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TR6 PI fuel pump/supply voltage


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I have seen a number of posts and comments over the years regarding the Lucas fuel pump and PRV performance and thought I would share what I have done. I used to get problems with a "hot" pump that would be unreliable after a run and then on stopping the car after a run the car would not start due to fuel supply. I used to clear the issue by loosening the union to the metering unit, run the pump, re-tighten and then proceed. This may have removed an air lock. This was a number of years ago and what I did was to purchase second hand pumps until the issue went away. On the third pump, with a cooling coil, I suspected that it may not be the pump (Doh!). Checking the pressure at the metering unit it was reading approximately 140-150 PSI. I took apart and cleaned the PRV and pressure reduced to approximately 110 PSI. The main problem went away although I was never happy heating up petrol with the cooling coil. So I looked harder and asked for more advice.

Where I am now. I had one of the old pumps and PRVs refurbished by a contact on this site although lost the contact number/name (please contact me again as I have a pump, PRV and metering unit that I would like refurbished). This person also had views on providing the battery voltage to the pump and I believe this to be sound advice.

Investigating further there are potential voltage drops across switches connectors etc. There is definitely a voltage drop across the ammeter (approx 1/2 volt) because this is how it works.

So my aim was to supply full battery voltage to pump and not make any changes to the original wiring loom. I am not an electrical motor specialist but understand some possible heating issues caused by over current (I). e.g I squared multiplied by the resistance (R) heating effects. Overhead power lines have a very high voltage to inversely reduce the I (I squared R losses).

So I have used a single pole relay (12V DC 40A rated) to activate the pump. This takes a fused direct feed from the battery +ve to the relay from the battery. Two other connections from the battery -ve to the relay coil and back to the pump in the boot.  The other side of the relay (when energised) is the direct +ve feed back to the pump. To energise the coil I have taken a "piggy backed" connection from the fuse box, that has an inline fuse, to the relay coil. Relay coil requires 100 mA so fitted a 2A fuse because of the physical size and rating of available fuses as these were the constraining factors.

In summary there is a direct -ve feed to the pump and a feed from the battery via a relay for the +ve feed to the pump.

The feed from the battery to supply the pump via the relay I have gone for a 10A fuse that is close to the requirements for the pump and if this proves an issue (no problems yet) I will go to a 15A fuse.

For the cable I have gone for the maximum  number of strands versus size at 44/0.3 to increase the current carrying capabilities.

New wires hand wrapped with self amalgamating tape as per original loom.1069455766_batteryconnection.thumb.jpg.03e8a6b0b8a726b2c1517a16d51604d9.jpg876182411_relay2.thumb.jpg.d9f634038dae37c1eb2d0fab97dbdae1.jpg

I have included a list of parts that might not make sense regarding quantities as I bought extras e.g. spare relay, fuse holders and fuses.

Very happy because because no issues with running, no hot petrol, can be reverted with no impact on originality of the loom and car runs so much better!

Any comments welcome.

.109919732_pumpwiring.thumb.jpg.e7d5b51881a5f622a921dbef8486ab7b.jpg

 

relay connections.jpg

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I used the trgb kit which sounds similar except it hasn’t got the direct earth to battery.  My relay is in the passenger footwell next to the indicator relay.   

 

You are right that the pump definitely likes as high a voltage as possible.  Ive been running it this way for 3 years now and all good.

 

Well done for sorting, give yourself a pat on the back.

 

cheers

 

dave

 

 

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Thanks for the feedback. Getting the relay close (electrically speaking) to the battery will remove small voltage drops although considering the distances this is probably academic as the the volt drop across the ammeter is possibly the largest drop. As for taking an earth in the boot; when I originally restored my car the earth wires from front to rear (black insulation) had had a "burn up" up at some point. These were replaced with a similar diameter although more strands (> I capability) and hand wrapped again with tape. Maybe academic again although my thinking is to keep the highest conductivity wire all the way through.

I did not know there were kits for this.

Just a thought and just make sure it is all well fused!

Regards Jonathan.

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In addition the the relay,mwhich in my case is in the boot, i ran a large earth cable to the boot area.

this terminated in an 8 way marine bus bar, which allows fresh earths for the rear lights whch they needed !

steve

 

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28 minutes ago, stuart said:

Hopefully you have made sure the inertia cut-out is still somewhere in circuit?

Stuart

Yes, Stuart is right, as always !

i think the best way is to use the original feed from the inertia cut-out to energise the relay that then feeds ‘clean’ power to the pump

in this way the cut-out still cuts the pump in the event of an accident, but it does not carry power to the pump it just energises the relay.

steve

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Hi Jonathon

Looks like you have done a good job there.  I had a kit from TRGB, but it didn't come with a new wire from the relay to the pump which was a bit odd. Only thing I would suggest looking into is your fuse rating.  Most of the modern fuses are continious rated and not Blow rated. This would mean if yours is continious rated at 10 amp it will only blow at 20 amp. In that case I wouldn't go up to 15 amp probably a bit high for a pump.

Roger

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On 12/24/2018 at 6:16 AM, Steves_TR6 said:

Yes, Stuart is right, as always !

i think the best way is to use the original feed from the inertia cut-out to energise the relay that then feeds ‘clean’ power to the pump

in this way the cut-out still cuts the pump in the event of an accident, but it does not carry power to the pump it just energises the relay.

steve

This makes the most sense to me too.  And takes the load and voltage drop away from what is doubtless an aging and infirm inertia switch.

I've also heard pro/con debates about boot-bomb setups with relays situated there vs hood vs passenger cabin.  IIRC modern relays are sealed so it a moot point from an ignition POV & that provides the easiest point to energise the coil from the pump feed without hacking into the loom.   I suspect my original loom and switches/contacts provide more sparking options in the boot than a modern relay does.

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Hi all,

No inertia cut out on my model of TR6 (UK car first registered May 1972) although this could be put in series with the +ve supply voltage or relay coil. Relay is in the engine bay away from pump so think I am okay with that. New connections to the pump are all soldered and plenty of heat shrink sleeve. As somebody said modern relays are encapsulated so sparking "should" not be an issue.

Understand that fuse blow current is *2 rating although cabling used for supply is rated at 22A (many strands so probably higher than specified) so hopefully should be okay. Slow blow or quick blow (higher rating) fuses would be nice although could not find those to suit an inline fuse.

Thanks for all your feedback.

Kind regards Jonathan.

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This is what I did when I rewired the fuel pump.

Relay x2 (1 spare just incase) deffo keep the relay out of the boot, even if the relay is sealed there is still a chance it can be an ignition source as they will not be ATEX approved, ie for use in a potential explosive atmosphere.

ran all new wiring to the pump from the engine bay soldered all connections then shrink wrapped them as a matter of course done over 10000 miles with this set up no issues thus far

 

 

 

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  • 2 weeks later...
  • 1 year later...

Hi All

Reading around on this subject as i've had some starting issues which were helped by using a dedicated 12v Batt in the boot for some initial starts. Seems once started and been running for a few mins it will start ok. But if left for a while, weeks, it struggles. So I’m going to go the dedicated 12v feed to the pump (Lucas on mine).

I absolutely get the theory behind the wiring but not being an auto-electrician I’d rather follow a wiring diag than wing it.

There was a post which referred to one but the link seemed to link back to the same post so I’m a little lost on that and was hoping someone had a diagram.

Q2 is what spec Relay do i need. Again fine with the theory of the Ign wiring telling the relay to connect the direct feed but all relays are not the same and i have no idea what spec I might need.

Cheers   Keith

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Hi Keith, a standard 30A rated relais will be fine. They are available with an integrated fuse too, that’s what I would do. If you do not have it, consider installing an impact switch as well.

Cheers,

Waldi

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You don't need anything fancy Keith, it just needs to be a 'single pole single throw' (SPST) relay - also sometimes called 'make/break'- with adequate contact rating for the pump current.  It's probably worth buying one from a reputable supplier rather than going for the cheapest from e-bay.

You can get relays which incorporate a fuse, which might be useful and save needing a separate fuseholder.  Examples are these:

https://www.autoelectricsupplies.co.uk/product/170/category/36

https://www.autoelectricsupplies.co.uk/product/169/category/36

 

edit -  I see Waldi beat me to it so that is a 'plus 1'.

Edited by RobH
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Any chance of a wiring diagram? Since fitting a new pump to mine it will not start straight away if left overnight, it was ok before the new pump. Once started it is ok. The company who sold me the pump said check starting voltage at the pump but not sure how to do this.

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8 hours ago, Phil Read said:

Any chance of a wiring diagram? Since fitting a new pump to mine it will not start straight away if left overnight, it was ok before the new pump. Once started it is ok. The company who sold me the pump said check starting voltage at the pump but not sure how to do this.

From Dan Masters:

https://www.google.com/url?client=internal-element-cse&cx=partner-pub-8168402389087911:7413808915&q=http://www.britishv8.org/Articles/FuelPumpInstallation.htm&sa=U&ved=2ahUKEwiS86u6wJHqAhW34jgGHTkxC0sQFjAAegQIAxAB&usg=AOvVaw12dqd-MMgiKzvGeIZk32VO

The cut out and warning buzzer are desirable but non essential options. 

You can check the voltage at the pump with a multimeter connected directly  onto its terminals. If you don't have an assistant you will need a multimeter extension lead that lets you see the multimeter from the cockpit ( I've made these up from 2 core lighting wire ) or a multimeter with clamp leads and a hold function.

 

Edited by Mike C
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Good conversation on the approaches to get the right electrical supply to those fuel pumps.  Just an fyi - Like many of you I fitted a direct, fused, feed to a relay in the boot and then used the original circuit to activate the relay.  The solution worked well.  

I also found an inexpensive display that allowed me to see how voltage and current fluctuates.   So when starting the motor I see a battery supplying 12+v to the relay and pump, and then once started I see 13+v and ~5amp.  An inexpensive add that i fitted primarily to monitor the current the pump was drawing on the basis that more current draw = more heat = more likelihood of problems.

One observation, I was surprised to climb into the car one morning only to find there was no pump whine.  I discovered the relay had failed, I'd assume relays were pretty indestructible but apparent not.

Jim

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Waldi, the meter model is PZEM-31.  Manufactures somewhere in china and sold through a number of online retailers.  I bought mine through Amazon and it was described as a...     DC 6.5-100V 0-20A LCD Display Digital Ammeter Voltmeter Multimeter Current Gauge with Built-in Shunt.    It sells for about 14 Dollars, I probably bought it for about 10 bucks eighteen months ago.  The built in shunt was an important factor in my purchase, lots of them come with an external shunt.  

A google search on "pzem-031 meter" will, I suspect, throw up a number of European retailers to pick from.

Hope that helps, Jim

 

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