Did you check the A/C fuse?

Well Bob, you and I finally agree. A test light is appropriate for some conditions, but not others. So when used properly, it does have a place in the tool box.

I have never advocated a test light for any electronic device, but for electrical issues, yes, it can be handy - but I'll caveat that with the user should have some idea of what they are doing.

Glad to help Bob on the issue of impedance. In the days before integrated circuits, when tubes, then transistors were the going thing, a high impedance meter was still necessary to properly take a voltage reading. So its really not anything new.

The industry standard in those days was 11 Meg Ohms. This was accomplished first by VTVMs (Vacuum Tube Volt Meters), then FET-VOMs (Field Effect Transistor - Volt Ohm Meters), and now days, it requires a rather expensive bench type Digital Voltmeter to match the performance of those VTVMs and FET-VOMs.

While a few expensive modern Digital hand held meters do have the performance of the bench voltmeters, most do not - mostly because they lack high-frequency response. I have not researched a lot of the newer hand held DVMs, so I am not sure if they match the standard 11 Meg Ohm impedance or not.

But any DVM impedance close to 11 Meg Ohms would be satisfactory for resistors, diodes, relays, and such. Even a 40 year old VTVM would be more than adequate for those components (as long as you can find a place to plug into 120VAC for the meter).

In the old days, with either tubes or transistors, the primary reason to have a high impedance meter was accuracy. The standard analog voltmeter in those days had an impedance of about 20 Kilo Ohms (per volt), which is considered a low-impedance device.

Electronic equipment, even tube or transistor based, has low signal levels, both in amplitude and current. If you placed a standard voltmeter on that circuit, you would essentially load the circuit down by the voltmeter's internal coil, and this would affect the accuracy of the reading. In many of the worst conditions, you could actually stop the circuit from working as the signal level of the device you were attempting to measure would be interrupted.

In contrast, a high impedance meter will not load the circuit down, and its attachment to the test point will not generally affect the voltage level of the circuit.

This can also be explained by ohm's law for parallel circuits. When using a voltmeter, test light, or any other device, you are basically creating a parallel circuit. The current requirement, and subsequently the voltage drop is then basically a ratio of the circuit's impedance and the voltmeter's impedance.

In a high impedance voltmeter, the ratio between the meter and the circuit is such that the circuit will typically be much lower (thousands of times much lower), so the change in resistance due to the parallel circuit you make when attaching the meter is nil.

But in a low impedance meter (or test light for that matter), the resultant resistance will likely be much lower than the original circuit's resistance. Not only will the voltage reading be inaccurate, this will affect the circuit's operation, and in some cases, make the operation cease.

Now to the modern day, with modern electronics. With the advent of integrated circuits, the circuit currents are quite low. This makes them much more susceptable to a change in condition if a low impedance device is used to measure them. In addition to everything I just said, it is also possible in the worst case scenario that you could fry a component by in effect forcing too much current through it by virtue of lowering the resistance by the parallel circuit you create when attaching the device.

The good news though is often these sensitive components are contained within a module or circuit board. Typically, all connections into or out of the module have interface circuits, called drivers, attached to them that boost the signal so that it is not quite as sensitive. In this sense, driver means a hardware circuit, but like a software driver, it provides an interface to other equipment.

One good example of this is an ethernet card. Internally, it has very sensitive components, all of which could be damaged by poking around with a test light or non-electronic meter. But there is a driver circuit between those components and the twisted pair cable so that it can transmit the required distances, withstand transient current spikes that occur with nearby electrical interference, and so on.

If you attached a meter to the twisted pair side, it would not damage the circuitry. Even if you attached a test light, I would expect the driver circuit to be sophisticated enough to limit the current that it would supply, so it should not damage the equipment. The circuit would quit working, but it should not be damaged.

So in closing, I would find it hard to believe that an automotive module would not have some sort of driver interface that would isolate and protect the internal electronics from the external components it attaches to, given the harsh conditions, transient voltages, and other interference that is typically found in that environment. Afterall, the Aerospace (and possibly the marine) industry is probably the only more harsh envrionment that one will find to run electronic equipment.

In this sense, automotive module design is conjecture on my part, but automotive electronics is just a specialized field of electronics, so good engineering practices should be used regardless of application. I would consider it poor engineering if interface circuitry did not exist. Its fairly standard in industry that when module building, there is some form of protection from the outside environment.

So Bob, I'll take you at your word that you are seeing schematics with exposed components, but unless this is contained within internal modules or circuit boards, I would submit that it is not a very good design, as all electronic modules should have interface circuitry or drivers that protect the internal circuitry.

In fact, I would expect to see any circuit board or module used in an automotive application to be potted (sealed) so that moisture nor any other contaminant be able to access those components. In those situations, there is generally no way to repair the module, as removal of the potting is all but impossible to do without destroying the module itself.

I haven't any idea if this is so, but, I suppose the auto industry is the epitome of consumer grade and cheaply made.

Now if you are talking about opening a module and poking around inside - then the neophyte is going to fry someting for sure.

Tis better to keep silent and have people think you a fool then to speak and remove all doubt......

My head hurts