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mechanics say my truck isnt getting power to fuel pump


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Its a 85 nissan 720 i just replaced fuel pump relay and fuel pump and just put on new hitcachi carb... Fuel pump seemed to be working before I dropped it off there, well I heard a clicking coming from it, what would cause it to not get power?



With the relay and the pump replaced you now have to possible causes. New does not mean it's going to work perfectly. Most replacement parts are total junk. Did you replace it, or did a mechanic replace it???... because a mechanic loves taking money for stuff you can easily replace and trouble shoot yourself.




You could just wire power from the ignition switch so the pump is on when the engine is running.

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I replaced it myself with one from orielly ive had to replace it several times the past year, i had it hardwired to the ignition for awhile before replacing relay and it seemed fine after replacing the relay so I took off hardwire.. Im gonna just tell them to do that, all I took it to the mechanic for was to get carb adjusted now they are telling me the fuel pump isnt getting power...

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Tow home and fix it yourself or it's going to get expensive. If you replaced the pump several times in the last year this should tell you something. On the next pump be sure to replace the filter between it and the tank.

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I replaced that a few months ago im willing for it to be somewhat expesnive as long as they fix whats wrong with it, do you think its likely my fuel pump? i figured it couldnt be that because it was shotting gas to carb and was literally brand new, but mechanics do say the float bowl is empty...

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Make sure your connections are solid under the pass seat. But im in agreement with the others, shit will get spendy quick. HF meter, $5. Wiring diagram, free. Your time, free. You do the math.

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Mechanic sees a boat payment coming his way.  But really, how many fuel pumps?  They should last, oh, 20 YEARS!  OK, so it's an Oreilly pump, so 20 months, but still.  Something's either killing pumps (rusty tank with no filter before the pump, maybe?)  or you have a wiring issue and have been replacing perfectly good pumps.  Which is expensive for a mechanic to diagnose, probably looking minimum $80 an hour for 4-8 hours "troubleshooting", which you could do in an hour with a voltmeter and a wiring diagram.

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OK.... if a "Mechanic" is having problems with a truck THIS simple, you need another mechanic.


The pump has a ground wire, and a plus to the relay.  It's super simple to troubleshoot.  Measure a few points with a meter and see if it's a fuse, the relay, or a missing signal from the alternator and oil sender.

The 720 is designed to shut off the fuel if the oil pressure drops or the engine stops.  (no alternator signal).


A single jumper in the fuel relay plug will run the pump right off the ignition switch.


To keep a 720 reliable:  Do NOT use aftermarket parts.  Mine is 28 years old, original fuel pump and relay.  New filters every few years, as a clogged one can burn up the pump from overworking it.

The only thing that goes wrong with the fuel pump relay is a bad solder connection.  Easy fix, and generally permanent.


Make sure the ground wires are good between the pump and the frame, and the frame and the battery.


BTW, I had to get a new account... my email changed.  I'm not as new as I seem.  Don't let the low post count scare you.  I DO know the 720 quite well in 28 years owning them.

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okay so the mechanic said my fuel pump is shit and suggested i get a new one, he also said he fixed the problem with it not getting power and says all in all about 2 hours time plus the cost of fuel pump, im pretty pleased thought it would be more time, he says hes owned my same truck and put over 300k miles on it so im feelin pretty good about it.

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  • 1 year later...

I am posting this because I have referred to this forum numerous times and it helped me, so I hope this helps someone else.  READ THIS BEFORE BUYING A NEW FUEL PUMP!! I now own 3 working Nissan 720 fuel pumps!  Here's why:


About 6 months ago, I am driving my '84 720 to work.  I barge into a snowdrift and it throws me 180°.  Weee what fun.  About the time I have to get out and lock in the hubs and get back in, the truck dies - WON'T restart.

I have someone from work pull me to the shop.  Troubleshooting the problem, we discover it's not pumping fuel (took the larger of the 2 fuel lines apart in the engine compartment (after cleaning out the packed snow).  No fuel being pumped there.  We test voltage at the fuel pump and it's OK.  Assume the fuel pump is bad.  Bought a new fuel pump.  I kept the connector plug in place and spliced the wires leaving the plug onto the new fuel pump so that in the future I could easily swap out the pump by simply unplugging the connector.  $60 later, I'm happy it's running.  I stow away the old fuel pump because I'm a hoarder.


Fast forward 6 months.  I'm driving along and notice the truck sputters occasionally, and often dies - won't restart.  It seems to happen on occasion, but getting more and more common.  At this point, I keep in the back of my mind this COULD be an intermittent electrical problem.  I found a forum that talked about checking the solder connections on the circuit board of the fuel relay (mounted to the right of the glove box).  I took it apart, and notice the solder connections are OK.


Finally I had my chance - I happened to be at home when the problem was occurring, and I had a few minutes to look at it.  When I take the fuel line off in the engine compartment and put the hose into a bottle, no fuel is pumping.  So I test voltage where the fuel pump is and I get +12V.  OK - the problem HAS to be either clogged fuel filters or another bad fuel pump.  Since I ran out of time, and I HAVE to get the truck going again, I order another new fuel pump.  This time I also order both filters along with it, because the warranty won't cover the pump unless the filters are also replaced.  Another $60 plus $20 worth of filters. 


The filters come one day before the pump, so I change out the filters (original ones were OLD).  Truck starts right up, runs for a few seconds then shuts off and won't restart.  I again test the voltage to the pump, and of course, it is OK.  Test fuel flow under the hood and again it is NOT flowing fuel.  I am confident it is a bad fuel pump.


Next day the fuel pump comes.  I put it in and the truck fires up, runs for a few seconds then shuts off.  Won't restart.  I ONCE MORE test the voltage at the fuel pump.  THIS TIME - NO VOLTAGE!!  When I jiggled the wires where the plug connector is at the fuel pump, I can get the power to come on-and-off.  Ah that stupid little plug connector!  Third fuel pump, numerous breakdowns, and lots of education about Nissan 720 fuel systems, and the problem the whole time was I didn't cut out that little plug connector.


So lesson learned: If you suspect your fuel pump has gone bad, first test the voltage BEFORE the little plug connector, and if it is GOOD there, put your test probes AFTER the plug connector and jiggle the wires at the plug and see if you can get the voltage to be intermittent.


Anyone need a slightly used Nissan 720 fuel pump?

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