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Wiring Question.


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#1 Ann-Wilson

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Posted 09 November 2019 - 10:02 PM

Wondering if someone can help ..

Situation is I have an mpi tank with a replacement fuel pump that seems to have broken..

Symptoms are inconsistent fuel pressure when engine warmed up...the system feeds an SC 5 port modification...so I need approx 45 psi.. anyway to get to the point..the car was originally a carb version so I needed to plump in wiring for a relay to operate the pump...including an online fuse and fuel cut out switch for safety. I have been told that the fuel pump may have failed as the cables in the wiring to the relay might have been too thin...I suppose meaning that the pump draws more current? Could this be the case? If so what wire gauge should I use? Is 20 amp a reasonable rating for the inline fuse? Any help appreciated.

#2 Ethel

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Posted 10 November 2019 - 12:01 AM

Assuming it's a blade type fuse it should be rated at more than the current drawn by the pump but less than the current carrying capacity of the wiring, typically wiring is rated at double the continuous current rating of the fuse.

 

chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/https://www.uscars.biz/uscars/mini-documents/Rave_CD_cdxn990e.pdf

 

It's on page 16  :-)



#3 Ann-Wilson

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Posted 24 November 2019 - 09:07 AM

Actually it's a glass fuss holder... Does this make a difference?

#4 Ethel

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Posted 24 November 2019 - 11:31 AM

Glass fuses show the amps that they blow at, blades the current they're intended to carry in normal use. In reality it's complicated by how rapidly the current increases.

 

For practical purposes just double the rating of a blade fuse to get the glass (Lucar) equivalent. 



#5 Ann-Wilson

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Posted 29 November 2019 - 10:06 AM

Thanks for this, helps alot:)

#6 Ethel

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Posted 29 November 2019 - 04:05 PM

You're welcome.

 

I'd best add the really important bit!

 

The problem is when halving a Lucar rating doesn't match an available blade e.g. there's no 17.5A blade to replace a 35A glass fuse. If you try and fudge it the next nearest, you could end up with fuses that blow when it's least convenient - or worse, one's that slowly melt instead allowing your loom to go up in smoke 'n all. The only solution is to swot up and do a full rewire, likely separating out, the circuits involved starting from +ve taps off the solenoid

 

.....or just stick with the original 35A fuse.






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