Finally an accurate fuel gauge...maybe

Was talking with a guy over on the Diesel Page and he is making a replacement fuel sender for the older GM trucks. Uses the capacitance type electronic probes. Asked him if he would entertain making them for us as I need one for the Caddy and it seems it is a common problem with all the "B"s and "D"s.

This system uses a small magnet on a doughnut float (moves up and down the sender tube) and is accurate to within .2%. He has asked me to send him a sender to look at as these tubes have to be custom built to suit for depth (truck ones are 12 inches) and the float requires a certain amount of clearance. Will try to find him both senders as the Caddy uses a different one than the B bodies.

He is selling the conversion kit for the trucks (same 0-90 ohm as our cars) for about $100.00 so it should be about the same for us. A little pricy but it will be the last sender you will ever have to buy, and they can be adjusted for accuracy.

Just wondering if this is something you guys would be interested in. I'm hoping he can make one for me but if we can put together a group buy, I'm sure we can make a deal.

Bill

I put the $15 electronic adjuster on my cars. It most definitely helps.

Tim:

The problem with the variable resistor (15 dollar adjuster) is that only addresses the full section of the tank. I'm more concerned about the other end of the scale. Don't care about how full the tank is, I want to know when it's empty.

This unit is accurate throughout the whole range.

Bill

What you are working on is definitely a better answer. I know the gauge well enough now so it is not a problem for me. I really use the trip meter for my estimate anyway.

Is there some sort of translator box that adjusts for the geometry of our gas tanks? How sensitive is it to fuel slosh? When you say it can be adjusted for accuracy, how much adjustment? Adjustment at full, middle, and empty? Or just one or two of those?

What is involved in removing the sender? I assume I need to drop the tank?

sherlock9c1 wrote:

Is there some sort of translator box that adjusts for the geometry of our gas tanks? How sensitive is it to fuel slosh? When you say it can be adjusted for accuracy, how much adjustment? Adjustment at full, middle, and empty? Or just one or two of those?

Really don't have all the info just yet but from what I have read about this kind of sender, the output is truly linear. Our float on arm systems are biased so that the middle part of the tank take forever while the extremes are very quick. Its a geometry/arc thing. These units are straight up and down and can be adjusted to exactly match the input requirements of the gauge (exactly 0 ohms (dead short) at empty to 88 ohms at full.

It will still probably take a while to come off full as the float is about an inch thick so it will top out on the mounting plate probably a couple of gallons shy of full. The same applies to our arm floats as they top out on the sender travel pre-maturely.

Will get more info from this guy over the next few days. Trying to find a loose sender from both a Cadillac and one from a wagon. Looks like the sedan Cappys and Buicks use another one altogether. Hope its just hose location and not total depth.

Will keep you posted.

Bill

convert2diesel wrote:

Will get more info from this guy over the next few days. Trying to find a loose sender from both a Cadillac and one from a wagon. Looks like the sedan Cappys and Buicks use another one altogether. Hope its just hose location and not total depth.

It might be, the sedan has a 23 gal tank and the wagon is 21.

Cadet57 wrote:

What is involved in removing the sender? I assume I need to drop the tank?

Yes. Ergo my frustration this summer. Couldn't get the gauge to read properly under 1/2. Couldn't predict when the engine was going to die. Anywhere from 1/8th to 1/2 a tank. Every time I played with it, I had to drop the tank. Got it down to 20 minutes, but its still a pain. Never had a fuel gauge go on me in 45 years. This summer had the Caddy and two trucks go south on me.

Bill

Cadet57 - the tank lateral geometry is different but I think the height is the same. Don't quote me on that.

Does Canada use the same garbage gasoline as America uses? I'll agree with you - the float wiper arm geometry relative to the resistor assembly is quite nonlinear.

Bill, not to discourage you on this, but my two cents is this: all I really care about is that the sender kicks on the low fuel light consistently at enough gallons to allow me to get to a gas station. I use the trip odometer for everything else. I never understood why people wanted to tweak the "FULL" side of their gas gauge; who cares? It's full! Nobody ever ran out of gas on a full tank! That said, after my well-publicized sender refurb over on the ISSF, for the last 3+ years and who knows how many miles, both of my sending units (I have two B-bodies) consistently kick the light on at around 3 or 4 gallons left. And the only cost was an $18 wiring harness from NAPA. No funny business with bending the float arm, electronic gimmicks, etc. And my fuel pumps sound better than ever even after 141k and 161k miles. The wagon's sender got erratic this summer but a bottle of fuel system cleaner fixed that.

I did the variable-resistor-to-ground mod because it was a lot closer to a useful fuel gauge than what we had.
I did a test by putting a gallon of gas in the back and running the tank down. The gauge was right at Empty
when the engine was juddering from lack of fuel. So both F and E were accurate, even if we don't have a
low-fuel light. I suppose that I could adjust the resistor so the low-fuel light turns on when there's a couple of
gallons left. But an accurate gauge is good enough for me. And I didn't have to drop the tank.

It's easy to create a linear output from a non-linear input . . . provided you can easily fit a math equation to
the curve of the input. Back when I was taking electronics and calculus classes, I could have done a simple
case in an hour. (Now I would have no clue.) Converting a float at the end of an arm in a cylindrical tank
would be a piece of cake. But if the surface area of the liquid in the tank is different at different fill
volumes, the math would get a lot more hairy.

Joel:

Canada may have tighter restrictions on cleanlinss and water content but essentially I think all our gas comes out of the same pipe, regardless of where we are on the continent. Don't think this is the problem. I started driving in the 60s and the gas then was horrible (yes it had lead in it) with no real consistancy from station to station, but I never had any gauge problems. If anything, the ethanol content should take care of alot of dirt related problems.

What started all this was, since I got the car, the bottom of the tank has always been unpredictable. You could never predict when it was going to die. I have run out of gas in this car more often than in the previous 40 years combined. The solution was to always keep the tank over 1/2 full. Pain in the butt on long trips, especially with the trailer out back.

First dropped the tank in July. Replaced the pump, followed your instructions and cleaned up the wiper and coil, soldered all the connections, tested it on the floor and it worked perfectly. Re-installed everything filled up the tank using a 1 gallon container and noted gauge location and when the pump picked up it's prime (low fuel light still on) and than continued, noteing gauge location as I continued to fill the tank. Great all is well. Three days later I ran out of gas at 1/4 tank. Arrrrrg!!!

Checked the rest of fuel system and charcoal canister and all is well so I dropped the tank again. Re-positioned float arm as it appeared to be hanging up on the anti-slosh container in the tank. Rechecked the sender and all is well. This time it quit at 3/8th of a tank. This is getting annoying.

A week before we leave to go visit the outlaws in the maritimes, dropped the tank again, redid everything in step one just on spec., hooked up a variable resister to the gauge to see if that was the problem, everything checked out so I buttoned everything back up again. Same problem.

Now with trailer in tow and stopping every 100 miles for gas we head on out to the maritimes. After third tank (we bought the gas in Maine so it is now US gasoline) the gauge went to full and has stayed there ever since. Must be an open circuit now.

If I'm going to do this again, I want to be sure it will be the last. Am really getting tired of dropping fuel tanks, laying on my back and stinking of gasoline (gawd I hate that smell). I am either going to buy a whole new unit or give this technology a try. Used these senders in the aviation game years ago and never had a problem and I now see the guys in the marine game are using it. Ergo this discussion.

Bill

Sounds like it is a worthy solution for you then. I would be curious to hear how it turns out if you proceed with it.

If I go ahead with this I will make it a point of documenting it with picts.

Bill