Fuel guage erratic

[wellsc1]

I suspected bad sending unit (shorted), but after reading a post similar to the subject it may be wiring. Fuel guage works at full, but somwhere around 3/4 full it pegged past fuel. Later in the trip it showe less than 1/2 full. Any suggestions where to look first - under the dash or at the tank?

I always start a trip with a full tank and keep tabs on how long I've been running, but like having a fuel guage to back me up.

 

[Paul Ryan]

Re: Fuel guage erratic

Look at the termial connection real close. Behind the gauge and at sender. Corroded and/or rusted and/or loose wires will give false, or intermit results. Sounds like a sender going out. Varnish and corrosion on the sender will give intermit reslut. Most can not be cleaned or fixed. New part. Start with the wires first. Let us know how it goes.

 

[tboltmike]

Re: Fuel guage erratic

No power the gauge reads full.
A full tank should have about 240 ohms.
And empty around 30.
You can check this with an ohm meter with no power to the sensor in the tank.
The sensor is a varible resistor of wound wire and a slider contact on the float. The winding may be jazzed. You can pull it out of the tank for inspction and a resistance check.

 

[Silvertip]

Re: Fuel guage erratic

Wrong, wrong, wrong!

No power = gauge reads empty -- not full. (Key off = no power = empty)
Full tank = 30 ohms -- not 240.
Empty tank = 240 ohms -- not 30
1/2 tank = 109 ohms (approximately)
The sender is indeed a resistive element but it consists of a wiper and two segmented resistive tracks on a printed circuit board. In other words it is not a wire wound resistor.

How do I know this is true? Experience -- and I happened to have a sender right next to me as read this thread so I checked it.

The test procedure for a fuel gauge is to ground the sender "S" terminal. That represents zero ohms and results in the gauge going full provided the gauge is not defective and that it does have +12v and ground present. The second lesson in this that if zero ohms = pegged, then 30 ohms = Full, not 240 ohms.

 

[tboltmike]

Re: Fuel guage erratic

Thanks the chastizement. some is deserved. I should have been up so late answering questions.

You are right about the tank level ohms.

However, my boat had a power switch to the gauge. If it was off, it read "full". Caused me to run out of gas once then I rewired to the ignition.

 

[Silvertip]

Re: Fuel guage erratic

Just curious but how does an OFF setting of a power switch cause a gauge to go to FULL? Without power, the gauge cannot go to full - with or without a sender attached.

 

[wellsc1]

Re: Fuel guage erratic

I finally got a moment to pull the sending unit. The resistive wiper seemed erratic when put on an ohm meter. I think I got it cleaned up enough to work. However, the ground lug was a bit corroded too. I cleaned that up, but may need to put a new female spade connector on it.

I'll know if it works on next trip out in a couple weeks. At least I have the part number on the sending unit now (Rochester #8341-7000).

For now, the gauge reads correctly full.