Temporary Fuel pump battery

[elvimto]

Any thoughts on this, when the fuel pump just does not have power enough to start pumping could you use a small 12 volt motor cycle battery connected direct to the pump to get you going something like this.
BATTERY TYPE: YTX5L-BS AGM battery with [- +] terminal is a rechargeable, power sport battery with 12V 4AH, 70 CCA, 48 Watts (H)4. 21" x (L)4. 45" x (W)2. 6"



This would be used just to get the car going when the main battery will not give you the 12 volts at the pump after it has been laid up. I have a Optima dry cell in the car but I have had times where the pump will not kick over. I had to put the car into storage they have no power and cannot put up a solar trickle charger. It would save me pulling the battery out to charge it. Your thoughts

[Phil S]

if the battery is that low it wont power the fuel pump, then it wont start the car either, or am I missing the point, not for me, a jump pack would be safer

[DJT]

If the fuel pump isn’t getting battery voltage, then there is something amiss with the wiring connections. The pump gets its supply from one of the ignition relays (white wire) via the inertia switch, so there aren’t many connections to check. Have you cleaned the contacts inside the inertia switch? 10 minute job. Could the ignition relay be faulty?

451382BA-330C-4F9A-B6AF-9BFF9E2C4AF1.jpeg

[richardthestag]

I am with Dave on this, I would guess that one of the connections inside the inertia switch has furred up and gone all high resistance on you.

Prove it by temporarily bridging the two terminals on the inertia switch together

[elvimto]

The inertia switch is good just been cleaned a few months ago and connections are good the battery is in good shape i.e. it will turn the engine over but sometimes the pump needs a push, When the car is laid up for a bit the small voltage drop will not kick the pump over. The last time this happened I ran a wire straight from the battery to kick it over and get it started once there it ran. Just looking to see if this would be an alternative way to go. The battery is $ 21 bucks but my main question is would it work?? I can make connectors to go from the battery to the pump the battery is small and easy to carry, I also have one of those new style jump packs.

[dasadrew]

I think I'd probably go this route if the wiring is OK, connectors to the fuel pump in the loom are ok and resistance along the line to the pump is ok:

https://www.ebay.co.uk/p/BEK347%2DMGB%2DMGC%2DV8%2D1968%2DON%2DGENUINE%2DSU%2DFUEL%2DPUMP%2DNEGATIVE%2DEARTH%2DAZX1307/1023053929?iid=283370605493&rt=nc

Drew

[StagJonno]

I would counsel strongly against a cludge with an additional battery, especially as the fuel pump circuit is completely unfused to the battery, with subsequent risk of fire. There's a connector in the wiring (under the carpet I think) that could be the culprit with corrosion introducing a high resistance. Also, as Drew is indicating, it could be that your fuel pump is getting lazy - refurb kits are available (or at least used to be) as well as the option for a new replacement.

Jonno

[Kithmo]

Have you measured the voltage at the pump ?

[wilf]

You could see full battery voltage at the pump, but if it isn't drawing any current at the time, you could still have a high resistance in the feed that will drop the volts as soon as it does.

[Phil S]

do what we did on TR5s add a relay in the boot switched from the original wiring, short run no drop

[Kithmo]

You could see full battery voltage at the pump, but if it isn't drawing any current at the time, you could still have a high resistance in the feed that will drop the volts as soon as it does.

Taking the connector off the pump and measuring the voltage, resistance in the wire would show as a loss in voltage. Same principle as the ballast resistor wire which drops the voltage to the ign coil from 12v to 6v.

[wilf]

Err, no. Sorry.

No current flow = no volt drop. V=IR, basic Ohms law.

[Kithmo]

Err, no. Sorry.

No current flow = no volt drop. V=IR, basic Ohms law.

Err, the current is flowing through the voltmeter.

[djtaylor]

Err, the current is flowing through the voltmeter.

Nope, it better not be otherwise it would be an ammeter measuring current and not voltage and even then, anything more than a tiny current will have a shunt in parallel where nearly all the current will flow through.

[Kithmo]

Nope, it better not be otherwise it would be an ammeter measuring current and not voltage and even then, anything more than a tiny current will have a shunt in parallel where nearly all the current will flow through.

Admittedly it is a tiny current, otherwise the voltmeter would not function, but this makes it possible to measure a voltage loss through the wire.
Put a voltmeter across the battery, then across the battery through a resistor, is it the same voltage ? I don't think so.