Digital fuel gauge reeeeeealy lies

Was out for a drive today and for the second time the Caddy left me on the side of the road. Ran out of gas with the digital gauge still showing over 1/4 tank. This is an on-going problem and the second time it has run out completely. Wouldn't mind if it was consistant but sometimes it will let me get down to 2 bars over empty and the next is like today...2 bars over 1/4.

Anyone have any ideas on how to diagnose this without pulling the tank? I'm thinking one of 4 possibilities:

1. Bad connections at the cluster (speedo, odo etc work fine)
2. Bad or flakey connection in wiring to tank
3. Defective sender and or gauge
4. Fuel pickup has dropped out of pump (possible as when this happens, car angle plays an important part in getting me to a gas station)

Any suggestions and or ideas would be appreciated as I really don't want to drop the tank if not necessary (why don't these things happen in the summer when I can work out in the driveway?).

Bill

Bill, it is likely the same problem that everyone suffers from with the mechanical gauge. You should be able to correct it with a gas gauge fix kit. Believe it is just a pot that goes on the wire from the sender to the gauge.

I am not a fan of the corrector kit because it only bandaids the problem, and it doesn't address the crappy fuel pump wiring at all.

I almost ran my 9C1 out of gas on saturday but it definitely wasn't the sender's fault (the needle was trying to touch the floorboards well before it started sputtering). With God's help I made it to a gas station before it just about died, despite having 1.5 gallons in the tank. B-body tanks are really lousy below 1/3 tank. Even the B-body drag-racers will race with the tank 3/4 full.

A couple of observations:
1. After a lot of miles, our stock sending units REALLY need some help, particularly on the fuel pump connections!
2. On both of my sending units, the resistor, even at empty, still read at least 9.8 ohms.
3. You can fix this for free (just gotta drop the tank) if you're good with a soldering iron by "recalibrating" the resistor to read zero while it's empty.

To diagnose: disconnect the fuel pump harness when the tank is just about empty and with an ohmmeter, probe the purple and black wires. Ideally it should be zero. Or, you can probe them with the tank absolutely full and ideally it should be 90. This tells you how far off your sender is, and whether the problem is right inside the sending unit.

I have a bunch of pictures from when I last refurbished a B-body sending unit. I've been meaning to email Mike another FAQ post.

sherlock9c1 wrote:

I have a bunch of pictures from when I last refurbished a B-body sending unit. I've been meaning to email Mike another FAQ post.

Would really appreciate that. Would like to know what the whole thing looks like. Do you have a picture of the pump and pickup as well?

Quote :

Bill, it is likely the same problem that everyone suffers from with the mechanical gauge. You should be able to correct it with a gas gauge fix kit. Believe it is just a pot that goes on the wire from the sender to the gauge.

May just do that as I have to drop the tank when I do the conversion to change the pump over to the low pressure version. Would be nice not to have to do it twice. Planned on replaceing everything at the same time. Going to have to re-route the return line so its close to the intake (helps to ungel diesel fuel in the winter) and fab up an anti slosh baffle. Diesels can't tolerate even momentary air bubbles in the fuel line.

Putting a variable resistor in the line makes sense except that this problem does not seem to be consistant. Will do the resistance check on the harness both empty and full to verify.

Thanks for the help.

Bill

I will, but Bill, given your expertise and mechanical know-how, I think you'd have no problem understanding what the thing does. If you can say "rheostat" and "wiper," you'll be good to go.

sherlock9c1 wrote:

I will, but Bill, given your expertise and mechanical know-how, I think you'd have no problem understanding what the thing does. If you can say "rheostat" and "wiper," you'll be good to go.

Obviously my knowledge and expertise has taken a little vacation without me. In the above post I was actually contemplating adding an extra resistor to the equation. As my problem is on the "empty" side (going towards zero), adding more resistance to the line will only make matters worse.

Looks like your description of bad harness wiring is more then likely the culprit. Is the harness wiring one piece from the sender to the rear plug, or is there another connection in between? Have never been a real fan of weather packs but I guess they are better then the older alternatives. Would be nice if we could afford to replace everything with Cannon connectors and be done with it but I guess that is just wishful thinking.

Thanks again.

Bill

These cars have fuel gauges?

Hunh...

I just use my trip. 300 miles around town and it's time to fill up, no matter WHAT the gauge says. I'll go almost 400 on the highway.

toomanytoyz wrote:

These cars have fuel gauges?

Hunh...

I just use my trip. 300 miles around town and it's time to fill up, no matter WHAT the gauge says. I'll go almost 400 on the highway.

Usually I don't let it get below 1/2. Today was the exception. On long trips the odo is still best but around town I just put it in $20.00 at a time. Guess it's my sub conscious aversion to buying the wrong fuel. Still find myself pulling up to the diesel pump and keeping a mental note on trips of what stations sell the good stuff.



Bill

toomanytoyz wrote:

These cars have fuel gauges? ... I just use my trip. 300 miles around town and it's time to fill up, no matter WHAT the gauge says. I'll go almost 400 on the highway.

Only if you fix them.
Yeah, on saturday I had 150 of city/crazy driving and 180 of highway driving and the car started sputtering at 330. The light kicked on at 300. The last time I ran that low on gas was with you, actually, on the way to the RAISSE dragday in 2005. That time I had 354 on the trip odometer, so that was my own dumb fault as well.

As a side note for those of you who know I have a Z28 cluster in my 9C1 and are wondering how I have a low fuel light, I used a '91-93 Caprice low fuel gauge circuitry. I modified the cluster circuitry of the little orange idiot light next to "E" and used that as my indicator. The tap off the gas gauge sender feed has a 2k ohm resistor in series to keep the light from blinking as the sender float bobs up and down with low fuel. I'd recommend a higher resistance; probably 4k ought to do the trick, because these B-body senders bounce around so much with low fuel that you really need a high resistance to filter that out from the low fuel light, otherwise it'll blink a lot. My initial attempt had just a straight connection with no resistance. That was obnoxious!

BillDiesel, on the sending unit, there are two harnesses: the external harness with the weatherpack connector that goes to the TOP of the sending unit (the "bulkhead") and then there's an internal harness about 9" long that goes from the bulkhead to the fuel pump and to the sender. The black is the ground, the grey is the fuel pump feed, and the purple is the sending unit connection, on both harnesses. It's not the weatherpack that's the problem; it's the internal harnesses. It's a decent design but not for the environment it's in. It's constantly getting hit with fuel slosh, getting shaken around, attached to a vibrating fuel pump, and exposed to varnish and moisture. In fact, even with a new harness and cleaned contacts, I was still seeing flucuations in resistance as I moved the wires around. That's why I soldered all of my connections on that internal harness. This harness is available from Napa, but frankly the stock wires are thicker. I have a friend at work who wants me to refurb the sending unit on his '95 Z28, and when I do that I will see if I can just cut off the burnt ends of the stock wires and then just strip a short length of clean wire on each end and solder that directly to the pins. Do not use shrinkwrap on the sending unit; most shrinkwrap I've found is dissolved by fuel. The connections won't touch, and there's no fire hazard inside the gas tank. I've done two sending units this way and haven't had a problem after two years on each one.

Thanks for all the replys. Sort of committed to dropping the tank and doing the fix. Only regret is I'll be doing it again during the conversion. Either that or looking for a fuel regulator that will knock down the high pressure pump to 6 to 9 lbs. Anyways, will measure the resistance first on the external connection to rule out anything up stream of it.

Quote :

Do not use shrinkwrap on the sending unit; most shrinkwrap I've found is dissolved by fuel. The connections won't touch, and there's no fire hazard inside the gas tank. I've done two sending units this way and haven't had a problem after two years on each one.

Just to be on the safe side, why not use a length of small fuel line (ie. fuel line from a lawn mower)? Should be flexible enough to allow you to do the connection and stiff enough to cover the finished job.

Bill

good points, but the terminals are fixed enough that they don't move anyway. Not that anything conductive will touch it.

I'm running a Nordskog digital dash cluster in Shammoo.

It is on the GM 0-90ohm setting currently, supposedly to match the sender unit. It reads percentage of full tank. When the tank is full it goes past a reading of ""99" to about "145" BUT since it is only a two digit gauge all I see is "45". There is also another setting from 0-180 that I thought would be useful. That way it would never show full at "99" but at least when it nears "00" I'll know it doesn't mean "(1)00".

Someone mentioned a resister kit, but wouldn't that make things worse? If you add a resistor is series, it would never reach 0 ohms even though the tank is empty. So tell me more about his supposed correction kit please.

One link is here: http://www.fuelgaugecalibrator.com/bbody/FAQbbody.html - all this does is connect a potentiometer (variable resistance) to the sending circuit and to ground. It adjusts the "full" end of your gas gauge but not the "empty" end, and he specifically says that in the FAQ.

Another, more active unit is here: https://www.impalasuperstore.com/naisso/superstore40/shopexd.asp?id=873. This one looks like it adjusts both the high and low side. But it's almost $70, and again, it doesn't solve the actual problem, only bandaids it.

My Caprice's sending unit was reading 13 ohms empty and 140 full. After I refurbed it, it was reading 0.3 empty and 91.2 full. This was without replacing any part of the sending unit! I replaced the fuel pump sock and the wiring harness while I was in there, but that was it.