1985 nissan 720 fuel pump running all the time

[mclendon]

Hello everyone. I am new to the forum.

 

1985 Nissan 720 4X4 Z24 motor

I have a problem with my fuel pump that I cannot get a grip on. I know that the fuel pump has 2 sources of power. I will refer to them as "Fuse block" and "Battery".

 

 

The source from the Battery was unplugged when I got the truck. The reason  is that when you plug it up the fuel pump comes on and the air mixer (don't know the proper name but you can hear it running in the front of the carburetor) in the carburetor comes on, this is with the truck tuned off and key out of the vehicle. 

So the battery supply must remain unplugged.

 

That being said, the Fuse Block source runs the truck. It does not shut off. As long as the truck is turned on or cranked the fuel pump runs from this source. No time-out, it runs continuously. (Power from battery is unplugged)

 

I have replaced the oil sending unit, The relay beside the battery, and the fuel pump module beside the blower motor inside the cab on the passenger side, Checked alternator voltage, Have found no evidence of any kind jumper wire that someone has installed.

 

I would like to get this corrected because the air mixer in the carb. does not work. So until the truck warms up it runs too rich and causes to smoke a little.

 

Any help would be greatly appreciated.

[datzenmike]

Is there a large round electrical connector at the back of the carb, with 5 or 6 wires going  to the carb?

[mclendon]

yes

It is the factory carb

[mclendon]

Is there a large round electrical connector at the back of the carb, with 5 or 6 wires going  to the carb?

Yes

It is the factory carb.

[datzenmike]

I'm sure it is but 6 wires or only 2. There are several carbs and even EFI on the '86. The 6 wire is an electronic feedback carb and different to troubleshoot than the two wire carb. The purring sound from the carb would suggest it's the feedback one.

[mclendon]

Yes datzenmike it is the 6 wire plug.

[datzenmike]

The power to the control unit should come from the 6th fuse over from the left hand side of the fuse box. This can only be on when the key is on.

[mclendon]

You are correct but the pump has 2 sources. Fuse box suppose to come on for a limited time to prime the carb. then the power source should come from the battery after the vehicle is cranked. In my description I described the problem I am having.

[datzenmike]

No where is there constant power to the pump relay. Both are switched one way or the other by the ignition key.

[mclendon]

Not once have I ever said that there was constant power to the relay. There are defiantly 2 sources that find their way to the fuel pump. One originates at the fuse panel (which should stay on for a limited time)  and the other at the battery (Which should come online after the truck is started). If I plug the one up at the battery the pump runs all the time even with the key out.

[Lockleaf]

From the factory, there is no direct run from the battery to the fuel pump. The fuel pump is run through ignition switch at all times with the factory wiring setup. By your description, your pump wiring has been modified (possibly done by previous owner to bypass failed fuel pump relay or something?)

[mclendon]

There is a single red wire connected to the positive pole on the battery with a fuse link. Do you know what this feeds?

I have 2 - 1985 trucks and both have this wire. One of the trucks doesn't run anymore but this red wire is plugged in on it. When I plug it up on the one I am referencing the fuel pup and air mixed in the carb comes on with the key off.

 

Also can you tell me the way the fuel pump suppose to work. I know my original 1985 truck when you turned the key on the fuel pump would come on for a few seconds then go off. It did not come back on until you cranked the truck. At least I think that is the way it worked. At this point I am not really sure of anything anymore.

[mclendon]

datzenmike Seems I am wrong about the 2 sources to the fuel pump. Time for me to eat a little Crow it seems.

I was told that the wire from the battery feed through the oil sending unit after the ruck was cranked.

[datzenmike]

First post third paragraph. Pump comes on with key out of ignition.

[mclendon]

I traced the red wire from the battery. it goes to the headlights in the fuse panel. so the headlights are only coming on when the ignition is turned on. I must be getting some kind of back feed.

 

when I plug the red wire up it is energizing the ignition fuses that is why the fuel pump is coming on. but it is only connected to the fuses for the headlights.

and that is why my lights are coming on only when I turn the ignition on. it is back feeding from the ignition section of the fuse panel.

 

is it possible that the fuse panel is shorted or is there some kind of relay I can look at. the red wire seems to run from the battery straight to the fuse panel.

 

Thanks for all your help guys

[datzenmike]

Yes while the Red wire does come from a Green fusible link, it does go to the battery. However it only powers the 2 far right hand side 10 amp fuses for the headlamps. I don't believe in 'shorts' they just don't happen. Someone has jumpered something into the headlamp supply.

 

 

Unplug the two fuses. If the pump stops then it's between the fuse box and the battery.

[mclendon]

datzenmike and lockleaf you guys are the BOSS. I found the problem. Someone had a jumper wire in the steering column running from one of the ignition wires to the red wire feeding the headlight switch. Disconnected jumper, connected the highlight power source from the battery and everything works as designed. I had to replace the fusible link at the battery.

 

Why the hell would someone do that?!

 

Thanks guys.

[Lockleaf]

Welcome to the wonderful world of previous owners.  Sometimes written PO, usually written WTF!!!

[datzenmike]

POs are definitely dick heads, I see this daily fixing Datsuns members have bought. I would imagine the Green fusible link to the headlights was the one that was blown. Easiest thing for a PO dick head would be to jumper power from the ignition directly to the headlamps. A real dick head would have stuffed four inches of 14 gauge house wiring in to replace the blown link.

[Charlie69]

POs are definitely dick heads, I see this daily fixing Datsuns members have bought. I would imagine the Green fusible link to the headlights was the one that was blown. Easiest thing for a PO dick head would be to jumper power from the ignition directly to the headlamps. A real dick head would have stuffed four inches of 14 gauge house wiring in to replace the blown link.

Mike are you not a PO also?  LOL

 

The electric fuel pumps on the 720 do run continuously when the truck is running.  They do not build pressure and shut off as conventional electric fuel pumps do.  The reason they do this is 1 because of the return line to the tank, 2 this provides cooler fuel to the carb than a conventional fuel system that does not have a return system.  This helps with not vapor locking in hotter climates.

 

This is why most of the after market electric fuel pumps fail in a short period of time.  They are not designed to run continuously.

 

I have found that the Facet pump is designed to run continuously and will work in the 720s.

 

http://www.ebay.com/itm/FACET-PUROLATOR-PRO-87SV-FEP-87SV-FUEL-PUMP-/172115974731?hash=item2812e95a4b:m:mMG-tFrnaCrgJuzHgdr4AIw&vxp=mtr

 

Best price I have found on the facet pump lately.  Ask Rhino how many pumps he went through before buying a Facet.

[datzenmike]

YES. We are all POs. (usually we also evolve) or Darwinism weeds us out from the dick head things we do.

[Meesh]

I’m a PO…S 😂😂😂