Home » Public Forums » GMCnet » Fuel sender question
Fuel sender question [message #311370] |
Fri, 09 December 2016 00:27 |
Atom Ant
Messages: 170 Registered: October 2016 Location: Austin, TX
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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?
1976 Palm Beach
Austin, TX
[Updated on: Fri, 09 December 2016 00:29] Report message to a moderator
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Re: Fuel sender question [message #311372 is a reply to message #311370] |
Fri, 09 December 2016 03:04 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
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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
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Re: Fuel sender question [message #311381 is a reply to message #311370] |
Fri, 09 December 2016 10:31 |
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Matt Colie
Messages: 8547 Registered: March 2007 Location: S.E. Michigan
Karma: 7
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Atom Ant wrote on Fri, 09 December 2016 01:27I'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 - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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Re: Fuel sender question [message #311383 is a reply to message #311370] |
Fri, 09 December 2016 10:48 |
Atom Ant
Messages: 170 Registered: October 2016 Location: Austin, TX
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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
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Re: Fuel sender question [message #311391 is a reply to message #311370] |
Fri, 09 December 2016 13:21 |
Atom Ant
Messages: 170 Registered: October 2016 Location: Austin, TX
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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
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Re: Fuel sender question [message #311392 is a reply to message #311391] |
Fri, 09 December 2016 13:57 |
A Hamilto
Messages: 4508 Registered: April 2011
Karma: 39
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Atom Ant wrote on Fri, 09 December 2016 13:21What 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.
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Re: Fuel sender question [message #311395 is a reply to message #311370] |
Fri, 09 December 2016 15:00 |
Atom Ant
Messages: 170 Registered: October 2016 Location: Austin, TX
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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.
1976 Palm Beach
Austin, TX
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Re: Fuel sender question [message #311397 is a reply to message #311395] |
Fri, 09 December 2016 15:50 |
A Hamilto
Messages: 4508 Registered: April 2011
Karma: 39
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Atom Ant wrote on Fri, 09 December 2016 15:00I 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.
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Re: Fuel sender question [message #311398 is a reply to message #311370] |
Fri, 09 December 2016 16:02 |
Atom Ant
Messages: 170 Registered: October 2016 Location: Austin, TX
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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
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Re: Fuel sender question [message #311406 is a reply to message #311370] |
Fri, 09 December 2016 20:50 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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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
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Re: Fuel sender question [message #311412 is a reply to message #311407] |
Sat, 10 December 2016 00:26 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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Atom Ant wrote on Fri, 09 December 2016 21:22Thanks 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
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Re: Fuel sender question [message #311417 is a reply to message #311407] |
Sat, 10 December 2016 07:26 |
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Matt Colie
Messages: 8547 Registered: March 2007 Location: S.E. Michigan
Karma: 7
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Atom Ant wrote on Fri, 09 December 2016 22:22Thanks 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 - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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