[BADCRUZ68]

I am about done digging into this one, the summer is flying by and at my pace its going to be over with before I figure this out. As a last ditch effort I am going to throw out a post in case anyone has feedback before I either tow it somewhere or get someone to make a house call. The fuse that is blowing is killing my fuel pump. I am temped to blame it on the fuel pump just because I can idle/rev it all day in the garage without problems but once I get it on the road under load it will blow in a few blocks. This weekend I moved the battery back to the front to solve some hot start issues most likely related to grounding. Well it fires up just fine now when its hot and the battery well grounded to the engine but still blows the fuse. I hoped it was all related. Below is a pic of the fuse box, the missing fuse is what is blowing. The switched wire you see is whats connected to the fuel pump power, thats been fine for years with no issues. I have also tried a fuse tap directly to where the missing fuse is. Any ideas before I break out the wallet and pay someone for this

 

[SchnorrCS]

My guess would be the way you have that fuel pump wired.
A fuel pump is a very high draw item, usually. What kind of pump do you have?

If I were you, I'd run it off a relay. Do a search here to find out how to make your own, or go to summit and order a fuel pump wiring kit from painless.

Guarantee that's your problem. The factory harness was never made to handle anything like an electric fuel pump.

 

[rcodenewf]

I agree totally with SchnorrCS. Wire it up to a relay and it should be fine. Now that the pump is getting older (and so is your factory wiring) it's probably drawing more current now and therefore putting more of a load on the electrical system....in particular that circuit/fuse. Or....you may have an intermittent short in the wiring causing the fuse to blow.

Check that pump and see what it draws. Most likely a 30 amp relay will suffice.

#30 will be wired directly to the battery feed.
#86 will be wired to ground
#85 will be wired to the back of the ignition switch or power source that is hot in run/crank.
#87 will be wired to your fuel pump.

Someone can chime in if my numbers aren't correct, but it should work.

Regards.....John

 

[BADCRUZ68]

The pump is a Holley Blue with probably 100 miles on it. It does have its own inline fuse, not sure the amps but that one is fine. I do see that the switched wire I am using and have used for a few years is smaller than the power wire from the fuel pump so I wondered about that. I am challenged with this car, its not my skillset so everything takes me 10 times as long. I will look into a relay starting with google to better understand how this can help, I appreciate the feedback!

BTW, the fuse that is blowing is a 20amp

 

[Explorer]

What else is running off that fuse? The fact that it is blowing and not the inline fuse means you are overloading that fuse. I would suggest finding the instructions and wiring it correctly, including the safety functions if not in place. The ground wire at the fuel pump could be getting corroded and making the pump draw more current than it did before, just enough to overload the fusebox fuse.

 

[Asm109]

If you have your own inline fuse, tap the fuel pump wire off the upstream side of the fuse. You don't need two fuses in line.

 

[BADCRUZ68]

Motor draws only 3 amps current
7 1/2 amp fuse recommended.

Those are the holly specs Holley Performance Products 110 GPH "Blue" Electric Fuel Pump With Regulator*12-802-1

Let me check spend some time today educating myself on relays and how I could use one to wire this for another go at it. I checked the inline fuse that came with the pump and it is 7.5 amp. I will keep an eye on this thread for anymore comments and see if I can get a relay in town without ordering online. I am not sure what if anything else is running off that fuse, I don't even have a radio in the car.

Edit
I dug out my ron francis catalog and see this part
http://www.ronfrancis.com/prodinfo.asp?number=FP%2D35

Comparing it to Summits part
http://www.summitracing.com/parts/SUM-890023/

Great suggestions though, I see the benefit now that I have done alittle research

 

[Woodchuck]

+1 for tapping it upstream but make sure you aren't tapping to a circuit fed by the headlight switch or you might end up with flashing light syndrome.

There is only one cause for blowing fuses and that is current draw in excess of the fuses rating. Startup surge draw can be managed with a slow-blow fuse; excess current in general should be fixed or split into 2 or more separate circuits.

Excess current draw can be caused by too much resistance in a circuit or too many devices attached to a circuit, just like in your house. If the rated amperage total of all the devices falls within the protection range of the circuit plus a safety margin you should be good to go. If not, something is drawing more than it should.

 

[Magnus]

According to my '67 shop manual, which ought to be identical to the '68, that blown fuse is supposed to be a 14A that runs:
Back-up lights
Radio
PRNDL light
Turn signal

Maybe one of those has a bare wire and is shorting out. If you don't have a radio, maybe that wire's hanging there somewhere and occasionally grounds out. Since the fuel pump fuse doesn't blow, maybe it's one of the other things causing your issues.

 

[SchnorrCS]

It's blowing because of the draw on the entire circuit not just the fuse. The fuse is only there for protection to prevent melted wiring and fires.
You really do need a relay. It's hard to explain in writing, but I have seen some schematics on this site that explain it well.
I would take power for the relay off the starter solenoid, and trigger it off the key on post on the solenoid itself. Run a dedicated relay for the fuel pump only.
Generally when aftermarket companies are writing their instruction sheets, they are for newer cars, like Dow body mustangs. Newer cars have many more electrical circuits in them from the factory, so it's easier to just tap into and existing circuit for power for things like fuel pumps. Cars as old as our mustangs were pretty close to tapped out for electrical power when stock.
I put a new harness from painless in my car. It was a total pita, but well worth it. Much more modern, and it will be easy to get power for stuff like, oh.... Electric fuel pumps when I go to EFI.
If you don't wanna redo the entire harness, do a relay. It'll work much much better than the setup you have now. Trust me.

 

[Paul1958]

Here is a 'typical' schematic for a electric fuel pump relay. You can disregard the wire colors shown. Where it shows the 'blade' fuse you should use that 7.5 amp inline fuse that came with the pump. Where it says 'red fuel pump power wire' use the wire from the fuse box that you previously used to run to the pump. Go to any decent auto parts store and ask for a 20 (or 25 or 30) amp power relay and socket. All quality relays use the same terminal identifying numbers (ie 30, 85, 86, 87, 87a). You can mount the relay anywhere but if you can minimize the length of the wires from the battery to the relay and from the relay to the pump that is best.

 

[BADCRUZ68]

Thanks for the suggestions everyone, I am alot more upbeat about getting this on the road next weekend

 

[tazzilla]

Here is a simple fix/test either way it will do the job. I would use a connection straight to the battery positive terminal with of course a inline fuse run that to a toggle switch so you can power the pump manually through the switch and bypass the fuel block all together. If the fuse block fuse keeps blowing then you have a short to ground somewhere if fuse does not blow then it was related to the fuel pump. Now it has been connected to the battery directly if that inline fuse blows then you know the fuel pump has some high resistance internally causing the issue, fix then is a new fuel pump. If nothing blows then you know that you were running too much off the circuit causing the issue. hope this helps. I am a retired ford master electrical tech with over 20 years experience trouble shooting problems just like yours. Taz

Note relay diagram shown above is a much better permanent option this is more of a fix/test option to diagnose what is the real issue. I would do this before wiring in a relay only to find more is going on.

 

[fstbk1968]

I had the same problem with my electric fuel pump. It was originally wired into the factory fuse block as the power source & it would blow the fuse while driving. Not fun. The others are correct in stating that it is the additional load to that fuse that is the main culprit. I ran a separate power line with it's own dedicated fuse and haven't had a problem with it since.

A relay is a better solution and I'll get around to putting one in someday.

 

[slim]

I had the same problem with my electric fuel pump. It was originally wired into the factory fuse block as the power source & it would blow the fuse while driving. Not fun. The others are correct in stating that it is the additional load to that fuse that is the main culprit. I ran a separate power line with it's own dedicated fuse and haven't had a problem with it since.

A relay is a better solution and I'll get around to putting one in someday.

I agree. I've probably missed something. Have you said what the rating is on the inline fuel pump fuse?

Thinking about our home circuits wiring/breakers. The circuits that are 115V are either 15amp or 20amp. No appliance is stated to draw more than 12amps. The major ones that even draw 12 amps have only a couple lights as added load. When a combination of things occurs a breaker blows. Since your fuel pump is separately fused why not connect it more directly like through a relay?


Slim

 

[BADCRUZ68]

Here is a simple fix/test either way it will do the job. I would use a connection straight to the battery positive terminal with of course a inline fuse run that to a toggle switch so you can power the pump manually through the switch and bypass the fuel block all together. If the fuse block fuse keeps blowing then you have a short to ground somewhere if fuse does not blow then it was related to the fuel pump. Now it has been connected to the battery directly if that inline fuse blows then you know the fuel pump has some high resistance internally causing the issue, fix then is a new fuel pump. If nothing blows then you know that you were running too much off the circuit causing the issue. hope this helps. I am a retired ford master electrical tech with over 20 years experience trouble shooting problems just like yours. Taz

Note relay diagram shown above is a much better permanent option this is more of a fix/test option to diagnose what is the real issue. I would do this before wiring in a relay only to find more is going on.

Thanks Taz, that is exactly my plan. I will hook up a relay power sourced from the battery or the pos side of the solenoid using my current switched power source as the trigger. I currently have a 7.5 amp inline fuse in place, that came with the Holley blue and that never blows.

 

[BADCRUZ68]

Well its been a solid month since my last post on this so you can see I have not been too motivated this summer for car work. I finally installed a relay. I went to Advanced Auto for the relay. Pretty simple setup, the relay had a connection for the switched power , the power source directly from the battery, a ground and then 2 ouput connections, one of which I used for the fuel pump. Again it ran fine in the garage and general slow driving up and down the driveway but as soon as I hit the streets and gave it some throttle she blew a fuse again. This time 2 of them (the top and bottom one in the initial post) No clue whats going on now but I think I will use a toggle switch and avoid the fuse box altogether at least for now to get some drive time before winter.

 

[Explorer]

When you get around to wiring it correctly, seriously consider installing a low oil pressure cutoff switch for safety. While running off a toggle switch is fine temporarily, it gets old fast. Can't remember how many times I've either forgotten to turn it on, or forgotten to turn it off on my 4X4 trail rig.

 

[tazzilla]

Run a separate circuit all together for just the fuel pump off a toggle switch and make sure to install a inline fuse. If it blows again the fuel pump has excessive resistance and that is why you keep blowing fuses. If it does not blow at this time you have something else going on with your original circuit. Good luck please let us know how it turns out ok Taz