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Massey Harris & Massey Ferguson Tractors Discussion Forum

MF 35 fuel guage

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Steve

07-31-2004 23:13:06

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I have the fuel guage out and the fuel gauge reads full and there is less then half a tank of fuel. When I touch the gruage to the dash it goes down to empty. The wire from the tank is on the right post as sitting on the seat. Any ideas?

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txblu

08-05-2004 07:36:46

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Re: MF 35 fuel guage in reply to Steve, 07-31-2004 23:13:06

You have a simple resistive circuit and the fuel gauge is measuring current and showing it as fuel level. The sending unit will vary from over 100 (don't remember exact value but my fuel gauge has the numbers in fine print on the lower part of the indicator beneath the needle pivot) ohms empty to around 35 ohms full. Additionally mine is for a 12v ckt; assume yours is too. 12v is applied to one side of the gauge via the ignition switch (or in some cases, a switch on the engine which closes when the engine is rotating) and the other goes to the sensor.

Fuel gauges use a bridge ckt which requires ground so when you touch the case to the tractor, you are affecting the balance of the bridge and the reading.

Wouldn't short the sending unit terminal of the gauge to ground as you will probably burn up the gauge bridge. It needs at least 35 ohms limiting the bridge current to a max of 300 milliamperes or thereabouts.

I have tried to go to NAPA and get a standard sending unit (like for a car) but in some tractors, the fine wire they use for the sensor resistance, is so fine that the bouncing around of the tractor wears it out in no time. Additionally, the gauge and sending unit need to be matched so that the low position equates to "E" and the upper position equates to "F". So, I go to OEM parts at the dealer and have no problems, and I find that the price is not that far from NAPA.

Mark

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TO35

08-01-2004 07:48:17

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Re: MF 35 fuel guage in reply to Steve, 07-31-2004 23:13:06

Steve, assuming your system is 12v neg gnd. and wiring is good when you ground the sending unit wire(removed from the post) it should go to past the full mark on you guage if not Bad guage. if it does ...remove the sending unit and hold it in your hand and reattach wiring making sure you have it grounded, then move the lever up to 1/4 it should read 1/4 on the guage....go all thru the ranges up to full...if it does not make it to full bad sending unit....sometimes you can remove the side cover on the unit and rebend the contact tang to make better contact and it will work.....most of the time the contact patch is worn thru and a replacement sender will be needed...

hope this helps..
best wishes TO35

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