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Inj Fuel pump relay and inertia switch


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Hi folks, i'm sure this subject has been done many times so please excuse my questions . 

The car (Tr5) is in the air and i've gradually worked through a list of jobs until i'm now actually looking for things to do to keep boredom at bay. 

I thought it would be an idea to fit a larger wire to the fuel pump straight from the battery via a relay and add an inertia switch, just because why wouldn't you ? 

My plan is to fit the relay near to the fuse box/ battery  (i dont like the idea of a permanent live running to the rear) and fit the inertia switch into the line from the fusebox to the relay . The inertia switch would then fit somewhere near the fuse box but i'm a bit concerned that the engine vibration etc may cause unwanted shut off ? 

It may be possible to use the existing fuel pump wires to fit the inertia switch in the boot which is a usual place on modern cars i believe.       

Anyone had experience of this or advice be appreciated .  Thanks ,  Dave.  

 

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You should never wire direct from the battery, always from a fuse. Normally you would have a large fuse from battery to fuse box to cover all possible current from fuse box covering both permanent live and ignition on requirements. On my kit car I have a 70 amp fuse but you would want less for a TR.

You need a fuse on the main fuse box that you can use for the fuel pump main feed. I would allow around 7 amps for the fuel pump so a 10 amp fuse but depends on the fuel pump you have - EFI pumps require more than carb and the 7 amps is agin my kit car EFI pump rating. I suppose if you do not have a spare fuse you could wire direct from the battery to a single fuse holder then on to the relay however I would not normally wire the pump from a permanent 12v source, usually an ignition on one but it perhaps does not matter in this case as you have a relay in the circuit.

You now want your ignition on 12v feed from one of the ignition on fuses to go to your inertia switch then on to the relay to trigger the fuel pump when you turn on the ignition. The inertia switch will then turn off the pump by cutting the 12v to the relay (which turns the pump on / off).

So your circuit is

1.) Battery > Fuse Box > (ignotion on) fuse of 10amps > Relay pin 30 + Relay pin 87 > fuel pum +ve + Fuel pump -ve > earth. This circuit provides your pump with the 12v to power it.

2.) Ignition on fuse (propably you current fuel pump connection) > inertia switch +ve + inertia switch -ve > Relay pin 86 + relay pin 85 > earth. This circuit trigger the pump when you turn on the key.

As for where to put inertia switch, probably does not matter as it works with a bump, not a vibration.

Overall, if your current wiring caters for the current for the fuel pump, why put in a relay? Just wire in the inertia switch in the cable going to the pump.

I suggest you always sketch out a hand drawn circuit so you are clear what you want to do and that there are no errors.

HTH.

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I also have this on my list to do, so a welcome thread, which I hope you don’t mind me joining Dave. My loom is new from AutoSparks with upgraded feed 17amp feed to a Bosch pump conversion which I believe is AMPle. I think that the pumps draws 13amps, is this correct? I also understand that most inertia switches are rated at 10amps? Assuming that this correct I guess a relay is needed.

Again, all info gratefully received!

Tim

... Dave is your pump Bosch or Lucas?

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Hi , thanks for the reply . 

I think that your suggestion pretty much mirrors my plans except i aim to come straight off the battery for the pump feed , (with an inline fuse  )  with a switched (ign) live for the relay feed off the fuse box. 

I believe the issue is that the original wire is only just sufficient and a thicker (3mm ?) wire direct from the battery = less resistance and more amps / volts at the pump which gives a higher fuel pressure , a better 'squirt' and a smoother running engine .  And less load through the ageing ignition . 

I'm guessing an inertia switch isn't really designed to handle the ampage of a high pressure pump so it would need a relay to fit it .   

I do have the upgraded Sytec pump /filter system which i believe draws slightly more amps than a bosch pump ? . 

Definitely agree on the sketch, often shows up a design flaw . 

D .    

 

 

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17 minutes ago, Tim T said:

I also have this on my list to do, so a welcome thread, which I hope you don’t mind me joining Dave. My loom is new from AutoSparks with upgraded feed 17amp feed to a Bosch pump conversion which I believe is AMPle. I think that the pumps draws 13amps, is this correct? I also understand that most inertia switches are rated at 10amps? Assuming that this correct I guess a relay is needed.

Again, all info gratefully received!

Tim

... Dave is your pump Bosch or Lucas?

Hi Tim, 

No problem, we can both learn together !

My pump is the Sytec system , i'm not sure what make the pump is assuming Sytec don't actually manufacture their own . 

Out of interest do you know how thick the pump wire is on your upgraded loom ?   I have some 2.8mm wire to use which looks ok for the job but i suppose should seek out a calculations table for the supply . 

D

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The above is a good plan but Dont Forget the Earth !

TR earthing can often be weak so I ran a heavy duty negative cable from the battery in addition to the relay fed positive, might as well whilst you are there!

i actually added a 0v bus is the bood and used it to earth all the lights etc too.

steve

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6 minutes ago, Steves_TR6 said:

The above is a good plan but Dont Forget the Earth !

TR earthing can often be weak so I ran a heavy duty negative cable from the battery in addition to the relay fed positive, might as well whilst you are there!

i actually added a 0v bus is the bood and used it to earth all the lights etc too.

steve

Hi Steve , 

Thanks for the info' 

I have a 2 core cable so yes i shall return the earth straight back to the battery (or there abouts.)

I just checked the wire capacity and for a 10 amp feed at 3 metres length  it is giving a 3.0% voltage drop .  

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Dave, my feed is 17amp, 2mm, 28 strands, braided,

cheers

Tim

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8 hours ago, Trumpy said:

Hi folks, i'm sure this subject has been done many times so please excuse my questions . 

The car (Tr5) is in the air and i've gradually worked through a list of jobs until i'm now actually looking for things to do to keep boredom at bay. 

I thought it would be an idea to fit a larger wire to the fuel pump straight from the battery via a relay and add an inertia switch, just because why wouldn't you ? 

My plan is to fit the relay near to the fuse box/ battery  (i dont like the idea of a permanent live running to the rear) and fit the inertia switch into the line from the fusebox to the relay . The inertia switch would then fit somewhere near the fuse box but i'm a bit concerned that the engine vibration etc may cause unwanted shut off ? 

It may be possible to use the existing fuel pump wires to fit the inertia switch in the boot which is a usual place on modern cars i believe.       

Anyone had experience of this or advice be appreciated .  Thanks ,  Dave.  

 

Hi Dave, you may find my wiring diagram helpful, not dissimilar. I would fit the inertia switch on the bulkhead as per TR6.TR6 CP KG Wiring Dia.pdf

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3 hours ago, Kevo_6 said:

Hi Dave, you may find my wiring diagram helpful, not dissimilar. I would fit the inertia switch on the bulkhead as per TR6.TR6 CP KG Wiring Dia.pdf

Brilliant thanks . If Tr6 inertia switch is on the bulkhead then obviously ok . And an immobiliser switch in the circuit. Sensible. 

Edited by Trumpy
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Dave, new loom has been fitted and went very well. Which inertia switch did you get and from where? Do you know what current rating it has?
Cheers

Tim 
 

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Ordered a switch off eBay , it seems to be a fairly standard type used for Peugot Landrover etc . I needed one with a plug though so this limited the choice somewhat. 
It didn’t specify an ampage rating but as it will be in the relay circuit I don’t think that matters to much . 
I’ve also ordered an isolator switch ( as Kevos diagram) with a key fob button as some added security against theft . 



 

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I'm not sure I understand all your high amps and earth.

I designed and made my loom for my Dax Rush and will do for my TR6.

First, my EFI pump for a Rover V8 EFI is a Sytech pump and equal rated to a Bosch and is 7.5 amps as I said above so a 10 amp fuse and cable is fine. My EFI pump is pumping continuously as it pumps fuel to the fuel rail and then back to the tank via a pressure regulator to ensure there is a high fuel pressure in my fuel rail. My engine is a 5 litre so could consume a lot of fuel. I doubt your pump pumps so much.

Almost all of today's cable is thin wall and will take more amps than old wire. Your 3mm cable should be fine but check its capacity. Loads on the internet but https://www.rpc.com.au/pdf/Wire_Chart.pdf is one.

17 amps for a pump is huge. Are you really sure?

Earth. All you do is wire to a convenient earth location such as a bolt / nut using a proper ring connector. If there is not one, simply drill a hole in the bodywork and put a bolt - MAKE SURE IT IS NOT A Stainless steel one - in the hole and attach the earth wire to it. Absolutely no need to wire back to the battery. Make sure bolt touches the metal so scrape away some paint.

Why a relay? You only need a relay if you are installing a pump that is much higher rating than what is there and requires a larger cable.

An inertia switch does not take any current. It is just a switch. If it says 10amps on it that means that it can only take a 10amp load through it, it does not consume 10 amps. It consume nothing.

An immobiliser (Thatcham style) cannot take any real current through it so do not just wire into the pump circuit. I this case you do need a relay where the immobiliser switches on the relay.

 

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Hi Richard,

I don’t disagree with what you say . 
I haven’t quoted any hi amp figures, I believe my Sytec pump draws about 8 amps so similar to your figures. 
Tim mentioned 17amps but I think he was referring to the cable rating in his new loom rather than the pump requirements. 
The fact his loom has an uprated  pump feed highlights a problem with the original wire . 
Earth wise Steve advised that Trs tend to have poor earth return through the chassis/ body . The cable I have is twin core so it is no problem to return the earth straight to the battery , otherwise totally agree , any bolt would do . 
A 53year old ignition is likely to have reduced load capacity and by using a relay I have reduced the load by 8 amps.  Maybe not necessary but easy enough to do and this allows me to add the inertia switch and an imobiliser. 
Fully understand that these switches dont require any amount of ampage to work , we are talking about the switching load capacity ( as you say). As they will only power a relay for me this isn’t an issue but I think Tim may be looking to fit an inertia switch directly into the pump feed wire , hence he asked if my switch had a rating . I’m sure it does but it’s not quoted in the advert. 
My pump appears to operate just fine at present and tbf I’m doing this to fill a couple of evenings rather than watch more rubbish tv but it does seem to be a popular upgrade that owners do and can only have a positive result , unless my immobiliser leaves me stuck at the road side ! A bypass wire in the glove box may be a good idea . 
thanks all .  D 

Edited by Trumpy
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Dave/Richard,

To clarify my concern. 
My feed to the pump is 17amp. Revingtons suggest the Bosch pump draws 13 amp so all is fine there? I was hoping to fit an inertia switch in the boot near the pump direct in the live supply. However I think that I had read that most inertia switches are rated at 10amp. Whilst I understand that the switch does not draw any current in its own right it’s components must cope with a current slightly greater than that which the pump draws, hence the need for a relay?
Or am I missing the point?
Many thanks one and all.

Tim

 

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41 minutes ago, Tim T said:

Dave/Richard,

To clarify my concern. 
My feed to the pump is 17amp. Revingtons suggest the Bosch pump draws 13 amp so all is fine there? I was hoping to fit an inertia switch in the boot near the pump direct in the live supply. However I think that I had read that most inertia switches are rated at 10amp. Whilst I understand that the switch does not draw any current in its own right it’s components must cope with a current slightly greater than that which the pump draws, hence the need for a relay?
Or am I missing the point?
Many thanks one and all.

Tim

 

Relay in the boot should work just fine with the inertia switch . (IMO) 
 

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16 minutes ago, Trumpy said:

Relay in the boot should work just fine with the inertia switch . (IMO) 
 

Thats what quite a few of us have done - retain the inertia switch but use it to drive a relay ( at either end of the car )

imho the innertia switch is ESSENTIAL for a fuel system running at 100+ psi , it’s there to cut the fuel pump in the event of a crash

sgeve

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Ok, still surprised at the 13 amps for a pump but sobeit and relay will solve that problem.

I would actually switch off the whole 'ignition on' circuits (or coil and fuel) as that will cut the engine too. The fuel line would still have a reasonable amount of pressure in it to keep the engine running.

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26 minutes ago, Richard Pope said:

Ok, still surprised at the 13 amps for a pump but sobeit and relay will solve that problem.

I would actually switch off the whole 'ignition on' circuits (or coil and fuel) as that will cut the engine too. The fuel line would still have a reasonable amount of pressure in it to keep the engine running.

Not so Richard, the injectors are effectively poppet valves so stay locked shut unless correct pressure is applied.

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All wired up . New cable running down the chassis alongside the fuel feed pipe . 
The pump has already been moved to the rear wheel arch but the wiring was thin .  The original white pump wire in the boot is a good thickness but it was poorly connected and the wires pulled apart v easily. 
I connected the new pump feed to fuse no2 which is a permanent live straight from the battery ( brown wire) and it conveniently had a spare spade connector. 
The switched relay feed is from fuse 1 ( green wire) 
I ran the pump for a couple of minutes through a 10amp fuse with no problem. 
Pump certainly sounds good , I’ll hopefully get a chance to give the car a run out tomorrow and see if there is any improvement. 

4839C512-E7CB-4181-A8AF-FB547229449B.jpeg

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17 hours ago, Trumpy said:

All wired up . New cable running down the chassis alongside the fuel feed pipe . 
The pump has already been moved to the rear wheel arch but the wiring was thin .  The original white pump wire in the boot is a good thickness but it was poorly connected and the wires pulled apart v easily. 
I connected the new pump feed to fuse no2 which is a permanent live straight from the battery ( brown wire) and it conveniently had a spare spade connector. 
The switched relay feed is from fuse 1 ( green wire) 
I ran the pump for a couple of minutes through a 10amp fuse with no problem. 
Pump certainly sounds good , I’ll hopefully get a chance to give the car a run out tomorrow and see if there is any improvement. 

 

Not a good place to run the wire Im afraid, too much possibility of damage from anything going under the car especially if it ruptured the fuel line at the same time, far safer to run it inside along side the main loom.

Stuart.

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6 hours ago, stuart said:

Not a good place to run the wire Im afraid, too much possibility of damage from anything going under the car especially if it ruptured the fuel line at the same time, far safer to run it inside along side the main loom.

Stuart.

+10 just about the worst place to put it 

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23 hours ago, Trumpy said:

All wired up . New cable running down the chassis alongside the fuel feed pipe .

Oops. Having gone thrugh an IVA test (that main and extensive test to register an individual built car) that's an immediate fail. I really suggest you move it well out of the way of fuel.

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It was the quick easy route tbf .  I did consider the risk of electric and petrol next door to each other and ran the wire through a conduit and being in the C section of the chassis is extremely well protected but I take on board the concerns above and will re route the wire for peace of mind . Thanks for the advice. 
I did take the car for a run today with no noticeable difference in performance but at least I now have an inertia switch fitted . 
Dave 

 

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