Voltage drop testing is a technique used to find very small resistances, especially when the circuit is under load. It's nothing more than Ohm's Law in action. It involves using the meter on the millivolt scale and measuring points along the same conductor rather than from one conductor to another (like from +12 to ground) as is typically done when measuring voltage. Any resistance between the two measured points will cause current to flow through the meter. The more resistance the higher the voltage will be.
Anything more than 100-200 mv needs to be corrected. Conversely, the lower the voltage the better the connection is. Perfect connections would have zero volts across them but it's near impossible to do that without using special termination techniques and materials. Gold comes to mind because it doesn't oxidize. For those that don't know the contacts in your ECU are gold flashed for this reason. The connector pins and receptacles (called a "pulselock" type by Toyota) are especially made for this purpose and of very high quality.
Anyway, lets take the main ground as an example. Set your meter to DC millivolts and place some load on the circuit ie; start the engine and turn on the headlights or heater blower. Place one meter probe (doesn't matter which) on the battery negative post. On the post mind you, not the battery clamp. Place the other probe on the engine block. Ignore polarity indication. If you get more than 200 mv work your way back along the main ground. When you find the location of the loss the voltage will decrease.
You can also start at the same point and move forward along the circuit path. For example place both probes on the battery negative post. You should get zero millivolts or very close to zero. Leaving one probe on the post move the other to the negative clamp then (if possible) to the wire exiting the clamp. Follow along to the next accessible point ie; the negative terminal at the block, then the head of the bolt connecting the terminal to the block, then the block itself. As you move along the voltage will increase in proportion to the amount of resistance found in the path. Anything over 100-200 mv should be corrected by cleaning, re-crimping, replacement, etc.
Course, once you find a "bad" connection and repair it you can place the meter probes on either side of it. You can do this from the git-go too. Say you find 500 mv between the negative cable terminal and the head of the bolt on the block, indicating unacceptable resistance in that connection. Remove and clean the surfaces then check it again by measuring directly across those two points. If you can't get the voltage below 200 mv the problem might be a bad crimp or corrosion in the wire/terminal interface itself. Check by measuring between the terminal and wire entering it if it's visible or by piercing the insulation if it's not.
Your goal is to make the voltage as low as possible or at least below 200 mv when measured between two points, including the entire circuit path ie; from the battery negative post to the block. I'm very anal about electrical stuff but during my last check I found 700 mv on my main ground. The problem was the interface between the negative terminal and the bolt on the block. Keep in mind the electrical system exhibited zero symptoms with that amount of loss. It's still unacceptable though because it would have gotten worse with time until symptoms did devolop. Long before the car would refuse to start the connection would have begun heating. Cleaning reduced the drop to 50 mv. I've yet to work on a car (any car) more than a few years old that didn't have excessive drop in it's main battery/charging circuit.
The technique works on any path. In his case (losses at the ECU connector) extend one meter probe using wire. Place the long probe on the battery positive and the other on the clamp. If that connection checks out you can move the positive post probe to the clamp so as to make it easier to attach. Take the other probe (along with the meter) and check any of the + connections at the ECU. If you find more than 200 mv work backwards.
Alternately, you can check across every connection using both probes. That way you won't have to extend one. It's best to test total drop from one end of the circuit to the other in the beginning though. That way you'll know right away if there's excessive drop. Needless to say doing drop testing requires accessing every point along the circuit where the conductor is "broken" by some device. Relays and their sockets, relay contacts, switches, terminal block, crimps, on and on. That's why it's best to measure total drop first. If it's OK you can move on to another circuit.
Another technique is to simply take a long jumper and bypass the path to see if the symptom you're having disappears. The method is crude however and doesn't pinpoint the fault. It's quick and dirty though.
The key things to remember when doing voltage drop testing are:
1) Make sure meter connections are electrically solid because any resistance in them will effect measurement. Don't be probing through dirt, oil, or grime for example. Make sure the meter probes are clean too. Scotchbrite them if need be, a good thing to do on occasion anyway.
2) The circuit must be under some load. The more the better. You can't do voltage drop testing without current flowing. When dealing with main power paths like the battery cables turn on the lights, blower, etc. With circuits like the ECU power, instruments, lighting, etc, the load they draw on their own will be enough.
3) Think about what you're doing. Be sure to probe every connection, say a relay's pin and it's socket. Any such "interruption" along a wire is a potential source of resistance and thus drop. Think in terms of moving the probes as close together as possible while still remaining on either side of a connection while it's under load. It's similar to how you'd do resistance testing except drop testing is capable of finding very small resistances your ohmmeter can't. (It takes a special type of ohmmeter to measure very low resistances).
If you find the drop across a wire itself (by probing or piercing it's insulation at either end) is more than 100 mv the wire is either bad or too small for the job. With stock wiring that's unlikely (even when old) but still possible. Put another way excessive drops (more than 100 mv) are almost always a connection problem along the conductor and not the conductor itself.
Voltage drop is a very useful form of electrical testing. Google the term for more details. The Net is more than a global pornography network you know -Jetjock
