Morris Minor Owners Club

Ive bought and fitted a heated front screen in my Traveller and after wiring it upvia a relay and switch Im not sure that it is working.
Is there a way of checking if the screen electrics are working other than leaving the car outside as it is immobile in my garage at present.
If anybody has fitted or had experience of the screens I would be intrested to hear.
many thanks

If you have an ammeter could you pop it into the circuit to check the screen is drawing current when switched on .

What's the Amperage of the screen?
I would expect it to be at least 10 amp and a regular Multimeter usually blows it's fuse at 10 Amp.
The one on my Ford gets warm pretty quickly, but that's 40A each for LH and RH halves of the screen.

I assume it's wired up via the ignition switch so it's supposed to be on only when the car is running and that you have an alternator fitted?

I further assume you haven't got an ammeter fitted to the car or you would hopefully have already thought of seeing what that tells you?

Do you have a Multimeter?

Connect Multimeter across the battery, on a suitable range to read the battery voltage.
With all other items such as radio, lights etc switched of...
Turn the Ignition on and check voltage, it will likely drop slightly as the Inst lights, Coil etc are now powered, perhaps only 0.1 or 0.2 of a volt.
Turn on the heated screen and if the screen is now powered, the battery voltage will drop by perhaps a volt depending on the current state of the battery.
But whatever the amperage of the screen, just powering it from the battery will result in a terminal voltage drop as the load is applied.
Just do it for a few seconds to witness the drop, that's all you need.
Then switch everything off again.

I've put an ammeter across the battery today and it's 12v4. As I turn the heated screen on the volts drop to 11v 8 so am I correct in thinking that the screen is working as it should please?

Well hopefully you didn't use an ammeter, but a multimeter....
Which is why I put Voltage in Bold on my instructions.


Yes, that sounds fine.
When you put a load on a battery, the terminal voltage drops, then depending on the size of the load and the capacity of the battery, it drops off towards the end.
Basic characteristics as shown on this chart I found.
https://batteryguy.com/kb/knowledge-bas ... e-voltage/

It's a characteristic that can be very important in equipment design.

When I fitted a heated front screen i used a 10amp fuse. It has never blown so thr operating current is presumably below this

mowogg wrote: ↑Sun Jan 31, 2021 2:10 pm When I fitted a heated front screen i used a 10amp fuse. It has never blown so thr operating current is presumably below this

The value of a fuse can (usually does) bear no resemblance to the current at which it will blow. Many fuses will take a 50% overload (but not for long) and continued short term overload can lead to premature failure, but may never blow at, say, 25% over its rating.

Potentially higher currents, than the instrument is designed for, are more safely calculated using Ohm’s Law.