amperage of fuel pump

[digitaldrifter91]

i have two questions. i have a fuel pump that i want to put a cut off switch on. though if im going to do it, i need to know the correct amperage so when i buy the switch i wont be under the amperage of the fuel pump....

i have a 1991 honda accord. i bought a 30 amp switch, and a 20 amp switch...both rated at 12vdc.

now is there a way to test how much amperage is going to the fuel pump?

i am doing this for added security

 

[SteveD2]

I would strongly suggest you use a relay rather than having the full current of the pump go through the switch.

 

[MolaKule]

Put a 10 Watt, 0.01 ohm power resistor in series with the "hot" lead to the pump and measure the voltage "across" the resistor with a meter capable of measuring down to the hundreth of a volt, such a DMM.

I = Voltage measured/resistor value.

Let's say for example your pump takes 10 amps, then the measured voltage across the resistor would be 0.10 volts, or I = 0.10 volts/0.01 ohms = 10 amps.

If the measured voltage across the resistor was 0.20 volts, then the pump has 20 amps flowing through it, etc.

 

[txmatt]

Look at your fuse box or owner's manual and see what amperage fuse is used for the fuel pump.

Sears sells a clamp-on AC/DC ammeter for $50. You can find it here...

http://makeashorterlink.com/?U3BE21407

or if that link gets corrupted it's Sears item #03482062000.

I bought one and it's great (400 amp max). These are great for measuring current without having to interrupt the circuit to put a resistor or meter in series.

Matt

 

[labman]

Keep it simple. Any switch rated for the fuse size or greater should do. If you are wanting theft deterrent, you could even put the switch in the circuit to the coil of the fuel pump relay.

You may not have a seperate fuel pump fuse. On my truck, it runs off the ignition fuse. If so, just make sure the switch is rated what that fuse is. Likely either of the switches you already bought, will work.

[ January 09, 2004, 08:31 PM: Message edited by: labman ]

 

[theguru]

I agree with labman. Use the circuit fuse as a starting point for your switch/ relay (prefered) and go a higher rating if possible. Reading the curent draw can be misleading as the pump will probably draw much more at start-up which will not be shown on a meter as these react too slowly. Also be aware that fuses on a car are slow blow and can take considerably more short time current than the rating before blowing.

On my car for instance most of the fuses are 15amp, but the relays are 30amp.

 

[digitaldrifter91]

ill look for my fuse if i have one, i do know where the relay is...its under the dash...yet id rather not have robbers see that area.

on the relay they have about 9 connectors...none of which i have any clue where they go haha

if nothing, ill go with a high amperage switch...but someone mentioned using a relay. sounds interesting...id like to know how that goes.

i kinda dont like the idea that im going to run a switch from all the way of the back of the car, to the front, and a line going back to the fuel pump....just that what if something goes wrong...
i do have a voltameter tester and i think it does amperages too...but i dont know how to use it...

ill try to use that formula that was given, i believe the voltage is 12.1v with the ACC on.

i want to be safe as possible. dont want to mess anything up at all....

 

[digitaldrifter91]

http://www.pcrealm.net/~howardh/sup...mp-security.htm

http://www.1300cc.com/howto/how2/rewire.htm

those seem to my advantage, any of them correct...i still dont understand some concepts of the relay hehe

 

[dkcase]

quote:

---

Sears sells a clamp-on AC/DC ammeter for $50

---

Be careful. A DC clamp-on ammeter is likely to cost more than this. Check the link more closely. AC -ammeter

 

[digitaldrifter91]

thats neat...now is that how a relay works...im soo lost

 

[digitaldrifter91]

sorrie if im whorin it up...but here is a switch and fuse method

http://www.civicland.com/diy/misc/fuelpupmpcutoffswitch.shtml

 

[theguru]

Frankly, if you are not sure what you are doing, get it done by someone who does!

Apart from a fire threatening life/ limb, I don't think your insurance company will be very impressed with claim due to you workmanship or lack thereof.

Sometimes it is quicker/ easier/ safer to use someone who knows what they are doing. Even if you had to go to an autoelectrician I doubt the cost would be too much. If you where getting an alarm/ immobilizer fitted they would as standard isolate the fuel pump.

 

[theguru]

quote:

---

Originally posted by digitaldrifter91:
sorrie if im whorin it up...but here is a switch and fuse method

http://www.civicland.com/diy/misc/fuelpupmpcutoffswitch.shtml

---

I have just had a read ove the article above and I am concerned about what this says to do. The author has upped his fuse rating to 25amps as he was blowing fuses. A REAL BIG NO NO!!!!! Fuses are rated according to the load and wire rating. They try to protect the wiring in case of a fault comdition.. If you put a higher rated fuse in then your wiring may then become the fuse!!! Most manufacturers wiring specifications are such that they will just be sufficient for the load. Japanese are probably the worst (or best depending on your point of view) as they allow no excess capacity at all.

Case study. My son recently upgarded his fuel pump on his Nissan 180sx to a higher rated Walpro unit. We were surprised how quiet it was, although they had a reputation for being quieter than standard. I checked the voltage across the pump and noted siginificantly lower voltages than normal. Also on feeling the wiring it was also warm. Obviously I could only feel the exposed parts. Checking back with the supplier and yes it needed upgraded wiring and without it the wiring was bound to burn through. Now, the standard fuse was not changed, nor did it blow!

Why was the voltage down and the wiring warm? Additional current draw over wiring that is offering too much resistance (eg too small a gauge) will cases heating - the greater current over a given rsistance = heat - and voltage drop.

So the solution. We had to rewire from front to back.

Now in the article you have quoted he is obviously drawing extra current as the standard fuse was blowing. A dangerous condition. As a standard fuse can continously take a large margin over the rating, burning of the wiring over time is a real possibility.

Sorry for my rant. Safety first. Car batteries are capable of huge amounts of current and I think sometimes people tend to forget this. They think it is just 12 volts, therefore it is safe.

[ January 09, 2004, 11:09 PM: Message edited by: theguru ]

 

[txmatt]

quote:

---

Originally posted by dkcase:

quote:

---

Sears sells a clamp-on AC/DC ammeter for $50

---

Be careful. A DC clamp-on ammeter is likely to cost more than this. Check the link more closely. AC -ammeter

---

The web page is confusing but you'll note the product description tab says "Accurately measures AC/DC current without opening or otherwise disturbing the circuit."

I have this clamp meter, and the manual lists the specification for DC current (DCA) as 400A with a resolution of 100mA and an accuracy of +/- 3%. Page 8 has the instructions for "DC Current Measurements".

1. Set the function switch to DCA.
2. Press DCA Zero Adj Button.
3. Clamp the Transformer Jaw around ONE of the conductors under test.
4. The display will indicate the current with decimal place and value.

I understand your thoughts, most DC clamp meters cost a minimum of 2x what this one does. But I can confirm this is a DC and AC clamp meter. I've used it to monitor charge current into my deep cycle 12V battery (it roughly matches what the charger says it's sourcing), measure current draw from my trolling motor, measure cuurent draw from a 12V battery while running an inverter, ...

Matt

 

[dkcase]

quote:

---

I can confirm this is a DC and AC clamp meter

---

Thanks for clarifying. That's a good unit then. I can use one. No wonder the link says they are sold out.

 

[digitaldrifter91]

thanks guys...

i think ill do the switch for now with an inline fuse using this formula to fiqure the amperage
I = Voltage measured/resistor value.

http://www.civicland.com/diy/misc/fuelpupmpcutoffswitch.shtml

ill use that, but the reason why he was blowing fuses he said was the switch that was lighted, ill test the current though and monitor it for a while for abnormalities...

ill posts pics monday...then later down the line i will work on a relay next to my existing relay

[ January 11, 2004, 04:07 AM: Message edited by: digitaldrifter91 ]

 

[Gary Allan]

I tried to filter through the entire thread ..but I think that everyone has over complicated this.

All that he has to do is interupt the existing FUSED/RELAYED wire to the pump. He can do this anywhere. He can also use a HEADLIGHT RELAY (or a 100 amp transfer switch for that matter) if he so desires and NOT suffer any loss in safety. He's not changing the "ampacity" of the circuit ONE milliamp. He's only installing a interupt. His only concern is that the relay (the one he installs) can handle the amp draw of the fuel pump.

I would naturally recommend a relay placed local to the line. That way the higher current carrying wire is short. Then he routes a nice safe low current capacity wire to the remote switch.

This shouldn't be too hard to do.

 

[terryr]

Check out The 12Volt.com. They have all sorts of diagrams. Join their forum and ask the pros.

http://www.the12volt.com/

 

[JohnBrowning]

So long as you use a good switch you should not have any problems. You just should not plan on buying it at Radio Shack or Localy. When I did car audio and worked with amature radio operators and illegal CB operators running insane power levels. I often used a switch. The deep cyclce batterys some of these guys used did not do well with standard automotive isolators. Some guys were useing alt./Gen. from diesel electric locomotives. So the soluton was to use high amperage switch's to switch current back to the deep cycle batterys as needed for chargeing!

I can not remember the name of the company I used to buy these from. They were made from copper, stainless steel and backalite.

I would use a switch rated for 3 times the continious draw of your fuel system. This gives you quite a margin of durability. I would also keep a spare switch in your glove box!

[ January 17, 2004, 06:02 PM: Message edited by: JohnBrowning ]

 

[eljefino]

Don't hondas have an access panel under the rear seat where you can get at the fuel pump? Maybe you can get at the wires easily there. I doubt the pump even draws 10 amps so either switch will work fine.