[Richard]

While fixing the broken fuel pickup tube as chronicled in the Emergency section, I am replacing the fuel sender. Easy to do while the tank is out of the coach. I had previously replaced the sender about 2 years ago but it never read correctly, always reading lower than what was actually in the tank. I assumed that I bought the wrong sender or the wrong length sender. 

So I bought not one but two more. One from VDO and one from KUS because I liked the external bobber on the sender. 

Installed both the them and same result. The tank reads much lower below 3/4 tank than it is. I could prove this because I still had the senders external to the tank. I suspected a wiring issue, so I wired the sender directly to the gauge in the cockpit. Same result. 

I suspected faulty ground, so I made a new ground. Same result. 

Hmm, order a new gauge. 

Then I pulled the old gauge out, and discovered a mysterious screw hole on the side of the gauge. Bingo, I wired it all up on the bench with the KUS sender, and was able to adjust the gauge to read very close the actual level of the float. Woo Hoo, who knew. The variable ohm lettering on the side was a clue that the gauge may be adjustable. 

So, if your gauge is off, it’s something to look into.

Hmmm, cancel order for new gauge.

[Richard]

So, one might ask, how would I go about calibrating the gauge as you did WITHOUT having a spare sender to play with.

The gauge registers full at 0.5 ohms, and empty at 90 ohms.

I would use a 45 ohm, and a 22 ohm resistor to check the gauge. You don’t have to disassemble the cockpit and remove the gauge. You can do it by snipping the leads on the fuel sender at the tank and substituting the resistors for the sender. Of course you will have to butt splice the wires back but no biggie. If you wanted to do it fancier you could use a 100 ohm potentiometer and your voltmeter to set whatever resistance you desire.

[BusNit]

Thank you for this! I never checked my gauge for the adjustment screw. I know someone replaced the senders but like yours, gauge was never accurate once it got to 1/2 tank or less.

[Latitude 28]

Richard,

Great find on the fuel gauge adjustment!  Mine is off a bit and will be adjusting it shortly.   I am curious, measuring at my fuel sender its (empty 0 ohms & full at 90 ohms) which is inverse of your sender unit....(full at 0.5 ohms, and empty at 90 ohms).   Would the fact the senders are opposite from each other mean the fuel gauge is a different part number?

[Richard]

Yes on different part number. VDO makes gauges with all manner of ranges including a 0 to 90 ohm and a 90 to 0 ohm. It’s confusing as all get out when trying to match gauges and senders.

https://gaugesvdo.com/products/947-301-1...-onThBOmfA

https://gaugesvdo.com/products/948-301-1...-onbRBOmfC

I always knew the original gauge was wonky, then it was still wonky with a new sender, and still wonky with the new sender that replaced the new sender. So I suppose it was not adjusted correctly from the factory. The give away was that 60 to 90 ohm was printed on the gauge. It was a big A Ha!

And I posted this because somewhere in my fog I remembered both you and Simon commenting on the gauge being “off”.

And this is way off base, but Simon had commented on a potentiometer that is located in the wiring bundles in the passenger footwell. There was speculation that potentiometer was used to trim the fuel gauge. I haven’t discovered yet what that pot does, but it is NOT part of the fuel sensing circuit.

[bestgenman]

When I replaced the fuel tank a couple of years back (a complete picture collage here on the site) , I replaced the sender.  Filled the tank with fuel and the gauge didn’t show full.  We’ll, I thought the wiring was compromised or the gauge was faulty but didn’t fuss over it.  I keep a log including miles traveled and fuel purchased and it gives me a somewhat accurate level of fuel.  Long generator runs can affect the accuracy but I pretty much know my generator’s consumption and can factor that in and, it is logged as well.

A couple of years pass and I’m disgusted the new sender didn’t work so, I purchase a new gauge.  Same result.  Then as I am pulling into the truck stop, the fog clears and the gauge is showing  full.  I sit down to log in the miles etc,  after filling,  and look at the gauge, it is now empty.  DUH, the adage “stupid is as stupid does” applies!  When I ordered the sender, I ordered the inverse resistance style.   I just have to remember full gauge is empty tank and empty gauge is full tank…. I could change the gauge again but I’m fine with being reminded of my mistake.

Sure does humble a guy when he makes a stupid mistake!

[BusNit]

Gordon, I've made many mistakes that I keep my face planted into. Sometimes I have to pass out several Benjamins to correct my deficient thinking.

Richard, I forgot about that potentiometer. I haven't gone back to play with it since I posted about it. Goes to show my mind is erasing things already...

[Latitude 28]

Thanks Richard for finding that adjustment. After fiddling with my instrument panel I gained access to the gauge and made an adjustment. The gauge was set mid point as I was able to go to the left and right a little over an 1/8 of a tank from the original setting. Great find!