Voltmeter -false low readings

[Nickol]

I want to install an aftermarket Voltmeter in my 69 Traveller.

However, when I connect it up via the incoming green wire to the voltage regulator, it only registered 10V. And no, it was not attached to the fuel gauge output.

So back to basics. Using an extension wire I connected it directly to the pos battery terminal. It read just under 13V as it should be. Next step was to connect to the live brown wire the "wrong" side of the fuse box. Again just under 13V. This proves that not only is the Voltmeter reading correctly but also that the earth connection is ok.

Now it gets interesting. Connecting to the white wire at the fuse box which is ignition switch controlled and with it switched on, 10V again.

My conclusion is that there is an internal resistance in the ignition switch - may be not enough though to effect the general running of the car once the engine is running.

As an afterthought , I have not yet started the car since fitting the voltmeter.

Is my logic nachvollziehbar.........a reasonable assumption.?????...or could there be another reason?

[geoberni]

Hi Nickol

So, referencing the diagram, to make sure I've understood your quite clear description.
A = 10v
B = 13v (good battery charge)
C = 13v
D = 10v

Nickol.JPG (150.14 KiB) Viewed 376 times

Yes, I'd say you have a poor connection at one or more of the points I've coloured yellow. Possibly at all of them just half a volt or so at each, or all 2+ volts at one point.
If the switch is original, it's likely the 55 year old switch contacts are showing their age.
I've never had the need to look at an Ign Sw, but you might just be able to clean it up with some switch cleaning fluid if it's not sealed.
.

[Nickol]

Hallo Berni,

thankyou for your suggestion - I will try to give the ignition switch a clean and see what happens. Your nice diagramme interpretation is perfect.

Incidentally a Brian Hawley, who may well be a MMOC member, has downloaded this excellent colour wiring diagramme.

https://www.dropbox.com/s/250kmt769vham ... rnator.pdf

Since my previous post I had another fiddle. I disconnected the ignition green wire feeds to the voltage stabiliser and connected up the voltmeter to them. Hey presto "nearly" 12V. Reconnecting these green wires to the stabiliser and back to 10V.

Need to have athink what that tells me.

[svenedin]

Errr....

The Smiths instrument voltage stabiliser is designed to provide 10 volts DC......

https://www.minimania.com/Smiths_Voltage_Stabilizers


Stephen

[Nickol]

yes, but only the "I" terminal is regulated to 10V, more or less.. The "B" terminal is just a connection point and is a nominal 12V..........only in my case it isn't..

[King Kenny]

I did mine using a momentary switch .

12v .jpg (48.31 KiB) Viewed 326 times

[Nickol]

Did you do this because you had similar problems to mine?

Anyway, this morning after having my think, I removed the ignition switch and as suggested cleaned the terminal tags. Before doing this however, i checked the Voltage with a multimeter which gave the proper readings of 12,6V on both the live input ( blue/brown wire from control box A1 terminal) as well as the two tags for the white wires, once the ignition was turned on.

However, at the fuse box only 10 , something volts. So presumably the wiring is at fault for the voltage drop. I do not have it in mind to replace the wiring harness which would be the proper solution. Everything else seems to work ok, lights, wipers, horn etc.

What I will try however is to connect the voltmeter to the ignition switch directly using a piggy back spade connector and see what reading that gives with the ignition on. If it gives a proper reading, I can then use an in line fuse as the main fuse box will be by passed.

[Nickol]

Sooooo, the plot thickens.

I used the piggy back so that the white wire from the ignition output was in place and the Voltmeter connected up. Only 11V !! So I disconnected the white wire to see what happens and the voltmeter read 12,5 V !!!

Thus for my logic there is some problem with the white wire which has some kind of resistance in it. It is nearly 55 years old, after all.

Thinking cap on again. Bad earth connection some where????

[geoberni]

Hallo Berni,

thankyou for your suggestion - I will try to give the ignition switch a clean and see what happens. Your nice diagramme interpretation is perfect.

Incidentally a Brian Hawley, who may well be a MMOC member, has downloaded this excellent colour wiring diagramme.

https://www.dropbox.com/s/250kmt769vham ... rnator.pdf

Since my previous post I had another fiddle. I disconnected the ignition green wire feeds to the voltage stabiliser and connected up the voltmeter to them. Hey presto "nearly" 12V. Reconnecting these green wires to the stabiliser and back to 10V.

Need to have athink what that tells me.

Yes, Brian is a member but seems to have completely abandoned this Forum in favour of the facebook group. He was last active here about the time the fb Group started in 2021.

I've just deleted everything else I had written here following your latest update , luckily I was distracted for a while so hadn't sent it.....

[geoberni]

Sooooo, the plot thickens.

I used the piggy back so that the white wire from the ignition output was in place and the Voltmeter connected up. Only 11V !! So I disconnected the white wire to see what happens and the voltmeter read 12,5 V !!!

Thus for my logic there is some problem with the white wire which has some kind of resistance in it. It is nearly 55 years old, after all.

Thinking cap on again. Bad earth connection some where????

Yes, it sounds like a poor connection, either at the Ign Switch or at the fuse box. I'd use the Multimeter set on Resistance and see what you get on that white wiring. It may just be some slight corrosion.

I don't believe it to be an Earth, it's a Supply issue.

[Nickol]

Yes, i am using a multimeter and am now following the circuit along. I think it is worth doing even though the original point of the exercise was just to fit a voltmeter. In effect, it is showing me that soemthing is amiss even though no symptoms are present e.g weak lights or wipers.

Have disconnected the white wires at the fusebox, turned on the ignition and measured 12,6 V. So the white wires are ok and also the ignition switch. That is a relief. Reconnected them, took out the fuse, still 12,6 V. Put in the fuse and only 10,1 V. I even bridged over the fuse connection in case the fuse itself was poorly connected but no difference, still 10,1 V. reading both sides of the fuse.

Next step is to remove each one of the fuse box connections, each of the green wires in turn and measure the voltage and see what I get. But it is such a nice day so we have gone for a walk in the woods. Here at 600 m above N.N spring comes a bit later than lower down.

[Nickol]

I have solved it!

I disconnected each of the 3 green wires in turn from the fuse box until with one removed, I got the 12V reading on the others with the multimeter and also with the voltmeter inside the car.

This rogue wire turned out to be the feed to the windscreen wipers. Now I have a bonnet extension fitted to the car and what a great asset it is when working on the engine area. However when it is fully open, it fouls the windscreen wipers. Due to my complete imcompetence I had inadvertedly switched them on and they must have jammed against the bonnet. Although I had noticed the switch in the "on" position and switched it off, the wipers were of course in park modus and thus drawing current - or at least trying to. Vollidiot.

Is there any hope I ask myself?

[geoberni]

Well that explains the Voltage Drop!!
Glad it's sorted.

[Nickol]

All a bit embarassing really. Asking the forum for help and advice when the solution was a new brain. But thanks for the feedback.

At least with the dashboard dismantled for access to the rear of the speedometer, it gave me a chance to replace the instrument lighting with brighter LEDs. Now to put it all back.