Morris Minor Owners Club

I picked up my new moggy yesterday, the problem is that the brake lights do not shine brightly. When i short out the brake switch the bulbs are very bright. I have connected a multimeter across the switch and when the brakes are applied the resistance drops to zero, i have cleaned up the terminals but the lights do not shine very brightly.. any thoughts or is it just get a new switch!<br><br>

It does point to a troubled switch!

Hello CMM,

to confirm that the switch is the problem, measure the voltage across the two brake switch terminals with a helper pushing the brake pedal. It should be near enough to zero (use the lowest range you have on the meter), if it is a volt or two then the switch is faulty.

The problem with using a meter on the ohms range is that they are not very accurate at low resistance measurement. A volt drop measurement is much more useful.

Alec

If the lights are bright with wires shorted - and dim when through switch - it's new switch time !

It could just be the lucar connectors. The first thing I'd do is to change them both for new ones.

sorry I should have said changing the connectors was the first thing I did.

I am about to go out and try measuring the voltage drop, thanks for the comments.

i have measured the voltage drop and i have a reading of 11.93 so either the switch is gone or i have done something wrong (very likely).. so off to buy a new switch

I think what Alec meant is that you are recording a drop in voltage, not actual voltage. So, for example, if you measure across the terminals and get 12v then press the brake pedal and you get 10.5v you have a switch fault. If the reading stays the same at 12v then the problem is somewhere else.... HTH

ok pedal depressed = 11.93 pedal not depressed = 11.94

i need to read a book on electricity!

Well, by that theory - nothing wrong with switch
Although, I would be expecting to see more than 11.9v ?

You could try a test light of the same wattage. disconnect both wires from the switch. Connect one side of the test light to earth and the other side to the live side of the switch (ignition on) The light will be on bright. Then connect the live side to the switch and the test light wire to the other side of the switch. Press the brake. if the light is dull its the switch. Make sure the spades on the switch are clean.

Good plan I'm thinking bad earth on the switch....

Hello Charlie,

from your voltage reading the switch is doing nothing. But just to clarify, you are measuring between the two terminals and not one terminal to the body?

What should happen with a good switch is system voltage across both terminals with the brake off, and virtually zero volts when the brake is pressed, i.e the voltage drop is minimal.

Incandescent lamps are very sensitive to voltage and a small loss causes a significant loss of light out put. This loss can be anywhere from the fuse to switch, poor contact in the lamp holder or simply a poor earth connection. Voltage measurements along the circuit can quickly pinpoint the trouble.

Alec

Alec yes i put them on the terminals of the switch.. i even reversed them as i could not tell which was the green and which was the green purple and got a negative reading so I swapped them back.

So a new switch it is then.

The switch isn't earthed. It is an in line switch on the live side (neg earth) Alec I don't understand your idea as putting a volt meter across the terminals with brake off will show low voltage as the bulb will be an in line resister. when the brake is applied on a good switch would the voltage not be the same on both sides and show 0 volts or not?

charlie_morris_minor wrote:ok pedal depressed = 11.93 pedal not depressed = 11.94

i need to read a book on electricity!

These readings are what you want if the switch is duff, The low volt reading will be because of the resistance (bulb) if the switch was good when you press the brake you would get 0 volts

Switch closed = volts across will be ZERO! Any volts reading indicates the switch is goosed! And yes, there is no 'earth' on the switch - it's just a switch!

Confirmed - Just checked mine and zero volts across switch (just goes to prove one should never assume anything )

THat's right Paul, but since the voltage did not change when the pedal was pushed it proves that the switch has failed.

a new switch has been bought..

thank you everyone for your help and suggestions

charlie