Sending Unit/Fuel Level Gauge Question

[MIDEVIL214]

Installed a new aftermarket EFI fuel tank with an in tank pump. Just put 6 gallons of fuel in and the fuel gauge doesn't even touch "E" line. I verified the suggested ohms (full/empty) before I installed the tank and they were real close to what the vendor wanted to see.

Checked voltage on + orange wire that connects to sending unit (2.5 volts). Checked voltage on fuel gauge post at back of gauge cluster (2.5 volts). Grounded orange sending unit wire and gauge shows all the way full.

Im at a loss because the oil pressure and thermostat gauges work fine. Wouldn't a bad cluster voltage regulator throw them all off? I assume the low reading off the back of the gauge is the reason my fuel level is so far off with fuel in it?

 

[DirtDonk]

I would think so too. But what is the voltage going into the gauge? That would tell you a lot more I think? What's coming out could be skewed.

Couple of things that come to mind. First it sounds like your grounding is good, since when you grounded the signal wire the gauge went to full. As it should.
Did it go well past the full line though? Or stop just near it? Should go way past it if I'm not mistaken. Maybe someone will know for sure.

I see you have a '74. Some newer info just came up recently and I think that might be one of the years with the different ohm range expected. I'd literally never heard of that until just this past year, but apparently Ford had two ohm ranges!
Don't know if it was just a fluke, or a running change across the entire lineup of Ford products, but while not a huge change, it's enough to give you a headache trying to track down the reason for strange readings.
You don't happen to still have your old sending unit by any chance, do you? Be great if you could verify it's readings. Especially if the gauge read correctly before the tank swap.

Don't know where your tank is from, but we verify that each and every sending unit falls into the ohm range at least for the ratings that we've come to expect as "stock" for all EB's. But if that info on the change is correct, then that would explain a lot of things.
If so, looks like someone is going to have to come up with a fix, or else everyone who has one of the changeover models is going to be dealing with odd readings. If I remember, the change was not a lot, so maybe it's as simple as adjusting the stops to hold the float arm up slightly when empty, and/or down slightly when full?

Is your IVR a standard type, or one of the new electronic ones? If the old type, you'll get a pulsing voltage reading coming out of it. If electronic, you would have a steady signal.
The pulse is very obvious on an old analog meter, but sometimes hard to tell with a digital one.
Did you also verify the voltage readings coming out of the other gauges? If different, that might lead you to a faulty gauge, rather than an incompatibility issue with the sending unit.

One thing I've found over the years is even with new sending units, the matching of the gauge and wiring to the unit was more important than just matching the book. I even had to adjust factory sending units on my '71 to get them to read correctly. Which they'd never done since almost new. A few tweaks and the gauges read perfectly after that.
Just thinking out loud here. Sounds like your wiring is in decent shape since you did not find a voltage drop between the output of the gauge and the sending unit (2.5 volts at both ends) but you never know.

Good luck.

Paul

 

[Viperwolf1]

Do you have the tank switch on the dash? The contacts tend to get dirty in it.

 

[DirtDonk]

That! What he said.
That switch is probably the most finicky of all the electrical stuff on a Bronco. Even the factory originals wore out fairly regularly. Hate to think how long the cheap new stuff we have these days lasts!

That said, the new stuff is at least made by the same company that made the originals for Ford. So perhaps we're not in bad shape there.
Or still are and just have to deal with it!

Paul

 

[Viperwolf1]

That! What he said.
That switch is probably the most finicky of all the electrical stuff on a Bronco. Even the factory originals wore out fairly regularly. Hate to think how long the cheap new stuff we have these days lasts!

That said, the new stuff is at least made by the same company that made the originals for Ford. So perhaps we're not in bad shape there.
Or still are and just have to deal with it!

Paul

The old switches are completely rebuildable. They don't wear much, just the copper contacts get a little corroded. Easy to clean up. Same for heater switch. Stuff was built to last forever back then.

 

[DirtDonk]

Funny you say that. I've seen some that could be cleaned up, but at least half of the failed switches I've taken apart literally had fallen apart inside. Some of those might still have been easily rebuildable of course, as I never bothered to check closely to see if things had just dislodged when pulled apart, or were well and truly broken inside.

I agree that things were intended to last a long time back then. Just that some didn't in spite of that head start.

If yours falls into that category MIDEVIL, have at it for sure. If not though, at least you know new ones are available. But definitely check it out. I'd totally forgotten that little gem inline with the sending units. That's what makes the fuel circuit different from the other two gauges.

If you only have one tank however, still check the little connector over in that corner of the dashboard. Ford made the harness the same whether two tanks or one were optioned. It can get rusty or fall apart after 40 years.
Follow the Orange wire...

Paul

 

[MIDEVIL214]

Lots of good questions here. The fuel gauge pegged way above the full line when grounded. Bronco never had a dual tank dash switch and came with one tank. The IVR is factory 74, I did notice slight voltage change/pulse with digital multi-meter. I will give the other gauges a test tomorrow. Interested in this little connector that is mentioned in the wiring harness. Is it lose and uncapped?

My last tank was a 23 gallon NW metal products tank and that never read completely correct. Never did any testing though and chalked it up to a bad sending unit. It would read empty with quite a bit of fuel as well come to think of it.

I'm working with the BC tank now that WH sells. Not sure if the sending unit comes from WH or BCs.

 

[Viperwolf1]

I would say it's a bad spot in the orange wire somewhere. Dual exhaust was notorious for melting the rear harness along the frame.

 

[DJs74]

Setting the electrical aspect aside for a second (which is probably the culprit somewhere between the tank and the gauge), did you verify the float level arm was swinging freely before you installed it in the tank? Any chance the new unit had the float arm fastened for shipping reasons (to prevent the arm / contact from sweeping the resistor card dry). Or maybe you have an obstruction between the float arm and tank preventing the arm from moving.
I spent about 10 years designing fuel level units for John Deere, Polaris, Bombardier and hot rod adjustable aftermarket and even with a pump involved, they are pretty plain units - not much to go wrong as long as the resistor card and contact point are touching. The new modern unit probably has a ceramic resistor card but maybe wire-wound?? might be nice if you could physically move the arm by hand and take it though the entire empty to full sweep while someone watches the gauge - but you'd need to take the unit back out of the tank

DJs74

 

[MIDEVIL214]

I would say it's a bad spot in the orange wire somewhere. Dual exhaust was notorious for melting the rear harness along the frame.

That wouldn't explain the low voltage off the gauge cluster though. Isn't there supposed to be 5-6 volts back to the sending unit?

I don't think the sending unit arm is obstructed. I got the proper ohm readings off the sending unit before the tank install with no fuel (tank upright, then upside-down).

 

[70_Steve]

LThe IVR is factory 74, I did notice slight voltage change/pulse with digital multi-meter.

That wouldn't explain the low voltage off the gauge cluster though. Isn't there supposed to be 5-6 volts back to the sending unit?

The stock or stock type replacement IVRs produce a pulsed voltage, normally pulsing at about twice a second. With a DVM you should see numbers between about 2 and 10.5 VDC on the display. All depends on how much damping is built into the DVM. An analog meter shows the pulsing much more clearly as the needle just keeps bouncing back and forth.

There are solid state aftermarket IVRs that produce a constant DC voltage.

You might want to pull the wire at either the temp or oil press sending unit and see what the DVM display looks like. Then you'd have something to compare it to when looking at the fuel level wiring.

 

[Broncobowsher]

Ground the wire for the sending unit, does the gauge more? I am guessing not.
Try it again at the gauge. Works at the gauge? Bad wiring/selector switch. Still doesn't work grounding at the gauge, swap power wires coming off one of the functional gauges. After that the gauge is probably bad.

If you can't find a good gas gauge you can swap face plates off a water temp gauge which are generally easier to find.

 

[MIDEVIL214]

Tested a few more things tonight. I grounded out the fuel gauge and it pegged full (does same with sending unit wire grounded farther back).

Im getting 12 volts to one side of the IVR and a little over two out the other side (some fluctuation). Oil pressure and coolant gauges both showed fluctuating 2-3 volts (briefly jumped to 5 once for half second). Same voltage readings at the actual pressure and temp sensor in the engine bay.

So why would the oil and coolant work on this voltage and not the sending unit/fuel gauge?

 

[70_Steve]

So why would the oil and coolant work on this voltage and not the sending unit/fuel gauge?

It looks like you have verified everything except the sending unit.

These sending units are a variable resistance. ~73 ohms is 0% of scale (Empty) and ~10 ohms is 100% of scale (Full). Since you have a DVM, put it on the 200 ohms scale and measure the resistance of the sending unit. Also verify the ground on the other side of the sending unit.

FWIW, it's pretty common to have the fuel level measurement be pretty inaccurate on these 23 gallon tanks, no matter the manufacturer/vendor. I have one of the earlier BC Bronco's tanks. When I fill up, the gauge will show Full, but by the time I drive the 5 miles home, it's down off of Full already. And when it gets to Empty, 12 gallons will fill it up.

 

[bronconut73]

I have an expensive Autometer fuel gauge. It is the proper 73-10 ohm gauge with a 73-10 ohm sending unit. One of those new "stock like" 13 gallon tanks.
While empty, if I put more than 8 gallons in it, it will read over full.
Obviously not really accurate on the full end of the scale.
But on the empty end of the scale it appears to be really accurate, and I guess that is OK with me.
Not perfect but I went without a functioning gas gauge for over 20 years so I am relatively happy.

 

[MIDEVIL214]

It looks like you have verified everything except the sending unit.

No, this was mentioned in the first post. WH said 74/10 Ohms. It was within an Ohm or two of that full/empty.

This isn't a case of it being off slightly. There are 7 gallons in it now and it is way below empty. I'd say I have 10 gallons of reserve once the gauge reaches the E line.

 

[70_Steve]

No, this was mentioned in the first post. WH said 74/10 Ohms. It was within an Ohm or two of that full/empty.

This isn't a case of it being off slightly. There are 7 gallons in it now and it is way below empty. I'd say I have 10 gallons of reserve once the gauge reaches the E line.

Yes, I can think of how you verified the sending unit with no fuel in the tank, and I'm sure the sending unit measured 73(ish) to 10 ohms. What I was suggesting is to measure the resistance of the sending unit now that there should be some measurable level.

You saw my comment on where my gauge reads.

I remember one (of the many) posts about the fuel level sending unit where one of the members removed the sending unit from the tank before installing the tank and made some modifications to the sending unit, including bending the arm, to get a "more accurate" reading. He did this by holding the sending unit to the outside of the tank and moving the arm manually.

BC Broncos now offers a replacement sending unit for my tank that is supposed to be more accurate. If I ever have a reason to drop the tank I may consider it. For now, like you, I will accept the 10 gal reserve.

 

[DirtDonk]

How do you know the other two gauges are reading correctly? If you only have an average of 2.5 volts or so, they're probably reading low as well and you may just not know it.
What are your oil pressure and water temp readings now?

Then again, reading that pulsing output with a digital voltmeter is problematic and maybe the reading you were getting was skewed a bit?

I know many oil pressure variations within different engines make that a hard thing to verify, but water temp is pretty consistent and has known levels to expect, so guessing you're seeing what you are expecting to see there?

The gauges are very heavily dampened too, so if enough voltage is getting through often enough, the readings will be somewhat accurate.

From what the others are saying, it does sound too as if your initial readings with the fuel gauge are maybe not so far off as we think. At least consistent with what some of the others have found with the aftermarket tanks in their cases. So maybe you're chasing the wrong culprit and these senders are just not set up well for our gauges.
I think we should send them separately too. Not only to avoid any banging around during shipping, but to make calibrating that much easier. It's always been my contention that whether a factory sender or not, you should match it to your specific gauge and electrical system, rather than relying simply on the sender meeting the factory specifications.
I had to adjust my factory original sending units way back when. They never read correctly from the factory as far as I know, but a half-hour or so with each sender and both tanks read just the way I expected them too. Just above full when full, and just below empty when empty. Loved it, but it took some work. Too bad I never actually measured the ohm range at that point to see what I was actually doing. I simply plugged the senders in while they were out of the tanks, and bent and tweaked the stops and arms until I got the readings I wanted.

I can understand installing them for shipping too though, as it's one less part banging around in a box, and (theoretically at least) one less thing for the customer to have to mess with before installing the tank.
Unfortunately it seems we have to mess with them anyway!

I'm not sure if I can come up with a working Bronco that someone will let me mess with, but if I can I'll take one of our sending units (different brands even in some cases) and see what they read on the gauge after we've verified the ohm range. See what it reads on a Bronco here. Not sure if that will help anything, but it will make me feel more confident talking about these things.

Good luck with yours MIDEVIL

Paul

 

[MIDEVIL214]

I went ahead an ordered a constant voltage IVR from WH. I figured it wouldn't hurt to replace it even if the problem is in the sending unit (which I hope its not). I agree that the oil pressure and temp may reading low as well but not as noticeable as the fuel gauge. Hopefully I will be reporting back soon with good news on the IVR.

 

[bronconut73]

I went ahead an ordered a constant voltage IVR from WH. I figured it wouldn't hurt to replace it even if the problem is in the sending unit (which I hope its not). I agree that the oil pressure and temp may reading low as well but not as noticeable as the fuel gauge. Hopefully I will be reporting back soon with good news on the IVR.

Did you get the stock type or the more expensive aftermarket IVR that WH sells?