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Bypass Fuel Pump Relay?


weldingrod

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Installed a new fuel pump relay and it failed within 1000 miles. Don't want to go through all that fuss again. Any reason why I can't take the hot line from the plug and connect it to the power line leading to the fuel pump? I assume pump runs full time but am not sure. Was thinking of installing a hidden switch as an antitheft device.

 

Checked fuse-good. Powered pump from 12 volt source-runs.

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The fuel pump relay is wired to shut off the fuel pump in case of an accident, and there is a possibility the fuel line is broken.  You do not need a fuel pump pumping fuel into a fire while you are trying to get away from a crashed truck.

 

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Drove older vehicles for years without that "life saving device" and I am still here. Thank you for your reply but it does not contain any technical information I can use.

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6 minutes ago, weldingrod said:

Still no help. These systems are recent in vehicles so are you saying that thousands of older, collectible and atiquevehicles on the road should be scrapped?

The older stuff never had electric pumps so if the engine stopped then the fuel stopped.....

I replaced my mechanical pump with an electric fuel pump and use an oil pressure sender/cuttoff...  if I loose oil pressure the fuel stop....

Screenshot-20181224-204232-e-Bay.jpg....

This takes a starter input and a 12v ignition input and outputs to the fuel pump..... 

 

You could just power it like you said but it's just not safe.... I'm not 100% sure on the 720 setup but your not the first I've read about with a fuel pump relay issue....  I'll elaborate if needed.....

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Could the failure be as simple as the replacement relay had too low of an amp rating? Say if the relay was only rated at 10a and your fuse was at 15a, the relay would blow firstif 10a was exceeded. You could technically do what you described, but the relay probably runs off battery power so if you connected it to the pump power it would constantly run without a switch. Your truck, but a relay circuit is pretty simple so I would try and find out what is going on. Could also be a defective relay. Kind of rare, but I have seen them bad right out of the box.

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No.... You referred to 'older vehicles'... I was saying it wouldn't apply to mechanical pumps that quit when the engine stops. Pretty sure all EFI have a fuel pump relay. Was not at all telling you to get it fixed, you can do as you like. 

 

Of 174,000 vehicle fires in 2015, only 3% are from an accident. Of this 3% only 0.66% of fatalities may have had fire as a contributing factor.

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3 hours ago, weldingrod said:

Installed a new fuel pump relay and it failed within 1000 miles. Don't want to go through all that fuss again. Any reason why I can't take the hot line from the plug and connect it to the power line leading to the fuel pump? I assume pump runs full time but am not sure. Was thinking of installing a hidden switch as an antitheft device.

 

Checked fuse-good. Powered pump from 12 volt source-runs.

So how do you know the relay failed?

Did you bench test it at all?

You may have an electrical issue with the system... bad ground or corroded connector..... 

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Datsunrides- used replacement relay and didn't change anything in the fusebox. Recommended fuse was installed. At $70per can't afford too many "bad out of the box". How did you hook up your system?

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Bought truck 2010, insured 2018 so not much that has been gone over. Last failure was diagnosed as relay, everything else good, only change new relay, so...................

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1 minute ago, weldingrod said:

Bought truck 2010, insured 2018 so not much that has been gone over. Last failure was diagnosed as relay, everything else good, only change new relay, so...................

How was it determined it was bad?

The old and the new ?

Are you looking to figure out why or just get around it and drive the truck?

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Crash- would like to keep it stock if possible, but need transportation. When I put this on the road a few months ago sold my venerable old '82 Dodge Diplomat spare wheels.

Will go out tomorrow recheck fuse values just in case. Take a couple weeks to get replacement relay so thinking might have to buy another beater/spare, pain in the butt.

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Determined old one faulty because new one came while I was finishing up a few loose ends, installed it, truck ran fine. Still searching for a return fuel line check valve.

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Well for the time being you can hot wire the pump like you were thinking so you can drive it.... 

But I would also check things like the ground to the pump and fuel filter.....

If you have killed 2 relays there is a reason..... 

Is there something special about the relay?

Can't you simply redo the plug end for a standard automotive relay? 

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3 hours ago, weldingrod said:

Still searching for a return fuel line check valve.

 

Just so you know, the check valve is on the gas tank vent line not the return. Gas is already in the return so it's sealed up and nothing can leak, but the vent is open into the charcoal canister and gas could leak out in a roll over.

 

 

 

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Mike- All I know is when I did some gas line repair the second line out of the carburetor was emitting gasoline, I assumed being a liquid should go back into the tank.

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 It's really hard to follow the fuel line drawings with all the bends but mine shows the check valve 17330-P7900  connected directly to the breather hose. 7335-U0110

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Right number for check valve. Haynes manual pages 4-4 and 4-5 shows lines at tank and....you are right.! One labeled fuel return line has no valve.It is the one to the rear. One just forward of it is labeled breather line and has check valve. Apologies! If I think it through the needle valve in the carburetor would stop the gas from siphoning back.

 

Don't tell me I am going to have to get a rebuilt carburetor! Could always dig into my stash, I guess (have three or four) and install one of those and see what happens. Either way, fguel relay will have to come out.

 

Any tips on bench testing it?

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2 hours ago, weldingrod said:

Right number for check valve. Haynes manual pages 4-4 and 4-5 shows lines at tank and....you are right.! One labeled fuel return line has no valve.It is the one to the rear. One just forward of it is labeled breather line and has check valve. Apologies! If I think it through the needle valve in the carburetor would stop the gas from siphoning back.

 

Don't tell me I am going to have to get a rebuilt carburetor! Could always dig into my stash, I guess (have three or four) and install one of those and see what happens. Either way, fguel relay will have to come out.

 

Any tips on bench testing it?

Try checking out this article....

https://axleaddict.com/auto-repair/The-Datsun-and-Nissan-Pickup-Fuel-Pump-Relay-What-the-Haynes-Automotive-Repair-Manual-Wont-Tell-You

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You can easily run without the relay. There is a how to in that article for making a by-pass jumper for the relay. This will peep everything stock until you find a replacement from another 720. Some day... or never.

 

8 hours ago, weldingrod said:

Mike- All I know is when I did some gas line repair the second line out of the carburetor was emitting gasoline, I assumed being a liquid should go back into the tank.

 

Yes. It's in a closed loop and sealed so no gas can normally get out. The pump pulls gas out of the tank and pushes it towards the carb. At the return line there is a pin hole to provide resistance so the pump can build up some pressure so it will go into the carburetor. If it was open gas would take the easier path and just return to the tank. Having constant flow through the return line removes hot gas and replaces it with cooler gas from the tank. This reduces re-start vapor lock certainly on hot days. Also it constantly filters the gas so less sediment collects in the tank and lines. Replace your fuel filter every 2-3 years and don't forget the one in the screw off base of the electric pump. It also has a magnet on it to collect ferrous metal.

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