THE INTERNATIONAL JAGUAR 'S'-TYPE REGISTER FORUM

See attached Lucas Service Bulletin
Cheers
David

Thank you gentlemen ! I think I will just convert to negative earth and use an Alternator...



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converse a

According to the wiring diagram there is a Brown/Yellow wire that went from the small terminal on the old dynamo to the old terminal D on the voltage regulator. D is directly contacted to W/LT on the regulator and this Brown/Yellow wire goes direct to the warning light. This is the wire W/LT that you disconnect from the voltage regulator and connect to the small terminal on the new Dynalite.
From the other side of the ignition warning light there is a White wire that runs via the Hand brake warning light to fuse 3.
If your warning light is not coming on you can do a couple of things.
1st check with the ignition on but engine off that there is a current running from the White wire going from fuse 3 to the ignition warning light. What I think happens is the White wire is power and the Small terminal on the Dynalite acts as an earth which is switched off when the alternator is generating power. If you ground out the Brown Yellow wire the ignition light should come on.
If the Brown Yellow wire is ground out but the ignition light does not come on, Check fuse 3. Check continuity between the fuse 3 and the ignition light. Check continuity between the ignition light Brown/Yellow wire and the small terminal end of that wire.
And as CSSType has suggested check the bulb in the ignition light.
Hope this makes sense.

CSSType wrote: ↑Mon Feb 22, 2021 12:02 am Connected to ignition light directly without the relay - nothing. It doesn’t light up. Swapped terminals on relay - nothing. So I don’t know. It’s frustrating.

Are you sure the IGN bulb is not blown?

See previous post for ammeter.

The 420 had an ammeter of higher rating. A correct S Type ammeter with standard dynamo should have a small 30 under the centre marking i.e. rated for 30 amps max. Likewise RB340 Regulators came in different ratings (originally marked on the face of the case with yellow crayon to differentiate them.

If you had enough modern accessories on the car you could indeed fry a 30 amp ammeter.

John, the reason why Ammeters disappeared once manufactures started to fit Alternators is that Alternators produce much more current than a Dynamo. To retain an ammeter would have required wiring of greater current capacity, I.e. much more expensive! All the current used by a car ( other than the starter motor) flows through the Ammeter.
I've known of several cars "upgraded" with an Alternator to burn out (literally melt) the innards of the original Ammeter, something to watch out for.

This probably is unrelated but I seem to recall that when Jaguar went to alternators such as the Series I XJ6, they also shifted from ammeters to volt meters. What did the 420 have? They must have felt that with an alternator a voltage reading was more useful.

Connected to ignition light directly without the relay - nothing. It doesn’t light up. Swapped terminals on relay - nothing. So I don’t know. It’s frustrating.

Yes ~ I think I need to measure exactly what is going on before I one day get stuck with a flat battery & not trust too much in the IGN light going out even at idle. I do appreciate the alternator difference.