Dual Tank Fuel Gauges Not Working

First post here. Just got a new truck and working on issues before title arrives in months!

I know, this again! Read the TM and searched this forum over and nowhere is it explained very clearly how to step by step troubleshoot this evil gremlin.

The TM mentions that battery voltage should be at the wire that plugs into the sender at the tank. Does that mean 24v? I have 5v at my left tank and 0v at my right. 24v into gauge (wire 27) but only 5v coming out of gauge to the selector switch. 5v coming out of selector to left tank and 0v going to right tank. Did continuity test on switch to find out it don't send anything to right tank wire and does indeed send to the left tank. Sounds to me like bad switch giving me not the 5v to right tank.

So is the guage supposed to have 24v come OUT of it (not wire 27) and into the selector switch? The daggone TM contradicts itself saying wire 28 goes one place on the swith and then says it goes on another post as well. Here is how they went on to confuse me. So does 28 go to pin A or B? Most importantly, has anyone tested the voltage at the tank sending unit with wire disconnected. Is it 24v there or the 5v I am getting?

Test 3. Test circuit wire of affected tank.

Step 1. Dieconnect lead 28 (left-hand tank, pin B) or lead 29 (right-hand tank, pin D) from fuel

selector switch.

Step 2. Place battery switch to ON position.

Step 3. Place fuel selector switch (if equipped) to affected fuel tank.

Step 4. Set multimeter on 50-volt range.

Step 5. Connect multimeter negative lead to chassis ground and positive lead to pin B or pin D

on selector switch. Multimeter should indicate battery voltage.

a. If battery voltage is indicated, repair lead from switch to sending unit (para. 3-131).

b. If battery voltage is not indicated, go to test 4.

Test 4. Test fuel selector switch.

Step 1. Dieconnect lead 28 from pin A of fuel selector switch.

Step 2. Place battery switch to ON position.

Step 3. Set multimeter to 50-volt range.

Step 4. Connect multimeter negative lead to vehicle chassis ground and positive lead to

connector of disconnected lead 28. Multimeter should indicate battery voltage.

a. If battery voltage is indicated, replace fuel selector switch (para. 3-109).

b. If battery voltage is not indicated, repair lead 28

Thanks, Jim

First thing I did on my M931A2 was to remove the ground leads (both ends) and clean everything down to bare metal...most likely you'll find a lot of rust/corrosion/gunk in there. Check both ends of the ground wires as well; I had to replace one as the insulation had cracked near the connector & allowed corrosion to take hold, so it was non-op.

You can paint over them after you have everything connected, the paint should help to seal up the area and keep corrosion out.

Neither of my fuel gauges worked, until I fixed the ground wires & connections. Now they work great.

gottaluvit said:

First post here. Just got a new truck and working on issues before title arrives in months!

I know, this again! Read the TM and searched this forum over and nowhere is it explained very clearly how to step by step troubleshoot this evil gremlin.

The TM mentions that battery voltage should be at the wire that plugs into the sender at the tank. Does that mean 24v? I have 5v at my left tank and 0v at my right. 24v into gauge (wire 27) but only 5v coming out of gauge to the selector switch. 5v coming out of selector to left tank and 0v going to right tank. Did continuity test on switch to find out it don't send anything to right tank wire and does indeed send to the left tank. Sounds to me like bad switch giving me not the 5v to right tank.

So is the guage supposed to have 24v come OUT of it (not wire 27) and into the selector switch? The daggone TM contradicts itself saying wire 28 goes one place on the swith and then says it goes on another post as well. Here is how they went on to confuse me. So does 28 go to pin A or B? Most importantly, has anyone tested the voltage at the tank sending unit with wire disconnected. Is it 24v there or the 5v I am getting?

Test 3. Test circuit wire of affected tank.

Step 1. Dieconnect lead 28 (left-hand tank, pin B) or lead 29 (right-hand tank, pin D) from fuel

selector switch.

Step 2. Place battery switch to ON position.

Step 3. Place fuel selector switch (if equipped) to affected fuel tank.

Step 4. Set multimeter on 50-volt range.

Step 5. Connect multimeter negative lead to chassis ground and positive lead to pin B or pin D

on selector switch. Multimeter should indicate battery voltage.

a. If battery voltage is indicated, repair lead from switch to sending unit (para. 3-131).

b. If battery voltage is not indicated, go to test 4.

Test 4. Test fuel selector switch.

Step 1. Dieconnect lead 28 from pin A of fuel selector switch.

Step 2. Place battery switch to ON position.

Step 3. Set multimeter to 50-volt range.

Step 4. Connect multimeter negative lead to vehicle chassis ground and positive lead to

connector of disconnected lead 28. Multimeter should indicate battery voltage.

a. If battery voltage is indicated, replace fuel selector switch (para. 3-109).

b. If battery voltage is not indicated, repair lead 28

Thanks, Jim

Click to expand...

Disconnect 28 or 29 without sending unit should be battery voltage, at the switch pins ,

silverstate55 said:

First thing I did on my M931A2 was to remove the ground leads (both ends) and clean everything down to bare metal...most likely you'll find a lot of rust/corrosion/gunk in there. Check both ends of the ground wires as well; I had to replace one as the insulation had cracked near the connector & allowed corrosion to take hold, so it was non-op.

You can paint over them after you have everything connected, the paint should help to seal up the area and keep corrosion out.

Neither of my fuel gauges worked, until I fixed the ground wires & connections. Now they work great.

Click to expand...

Checked all the grounds with continuity tester and the sending units and the gauge is grounded. I was gonna throw that in the first post but forgot. Thanks though. When I first got the truck I thought that would no doubt be the issue and to my credit card's demise, it isn't.

glassk said:

Disconnect 28 or 29 without sending unit should be battery voltage, at the switch pins ,

Click to expand...

So you're saying it should be 24v and not the 5v I get going to wire 28 (left tank)? So is this a faulty guage? I know half of the switch isn't working with the continuity test as I flipped the switch.

Well, I ordered a new tank selector switch and gauge. Let's see how that goes. Hope to learn something from using the voltmeter after the gauge install and will share the results and voltage from the gauge to the sending unit. With the reputation these trucks have on fuel gauges not working, it couldn't hurt to have an extra part laying around anyway if it in fact wasn't the gauge. I know basic electricity so the switch is definitely bad, and it sure seems the gauge is too considering the TM and glassk say it should be battery voltage.

Now for the wait that I hurried myself into.

Okay, the new gauge and tank switch came today. Changed the gauge and same 5-6v coming from gauge. So apparently that's the norm and not FULL battery voltage. My switch was bad so that was a good purchase. So I decide its time to pull the senders since both tanks read empty. Both floats were busted paper thin brass. Moved the lever and shazzam, they both work according to where I placed the float lever. So I need floats, have an extra gauge now, and have learned what voltage actually comes out of a working gauge. Hope this helps someone.

Jim

gottaluvit said:

Okay, the new gauge and tank switch came today. Changed the gauge and same 5-6v coming from gauge. So apparently that's the norm and not FULL battery voltage. My switch was bad so that was a good purchase. So I decide its time to pull the senders since both tanks read empty. Both floats were busted paper thin brass. Moved the lever and shazzam, they both work according to where I placed the float lever. So I need floats, have an extra gauge now, and have learned what voltage actually comes out of a working gauge. Hope this helps someone.

Jim

Click to expand...

It helps more than you think, thank you for posting this up!!

So apparently that's the norm and not FULL battery voltage.

Guess you need to fill one of these out,...

Glassk: I thought only those currently in the military were to do that. I'm just an old washed up number now.

As for the floats. Couldn't find any on MV websites and looked at all the classifieds on here. Found brass ones at Summit for 6 something each for a mustang and the plastic ones at jeephut.com for $3.20 each for '55 - '86 CJs. Looks like the same thing as what's in my 5 ton. Plastic is what I wanted since the brass ones both collapsed from the clip that holds onto them crushing them so I went with the jeep ones. Not 100% sure if exact match but they both have the groove for the arm to snap onto them and I figured if it's 1/4" off or so I can bend that clip to make it work. Might want to bend that out anyway to weaken it a bit and not crush the next ones.

Hope this helps anyone looking for floats.

Jim

Well, I got the floats from jeephut yesterday and they were a perfect fit. They seem well made of a heavy plastic. Both gauges now work. I think I may just order one extra for on down the road just in case one fails in a few years.

Jim