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FridgeFreezer

Series / Defender Ignition switch 551508 current rating specs

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A slight detour but I thought the part numbers forum might know this or have a dusty old Lucas catalogue kicking around...

While shopping for a new ignition switch, one of the few places that actually lists anything technical for them sees to specify that the IGN terminal is only rated to 5A which seems very low to me, given that on some models the coil & fuel pump & a few other things are all run through the one switch.

Has anyone got a decent technical spec / more detail on the ignition switch?

Other part numbers for the same thing, apparently: 551508, 34680, BR2057G, SPB501, BR2057, BR 2057, 54335169, 54316731, 30608, 35351, WW10145, 127651, 47SA, 31912A, 31912B, 31912

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AES are the guys I was describing, they don't appear to stock the standard Series/Defender petrol ignition switch but this is the closest:

https://www.autoelectricsupplies.co.uk/product/857/category/253

Which shows a 5A rating on IGN, albeit with a 15A rating on AUX, but an odd sequence that means AUX is disabled when cranking so no use for running ignition loads.

The one you posted is for diesels so has an extra sprung stage for the glow-plugs which I don't really want.

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That is what happens with the ignition on my V8, everything else on the truck is switched off while the engine is cranking and the engine fires after releasing the ignition key.

In ignition position it energises a relay which engages the starter solenoid

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And really, just run relays.  All these Lucas switches are carp and they all die.

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Yes but no - I need to run more than 5A of load from the IGN terminal to run the EFI which must be live whilst cranking, whereas running it from the AUX it would never start because it would kill the ECU while cranking (or require more wiring/relays)...

And the reasons for not running it from a relay is to provide some load on the ignition live to prevent the alternator back-feeding and keeping the relay energised when you switch the ignition off... I could re-arrange the system / add a diode but I'd rather not.

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19 hours ago, Red90 said:

You should always have a diode.

No, ideally you shouldn't, even if LR fit them as a bodge.

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It is not a bodge.  Every vehicle in the world uses them for the last 40 years.  It prevents back feed through the exciter circuit.  It is proper electrical design.

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Except if the alternator diodes fail you don't get the warning light lit with the ignition off, you just eventually get an alternator on fire.

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I've seen Discovery with failed alternator diodes  a few times, the red charge light was on.with no keys in ignition, had the fault confirmed by a rac patrol, we both had the same answer new or rebuilt alternator required. 

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14 hours ago, FridgeFreezer said:

Except if the alternator diodes fail you don't get the warning light lit with the ignition off, you just eventually get an alternator on fire.

The light still shows when the engine is running.   I'm not sure what use is a light when nobody is in the car and how that is going to stop the issue.

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Depends which diode(s) fail, it may only light when the igniton's off, and that's a clue.

Bosch don't mention anything about fitting a diode in the lamp circuit in their big book of electrical systems, and they go into enough detail about every other diode and component in the charging system.

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It always lights on a diode failure when running as you get an AC voltage on one phase.

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