Fuel sender question

A

adam metzger

New member
Oct 12, 2016
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I've rad that both the main and aux tanks should read the same level until about a 1/4 tank, then show different, and this is used to justify having
one working sender.

My tanks are showing between half and 3/4, - and show movement on the gauge between mani and aux. less than a quarter difference, but enough to see
movement.

Can someone explain these two things to me? Wht am I missing?
--
1976 Palm Beach
Austin, TX
 
Adam,
First of all, the senders are not that accurate.
First half of each tank lowers at the same time as the large fill tube is
connected to both tanks at half height.
So as the fuel is pulled out of one tank, the other tank will hold about or
slightly below half until you hit the Aux switch, then begin pulling from
the half aux tank.
Since I drive several coaches, I see that each varies and use the gage as
an indicator, not an instrument.

> I've rad that both the main and aux tanks should read the same level until
> about a 1/4 tank, then show different, and this is used to justify having
> one working sender.
>
> My tanks are showing between half and 3/4, - and show movement on the
> gauge between mani and aux. less than a quarter difference, but enough to
> see
> movement.
>
> Can someone explain these two things to me? Wht am I missing?
> --
> 1976 Palm Beach
> Austin, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
I have to assume from your symptoms that one of the senders is slightly off. It also could be a bad ground on one of the tanks or a dirty connection
on the fuel selection switch.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
> I've read that both the main and aux tanks should read the same level until about a 1/4 tank, then show different, and this is used to justify
> having one working sender.
>
> My tanks are showing between half and 3/4, - and show movement on the gauge between main and aux. less than a quarter difference, but enough to
> see movement.
>
> Can someone explain these two things to me? What am I missing?

Adam,

If you understand that the tanks are essentially common until the half tank level, then you have part of your answer.
Could one be an aftermarket or different?
Sure, I'm not sure I understand why you might have thought them to be identical to start with - Much less after 40 years and alcohol.
Then there is also the fact that unless the coach was dead-assed level and above the common fills, they will be different.

I always find it comforting when I see the needle move when I switch tanks, that means the dashboard gage is actually working. One of the things not
good about Delco air-core gages (what GMCs have) is that they lock at last reading when powered down. The old Intros always went to low.

When we are traveling, and the driver sees that the main is at half, we switch tanks. There are two reasons for this. The first is to pump down the
auxiliary tank so it gets new fuel at the next fill. The second is to keep fuel available for the APU should we need it. It also serves as a nice
reminder to the navigator to check that we are on track for the next planned fuel stop. But then, we do work the tanks closer than most do. We also
write the mileage on each fuel slip. This helps in tracking.

I suggest that leave it all alone until you are sure what it is telling you. Then if you don't like it, change it.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit
 
Thanks Matt-

Certainly after seeing some of the pics of my coach, you can understand how I might think things are wonky. either because they have been touched too
much, or not touched at all.

I agree, it's comforting that it moves, because that means it's working. at least partially, anyway. I hadn't considered the level ground aspect.

I want to have two working senders, I had read that you only need one, and I was just curious about the thought process behind this.
--
1976 Palm Beach
Austin, TX
 
Read here
http://bdub.net/gmcmotorhome.info/tank.html

> Thanks Matt-
>
>
>
> Certainly after seeing some of the pics of my coach, you can understand
> how I might think things are wonky. either because they have been touched
> too
>
> much, or not touched at all.
>
>
>
> I agree, it's comforting that it moves, because that means it's working.
> at least partially, anyway. I hadn't considered the level ground aspect.
>
>
>
> I want to have two working senders, I had read that you only need one, and
> I was just curious about the thought process behind this.
>
> --
>
> 1976 Palm Beach
>
> Austin, TX
>
>
>
> _______________________________________________
>
> GMCnet mailing list
>
> Unsubscribe or Change List Options:
>
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
 
> I've read that both the main and aux tanks should read the same level until about a 1/4 tank, then show different, and this is used to justify
> having one working sender.
>
> My tanks are showing between half and 3/4, - and show movement on the gauge between main and aux. less than a quarter difference, but enough to
> see movement.
>
> Can someone explain these two things to me? Wht am I missing?
If you mean movement while you are driving, especially on hills, gas moves back and forth between the tanks.

If you are sitting in your driveway and the gauges are moving, your GMC is possessed.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
So, sitting still on my drive. Main reads one level, switch to aux, reads another level.
--
1976 Palm Beach
Austin, TX
 
> So, sitting still on my drive. Main reads one level, switch to aux, reads another level.
Lots of stuff can account for that. Corrosion at any of the connectors in the electrical part. Sticky floats. Gunk on the potentiometers that the
floats slide over, etc. And the potentiometers are quite a ways from perfect 0 - 90 ohms.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
What I'm not clear on, is can this be normal?

Can this happen, with nothing actually wrong with the system?

Or do I need to start tracing down a problem because there are different readings?

In a perfectly functional system, is this normal, or a possibility?
--
1976 Palm Beach
Austin, TX
 
> What I'm not clear on, is can this be normal?
> Can this happen, with nothing actually wrong with the system?
>
> Or do I need to start tracing down a problem because there are different readings?
>
> In a perfectly functional system, is this normal, or a possibility?
"Normal" for a 40 year old vehicle is different than for a new car. When it was new, both gauges probably read the same down to a quarter tank. That
is the point where fuel stops equalizing via the fill plumbing.

"Wrong" is also kind of subjective. Your system has (age related) glitches in it. If you want to spend the time, resources, and effort to chase it
down and fix it is up to you. It might end up being the sending unit(s) in the tank(s). Then you either have to cut holes in the floor to get to them,
or drop the tanks.

Clean all the 12v and ground connections you can get to easily and see what changes.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
> I am about to drop the tanks and re hose and refresh the fuel system. This is why I'm asking, so I can take the time to fix this too while
> they're down. or try to, any way.
>
> if, in a new system, it worked that the main and aux tanks read exactly the same until less than a 1/4 tank, and mine are reading differently at
> 1/2 to 3/4, then I can assume there is something amiss with my system.
>
> if this (different readings) cannot be because there are different amounts of fuel in the different tanks, because they self level, then I will
> again assume there is something amiss with my system.
>
> are my assumptions correct?
>
> thanks again.
If no one has messed with the fill plumbing or the venting, on level ground, the tanks will stay at the same level until they drop below the fill
inlet level. Then the pump will pull from whatever tank is selected until it is empty, or you select the other tank. The point where they stop
"self-leveling" is around 6 gallons in each tank.

If your system is stock, your assumptions are correct. Most likely what is "amiss" in your system is a float or two, or a potentiometer or two, or
connections, or some combination of those.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
There is certainly something wrong with my venting system, as the PO bypassed the vapor separator, it took me way too long to fill the tanks, and was
leaking fuel from hoses and the tank (maybe gasket) on my first voyage. My tanks are connected through the drain plugs, the selector valve is not
hooked up properly, there's a fuel pump in the system with wiring to a switch that doesnt work, and a number of hoses blocked off with bolts and hose
clamps.

boy howdy.

--
1976 Palm Beach
Austin, TX
 
I would do 3 simple things first:

1. Disassemble, clean, and lube the fuel selector switch.
2. Check the grounds to both tank sending units.
3. Do not worry about it.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Adam,
The practice to combine the two tanks is not all bad as you'l not have to e
concerned about the tank selector .
It sounds like your needing to drop the tanks and rehose all the fuel and
vent lines.

> There is certainly something wrong with my venting system, as the PO
> bypassed the vapor separator, it took me way too long to fill the tanks,
> and was
> leaking fuel from hoses and the tank (maybe gasket) on my first voyage.
> My tanks are connected through the drain plugs, the selector valve is not
> hooked up properly, there's a fuel pump in the system with wiring to a
> switch that doesnt work, and a number of hoses blocked off with bolts and
> hose
> clamps.
>
> boy howdy.
>
>
> --
> 1976 Palm Beach
> Austin, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
> Thanks Ken. I'll do 3 first.

OK, but you were suppose to #1 and #2 first.

You sound like my wife. I'll ask her for 3 things from the kitchen. She will show up with #3 and forget #1 and #2.

Good Luck on your projects.

Ken B.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
> Thanks Ken. I'll do 3 first.

Adam,

If you do not take Ken's advice, you may end up dead on the road with gas in inventory that you cannot access. The switches are old and the grease in
many has hardened. Disassemble, clean and relubricate is not a big job, but its importance should not be undervalued. The interesting part is that
the switch for the valve and instrument are separate elements of the same switch. If the selector side goes bad and the instrument side is still
functional, the the gauge can show you how much fuel you can't use.

Oh, be careful when you pop the switch open, there are little pieces that springs can propel.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit
 
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