Testing sending units

[R8rPhan]

Is there a way to test if the sending unit in a tank is working, without emptying and dropping the tank?

Do they normally fail open reading??

Will measuring the resistance of the sender wire to ground tell me if the sending unit is good or not, even with an unknown amount of gas in the tank?

I have no idea what resistance they should read full, empty, or anywhere in between..

Tanks ,
Mark

 

[gmachinz]

Is there a way to test if the sending unit in a tank is working, without emptying and dropping the tank?

Do they normally fail open reading??

Will measuring the resistance of the sender wire to ground tell me if the sending unit is good or not, even with an unknown amount of gas in the tank?

Tanks ,
Mark

Not really. You need to pull the sender out so you can slowly move the float arm up/down. There is a resistor strip that the metal tip wears on-the resistance value changes as it moves and over time-a path gets worn into it and the reading become erratic or non existent. To test, set multi meter to OHMs and ground one lead to the sender metal body-the other lead goes to sender stud-then watch the readings as you move the arm up/down. Sometimes (I do get this problem too) the metal tip loses contact with the resistance strip and the readings goes way off. I have been able to gently bend the arm so that it has constant contact for full up or down. Ohms should be 0-90 range....if the strip is worn away nothing can be done to save it-time for a new sending unit then.

 

[gmachinz]

Also, most people drive around with a half or less amount of gas-this is the spot most worn away on the resistor strip-this is the reason why on a full tank, most of the time the gas gauge works well until the tank gets down to around half full-just an observation I've noticed over the years on vehicles in general.

 

[R8rPhan]

Also, most people drive around with a half or less amount of gas-this is the spot most worn away on the resistor strip-this is the reason why on a full tank, most of the time the gas gauge works well until the tank gets down to around half full-just an observation I've noticed over the years on vehicles in general.

Well, if I have to drop the tank to test it, might as well replace it while it's out..

Looks like the one on my donor 2nd tank is bad (not yet installed) .. Reading all over the place in with my fluke meter.. from 3-30 MegaOhm... With my cheap analog meter, shows open... There is some fuel in that tank too...

So, there goes another 80 bucks! Plus $50 or whatever for the selector valve (missing), plus hoses/clamps, etc..

Cha ching!

Still a lot cheaper than buying a newer truck with all the smog crap.... lol

Tanks!

 

[R8rPhan]

Well, I finished prewiring under the truck for when I install the selector valve and second tank..

I hooked up the fuel gauge to the existing tank and then when in the cab and measured the wire to ground and it said 60 ohms.. I then put 5 gallons more gas and measured 90 ohns.. So I guess that one is working...

I got the leak stopped and I'm good for now...

Finally got all the diagrams and schematics I need so I can start collecting parts...

Might pull the bed off when I get around to doing the fuel system.. Make it a lot easier to run the lines across the frame to the other tank and all...

 

[chengny]

Is there a way to test if the sending unit in a tank is working, without emptying and dropping the tank?

Do they normally fail open reading??

Will measuring the resistance of the sender wire to ground tell me if the sending unit is good or not, even with an unknown amount of gas in the tank?

I have no idea what resistance they should read full, empty, or anywhere in between..

Tanks ,
Mark

I know you are all set on this, but for someone else:

FUEL LEVEL GAUGE PARAMETERS

For Truck Years: 1967 - 1990

The Fuel Gauge Needle Should Point to:
Left Line (Empty) when sender resistance = ~1 Ω
Middle Line (Half) when sender resistance = 44 Ω
Right Line (Full) when sender resistance = 88 Ω

The corollary to that is:

Those resistance values are what should be observed across the sender when the tank is at the corresponding levels (i.e 88 ohms when full, 44 ohms with 1/2 a tank, etc.)