TC45D FUEL GAUGE PROBLEM

[spankyscamp]

Fuel tank full, fuel gauge reads 0. Fuel gauge reads full if you put the 2 wires together at sender. Then gauge will read fuel until you start engine, rev it up and down a few times, then gauge reads 0. Have 6.5 volts at sender when reads full (not 12 volts) then reads 3.2 volts when gauges reads 0. Disconnected wires and ohms, checks OK. After I short wires at sender gauge reads full, tap on gauge doesn't move until you start engine and rev up then reads 0. Can anybody give me any help. Dealer says I have everything that they would have done except change out gauge, but they say it should be OK from when the wires or shorted out at sender and gauge gos to full.

 

[jinman]

New Holland's Repair Manual says that that the fuel sending unit in the tank is a variable resistor. When the tank is empty, it should read 245 to 249 ohms. When the tank is full, it should read 32 to 34 ohms. If you want to make a test jig, stop by Radio Shack and buy a variable resistor that goes from zero to 500 ohms or zero to 1000 ohms. (Zero to 250 ohms would be perfect, but they may not have one of those in stock.) Set the resistor to 35 ohms and then remove the plug to the tank float and jumper the resistor to pins A and B into the wire connector going to the gage. Turn the key on and the gage should go to full and stay there. If it goes to full and slowly drops down to zero, then you know the gage is faulty. I think there is a chance that you have a dirty or loose connector. It's a remote possibility, but I'd take out the instrument panel and disconnect/reconnect the connector to make sure it is clean.

BTW: You can also use your VOM to check the resistance of the float. If you hook it to the wires going into the tank with the connector removed, you should be able to watch the resistance decrease as you fill the tank. It may not be completely linear, but you can estimate that the resistance should be about 140 ohms when the tank is half full (249-34=215, 215/2=107.5, 107.5+34=141.5).

If you decide to remove the gage from the instrument panel, check out this thread where I replaced my temperature gage. Also, if you want to do a test of the gage after it is removed, post back and I'll give you instructions for making resistance checks. It's a simple check once the gage is removed.

 

[spankyscamp]

I have checked the ohms of the sending unit of the new and old and everything is ok.With the sending unit out of the tank and wired up, the gauge will respond correctly. Took gauge cluster apart, pulled out fuel gauge cleaned contacts with fine sand paper and WD40. Put all back together, cleaned multi plug and sprayed with WD40. Started engine, everything ok until I revd it up then gauge went to 0. I have replaced the sending with a new one and it all still works the same way. How do you go about checking out the gauge it self. Maybe there is something in the gauge that is going bad and shows up when you get vidration from the engine.

 

[barbqranch]

I took apart the temperature gauge on my TC29D, and I expect the fuel gauge is similar. It consists of two coils of wire, at right angles, with, I believe from memory one common lead and two others. Between them on a shaft connected to the needle is a magnetized disk, such that if only one coil has current, the needle points one way, and if the other had the only current, it would swing around 90 degrees. If you have a bad connection in any of the wires going to the gauge, it could do what you describe. An unlikely broken wire in the gauge that made contact part of the time could do the same. If it were mine, I would take apart the cluster and solder small wires to the gauge power leads and lead them out so I could check the voltage to ground on each.

 

[barbqranch]

I should also mention, I believe one coil always has 12 volts on it when the ignition is on, but I didn't check that.

 

[RickB]

Those gauges fail at least as often as that particular sender.

 

[jinman]

Maybe there is something in the gauge that is going bad and shows up when you get vidration from the engine.

I think you might be right about vibration, but it may not be the instrument cluster. It might be the wire from the sending unit to the cluster is opening due to vibration. I'd get a couple of long wires and jumper directly to the plug going into the instrument cluster. I'll look and see what the pin numbers are. Since the wire is obviously opening, it won't hurt to splice into the wires with your jumper directly from the sending unit. That way you can isolate the problem to the harness or to the instrument cluster. Is that clear as mud? I'll check and get back with the additional info in a few minutes.

EDIT: Well, well, well. Things got a bit exciting after I posted this. We had a sudden rainstorm/downpour and multiple electrical losses including one final brownout that just hung there trying to fry everything in my house. I scrambled for the master breaker and shut the whole house down. Three hours later and we are back online with power. I guess we had a lightning strike that took out this whole end of the county.

Anyhow, here is what I was typing when all heck broke loose. . .
Okay, the brown wire from the sending unit is the signal lead and the black wire is connected to ground at a tie point on the LH hydraulic housing. The brown wire exits the sending unit into the harness and goes to connector C2 somewhere in the harness (probably below the instrument cluster around the relay assy). From that connector, the brown wire goes to pin #14 on the back of the instrument cluster. Power to the gage comes in on pin #21. It's the same power that runs your tachometer, temp gage, and 540/600 rpm lights on the instrument panel. If all those other instruments are working properly, you are probably not having input power problems. If you want to be sure that the tach signal is not interferring (very remote) then disconnect the connector with the pink wire on the back of your alternator. Thats the tach signal that would increase when you rev the engine. I think the most likely issue is a loose connector C2 or a pinched and broken wire making only slight contact that breaks when you rev the engine and cause vibration.

 

[spankyscamp]

Check for open wires from sending unit to ground and to the gauge (#69 wire) and also ran another wire from sending unit to gauge just to make sure and it still did the same thing. The gauge as 3 pins that plugs into gauge cluster. I checked those points back to the multi plug and seam ok. Would you know how to check gauge if you have the gauge in your hand. There are 3 wires at the multi plug that have 12 volts. All other gauges, 540/600 light work ok with no problems. There apers to be something that cuts down the voltage down at the sending unit because it will go from 12 volts to 6.5 volts the to 3.2 volts when you rev up the engine. I have disconnected the hot wire not the ground at the sending unit and attached my meter and watched the voltage change when I revd up the engine.

 

[jinman]

Here are the repair manual pages with the troubleshooting info.

Oh yes, one more thing. . . If you rev the engine and the gage goes to zero, does the gage go back up when you lower engine rpm? How about if you shut the engine off and then turn the key to ON? Have you looked for AC voltage on the wire? Perhaps an isolation circuit or poor ground in the tachometer circuit is causing AC voltage to be applied to your DC signal to the sending unit. If you are measuring a voltage drop to the sending unit, it would tend to indicate a problem with the bridge circuit for the fuel gage. Are you familiar with how a Wheatstone Bridge works? The sending unit would imbalance one leg of the bridge around the meter and cause current to flow through the meter movement. The fuel gage is not a true Wheatstone Bridge, but the principle is closely related. Here is a link to a description of a fuel gage that is probably more accurate than my description.

 

[spankyscamp]

Thanks Jim. Just was able to check out the gauge last night. The two terminals that should have between 302 and 352, read in between there then I moved the gauge with the meter still on it and it went to 0. It appers to have a bad connection in the gauge so I should replace it. The other two terminals checked with in the range OK. Thanks again for the great help. Rich