zero current relay?

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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=114587
Printed Date: May 04, 2025 at 6:33 AM

Topic: zero current relay?

Posted By: timberframer1
Subject: zero current relay?
Date Posted: June 20, 2009 at 9:06 PM

I am wiring my diesel truck with two fuel supply pumps. One comes on w/ the ignition and w/ a SPST (SPDT?)switch. the other is engaged while the first one is turned off.

My question is how do I wire the system so that if(when) the second pump burns out or blows a fuse, it automatically reverts back to the original pump so I don't damage my motor?. Also, possibly add a red light when this happens so I know its failed..
Thanks

Replies:

Posted By: dualsport
Date Posted: June 21, 2009 at 11:33 AM

Is it common for your second fuel pump to burn out? Was there a problem with the original fuel pump that you need to wire in the second pump?
Are the two pumps working in line with each other or in parallel? If one pump is locked up, will the other pump be able to pump through it?

You'll have to be able to identify the conditions that you can detect when the pump fails. Is it only if the pump current draw drops to zero?
You could detect the difference between that and the normal current draw, and trigger a relay if the current drops below some threshold.
If it could fail but still draw current, then this wouldn't cover that condition.

Maybe if you have a fuel pressure sensor for triggering it'd cover most of the failure modes.

Posted By: timberframer1
Date Posted: June 21, 2009 at 7:58 PM

There are no problems w/ the original pump. The second pump pumps veg. oil. Once the truck heats up I switch over to the veg. oil. The two pumps have two independant fuel tanks and fuel lines up to the motor.

The veg. oil pumps fail because of over heating (I think) because of the thicker oil. They also blow fuses, usually the inline ones.

I'm not sure what kind of relay is used if one pump has no current it causes the relay to activate the other pump??

Posted By: KPierson
Date Posted: June 21, 2009 at 8:49 PM

I, too, think a fuel pressure switch would be the best bet here.

Or, use the right stuff in the first place so the motor doesn't overheat and blow fuses. :)

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Kevin Pierson

Posted By: tommy...
Date Posted: June 22, 2009 at 5:54 PM

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I am using a dual gas tank solenoid from a Ford pickup. You just flip a switch and the other tank starts flowing. You can even wire it so that the fuel gauge works when you flip the switch. (some stuff i found on adding a 2nd tank...)

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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

Posted By: tommy...
Date Posted: June 22, 2009 at 6:00 PM

An oil pressure switch in line is a very good idea as This will kill The pump in The event of a roll over or serious accident.

There's yet another aspect to consider. With the increasing use of home-brewed biodiesel, having more capacity allows you to load up on this friendlier fuel if you don't have access to it on a long trip. Todd Smith of Extreme Biodiesel decided to add a 100-gallon tank to his rig. The tanks has a toolbox integrated into the top, so now he has plenty of biodiesel for his trips to the desert with a toy-hauler in tow.

Whatever the reason for adding a second tank, Aero Enterprises recommends using manual valves made of brass and Teflon to control the flow. After years of fabricating fuel tanks of all sorts--including replacement tanks for every domestic make of truck--the company feels that manual valves are the safest, longest-lasting and most dependable way to operate a fuel system. You control which tank you use first, and you'll always have a way to get home on one tank or another. And you'll also know how much fuel you have onboard because there's a separate gauge for the auxiliary tank.

Aero's Jake Jackson feels that the drawback to using a gravity-feed system into your factory tank comes if there's a problem with fuel contamination or clogging. Once that happens, it affects all of fuel. Also, with an electric valve, it's better to install a pair because there's no advance warning when one fails.

Another point in favor of manual valves is the warranty. When used with diesel fuel, electric valves have no warranty. Aero has a one-year warranty, but Jackson says he hasn't seen them fail even after 10 or more years of use.

As an extra measure of protection, Aero supplies a large in-line Fram fuel filter/water separator with its bed tank. In addition, the company's toolbox/tank combinations are made entirely of aluminum (not a steel tank with an aluminum shell). Prices range from $900 to $1700, and that's with installation included. All of Aero's tanks are welded in-house with internal baffles, and then pressure tested with air prior to installation. Drain plugs are included as well, should you need to clean out the tank.

Aero offers tanks for wide variety of applications, in addition to factory replacements. So even if you need a custom setup, the company can probably meet your need for extra fuel capacity.

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An optional pump-out system can be installed as well for providing fuel to other diesel engines, or to simply transfer fuel from the auxiliary to the main tank if you don't have a valve system. Another option is to fill the main tank with conventional petroleum diesel and put biodiesel in the other tank. That way, you can add the percentage of biodiesel you prefer as needed.

(another part of the article)

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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!