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1947Mk6

Auto elec ammeter wiring

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1947Mk6

I'm an old ex-motor mechanic with basic auto elec experience. Have an old Bentley Mark VI (1947) and have fitted a one-wire, neg earth alternator conversion (which retains the external appearance of the original dynamo); the alternator puts out 40 amps. At the same time I replaced virtually all of the under bonnet wiring with new colour-coded, including upgraded alternator cable, but left the original Lucas RB106 regulator in place (for authenticity) and retained the original wires as connected to the regulator's "A1" and "A" terminals as per the original spec. Vehicle has since been run and is performing well electrically.

The original 20/0/20 ammeter is part of a 4-in-1 gauge cluster i.e. inside one round housing, and a similar 40 or 50 amp unit isn't  available to substitute, so a higher capacity ammeter is not an option from the appearance point of view. Hence, I ran the new (fused) alternator lead straight to the battery terminal on the starter solenoid, where it is connected in conjunction with the existing power wire from the car's original  circuitry.

Obviously, though, the ammeter only registers the power consumption, so when fuel pump, ignition, headlights and a couple of accessories are operational, the ammeter needle is hard over on the discharge side.

At last to my question. Apart from adding an external shunt to the ammeter (and attaching the alternator charge cable via that) is there any other way (e.g. different or additional wiring) to register all , or even a good partial amount, of the charging amps on the existing 20/0/20 ammeter?

Any help would be greatly appreciated.

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Geoff1946

Why don't you want to use a shunt?  It seems to me the obvious way and I can't immediately think of an alternative.

Car ammeters I've messed with were very primitive, "moving iron", items comprising just a loop of copper inside.  I would imagine that even a Bentley will be similar. 

 

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1947Mk6

Hi Geoff1946 and thanks for your reply. Re my preference for something besides a shunt, I believe that a shunt has to be fitted externally on the back of the ammeter, so it's totally exposed and live . The dashboard on this car is timber on a steel frame and the wood surrounds the 4-in-1 gauge cluster closely on 3 sides, so I'm  a bit paranoid about an electrical short, especially if some conductive material contacts the exposed shunt and quickly develops into a  fire (there won't be much warning in this case).

A couple of queries, though, if you don't mind. How is the rating and composition of the shunt calculated? Also, do you know if a gauge specialist would be able to fit a shunt that has some sort of insulated shield protection to prevent accidental shorting?

Ron

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Geoff1946

Yes, good points, you don't want to be creating a fire hazard. If your meter is constructed as I described it will be VERY low resistance. It would be great if you had the means to measure it, but I guess that unlikely.

Do you have any electrical measurement instruments, either to determine the meter resistance or to measure current in the ranges we are discussing?  

 

Now a shunt to double the range, i.e. make actual 40 amps read as 20 on the dial, the shunt would be the same resistance as the meter. So, in practice you are probably talking about a length of cable, and a very low power loss/heat generation. In fact the power dissipated between the shunt and meter will be exactly what is being dissipated inside the meter now.

Hence your fire hazard is only from a short circuit, not from normal operation.

If you are concerned to reduce the overall fire risk on your car, consider adding a fusible link by the battery as is modern car practice.

 

How is the meter wired physically? Do both leads go into the engine compartment, or is the output connected to the rest of the dash/car systems?  (I have an outline of an idea in mind dependant on the answer to that)

 

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1947Mk6

Hi Geoff1946 and thanks again for your time.

Both leads from the ammeter eventually end up in the engine compartment - one going directly to the  "A" terminal on the voltage regulator, the other going via the isolating Switchbox in dash to the main fuse panel (also in engine compartment, located immediately underneath the voltage regulator).

The wiring is essentially still as per the manufacturer's original wiring scheme, but with the polarity changed to neg earth. I've tried to attach the manufacturer's wiring diagram to this message for clarity, but it appears I can't do so in this box; please let me know if there is some trick I'm missing. Otherwise, there may be a site administrator's email I could send it to; it's a PDF file (680 KB).

The original charge lead (between the dynamo and the voltage regulator has been disconnected/discarded , as has the field wire between the dynamo and regulator "F" terminal. The "Ind" wire from the new alternator is now connected  (soldered) to the  wire from the dash charge indicator light (prev. connected to the reg "F" terminal) and the indicator light is operating as it should.

As mentioned earlier, the charge wire from the one-wire alternator has been upgraded (8B&S) and goes (via a newly fitted auto-reset fuse) directly to the battery supply terminal on the starter solenoid. 

Kind regards, Ron

 

 

 

 

 

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roys

No attachments until you have 10 posts, it is an anti spam thing.

Unless admin see the posts and allow before hand. I have let admin know, so try again soon.

Edited by roys
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Geoff1946

Firstly, you need to be confident that the meter is as I suggested in the earlier message. i.e. a simple moving iron movement, carrying the full load through the instrument without any shunts or similar.

I suggest that you may be able to shunt the meter  with just a piece of cable.  To double the range, as you would like, this piece of cable needs to be the same resistance as the internals of the meter. 

I assume that you have no access to measuring equipment, so perhaps a bit of trial and error is needed? 

Let's assume the meter has a resistance of 5 milliohms. (That's a guess!) 

Some 2.5mm cross section copper wire has a resistance of about 7.5 milliohms per metre, so a length of about 2/3 of a metre would be equal.  

Connect such a piece of cable directly across the meter terminals and see what effect it has, perhaps using your headlights as a test load. You won't harm anything doing this, whether or not it works. 

If the meter still reads higher than you wish, shorten the piece of cable.  If it reads too low, lengthen it.

 

Now all this will only give you an indication of the load from your accessories.  If you want to show battery charge / discharge, as the original system would,  you will have to route your new alternator output via the meter, BUT beware overloading the wiring, as your alternator can probably supply twice or more the current of the old dynamo. 

My choice would be to short out the old meter, leaving it in situ for appearances, and install a discrete  battery condition indicator, which can be a simple as an led indicator. 

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1947Mk6

Hi, Geoff1946.

I really like both of your suggestions in your latest post, so will be utilising each one. I have a couple of "reasonable" multimeters, so can measure resistance of the ammeter and should be able to play around and make an appropriate shunt using the 2.5mm cross section wire you suggested; even though it's insulated, I'll also enclose it inside a piece of automotive loom as a bit of extra insulation/"insurance".

I was baulking at a bulky battery volts gauge hanging off the underside of the original wooden dash, but an LED battery condition indicator, nicely tucked away, will give me the information I need (I'd only seen the LEDs used as fuel gauges in aftermarket LP gas conversions - didn't even think about them being available as battery meters - I'll have to get out more!).

Love this site and sincerely appreciate the comprehensive and professional advice, prompt replies and great suggestions. You couldn't have solved my problem any better, as far as I'm concerned. Added to that, you've alleviated all my concerns regarding accidental shorting of the shunt  as a bonus. Many, many thanks, once again.

Kind regards, Ron

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Geoff1946

It's a pleasure to try to help. Let us know how you get on. If you find you can post pictures pop up a picture of the car.

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