Morris Minor Owners Club

I have a 1961 Morris Convertible (original factory car) which is currently on positive earth. I would like to convert it to negative earth to fit an alternator and negative earth fuel pump. However, the car has working semaphore indicators / trafficators and I'm concerned whether, firstly, these units could be damaged by a reverse in polarity and, secondly, whether they will still operate. Any advice in this matter would be very much appreciated. Thank you.

To answer this question we need to consider how the Trafficator works. There is an iron plunger and a coil of wire wrapped around the plunger. When current flows through the coil a magnetic field is generated which then acts on the iron plunger. The iron plunger does not care which way round the magnetic field is, a piece of iron is still attracted to a magnet whether it is the North or South Pole of the magnet.

Stephen

Hi DeeRo

I can understand your concern, but Trafficators are not polarity sensitive. Simple as that.

svenedin wrote: ↑Fri Jun 13, 2025 9:30 am The only problem might be if the iron core has become magnetised from long use with a certain polarity. This is quite likely I think and the result would be Trafficators that would work poorly, if at all with reverse polarity. The solution would be to demagnetise the iron core which I would have thought would happen anyway after a period of use........This is entirely hypothetical, I have never tried what you are suggesting.

Stephen

I can understand your thinking, but it doesn't seem to happen. I don't think the duration of activation compared to the time spent un-powered allows for sufficient permanent magnet effect to be established.
Or it may simply be that the field strength when operated with a change of polarity is sufficient to overcome any such weak permanent field.

geoberni wrote: ↑Fri Jun 13, 2025 9:30 am Hi DeeRo

I can understand your concern, but Trafficators are not polarity sensitive. Simple as that.

svenedin wrote: ↑Fri Jun 13, 2025 9:30 am The only problem might be if the iron core has become magnetised from long use with a certain polarity. This is quite likely I think and the result would be Trafficators that would work poorly, if at all with reverse polarity. The solution would be to demagnetise the iron core which I would have thought would happen anyway after a period of use........This is entirely hypothetical, I have never tried what you are suggesting.

Stephen

I can understand your thinking, but it doesn't seem to happen. I don't think the duration of activation compared to the time spent un-powered allows for sufficient permanent magnet effect to be established.
Or it may simply be that the field strength when operated with a change of polarity is sufficient to overcome any such weak permanent field.

Agreed. It is solenoid isn't it? I've never heard of people having trouble with the starter solenoid when switching polarity so why would the solenoid in the Trafficator be any different.

PS I had changed my mind so I edited my post but you had already quoted me!

Stephen

svenedin wrote: ↑Fri Jun 13, 2025 9:40 am
Agreed. It is solenoid isn't it? I've never heard of people having trouble with the starter solenoid when switching polarity so why would the solenoid in the Trafficator be any different.

PS I had changed my mind so I edited my post but you had already quoted me!

Stephen

It was a valid theoretical point, but generally speaking any residual magnetism can be easily overcome.

Even Flashing the Dynamo isn't that essential, it's just 'best practice' to help the reversal of the system to occur.
I've seen reference to flashing a 'new' dynamo when fitting it, but I bet plenty of people over the years have just fitted a new one and started their car up. The Polarity of the connections will eventually force it to what is needed.

To confirm - the trafficators will work fine on a negative earth car. I have had such a car in the past.......

geoberni wrote: ↑Fri Jun 13, 2025 10:07 am

svenedin wrote: ↑Fri Jun 13, 2025 9:40 am
Agreed. It is solenoid isn't it? I've never heard of people having trouble with the starter solenoid when switching polarity so why would the solenoid in the Trafficator be any different.

PS I had changed my mind so I edited my post but you had already quoted me!

Stephen

It was a valid theoretical point, but generally speaking any residual magnetism can be easily overcome.

Even Flashing the Dynamo isn't that essential, it's just 'best practice' to help the reversal of the system to occur.
I've seen reference to flashing a 'new' dynamo when fitting it, but I bet plenty of people over the years have just fitted a new one and started their car up. The Polarity of the connections will eventually force it to what is needed.

Yes I just remembered a schoolboy experiment when we wrapped a coil of wire around a screwdriver shaft, connected the coil to a battery and made an electromagnet. When the wire was removed, the screwdriver was magnetised but that is something made of steel, not soft iron.

Stephen