since you already pulled the "gauge panel", remove the fuel gauge and also the fuel sending unit from the tank (not very difficult) and bench test both together.
The sending unit is nothing but a variable resistace to ground.

 

[cranetruck]

RE: Fuel Gauge Dagnostic Problem -HELP

BTW, the fuel gauge sending unit ground path is as follows:
From the sending unit housing to the fuel pump housing via mounting screws and from the pump housing to frame ground via wire #79. The fuel tank is insulated from ground if it wasn't for that one wire from the pump connector to ground.
With your VOM, check/confirm thet the fuel sending unt is grounded.

 

[Recovry4x4]

RE: Fuel Gauge Dagnostic Problem -HELP

I think you're right on but I would triple check grounding of that gauge. it its a nylon gauge, one of the bolts used to clamp it down is marked ground. If you're certain that the gauge is grounded, sounds like a bad gauge. I have the same issue on the tractor and too lazy to fix it. I keep a stick under the seat to measure the tank. LOL

 

[houdel]

I have one half of a yard stick under the seat also. I even calibrated it in gallons and then varnished it so I can get stick the tank and know exactly how much fuel I have left.

I did check, clean and recheck the fuel tank ground. I even used an ohmmeter to check the sender, forgot the exact reading, but it seemed about right for the amount of fuel I had in the tank. The fuel sender reads 0 ohms empty and 31.5 ohms full.

I'm pretty sure the problem is the fuel gauge itself. According to the diagnostic, you should have "battery voltage", 24-28V, on wire 28 when it is disconnected from the sender. Logic would dictate that since this is a direct lead to the fuel gauge, you should also have 24-28V at the pin on the gauge where wire 28 connects. I had 25V going into the gauge, a good ground from the gauge mounting stud to the chassis, but only 2.4V on the pin where wire 28 connects. Sounds to me like the guage is crap. Time to call Saturn!

Yes, I know how sensitive the Deuce is to grounding. When anything electrical starts acting odd, my first step is to check the ground. In a few cases I've even added a redundant ground just in case. Like on my turn signal flasher!

 

[CGarbee]

There is a discussion on the M38 forum at www.g503.com concerning fuel guage diagnostics that is applicable to the ones in the M35, so I thought that I would post a link to the thread here...
See:
http://www.g503.com/forums/viewtopic.php?t=75261

Good luck.

PS: The guage in my M35A2C reads between 1/4 and 3/4 full... I went to pull the sender one day and realized that the screw closest to the frame has a buggered up head, so it will be a while before I pull it... Meanwhile, I use the dipstick and bladder method (dipstick to see how much fuel I have, bladder to tell me how often to stop and check the fuel level, among other things...).

 

[houdel]

Cabell - I have been using the dipstick method since I got my Deuce in February. Even calibrated it in gallons, taking into consideration that the truck runs out of fuel with 10 gallons still left in the tank. I guess the military decided to mount the pump pickup about 2" above the bottom of the tank for a little safety margin to allow water & other crud to collect when refueling under less than ideal tactical conditions.

I woudn't be in too great of a rush to pull that fuel sender. I did a quicky run through the diagnostics procedure the first time and came to the erroneous (I believe) conclusion I had a bad fuel sender. The second time through I was a bit more carefull in my procedure, and now believe the problem is in the gauge itself. I just ordered a new fuel guage from Saturn, so I'll know in a couple of days if I was right about the gauge being defective. I assume you've also run the diagnostics, but here is my comprehensive, but fairly simple to perform process to assess if the problem is the gauge or sender:

1. Ignition on, remove wire 28 from the sender. Fuel gauge should read "Full". Ground wire 28 to the negative battery terminal. Fuel gauge should read "Empty". Measure voltage at wire 28, should be 24-28V. Measure resistance from sender to the negative battery terminal, should be nominally 0 Ohms.

2. Get a 0-30 Ohm potentiometer (you can use a higher rated potentiometer, but 31.5 Ohms is the max resistance of the sender unit, anything higher will give a "Full" reading on the fuel gauge) (or use individual resistors, 10 Ohm, 20 Ohm and 30 Ohms). Ignition on, connect one lead to wire 28, the other lead to the negative battery terminal. Vary the resistance from 0 to 30 Ohms, the fuel guage should read proportionaly to the resistance in the circuit from "Empty" to "Full". If the fuel gauge responds proportionally to the resistance used, the gauge most likely is OK. If it doesn't, the gauge is probably bad.

3. If you didn't get 0 Ohms between the sender and the negative battery terminal, you have a bad ground to the sender. Repair and retest.

4. If you didn't get 24-28V at wire 28, time to pull out the gauge panel and check the continuety of wire 28. Disconnect wire 28 from the fuel gage and the sender. Measure the resistance of wire 28 from end to end, it should be nominally 0 Ohms (mine read 0.3 Ohms). If wire 28 shows any significant resistance, repair or replace it. If the continuety is good, time to check the fuel gauge itself.

5. Ignition on, pull wire 27 from the fuel gauge and measure the voltage, you should have 24-28 V. If not, find the problem and fix it. If you do have 24-28V present at wire 27, reconnect it to the fuel gauge and run a jumper wire from one of the gauge mounting studs to a good, reliable ground, like the negative battery terminal. Check the voltage at pin 28 of the fuel gauge, you should have 24-28V. If you do, the gauge is probably good and the problem is in the sender. If you don't have 24-28V at pin 28, most likely the gauge is bad.

Hopes this helps, maybe you won't have to pull that fuel sender after all.

 

[houdel]

Got my my new fuel gauge from Saturn and installed it today. Just as a matter of curiosity and thoroughness, I did some voltage checks during the installation. Measured the voltage at wire 27 (power supply to the fuel gauge) got 25V as expected. Connected wire 27 to the fuel gauge and read the voltage at pin 28 of the gauge (the fuel sender connection) - only got got 6.65V, this is NOT what I expected! According to the diagnostic in the -361 TM, wire 28 should read "battery voltage". However, my old gauge, which would only read between 5/8s and full, read only 2.4V at pin 28, so I guess more is better! The new gauge read full with wire 28 disconnected from the sender and empty with wire 28 grounded, both proper readings according to the TM.

When I connected wire 28 to the sender, the fuel gauge read 3/8th full. I sticked the tank and estimated I had 20.6 gallons of fuel in the tank - close enough to 3/8th of a tank, if there were 50 USEABLE gallons of fuel in the tank. So I guess in that respect my new fuel gauge is reading correctly and my problem is solved.

HOWEVER, my in tank fuel pump runs out of fuel when there still is 10 gallons in the bottom of the tank, so I only have 40 USEABLE gallons when the tank is filled to the mark on the outside of the tank. When my gauge gets down to 1/4 tank I still have 12.5 gallons in the tank, but only 2.5 USEABLE gallons. So I guess I have to either fuel up when I get down to 1/4 tank on the gauge, or figure out a way to get my tank pickup about 2" lower in the tank so I can make use of that extra 10 gallons in the bottom of my tank!

 

[bomar76]

[quote

 

[Recovry4x4]

The pump assy depth is adjustable. When it is adjusted correctly you should have around 10 gallons left

 

[houdel]

How can that be, as the pump motor is above the pickup and often exposed to air when the fuel level is low - say 1/3 of a tank or so? And when the fuel level gets low enough that the pickup is sucking air, the entire pump assembly is above the remaining fuel level?