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Old 10-20-2019, 02:41 PM   #1
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Seneca M2 Dual Diesel Tank Crossover Function

Crawled under today and can see the two fuel lines running between both tanks. Used my endoscope to peek into the passenger side tank and could see a pick-up line going down to the bottom of the tank. I assuming that the transfer of diesel from the passenger tank to the driver tank is both a gravity feed and a pump (pull action from the driver tank). If the engine is running, the driver side pump mechanism (or something) pulls diesel from the passenger side. If the engine is off and the passenger side diesel level is higher than that of the driver side (ie. parked at a slant where the driver side is lower than the passenger side), diesel will flow to the driver side until both are equal.

Appreciate it if someone could confirm this or educate me on how this works.

Thanks!
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Old 10-20-2019, 03:08 PM   #2
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It is my understanding that this is a dual feed dual return system. It pulls from both tanks and whatever fuel is unburned is returned to both tanks,. There is no gravity feed. Nothing happens when the motor is off. In the old days there was a crossover hose. DOT did away with those years ago. Its not a perfect system and air bubbles, trash in the tank and kinks keeps the system from doing everything evenly. Also fuel gauge is in one tank only and the generator will pull from one of the tanks.
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Old 10-20-2019, 08:34 PM   #3
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I also have a 2017 Seneca and i have experenced several times where my fuel gage has read a higher level the next day. I asume this was caused by gravity.
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Old 10-20-2019, 09:51 PM   #4
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On our 2015... We parked for about 1.5 hours on the side of a road where the slope was significant to the passenger side. We had about 3/4 tank when we stopped on the fuel gauge.

As we were starting back up to leave our stop... The fuel gauge showed 1/4 or less (I freaked out) and once we were back on the highway, the fuel gauge slowly came back to 3/4.

I firmly believe that there is a small hose that allows equaliziation of fuel between the two tanks, but not large enough to allow transfer of fuel from one to the other during fueling. All based on our experience (yes we are out on the road right now)
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Old 10-21-2019, 05:07 AM   #5
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Found this on the "truckersreport.com"

"...the crossover line is basically a 3/8" line between the two tanks that has a T in the middle to send fuel to the engine. If you fill one tank, gravity of the greater amount in the one side will force the fuel thru the line into the other tank until they are even. But 3/8" will take a while!"

So the pull comes from the engine fuel pump pulling equally from each thru a "T" or valve. Makes sense. I think Rob (robbbyr) posted a schematic a while back with an explanation. Not sure of the thread...I'll search some more for it.
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Old 10-21-2019, 06:40 AM   #6
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I stand corrected on part of my response. There is no crossover line anymore, however I saw this on the troubleshooting guide:

"Equalization: If full of fuel (bubble free), the lines will act as a siphon to level the fuel in the two tanks, just like a crossover line. An
air bubble can break the siphon if trapped in an upward loop, and
stop flow from one tank."
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Old 10-21-2019, 06:42 AM   #7
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Found this PDF.
Attached Files
File Type: pdf Fuel transfer system 2.pdf (143.1 KB, 45 views)
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Old 10-21-2019, 06:04 PM   #8
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Here is a link to a fellow Seneca owner's website where he talks about our Seneca's fuel feed/equalization system: Jayco Seneca Freightliner M2 Two Separate Fuel Tanks and Equiflo, How the hell does this work? | RV Underground
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Old 10-22-2019, 06:04 AM   #9
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Quote:
Originally Posted by houstonstroker View Post
I stand corrected on part of my response. There is no crossover line anymore, however I saw this on the troubleshooting guide:

"Equalization: If full of fuel (bubble free), the lines will act as a siphon to level the fuel in the two tanks, just like a crossover line. An
air bubble can break the siphon if trapped in an upward loop, and
stop flow from one tank."
I think the term "crossover" is now used in a generic sense. The pdf you supplied helps explain how it draws from and keeps both tanks equalized. Thanks!
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Old 12-05-2020, 03:04 PM   #10
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Has anyone added a crossover fuel line?
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Old 12-05-2020, 06:23 PM   #11
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Quote:
Originally Posted by egmhe36 View Post
Has anyone added a crossover fuel line?
To function adequately enough to allow filling both tanks from just one side the line would have to be fairly large. Damage to such a line would lead to substantial leakage which would lead to a large cleanup bill if you have a spill. Such crossover lines are subject to damage from road debris, I saw many during my career as a firefighter.

That is why newer heavy trucks with saddle tanks use the Equiflo system like our Senecas have. Nothing hanging low vulnerable to getting torn off when you hit the road alligator that you didn't see until the last second!
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