To Fuse or not to fuse - PI Bosch Pump relay

[MilesA]

Just doing some more background reading for the addition of various relays. 

In relation to the pump, the original un-relayed supply came from the ignition and was unfused. This is how it appears in my early black and white wiring diagram and this is what appears to be the case in practice on my car. The pump supply comes off the same side of the fuse box as the supply in from the ignition; it does not come from the fused supply from the other side of the fuse box. Or have I got that wrong?

Having read all the past posts on this, the common view is that the supply to the pump via a relay should be from a fused non-ignition supply (the original supply being used to power the relay coil). This makes sense but assuming I have understood the original wiring concept correctly, why the later addition of a fuse when using a non-ignition supply supply via a relay? Or is this similar to the debate about whether one should retain the original unfused supply for headlamps (not a debate I intend to reactivate). 

Partly curiosity (given that Triumph did not appear to think a fused supply was appropriate) and partly practical as I propose adding an additional fuse box or a single larger fuse box (given the additional electrical components) so need to work out where best to take the pump supply from. The safest approach is what I am after.

Miles

[RobH]

Yes same debate Miles. The original thinking was probably that if a fuse blows the car stops, but of course the fuse should only blow if there is a fault  - in which case the car will probably stop anyway and without a fuse there may be the added excitement of flames. 

All modern cars have everything fused, so the makers' thinking has changed.......  

 

 

Edited March 29, 2023 by RobH

[Mk2 Chopper]

Do you have an ammeter or volt meter gauge? An ammeter will show the draw of the pump if you connect straight off the battery rather than return to the middle as normal. If you have this set up, you can take your supply from the alternator instead and then the ammeter will operate as it used to. Adding a fuse, well why not? 

Gareth

[Paul Hill]

My Bosch pump is wired from the battery with an inline fuse into the relay then down to the pump. Carl Fitchett (TR Trader) said it’s the best way. 

[RobH]

8 minutes ago, Paul Hill said:

My Bosch pump is wired from the battery with an inline fuse into the relay then down to the pump. Carl Fitchett (TR Trader) said it’s the best way. 

It is OK if your car doesn't have an ammeter, because the battery and alternator output are directly connected together.  If it does have an ammeter then connecting like that will result in a constant 'charge' indication as the alternator powers the pump.  With an ammeter, the right place to connect is at the alternator output as Gareth says. 

While the engine is running, the current for the pump comes from the alternator, not from the battery. 

 

 

Edited March 29, 2023 by RobH

[Nigel Triumph]

I've fitted a fuse in the power supply to the relay for my Bosch pump. If that circuit fails, the engine will stop. I would prefer it simply stops, without an electrical fire from overloaded wiring.

Nigel

[Mike C]

2 hours ago, Paul Hill said:

My Bosch pump is wired from the battery with an inline fuse into the relay then down to the pump. Carl Fitchett (TR Trader) said it’s the best way. 

+1. With a large wire and generously sized  fuse. Happy to take a large load off the dashboard wiring and feed the pump on a reliable 12V supply

I'm used to what the ammeter readings should be during start/run/lights on operation with this set up.

[KiwiTR6]

3 hours ago, Nigel Triumph said:

I've fitted a fuse in the power supply to the relay for my Bosch pump. If that circuit fails, the engine will stop. I would prefer it simply stops, without an electrical fire from overloaded wiring.

Nigel

+1.  In fact, I've fused pretty much everything that wasn't fused on the original car.  The last thing I want is a burnt out shell!

[MilesA]

Thanks all. Fused supply it is.

Presumably, the same reasoning apples to the coil and distributor supply which was also originally (and remains on my car) fed by the unfused ignition supply like the pump.

If I adopt a second fuse box, I was proposing to replicate the original supply lines and (a) take the non-ignition supply for a second fuse box from one of the 2 spare connections on the 4 way block and (b) the ignition supply from a spur off the supply to the existing fuse box. Most of the load on the additional ignition supply will be for coil side of the new relays.  I have an ammeter. 

Miles

Edited March 30, 2023 by MilesA

[astontr6]

12 hours ago, Paul Hill said:

My Bosch pump is wired from the battery with an inline fuse into the relay then down to the pump. Carl Fitchett (TR Trader) said it’s the best way. 

I just use a combined fused relay' saves all the sodding about with a inline fuse and adapting the wiring. Very easy to get to as it is on the inside wing.

Bruce. 

[John Bracher]

If reworking a pump or wiring etc, why would you NOT fit a fuse, either as an inline one or with the relay......??

I think an inline fuse holder is about £1 odd??

[Andy Moltu]

I put a fused relay I'm mine.

If there's a fault that would blow a fuse, there's a problem. Problems that blow fuses likely result in a puff of Lucas' finest smoke or worse.

 

[ed_h]

This is a similar philosophical question to whether to fuse headlight circuits.   It's been done both ways, both based on reason.

Ed

[MilesA]

Thanks for the feedback all as ever. Not concerned about cost, just want to get it right (and possibly understand why!). New plan evolving!

Miles

[Tom B]

Don't forget the inertia switch - probably more important than the fuse. This is my implementation:

 

 

The early TR6's had a connector block on the bulkhead which is useful. 

Regards

Tom

 

[MilesA]

Thanks Tom

Absolutely agree and already have one in my spares box to complement the firewall I installed recently. 

Miles