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Fuel Pump Problem


Best Answer phillrulz , 08 June 2019 - 10:12 PM

Phil you're totally right. Ended up being the fuse corroded. Sanded them and it works.

 

 

No problem, just encase your not electrically minded, what lead me to say high resistance was:

 

When the pump was disconnected there was no current flow so voltage was max as the multimeter will only draw like uA or pA to measure voltage ,

 

When plugged into the pump there is low resistance to ground so all the current the high resistance prior to the pump will allow to flow will flow therefore no voltage drop across pump ( or atleast minimal). 

 

Effectively you have a voltage divider with say 20kOhms on your dirty fuse and 1Ohm on your pump.  

 

Say battery voltage was 12v, your dirty fuse contact was 20k Ohms and your pump is 1 Ohm, as you said, This as a voltage divider you would see 0.001V across the pump. or 1mV.....

 

 

EDIT:

 

Forgot to say, what made me say it was before the pump was if the high resistance was after the pump you would measure battery voltage on both the + and - of the pump with the plug connected as the high resistance after the pump would effectively stop the voltage "draining" away if you look at in a water related analogy. 

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#1 wassupcrew

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Posted 08 June 2019 - 03:41 PM

1996 spi. Just finishing a rebuild and I cannot get the car to start. I have checked the electrics and this is what I have found. Main relay works. Fuel pump relay works starter relay works. When I turn car over I have spark. The fuel pump does not prime. I have checked with a multimeter and have found I have 12.4 volts at the fuel pump plug when the pump is disconnected. When I connect the pump I get no voltage. The pump is brand new because the tank was filthy. I have checked with the old pump and the same no voltage when the pump is connected.has anyone got any ideas what the problem could be. The resistance across the pump connection was about 1ohm if that helps. Battery is brand new and says fully charged when I connect my charger to it but I have left it on for a bit incase it's low

#2 phillrulz

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Posted 08 June 2019 - 06:01 PM

Sounds like you have high resistance , where are you measuring with your negative probe? If its on chassis then it sounds like there is high resistance before the pump. 



#3 wassupcrew

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Posted 08 June 2019 - 07:26 PM

Phil you're totally right. Ended up being the fuse corroded. Sanded them and it works.

#4 phillrulz

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Posted 08 June 2019 - 10:12 PM   Best Answer

Phil you're totally right. Ended up being the fuse corroded. Sanded them and it works.

 

 

No problem, just encase your not electrically minded, what lead me to say high resistance was:

 

When the pump was disconnected there was no current flow so voltage was max as the multimeter will only draw like uA or pA to measure voltage ,

 

When plugged into the pump there is low resistance to ground so all the current the high resistance prior to the pump will allow to flow will flow therefore no voltage drop across pump ( or atleast minimal). 

 

Effectively you have a voltage divider with say 20kOhms on your dirty fuse and 1Ohm on your pump.  

 

Say battery voltage was 12v, your dirty fuse contact was 20k Ohms and your pump is 1 Ohm, as you said, This as a voltage divider you would see 0.001V across the pump. or 1mV.....

 

 

EDIT:

 

Forgot to say, what made me say it was before the pump was if the high resistance was after the pump you would measure battery voltage on both the + and - of the pump with the plug connected as the high resistance after the pump would effectively stop the voltage "draining" away if you look at in a water related analogy. 


Edited by phillrulz, 08 June 2019 - 10:22 PM.





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