Testing Diode board - Multimeter vs test light

[unclepete]

Bought a used diode board recently to have as a spare . Got it cheap ; guy upgraded to 400w system ; reported to work well .
Was checking it with a digital multimeter and all checks ok except one of the exciter diodes shows continuity in both directions , at different values .
So I checked with 12V battery and test light . With test light all diodes test as expected ; light up in one direction and with polarity reversed , do not .
I wonder if my multimeter is beginning to fail . 

[Kiwi_Roy]

One thing about diodes, they seldom half fail.
Your exciter diodes may be ok, perhaps something else in the circuit?
Try comparing the readings with the other one you have.
By Exciter diodes do you mean the ones from UVW to 61?
What were the different readings?

[unclepete]

Testing as outlined in Rick Jones' book Classic Boxer Charging . Board is a Bosch .
From B+ terminal to the three AC terminals on the back of the board I get no continuity in one direction and 6 , 4.5 , and 5 M ohms , which seems normal .
To test exciter diodes : From the D+ terminal to the three AC terminals I get 4.6 , 7.7, and .7 M ohms , reverse polarity and I get 0 , 0 , and 20 M ohms . The last diode according to my multimeter seems to allow current to pass in both directions , at values outside of normal . 
With test light and battery , all diodes test good ; light in one direction , no light with polarity reversed .
I suspect my multimeter . Plan to replace batteries , flush diode board with electrical parts cleaner , clean all terminals , and try again .   

[StuCorpe]

Diode is OK. We were told (with analog meters) a value of at least 10 to 1 (back resistance to forward resistance) was OK. You should use the diode test function on a digital meter which actually biases the diode on and measures the voltage drop across the junction. Very high backwards and .2 to .5 volts forward.

[unclepete]

Thanks guys . All is well . I had not selected the diode function on the meter . Don't do this enough to keep my head wrapped around it ; a good thing I suppose .

[Kiwi_Roy]

As StuCorpe says, use the diode test range, it passes a current thru the diode in the forward direction and tells you the forward bias Voltage.
The resistance values you see are very high and the resistance differences insignificant at the Voltages involved.

I generally find large diodes around 0.5 Volts, the smaller ones around 0.6



For those that don't have a diode test on their meter take a 1.5 Volt battery in series with a 1 K resistor and connect that to the diode, measure the Voltage across it
It should read 0.5 to 0.6 one way (forward biased) 1.5 the other (reverse biased), if it reads zero the diodes toast.
The resistor value is not critical, you just need a few milliamps.

[Rough Edge racing]

 I used to get paid to rebuild truck alternators...I tested diodes with a simple 9V battery powered test light.. Diodes can become conductive leaking AC into the system that can cause gremlins with modern electronics, can be the cause of strange instrument gauge readings ... The AC leakage should be no more that .25 volts as measured close to the linked diode DC out put...