Troubleshooting circuits

[john zibell]

I've seen may requests for help with circuits. Several years ago I wrote an article for a magazine on a basic system for finding faults. Hopefully with this basic instruction and help from Carl Allison's schematics,http://forum.guzzitech.com/tech/electrical/81-carl-a-wiring-diagrams.html , you will be able to locate the gremlins.

[kiwi dave]

Good advice. One issue I would warn about would be using a voltmeter or multimeter to measure voltages. Because of the high input impedance of these devices, they tend to find "phantom voltages" which may disappear when any load is applied.

I prefer the light bult (low wattage) tester, especially those that can penetrate insulation. They don't tell so many lies.

[john zibell]

kiwi dave wrote:

Good advice. One issue I would warn about would be using a voltmeter or multimeter to measure voltages. Because of the high input impedance of these devices, they tend to find "phantom voltages" which may disappear when any load is applied.

I prefer the light bult (low wattage) tester, especially those that can penetrate insulation. They don't tell so many lies.

Yes a test light is good for tracing 12 volts, but if it is a control circuit, say 5 volts like the TPS supplied voltage, the light may not light.

[Brian UK]

Also a control circuit is likely to be a high impedance load anyway, so putting a low impedance light across it will load the circuit to the point that it definitely won't work.
Correctly used, a voltmeter will give a lot of information, but always check voltages under load.
By this I mean, if a light is dim, don't disconnect it before measuring the voltage. If you do, then you will not be able to find a high resistance joint.
Remember, you can also use a voltmeter to check voltage drop along a cable.

[Wayne Orwig]

Good advice. One issue I would warn about would be using a voltmeter or multimeter to measure voltages. Because of the high input impedance of these devices, they tend to find "phantom voltages" which may disappear when any load is applied.

I prefer the light bult (low wattage) tester, especially those that can penetrate insulation. They don't tell so many lies.

I carry a 12 volt bulb with wires soldered to it and clips on the end for this very reason. I have seen too many people mistake a poor high impedance connection for a good connection because they use a fancy digital meter that isn't loading the circuit.

Plus the light bulb can come in handy along the road at night as a trouble light.

Not very useful for setting a TPS, but great for most other things.

[Cat City Rider]

As a retired (retarded?) Telephone Switch Maintainer who went from tin cans and strings to fiber optics over a 41 year career I feel that I can say that there is nothing better than using a simple "bug lamp" or trouble shooting lamp for most things where electronics are not involved. I emphasized the electronics part as we do not wish to blow any diodes or chips in the circuit.
A dim bulb can say a lot if there is a poor connection and will help you find trouble when it is easy to wiggle wires and connections while looking for changes. A digital multimeter will have numbers rolling all over the place and will be impossible to read, an analog multimeter will at least show trends up or down.
An over bright bulb obviously will demonstrate excess voltages.
There is a term called step by step trouble shooting meaning that you can break down each questionable section to help isolate the problem. Chasing a problem through a multi story building with no elevator is different than a bike, but the same idea applies. My wife tells me that there is a French expression that says, "if you don't have good brains, than you better have good legs". I have good legs.
A nice sharp almost needle like probe on the end of a lead is good for poking into wires and minimizing any damage that a chunky stock multimeter probe may cause.