Any problem powering Fuel Pump relay from Main relay output?

[twincam]

I'd like to limit the load on my ignition switch (British car syndrome). All the schematics I look at show the Main and FP relay coils supplied from the ignition switch. Would there be any problem driving only one of the relay coils from the ignition switch (we'll make that the Main relay) and feed the second relay's coil (we'll make that the FP relay) from the power contacts of the 1st relay. This would halve the relay load on the ignition switch (150ma less current). I realize that's not very much at all but, Lucas electrics.

[DaveEFI]

It's by far the best way to run in a new feed direct from the battery for the injection. No harm in doing a second one for the ignition too - as modern systems can take more current than the original. And then control the main relay from the ignition switch. It's a good practice to fit a fuseable link on the new feed(s) close to the battery.

[dontz125]

Dave - I think OP is asking about just the switching coil. The feed to the contacts is a different matter.

Twincam - not sure when the relay arrangement changed, but this is what it used to look like. I agree the FPR contacts should be powered from the battery, but I see no issue with the FPR coils being powered by the main relay. Considering the MR is only feeding the ECU under the new wiring diagram, it's pretty darn lightly loaded!

[DaveEFI]

Ah - right. Read it more carefully. Though it must be a joke worrying about a 150mA load on an ignition switch.

[twincam]

Yes, the schematic from dontz125 is the way I do want to wire it.

All the other later schematics I've looked at drive both relay coils direct from the ignition switch. Wonder why the newer schematics changed it. Maybe it made powering the relay coils more understandable to newbies like myself.

In any case, the 1960's Mini ignition switch I'm dealing with thanks you.

G

[DaveEFI]

Having had countless Minis, the ignition switch isn't a known problem area.

[billr]

Yeah, any switch that could handle the 5-10A of a '60s simple ignition circuit should be able to pass current for a few relay coils with no problem.

[dontz125]

I believe the OP's concern is that a 50-yo switch by The Prince of Darkness may NOT handle the relay coil current, let alone the spark coil current ...

[DaveEFI]

Yeah, any switch that could handle the 5-10A of a '60s simple ignition circuit should be able to pass current for a few relay coils with no problem.

The original Mini ignition switch handled a lot more than just the ignition. Everything switched by the ignition - wipers, heater, fuel pump etc. Think it was rated at 30 amps or so. Also it was a generic part used on many other models. So unless simply worn out through 50 years of use, no intrinsic problems.

[twincam]

The original Mini ignition switch handled a lot more than just the ignition. Everything switched by the ignition - wipers, heater, fuel pump etc. Think it was rated at 30 amps or so. Also it was a generic part used on many other models. So unless simply worn out through 50 years of use, no intrinsic problems.

Yes this is an old switch with unknown history. I agree the switches are quite robust but having seen way too many Lucas quick disconnect (spade connectors) go annealed dead soft and loose all their ability to maintain grip and thereby make a very poor electrical connection (root cause I believe of all the Lucas Prince of Darkness jokes), I'd like to minimize the load, especially on difficult to source parts (Classic Minis haven't been sold in US since 1967). Unless there's a specific reason to drive both relay coils direct from the ignition switch, it makes more sense to me to cascade the second coil drive.

G