fuel pump problem

[CHB]

i have an external fuel pump problem (not sure which car model it came from). it is similar with VL external pump. it keep on melting the fuel pump fuse 5A but never blow it. there is no short circuit from fuel pump relay all the way to fuel pump +ve terminal.
the resistance across the +ve and - terminal is 2.5ohm, i've compare with a known functional of same pump it is 5.0ohm.
it still pumping with good pressure, only problem is it draw too much current and keep on melting the pump fuse.
any one has experience on this weird problem b4?

[Kyosho]

If it's got a measurement of 2.5Ohm, then the resistance is too low.
Likely to be, that the internal winding has short circuited half way through. Hence half the resistance, hence melting the fuse.

Replace it with a new pump.

[CHB]

if internal winding shorted at half way, the pump should be running slow, hence not able to reach the pressure right? it happens that pressure is ok at preset 40psi.

[af300e]

Are you testing the pump impedance with the pump removed? Bear in mind that 2.5 ohm is almost no resistance and 5 ohm is bugger all more. Even a poor connection to your multimeter can cause this sort of variation in the reading.
The correct resistance is probably given as a range rather than a value so your 2.5 ohm reading may not mean the pump is buggered.

Also, I'm quite sure that many of those bosch pumps run 7-8amps (test draw for mercedes benz which use a very similar unit) so if that is the case then it's little wonder that your 5 amp fuse keeps failing. It's difficult not seeing the exact application but it does sound as though your fuse and perhaps circuit cannot support the draw of the pump.

[Cruiser97_80]

If you are melting the fuse and not blowing it check your wiring and connections. A dodgy connection creates heat and also poor wiring and wiring that is too small to carry the load.

[CHB]

yes 2.5ohm when the pump wiring is disconnected.
i've being using this pump all the while since Dec last year and it works ok until last week when i heard the pump hissing. when i open my boot and i found out the fuse holder was nearly melted and the fuse inside was swallen/melted. i always have a practice to use thick wires for all the wiring additional wiring in my KE70. by the way, all my added electrical wiring circuit run on relays, so i can isolate it from my stock KE70 wiring.
i just cannot understand why the bugger fuse does not blow out instead of melting? i just tried to run the pump "manually" the same problem happened. wires get hot but damn fuse was not blown

[oldcorollas]

errr..

V=IR
12=Ix2.5
I = 12/2.5 = 4.8amps (edited )

since when car is running it should be say 14-14.5V
14.5/2.5 = 5.8amps

no wonder the fuse is melting... it is on the limit fo what the fuse can take.

surely a 10 amp fuse would be a better idea?

edit, 10amps for a fuel pump is not unusual. most should be between 5 and 10 or 12 amps...

edit2: if your wires are getting hot with only 5 amps runnign thru them, then they are either of poor quality, or too small...

edit 3... 15-20amp capable wire and a 10 amp fuse to begin with....

[PlacentaJuan]

one the pump is spinning, doesnt back emf increase the resistance?

[oldcorollas]

bosch 044's draw 13 amps
walbro, typically up to 10 amps (might be lucky to only just keep it at 5amps )http://www.autoperformanceengineerin...l/fpspecs.html

still. 5 amp fuse and cabling is too small for a pump...

[Gabe]

one the pump is spinning, doesnt back emf increase the resistance?

You'll only get back emf on AC circuits.
Not DC.

[CHB]

well, during my troubleshooting, i use 20A fuse just to check if it melt and it does!!

[SillyCarS]

You'll only get back emf on AC circuits.
Not DC.

i think he means the back emf that exists within the motor, ie eddies that control its final speed

[af300e]

well, during my troubleshooting, i use 20A fuse just to check if it melt and it does!!

Because the everything else is also likely too small and heats up the fuseholder, wiring etc.

OC is right although his calculator is broken

V=IR, at 12 volts current draw is = 12/2.5 = 4.8 amps

Once it loads up though (and at 14 volts), it's going to draw more current than at free spin so I'd take that into account when you upgrade and use OC's edit number 3, 20amp wiring with 10 amp fuse.

PS. Measure the current with the engine idling and let us know what it is. Just take out the fuse and bridge the terminals with your multimeter set on it's 20A setting, then start the car and read the draw. Do it straight after start up so that the wiring is still cool and the resistance is at it's lowest.

[oldcorollas]

thanks.. calculator is fine.. is the brain and fingers connecting to put things on screen that have a problem


so you are saying the 5amp fuse doesn't melt... and the 20 amp fuse does melt? maybe the 5amp blew very slowly, so it had time to stretch and allow a conductive track of metal still?

if a 20amp fuse blows on the fuel pump circuit, and the pump is only 2.5ohms, then you have a short circuit somewhere.. cos it should only be pulling 5-6amps

[CHB]

all fuses i tried were melted, be it 5A, 20A and even 30A.
i'll try to measure the current when i get home..