Calibrating Fuel Senders

J

justin brady1

New member
May 4, 2015
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Is there any way to calibrate the fuel senders?
Just returned from a 5000 mile trip (full report later).
What I found was that when my gauges read dead empty (both main and aux) the tanks are still half full.
At dead empty on both gauges the coach takes 25ish gallons to full again (confirmed full, not from gauge)

I know the general rule of drive 300 miles and fill, but that does not help my brain be okay with driving under empty in the middle of the desert when
it's 100+ outside. This also makes it tough to know when to switch to AUX.

I know how the tanks work and the crossovers etc, I'm just asking about the senders themselves, is there a way to calibrate them to read empty?
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
> Is there any way to calibrate the fuel senders?
> Just returned from a 5000 mile trip (full report later).
> What I found was that when my gauges read dead empty (both main and aux) the tanks are still half full.
> At dead empty on both gauges the coach takes 25ish gallons to full again (confirmed full, not from gauge)
>
> I know the general rule of drive 300 miles and fill, but that does not help my brain be okay with driving under empty in the middle of the desert
> when it's 100+ outside. This also makes it tough to know when to switch to AUX.
>
> I know how the tanks work and the crossovers etc, I'm just asking about the senders themselves, is there a way to calibrate them to read empty?

Justin,

The simple answer is - Yes
But you need to know that it is not simple or easy and pretty much requires that you get the tanks down.

This is not the usual failure. As low fuel is also the low resistance and bad joints create high resistance, the other failure of reading more than
is there is the vary common mode.

If the pointers are moving, but get to the bottom too soon, that would require either the dashboard instrument be at fault or you have some very
specific wiring problems. The only way I can think of the the actual sender may be the issue is if the float sank. I would bet against that if there
is still any reading above empty.

From what you are saying, I would check the calibration of the dashboard instrument first. Particularly if both are doing the same thing. If that is
out of calibration, I can not begin to tell you how to correct it.

I did calibrate mine when I rebuilt the senders. But, I use the panel gauge as the standard. (That was the fifth time I had the tanks down.)
(Speaking of that, if you have to have the tanks down, search here for "lifting fuel tanks". I got real good at it.)

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
Thanks Matt,
I too am confused by this. Doesn't seem to be a failure that I can come up with to cause it. All other gauges are fine, and the fuel gauge operates
exactly as it should, just when it's empty I'm only 25 gallons down!
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
I would look at all external connection first.
Then pull out the float and use a ohm meter and check out the reading as
you move it up and down and record ohm vs. height.
Hard to believe both are doing the same.

> Is there any way to calibrate the fuel senders?
> Just returned from a 5000 mile trip (full report later).
> What I found was that when my gauges read dead empty (both main and aux)
> the tanks are still half full.
> At dead empty on both gauges the coach takes 25ish gallons to full again
> (confirmed full, not from gauge)
>
> I know the general rule of drive 300 miles and fill, but that does not
> help my brain be okay with driving under empty in the middle of the desert
> when
> it's 100+ outside. This also makes it tough to know when to switch to AUX.
>
> I know how the tanks work and the crossovers etc, I'm just asking about
> the senders themselves, is there a way to calibrate them to read empty?
> --
> Justin Brady
> http://www.thegmcrv.com/
> 1976 Palm Beach 455
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
Since my dash is fully custom, I do have the wiring run differently than stock, specifically using relays in series with the senders, perhaps that has
changed the resistance enough to alter the gauges?
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
Since 0 ohms should show an empty fuel gage if you ground the sender lug on the fuel gage it should read empty. It it does you should try to tie into
the fuel sender wires as close to the tanks as possible and ground each. If they don't show empty do as Jim suggested and access the senders so you
can repair or replace. If grounding these wires shows you have fuel there is a poor connection between this point and the gage.
--
Roy Keen
Minden,NV
76 X Glenbrook
 
A relay shoyuldn't matter - the sender is 90 Ohms. A contact which was very dirty might show an Ohm. Unless you have access panels aboove the
senders, you're prolly gonna have to let at least one and probably both tanks down. Are you sure you have the correct senders? Then measure their
resistance >in the tank with the fuel out of the tank
 
Sounds to me like someone replaced the tank sending units with ones that
did not have the same range of travel as the GMC senders. They have the
same electrical range, just not the same mechanical range. I ran into the
same thing when I built a custom tank for my Dodge pick up to move it from
behind the seat to the rear of the frame. The Stewart Warner sender had the
same electrical specs as the Dodge, but, my tank was deeper than the
mechanical range of the float. So, it shows empty on the gage, long before
I am out of fuel. Fix? Longer arm on the float that mirrors the depth of
the tank. P.I.T.A. when you have 2 tanks.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> Since 0 ohms should show an empty fuel gage if you ground the sender lug
> on the fuel gage it should read empty. It it does you should try to tie into
> the fuel sender wires as close to the tanks as possible and ground each.
> If they don't show empty do as Jim suggested and access the senders so you
> can repair or replace. If grounding these wires shows you have fuel there
> is a poor connection between this point and the gage.
> --
> Roy Keen
> Minden,NV
> 76 X Glenbrook
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
That's a thought, but I doubt it honestly. It's got 50,000 miles on it and nothing else had been replaced when I got it.
Hmm. I'll start with the resistance I suppose and go from there.
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455
 
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