Jump to content
weldingrod

Bypass Fuel Pump Relay?

Recommended Posts

1 hour ago, datzenmike said:

 

 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.

 

Share this post


Link to post

Seems other relay with fuel pump relay is the automatic choke relay. Seriously thinking of relocating pump relay if I have to go in there again.

Share this post


Link to post

PS. Mike, haven't seen an early 80's era 720 at a wreckers for years. Salal pickers bought all small import 4x4's up years ago, beat the hell out of them and then bought newer ones when they could afford to. Had 4 in the yard here a few years ago and it appears I should have saved way more parts than I did.

Share this post


Link to post

And it may not even be relay. If alternator wire or oil pressure switch wire not powered when running shuts off relay. Can't run truck, so can't test for power.

Share this post


Link to post

Or will just cranking the engine power those wires?

Share this post


Link to post
On 12/25/2018 at 2:37 PM, 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.

 

Probably the easiest thing to do

Share this post


Link to post
On 12/26/2018 at 2:40 PM, datzenmike said:

 

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..

Not all models and years of the 720 have the dome light timer.  Just higher end models.

Share this post


Link to post

Guess not an issue with me because mine doesn't work but maybe a good idea to check that circuit while I have the passenger seat out to remove fuel pump relay.

  • Like 1

Share this post


Link to post
On 12/25/2018 at 2:42 PM, Crashtd420 said:

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....

Using that schematic of wiring plug, for bench test, oil switch power should power pump. accessory power should power pump. power from fuse 5 should power pump. power from alternator should power pump. This should diagnose all circuits in relay. Is that right?

  • Like 1

Share this post


Link to post
On 12/25/2018 at 5:37 PM, 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.

 

That pic is probably showing you the orientation of those wires in the plug....

 

Ground = ground

 

Oil pressure = ground , but when it activates it shuts the ground off to turn the idiot light off.... 

This could be different on the 720, I'm not sure.. so verify polarity and function of your oil sender..... 

 

Acc= 12v switched from key....

 

Fuel pump= where your sending the power......

 

Fuse #6 = this is the power for the pump.....

 

Altenator =positive,  I think it's the lamp system so its 12v while truck is running... 

 

So the goal is to connect fuel pump to fuel #6..... if you connect ground to the ground and power to the acc you should hear a click, if not add power to the alternator then, then add in the oil pressure switch, I'm not sure what the relay is looking for from the oil pressure switch, maybe if it sees ground it wont activate.... so maybe that stays off...

check continuity between the fuel pump and fuse wire.... that's how I would go after it.... 

Feel free to comment and correct this if I'm off track.... 

  • Like 1

Share this post


Link to post

Also the dome light timer on the 85/86 720 is in the passenger side panel under the King cab 1/4 window.

Share this post


Link to post

 

 

Black wire is almost always ground.

 

I think the alternator input is connected to one side of a relay coil, the other to a reference 12v from the battery

 

12v battery........ relay coil.... 12v alternator (running) no current flow)

12v battery>>>> relay coil>>> alternator (not charging) current flow to ground turns a relay to ON.

 

The same applies to the oil pressure switch and another? relay for it.

 

Starter input

Ignition ON input. These two probably start a timer. The other two keep it going or turn it off.

 

 

  • Like 1

Share this post


Link to post

OK, new relay installed, checked pump with 12volt source and it works,  checked wiring from  relay to pump and getting 12 volts,  checked fuses all good, still no joy! Jumped relay plug, no action from pump.

 

Hate to tow it to a shop but looking like only option at this point.

  • Like 1

Share this post


Link to post

You have 12v at the pump? It should run. Do you hear it running?

 

If you have power at the pump and it isn't running it can only be a bad pump or the ground is no good.

 

If pump making sound but no gas, the tank is either empty, the line to the pump is plugged or the line to the carb is plugged..... this isn't that hard.

  • Like 1

Share this post


Link to post

Pump runs off battery, new relay, tank full. Many years ago my Father taught me if something won't start/run the first thing you check is the fuel. New lines pump to carb and return. According to the idiot test jumpers at relay plug are side by side, right?

  • Like 1

Share this post


Link to post

2238132.jpg

 

Looks like the W (white) to WB (white/Black) will send 12 v to the pump. Yes

 

Have you made this jumper? Does the pump make any sound? Is it running or not???????????

  • Like 1

Share this post


Link to post

Used jumper and pump not running. Pump runs when hooked up to a battery. All fuses good. New relay installed. Test shows 12 volts at pump connection. Pump has been stripped, filter cleaned, reassembled and bench tested before reinstall. All new fuel lines tank to carb.

  • Like 1

Share this post


Link to post

2238132.jpg

 

 Is there 12 v at the relay on the jumper wire? W and WB ?

 

If yes then there should be 12 v at the pump. The WB wire goes to the pump.

 

 

 

Is there 12 v at the pump when the jumper in place?

 

If no then the wire is broken or the 10 pin connector under the passenger seat isn't connecting.  This 10 pin connector joins the external harness under the truck to the rear,  to the cab harness.

  • Like 1

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use.