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I have an RV with a 06 F750 chassis that has a Cat C7 diesel

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transfer issue. There's a 50...
I have an RV with a 06 F750 chassis that has a Cat C7 diesel transfer issue. There's a 50 gallon tank on Left side and 50 gallon tank on the Right side. The engine and generator pull from the Right side and 99% of the time my Right side tank is empty and there's still 15-30 gallons in the Left side tank that hasn't transferred over to the Right side tank. The first time my Heavy Duty Ford shop replaced the fuel pump assembly - that didn't work. The second time they replaced the two sender assemblies - that didn't work. The third time they said the alternator was defective giving excessive AC voltage affecting the transfer pump so they replaced the alternator - that didn't work so they replaced the transfer pump on warranty. We recently came back from an RV trip where we had to fill up both tanks 7 times and only the first fill up showed approximately 5-8 gallons in both tanks at time of fill. The last 6 times the Right side tank was 0-5 gallons full and the Left side tank still had 15-30 gallons in it. I average 8 miles/gallon and have 100 gallons with both tanks. When I first purchased this used 07 RV a few years ago I noticed that after both tanks were full that the fuel gauge would stay at full until I drove approximately 400 miles (half of the average miles on both full tanks). Then fuel gauge needle would drop as I continued to drive. After the first repairs mentioned above the fuel gauge would start to drop as soon as i started to drive like any normal vehicle. That being said I'm quite certain that before any repairs that the Right side tank would always remain full while constantly drawing from the Left tank. I remember filling up once after I drove approximately 400 miles on two full tanks and the Left side tank was empty and the Left side tank was full. For all i know is that these tanks never had a problem and that the Right side tank would completely transfer to the Left side tank until the Right side was empty. I look forward to your response. Thanks, Bryan
JA: Have you ever replaced the fuel pump, or is this the original?
Customer: As per my original message it was replaced the first time then again on warranty
JA: What is the model of your '06 Ford?
Customer: I have an 07 RV - Thor Four Winds Fun Mover 39D with an 06 Ford F750 chassis
JA: Are you fixing your F150 yourself? What have you tried so far?
Customer: Judging by your questions it doesn't appear that you are reading my messages. Have you read my message?
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 1 year ago.Category: RV
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Answered in 7 days by:
10/26/2016
RV Mechanic: Jeremy, Certified Diesel Technician/Electrician replied 1 year ago
Jeremy
Jeremy, Certified Diesel Technician/Electrician
Category: RV
Satisfied Customers: 3,483
Experience: ASE Medium/Heavy Duty Technician, Hydraulics, Brakes, Electrical Certified
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The way these systems work is the senders tell the ECM when to turn the fuel transfer pump on to keep the fuel at about 2-3 gallons apart from each other. If at anytime, you get a 5 gallon difference, the fuel transfer pump will shut off for the ECM will tell it to not pump thinking the second tank has a leak. If you have the generator feeding off the same tank that the engine does, this is wrong. The reason is the tanks will never balance out as you are driving after running the generator all night long. The generator is supposed to be running off the second tank, not the primary one the engine feeds from.

Now if you fill both tanks up and then not use the generator at all and are still having transfer issues, there is then either an issue with the sending units (new ones can have a dead spot in them and should always be tested prior to install) or the wiring to the ECM. Most times if it is a wiring issue, it is at the plug next to the transfer pump. It gets moisture in it and corrodes. Check to make sure there is no corrosion there and then check for dead spots on the sender units with a ohm meter. Also, transfer that generator fuel line to feed off the second tank. This way after using the generator for the night, the tanks will balance out and then the transfer tank pump will then activate to keep the tanks balanced as they need after driving for a little bit.

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Customer reply replied 1 year ago
Hi Jeremy, thanks for contacting me.After reading your answer I'm troubled with parts of it.You wrote, "If at anytime, you get a 5 gallon difference, the fuel transfer pump will shut off for the ECM will tell it to not pump thinking the second tank has a leak. If you have the generator feeding off the same tank that the engine does, this is wrong. The reason is the tanks will never balance out as you are driving after running the generator all night long. The generator is supposed to be running off the second tank, not the primary one the engine feeds from."If I'm understanding you correctly the system when functioning as designed will cut off when there is more than a 5 gallon difference between the tanks because of a leak. Whether the generator is feeding from either the left or right tank there will always be more than a 5 gallon difference if the generator uses 5 gallons or more at a time. Either way I don't see how or why the manufacturer would swap the fuel connection from the left to the right tank...To be clear can you use left and right tank to refer to the tanks so that there's no confusion? thanks, Bryan
RV Mechanic: Jeremy, Certified Diesel Technician/Electrician replied 1 year ago

It will shut off "in case there is a leak" because that is usually only the time the tanks are supposed to see a difference like that. It is Ford's prevention of a fuel loss with the transfer pump.

The right tank is supposed to be the main tank. Left is the feed. Most times the generators on RVs on the Ford chassis run the fuel lines from the left tank since the truck runs on the right tank. This is to keep the fuel tanks balanced for when the main engine is running to bring balance to the tanks and then the transfer pump works as designed.

The manufacturer isn't usually the one that installs the generators. The body builder does. Ford built the chassis only. The body or the living space is done by the body manufacturer. I get a lot of the Fords in with this issue and I nearly always have to run the generator lines to the left tank to resolve that problem.

Now, there are ECM changes that can be made to have the transfer pump operate at a higher threshold up to a 15 gallon difference. With this having the CAT engine, CAT would probably need to be the one to change that parameter with the CAT ET program. That will resolve this issue for a generator burns about 8 to 9 gallons in 12 hours on average. Most generators will run at approx 1/2 to 3/4 of a gallon an hour and that is at full operating capacity.

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RV Mechanic: Jeremy, Certified Diesel Technician/Electrician replied 1 year ago

Good afternoon. I see you viewed this with no response or positive rating given, therefore the question is still showing as an open question.

Did you still need assistance? Were you able to understand the information I provided above? If you need help, I'm willing to assist more. I just don't want the question to time out due to no reply when help is still needed. If no help is needed, you can reply that as well that you are satisfied with the response and I can then know that this question is resolved for right now.

Thanks!

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Jeremy
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Jeremy, Certified Diesel Technician/Electrician
Category: RV
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