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weldingrod

Bypass Fuel Pump Relay?

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My 86 King cab had a similar issue and I thought it was the fuel pump relay and it turned out to be a bad ignition switch.

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Carb build kit, a good one, is around $30. It should have go, no-go gauges and detailed instructions for initial settings like float drop and choke pull off. The factory service manual (FSM) is also great for this. It's slow picky work for a couple of hours so preparation is vital. Have lots of space and tools, cloths, carb spray and a camera so you can re-assembel it.

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Thanks Mike. Actually disassembled and reassembled fuel pump as part of getting truck road worthy. Those pumpbases are a bitch to get off on a 38 year old truck.

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Where can I find that relay bypass article? Will order a new relay but at least this will let me use the truck while I am waiting.

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1 hour ago, weldingrod said:

Where can I find that relay bypass article? Will order a new relay but at least this will let me use the truck while I am waiting.

I posted it a page back..... I think it was about half way down the article....

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 2238132.jpg

 

Jumper the L (Blue) wire to the WB (White/Black) wire. This will send 12 volts to the pump any time the ignition is on.

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3 minutes ago, datzenmike said:

 

 2238132.jpg

 

Jumper the L (Blue) wire to the WB (White/Black) wire. This will send 12 volts to the pump any time the ignition is on.

Wont that cut off during crank?

he may have to give it a 3 count and let the carb fill up, and then crank the engine.... since it is a daily driver for him there should be enough fuel in the carb to get it started....

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I think there was a loose connection that was re-soldered but they can't all fail for the same reason.

 

The reason I mention timer is that the room lamp timers in the 720 look exactly the same. So does the variable delay wiper timer amplifier.

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Well if they are $70 like he said I would be opening it up and seeing what's going on.... or finding a cheaper way to do it while still keeping safety in mind.. 

I still think he has an underlying issue causing the failure..... 

Maybe opening it will give some indication about what part of the circuit is failing..... that's just me I hate replacing a part unless I know its bad and why..... but electronics can be a fussy pain in the ass....

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Niissan Datsun didn't make the relay. This can be sourced somewhere else for the fraction of the cost. It may not plug in but with female/male spade jumpers can be made to work. We have ancient simple crap, there are all kinds of replacement parts out there if you are willing to search. There is a name for that style relay somewhere which I apologize I don't know, but a couple of standard relays and some jumpers could work, Sorry for being vouge but there are all kinds of other applications out there that use the same relay. Just have to find it. Like I said sorry I don't have the answer but that same part exists for the fraction of the Nissan price. 

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Most EFI cars had one. Similar looking too. I have one (somewhere) from a 200sx and one from an S13

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See them out there from $35 to $80, so either I buy two cheap ones and cross my fingers or an expensive one and hope it lasts. Saw article on repairing, but that seemed to apply to high mileage units, but will open this one up and have a look. Looked like mostly solder cracks at terminal.

 

Only way I can fit to get relay out is to remove passenger seat, live on Vancouver Island on the Wet Coast and can't get truck into shop for obvious reasons so will work between showers and keep you posted.

 

Crash, you mentioned something about bench testing it- I am a welder and when I play with electricity smoke and flames are normal. Guessing not so much with electrical diagnosis. What would be the procedure to bench test it? And what is that other smaller relay next to it for? Maybe that one blew and affected bigger relay?

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Checked all fuses today for correct amperage, cleaned terminals and put back in. Just a keep busy thing, didn't get to yanking relay. Will check tomorrow to see if Lectric Gods did something miraculous at now it works.

 

I rather work with hydraulics, at least when they leak you can find it.

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The box under the passenger seat can cause this issue. I don't remember if that's the room dimmer or fuel pump relay. I sold my truck a long time ago.. but I had an issue similar.

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Fuel pump relay under dashboard passenger side, room dimmer simple rheostat on switch.

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Dash lights are dim-able with a dash switch. I was referring to the interior light delay when walking away from your truck. Interior light stays on for 20 second then turns off. The delay timer is under the passenger seat... if so equipped.

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Auto manufacturers don't just add superfluous relays. I doubt the accessory wire is rated to handle the pump current draw

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24 minutes ago, thisismatt said:

Auto manufacturers don't just add superfluous relays. I doubt the accessory wire is rated to handle the pump current draw

He should use a relay to switch the power between the fuel pump and the 12v fused source.....

Use the ground and acc to trip the relay...

Standard bosch relay will do the trick. and i think they are only $5.....

You might even be able to use the oil pressure switch somehow as a safety but I'm not sure how for that vehicle.. I would personally need to verify things before I would claim its possible.....

 

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50 minutes ago, thisismatt said:

Auto manufacturers don't just add superfluous relays. I doubt the accessory wire is rated to handle the pump current draw

 

 On the 720, the accessory side of the ignition switch powers two 15 amp fuses. One for the radio and cigar lighter, the other for the wiper/washer and air conditioning as well as the fuel pump control. So wiring the pump directly is no problem for it.

 

You could do this with on Nissan relay. Wire the starter signal to the fuel pump. Wire the ignition through a normally closed relay switch also to the fuel pump. On the coil side of this relay wire the one side to the ignition input and the other to the oil pressure sender.

 

Turn ignition on, the oil pressure sender is grounded and the relay it tripped to open position, so no power to the fuel pump.

In the start position power is supplied to the fuel pump and it runs as long as the key is in start.

Engine fires up and the start signal removed but now the oil pressure sender is not grounded and the relay trips to it's closed or rest position and power is supplied to the fuel pump as long as there is oil pressure.

 

Crude but would work. Only down side is if the carburetor is completely empty... you would have to hold the starter on till the fuel reached the carb. Most carburetors hold enough gas to start without any pump and drive down the street. This relay is already used on 720 so just get one off a wreck. The auto choke relay is perfect for this.

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Here we go reinventing the wheel.  If the fuel pump relay goes out find out why.  I would hate to see that some one burned to death because of a half assed repair!

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Have a box under passenger seat, but not delay lights. If I leave lights on they stay on.

 

Wonder what mine does? Haynes manual not much help. Sounds as though putting in another pump relay is going to be easier than any work around. Is there any way to test old relay before I buy a new one?

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4 minutes ago, Charlie69 said:

Here we go reinventing the wheel.  If the fuel pump relay goes out find out why.  I would hate to see that some one burned to death because of a half assed repair!

Been trying to find out why. Proper fuse in circuit, good ground, pump filter cleaned. Any ideas?

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Loose connection will do it.  Are you running a stock OEM electric fuel pump?

 

720 Factory pump installed on my 66 520.

 

DSC02360.jpg

 

 

Edited by Charlie69

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15 minutes ago, weldingrod said:

Have a box under passenger seat, but not delay lights. If I leave lights on they stay on.

 

Wonder what mine does? Haynes manual not much help. Sounds as though putting in another pump relay is going to be easier than any work around. Is there any way to test old relay before I buy a new one?

 

Interior light.

 

Comes on when you open the drivers door. Stays on for about 20 seconds after closing the door an leaving unless the ignition key is in. Not all trucks may have it but the amplifier is under the passenger side seat although the cruise control has something there as well..

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