6v wipers on 12v

[Nick Salmon]

I know this has been covered before but...

My pal has a Big 7 and when the thing was altered to 12v the P/O failed to do anything to the 6v wiper motor. Could someone kindly tell me what resistor is needed and how it should be fitted - any pics would be v welcome.

Many thanks

["Slack Alice" Simon]

Resistance = Voltage over Current    R = V/I

Let's guess the wiper motor takes 2 amps, and you want to drop 6 volts (from 12 to 6)

Resistance required is 6/2 = 3 ohms

The drop is lost in the form of heat, resistors are rated in Watts, Wattage = Volts x Amps = 6 x 2 = 12 watts

I would use something with a greater wattage rating.

I have a 4.7 ohm 50 watt resistor on the shelf, which I would try first - erring on the cautious side, and remembering the amperage was a guess.

If I didn't have a resistor, the standard bodge is to use a bulb as a resistor - if I don't have a 6 volt charger and need to charge a battery, I stick a 6 volt headlamp bulb in series and carry on.

You are welcome to the resistor if you think it would do. 

Try a bulb first, to see what sort of result you get.

Cheers, Simon

[andrew34ruby]

I did the same calculation and came up with this 2 ohm resistor. It worked but the wiper was a bit slow so I changed to a lower resistance, maybe 1.5 ohms. I would need to take the car apart to check, but somewhere around 1.5 is a good starting point. The keys are just for scale. There are plenty available on the internet.
   

[John Cornforth]

Hi

For the slightly later CW model, Lucas quote 3 to 4 amps for normal operation (wiper on wet screen) rising to 4.4 to 5.5 Amps stalled. Taking 4 Amps, that needs 1.5 ohms to drop 6 volts as Andrew notes above.

The resistor dissipation will be about 10 Watts, a "metalclad" type needs to be bolted to some metalwork to take the heat away.

[Nick Salmon]

Many thanks for the helpful replies.

[Dave Mann]

I ran a 6 volt MT7 on 12 volts for about 10 years.

[Parazine]

Running the wiper through a resistor is rather poor engineering and could be considered a "bodge". As the load increases (e.g. dry screen, snow etc) the current draw increases and the voltage drops, leading to a stalled motor and overheated resistor.

A much better and more professional solution would be finding a 12V Lucas DW3 (or even a post-war CWX) and fitting that. There are plenty about as 12V cars were becoming common place in the late thirties. The CWX type was fitted to Land Rovers post war and is readily available as a 12V option.

[Dave Mann]

The DW3 is best      as the CWX is painfully slow.