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Lucas fuel pump on my 72 CP


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On 5/23/2022 at 3:24 PM, Mk2 Chopper said:

If you are siting the new relay in the boot, you may as well use the original feed wire and earth to switch the relay. Then add a new earth wire for the pump along with your new pump feed wire. I chose to earth the pump in the boot, I don't see any advantage to running one back to the battery. 

Gareth

Then this negates running a new thicker wire from battery to pump, just from the relay to the pump? If so, wonder why Steve Denton didn’t do that?

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15 minutes ago, johnwill said:

Then this negates running a new thicker wire from battery to pump, just from the relay to the pump?

No, you misunderstand.

The original wire is just used to power the coil of the relay since you don't then need to change the connection from the ignition switch.  You still need the new thick wire for the pump supply, to the relay contacts and then on to the pump. The whole reason for using the relay is to allow a heavier higher current cable to be used and to relieve the ignition switch of having to carry that current.

 

 

Edited by RobH
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24 minutes ago, johnwill said:

Then this negates running a new thicker wire from battery to pump, just from the relay to the pump? If so, wonder why Steve Denton didn’t do that?

There are four connections on the relay, two are the low power to switch the relay (via your original wires) and the other two are the higher power that gets switched on and passes that power to the pump (from your new thicker wire from the battery, through the relay and onto the pump). 

Hope that makes sense. 

Gareth

Edited by Mk2 Chopper
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21 hours ago, RobH said:

No, you misunderstand.

The original wire is just used to power the coil of the relay since you don't then need to change the connection from the ignition switch.  You still need the new thick wire for the pump supply, to the relay contacts and then on to the pump. The whole reason for using the relay is to allow a heavier higher current cable to be used and to relieve the ignition switch of having to carry that current.

 

 

Thanks Rob & Gareth

another question. Best route to run new positive cable from front to back, and what’s involved? Need to remove anything ?

I assume LH side of car

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5 minutes ago, johnwill said:

Thanks Rob & Gareth

another question. Best route to run new positive cable from front to back, and what’s involved? Need to remove anything ?

I assume LH side of car

The main loom enters through a grommet near the fuse box area into the passenger area, you'll need to peel back some carpet and the trim panel over the rear wheel arch to get the wire into the boot. 

Gareth

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  • 2 weeks later...
On 5/16/2022 at 6:51 PM, stuart said:

Dont forget the collision switch though.

Stuart

Stuart

 

I will be following the advice from you and others, and wiring as per Gareth’s suggestion. 
 

eg new wire from battery to spare fuse in fuse box, then from there to collision switch, and from that to pump. I will use existing wiring to power relay. 
 

a question

my 73 CR has white and green/white on the collision switch. The diagram I’ve seen shows white from the ignition switch, green/white to pump. 
 

does that seem correct? Is it crucial which way round the wires are attached to the switch? Are there markings on it that I may have missed?

thanks

John

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2 hours ago, johnwill said:

Stuart

 

I will be following the advice from you and others, and wiring as per Gareth’s suggestion. 
 

eg new wire from battery to spare fuse in fuse box, then from there to collision switch, and from that to pump. I will use existing wiring to power relay. 
 

a question

my 73 CR has white and green/white on the collision switch. The diagram I’ve seen shows white from the ignition switch, green/white to pump. 
 

does that seem correct? Is it crucial which way round the wires are attached to the switch? Are there markings on it that I may have missed?

thanks

John

Its just a simple through pass switch so it doesnt really matter which way round its connected.

Stuart.

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Going back to this one again. 

Is there any reason why you can't use the junction block next to the fuse box as the new feed for the fuel pump and then pass it through the spare fuse?

IMG_4907.thumb.jpg.5e3da0539b743a0cd81789599a77c592.jpgIt's no. 22 on the wiring diagram and takes a feed off the starter to feed number 2 fuse as well as the alternator. Uses big 9.5mm spade connectors. 

See attached picture. 

Seems a better idea than a messy wire off the battery. I'm intending to use the original wires to power a new is the boot so no need to pass the wire through the crash switch.  

Any thoughts ? 

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You can do that but as discussed earlier in the thread, if your car has an ammeter the pump current will register as a 'charge' all the time because the junction box is on the battery side of the ammeter.

 

 

 

 

Edited by RobH
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