multimeter reading on diode test ~ Q?

steptoe · Post by **steptoe** » Tue Sep 22, 2009 1:33 pm

I use a multimeter with limited ability.

I am testing the reverse switch wires on L 5 speed. Normally 12V + goes through this switch. Mine is not connected due to an auto to manual conversion and the auot switched the reverse lights on from the selector switch at the base of the t bar.. When using the diode test mode of the DMM, instead of getting the continuity? buzz I get a reading of 0.065 when the box is in reverse gear (indicating I have a circuit) and nothing, as expected, when out of reverse gear. At least since my electrical disaster weeks back the wire at the auto selector loom does not take 12V+ to the reverse lights, it shorts and blows the fuse10. Not looking for the short, happy to wire up a new circuit but not sure if switch is working the way it should. It did in the box donor car.

What does this 0.065 mean? Is it a resistance acros the closed switch? which I suspect should not be there. I am going fromt he round white connector with six wires, on the Green and Green/white trace so I got the correct wires for the switch

vincentvega · Post by **vincentvega** » Tue Sep 22, 2009 5:09 pm

it is there to measure the forward voltage of the diode.

0.065 is the voltage it is measuring across the switch.

use the lowest ohms setting on your meter to check the switch. diode test is for diodes...

Gannon · Post by **Gannon** » Tue Sep 22, 2009 5:12 pm

As a general rule, the diode test on a digital multimeter indicates the voltage drop across the diode when it is conducting (because diodes dont have a linear resistance)

The reading is usually one of mV but can sometimes be in volts too. So your reading of .065 is rather small at 65mV. A more plausible answer would be 650mV because most power diodes have a rated voltage drop of 0.6V

I just measured a diode i had lying about and got a reading of 578

The easiest way to tell if it is a diode is to reverse the test leads, you should get no reading if it is.

fredsub · Post by **fredsub** » Tue Sep 22, 2009 7:14 pm

very good points replies above.

Additionally I would add that if your multimeter is on the ohms range
or diode or continuity range, you should ensure the circuit/wires have no other voltage source on them, otherwise it may result in damaged reading,
and damaged multimeter likely.

easy to check correctness of ohms range by using known resistor values (use 1% metal film types)

for diode, obtain a known working diode. Forward biased should indicate ~ 0.6V.

steptoe, are you expecting a diode in that circuit?

steptoe · Post by **steptoe** » Tue Sep 22, 2009 7:53 pm

You guys started to lose me a bit. I now know I should not use the diode test for a closed/open circuit test, sticking the black on body earth and poke the red point about a bit. I was not expecting any diode or resistance , just an on or off condition. For me, the connector is in a hard to reach spot with my LPG stuff getting in the way. A search for the connectors mate and some wires and wire it up as a switch will prove one way or the other what I was looking for. I half expected these switches on the L series gearbox to only switch NEG but the diagrams indicate we carry POS+ onto the box and back again down to the reverse lights.

Dirty or greasy switch internals cause my reading instead of diode test buzz?

The easiest way to tell if it is a diode is to reverse the test leads, you should get no reading if it is. This is one thing I learnt not so long ago, was not aware you could use them in a 12V car circuit, thought it was more for electronic PC boards etc.