Sending unit testing

[Speedrdr]

Quick question. Did a site-wide search and found that a reading of 73 ohms empty and 10 ohms full is expected. No mention of whether it’s the main or the auxiliary tank…or both.
Anyone know as it wasn’t specified in the 77 service manual.

Randy

 

[Oldtimer]

Those values are what the gage expects to see, from either tanks sending unit, or any aftermarket sending unit.

 

[Speedrdr]

Those values are what the gage expects to see, from either tanks sending unit, or any aftermarket sending unit.

Awesome! I was just wanting to make dang sure the sending units were in proper working order before they were hung under the EB and full of fuel. Appreciate the reply, @Oldtimer

Randy

 

[DirtDonk]

In that case (and good on you for even thinking about it beforehand!), test them while connected to the system and energized.
In other words, don't just go by the ohm reading being within spec. Actually connect them to the harness and run them through their range while you watch the gauge.
It's one thing to have the correct range, but sometimes it's another to read properly on the gauge. Should NOT be, but it often is.
You know, we just call it a Bronco thing. Don't know why that should be, and going by the book should be enough.
So you can trust, but I'd still verify.

Paul

 

[Speedrdr]

In that case (and good on you for even thinking about it beforehand!), test them while connected to the system and energized.
In other words, don't just go by the ohm reading being within spec. Actually connect them to the harness and run them through their range while you watch the gauge.
It's one thing to have the correct range, but sometimes it's another to read properly on the gauge. Should NOT be, but it often is.
You know, we just call it a Bronco thing. Don't know why that should be, and going by the book should be enough.
So you can trust, but I'd still verify.

Paul

Can I hook up the pigtails to the sending unit and run to battery and ground and ch check it that way? My wiring harness is kinda still in the box, so…

 

[Oldtimer]

You will need the instrument voltage regulator (IVR) in the circuit. The gages run on 5ish volts, not 12 volts.

 

[Speedrdr]

You will need the instrument voltage regulator (IVR) in the circuit. The gages run on 5ish volts, not 12 volts.

Thanks! Guess that means I’ve got wiring to do before I fill the tanks. Was hoping I could hook up some AA batteries in series to get enough ‘juice’ to get a good reading but it is what it is.

Randy

 

[DirtDonk]

Yeah. Don’t bother to test any sending units until it’s fully wired. You wanna make sure that it’s working properly for the full set up. With all the wiring length set to what they will be when it’s finished.
Then connect the sending units to your existing wiring, which in turn should work the gauges correctly.
Then you can tweak the sending units, if needed, to make the gauges read properly.
And don’t be complacent. Tweaking of the sending units is necessary more often than anyone would like.

 

[DirtDonk]

Should it work fine just with a couple of wires and the IVR connected? Sure!
But does it? Not worth going to the trouble to find out.

 

[Speedrdr]

Should it work fine just with a couple of wires and the IVR connected? Sure!
But does it? Not worth going to the trouble to find out.

Paul, that’s truly sage advice there. Could’ve would’ve should’ve…but I don’t believe I’ll push my luck. I can always use my temporary gas tank and drop the empty tanks when the appropriate time comes to check the sending units for accuracy.

Randy