Hmmm. I knew some of this but not all of it. Just in case:

 

 

WHoah!

 

Did you know that if you use a trouble light instead of the properly set electronic meter to check a circuit that's connected to the ECU, you can damage the ECU?

 

(I did, but I work on computers -- and most computer techs don't give half a damn about that kind of problem either -- the problem is that when you damage a semiconductor it just changes the spec for when it switches from conductor to insulator, so it goes off spec-- making an intermittent failure.)

 

But I didn't know this part:

 

DIGITAL VOLT/OHM METER

Use a digital volt/ohm meter (DVOM) with a minimum 10 mega-ohms internal impedance when testing an ECU or related components, unless use of an analog meter is specified by a testing procedure. Analog meters have low internal impedance and cause circuit "loading" in low voltage circuits, resulting in inaccurate measurements and possible damage to components. Use of a DVOM is necessary to obtain accurate measurements and avoid damaging low voltage ECU circuits and components.

 

TESTING PROCEDURES

Always follow recommended testing procedures. Never ground or apply voltage to a circuit unless specified by a testing procedure. ECU's provide a regulated constant low voltage source to some circuits, which may require a minimum resistance (impedance) to operate. Directly grounding or applying battery power to these circuits could damage the ECU and cause a system failure.

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You make some good points on electronic testing Hank and I agree with you. Using a digital meter with its' high input impedance is a good rule when working on solid state circuits. I will say though that using a good analog meter with a 20,000 ohms per volt rating (some members will know what a Simpson 260 meter is) will work fine for most testing on circuits in a car at least. Most circuits external to the ECU don't have impedances high enough to really worry about voltage drop error using an analog meter. Some problems can be seen better using a analog meter over a digital meter. Knowing the type of circuit design you are working on really helps.

All good advice and common sense, I just thought I'd mention that is why there is the logic probe - test light of the digital world. Working TTL or similar such as the ECU generally you have 15-20ma source and sink to work with on average. Very dangerous to use low impedance(resitance) test measures without knowing just what you are hooking those leads to...