Dynamo conversion to positive earth Alternator/Dynamator

[cass3958]

I agree with John that Negative earth should have been the route to take but you will have your own thoughts on this and I respect your decision.

If you go to my post at http://www.jagstyperegister.com/forum_n ... earth#p802 you can see what wires go where. There is no need to go behind the dash as all the wires start at the RB340 control box. My post details and identifies which wires go where, which ones are required and which ones are not. Once the wires have been sorted you can then either remove the RB340 or leave it in place with dummy wires attached but it is no longer required once the alternator is up and running.
Although my post is talking about converting to Negative earth it is still talking about fitting an alternator in place of the generator.

[CSSType]

Thanks guys for comments. I went for positive earth as I am not feeling confident enough to change all polarities throughout the car, but once I understand the system better I may change it down the track. Now I have converted it, the new alternator fits nicely and wiring has been done as per Dynalite instructions. The small terminal from Dynalite is attached to warning(ignition) light and large terminal to B (To battery). Car starts well, runs well and battery is getting 14.3 volts (checked by multimeter at battery terminals) when engine is running. Suddenly there seems to be a smoothness in the ride. Up to this point I am happy.
But.... (and there is always a but !)
Now as you may (or may not) know that with the positive earth Dynalite there is a relay fitted between the Ignition light and Ignition, which I fitted as per instructions. However the ignition light is not coming on, not when key is turned and not when engine is running (and battery getting 14.3 volts). Also the Amp meter showing continuous negative deflection (towards D). When the headlights are turned on it deflects more (while battery still getting 14 volt charge in +ve). I didn't touch the amp meter at all. I presumed that it should show positive (towards C) deflection at some stage?
So is there an issue or is it normal ?
Battery is fully charged.
Thanks

idling

Amp 1.jpg (243.8 KiB) Viewed 1347 times

With headlights on

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[cass3958]

When I fitted mine I did not have a relay between the Dynalite and the ignition light. (maybe a new thing) so I would try removing the relay and just have a straight through wire as per the original. If you go back to my fitting instructions http://www.jagstyperegister.com/forum_n ... earth#p802 there was a wire on the RB340 terminal marked W/LT which runs direct to tthe ignition warning light. All I did was connect this wire direct to the small terminal of the Dynalite and the ignition warning light works as it should.

With the Ammeter "C" stands for charge and "D" stands for discharge so if you have a full healthy battery it should very rarely go in to the "C" range on the dial as the battery is always being topped up on a trickle charge. At the same time when you start to drain the battery by using the lights, radio, heater fans, wipers and driving hard then the Dynalite will be pushing more power to the battery to keep it topped up and I would expect the needle on the dial to dip into the "D" range so consider this normal.

[Glyn Ruck]

Cass ~ out of interest. When my car is not in use e,g. lockdown my battery stays on a maintenance charger. I have never run down my battery.

Question: After the draw of the starter motor on start up does your gauge ever go into the charge zone, even briefly? When we were messing around with semi charged batteries in the build phase it used to display charge but with a new battery it sits dead centre & never seems to display charge ~ only discharge. Charge/IGN light always goes out as it should.

Do you consider this normal?

As I say I've never experienced a low battery. I just find the ammeter behavior strange. Dynamo rebuilt, regulator new. After cranking to start I would expect a brief period displaying charge as my Alfa's did but not the Jag. We did suffer an ammeter failure during build. Overheated & glass went white. Removed it for a replacement & all has been well. (no change in wiring)

For what it's worth my car remains positive earth.

[cass3958]

Glyn I think with the older Dynamo the Ammeter was possibly more important then if you have an alternator fitted. Modern batteries might help as would a new or fully rebuilt dynamo unit as well.

My old dynamo would only really start exceeding 12 volts when the engine was producing 1000 to 1500 RPM. Quiet often when I had the dynamo fitted when the car was on tickover the red light would be on and the ammeter would be showing towards "D" meaning the battery was running all the systems and very little extra voltage was being produced to charge the battery. Once the revs were over 1500 the dynamo would start to produce more than 12 volts possibly up to 15v to run all the systems and recharge the battery so the needle would swing over to "C". The dynamo as I understand it varied its out put depending on the speed of the engine from below 12v on tickover to over 15v at full gallop but the RB340 voltage regulator would only allow a little over 12v in to the system or fuses would start to blow.

The alternator produces a constant 13 to 15v so even on tickover it is producing enough power to run the systems and charge the battery so there is very little movement in the needle on the ammeter unless the auxiliary belt snapped or the battery died on you.

The needle on my ammeter sits as your does smack in the middle and I have often wondered if it was working or not. You could always test it by starting the engine then disconnecting the battery to see if the ammeter moves. Or do it the other way and disconnect the wire on the dynamo then start the car off the battery and see where the needle goes?

[Glyn Ruck]

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.

[CSSType]

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.

[John Quilter]

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.

[Treetrimmer]

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.

[Glyn Ruck]

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.

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