Jump to content

Help me understand problem with the fuel pump/sender cap causing fuse to blow..


TheRadBaron
 Share

Recommended Posts

So my '87 recently started blowing the fuel pump fuse. I at first figured that it was the fuel pump going bad but I've done some reading here and now it seems that the problem might be caused by bad wire connections on the top of the fuel pump and fuel gauge sending unit assembly.

Here's a post that details the problem. http://www.nissanpathfinders.net/forum/topic/22462-fuel-sending-unit/

A summary is that the wires to the fuel level sending unit corrode where they pass through the metal cap and that causes them to short out and blow the fuel pump fuse.

What I don't understand is how this would happen since I don't think that the fuel gauge has anything to do with the fuse that runs the actual fuel pump. I studied the wiring diagram for a while and the fuel gauge fuse seems to be included with the rest of the instrument cluster and totally separate from the fuel pump.

Plus, when the tank is full I believe the fuel level sender is basically shorted to ground anyway. This causes the gauge to read "full" and the resistance increases as the fuel level drops.

Am I thinking about this correctly or am I missing something major here? When my fuse was blowing it was raining hard and the top of the fuel tank (along with the rest of the truck) was soaking wet. Then I replaced the fuse and it didn't blow but the pump wouldn't run either. Once the truck dried out the pump ran again and the truck ran fine.

I'm tempted to just buy a new fuel pump and sending unit assembly but I don't really like to just throw new parts and money at a problem without understanding what the problem is. Thanks for any insight.

Link to comment
Share on other sites

Well generally speaking, electrics don't like water so when it rained and those wires got wet, of course the fuse would go bye bye. That's it doing it's job preventing a fire. My only question is how did water manage to get in there? In the back above the fuel tank, those wires are covered by carpeting so no water should be able to get there unless you washed the back carpet.

Link to comment
Share on other sites

Wires and connections simply getting wet isn't usually enough to cause a fuse to blow in a 12V DC system. I agree that electricity and water don't agree with one another but if a fuse blew every time a wire or connection got wet it would be almost impossible to drive in the rain. If the water IS a factor in making the fuse blow there's something else wrong.

There's enough road spray under the truck to totally drench the top of the fuel tank and fuel pump when driving on the highway in the rain. That's how mine got wet.

Link to comment
Share on other sites

Maybe the electrical tape that was there has crumbled and fallen off. I'd get under there and tape everything up then even put shrink tubing on the wires to make it more waterproof. Spray everything with WD 40 to keep the water off.

Link to comment
Share on other sites

I haven't dealt with this personally, but I would suspect that it's not just water conducting electricity. Water, road dirt, corrosion from the wires where water's gotten into the cracked insulators, seems like you could pass some current through that. I seem to remember an ETCG video where he found some current flow through the layer of dirt on top of his car battery. I haven't stared at it recently, but IIRC all the fuel pump and sender connections are fairly close together on top of the pump. It wouldn't have to make a dead short, it just has to draw enough juice to add onto the fuel pump's current draw and overwhelm the fuse.

 

It is interesting that the pump didn't come back immediately with a new fuse, but didn't blow the new fuse either. That would have me suspecting either a loose connection or a fault with the pump itself. Have you pulled up the access hatch to see if there's anything obvious?

 

I did deal with a bad fuel pump connection once on a friend's Alfa. We'd drive a ways, the engine would quit, we'd check various things, and then it would restart and we'd think we'd fixed it. A few miles later it would crap out again. Near as I can tell, some part of the fuel pump harness was getting hot and losing contact, then regaining contact as it cooled. When we finally checked for power at the pump and found none, we hotwired the pump from the headlight circuit and all was well (until the headlight switch melted). Interestingly, my friend re-wired the pump later and found that it blew fuses, suggesting that the power demands of the pump might've been part of the problem.

Link to comment
Share on other sites

  • 2 weeks later...

the pump is likely fine. the sender is shorted out. not recommended but u can put a 15a fuse in and i bet it runs for a mile before it pops again haha. get a new or junk sender will work just fine, pump is not needed just swap. it to the new assembly

 

cheers

Link to comment
Share on other sites

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
 Share

×
×
  • Create New...