Yes, I am
certain.... I have degree in electronics and have been troubleshooting/repairing equipment for over 50 years.
An ohmmeter uses VERY small current to measure resistance. This IS NOT a good indicator of current-carrying capability.
Many
many times, I have troubleshot 'bad connections' which an ohmmeter showed low resistance... but when current was flowing there was a large voltage drop. This is explained by OHMS LAW. (more current = more voltage drop)
- I have seen tight bolts NOT be able to carry current due to corrosion on the threads.
- I have seen differential-bearings fried because all the current from the alternator was passing thru it. (broken ground on engine-case)
- I have seen the charge-voltage at the battery be 16volts because the regulator ground was 2v above battery-ground. (relying on the frame to carry current... the regulator was working fine, it just had a bad reference.)
- Many head-gasket failures can be traced to poor grounding forcing the ignition (spark) energy to flow thru the headbolt-threads. This results in arcing THROUGH the head-gasket.
There is a
reason that equipment-makers NEVER rely on frame to conduct current.
You're almost correct, however, the automotive industry has been using the frame/body for the ground for a very long time. Pretty much since batteries started appearing in cars. It's more cost effective, which is the primary reason for most anything designed for cars. It makes more sense than running two wires when there's no way that battery can put enough current out to melt a transmission housing. The mass of metal in the frame and drive train of cars, trucks, tractors, etc is the primary reason it's used that way. What other metals lack in conductivity versus copper is more than offset by conductor size in terms of Cross Sectional Area, which is exactly what wire size indicates.
What was actually happening in your bad connection anecdotes was that the cold connection would carry the small current of the Ohmmeter just fine. Once it heated up because of higher current flow through a bad connection (resistance), though, the resistance goes up. More resistance, more voltage drop, more watts dissipated at the bad connection, meaning more heat, more resistance, etc, etc. The resistance goes up exponentially with temperature, with voltage drop following it. I've used an infrared camera to find bad connections in bus bar that's 3/4 inch thick and 8 inches wide in outdoor substations. They heat up and tell on themselves. Even a slight temperature rise over other connections indicates a problem.
The Ohmmeter reading was VERY correct for the current it was putting through the connection, but it doesn't quite tell all the story. If you're measuring [expected] low resistances intended to carry large currents, an inexpensive ohmmeter may not be your best choice. A Low Resistance Ohmmeter (also know as a Conduction meter or Mhometer) uses a much higher (10A or 100A) current source (only millivolt output at max current) and voltage drop (as you said) across the connection, and measures resistance in milliOhms, the inverse of which is Mhos (1/R) (yes, Ohm spelled backward) or the European unit, Siemens. These instruments are used in medium and low voltage circuit breaker maintenance. I have 2 in my truck as we speak, both digital units. They're also used for Earth Resistance testing in substation construction and a host of other applications where lower resistance is better. Anything that conducts high current needs low resistance to prevent heat.
I've been a field engineer in (electrical) industrial services for 40+ years with most of that in power delivery equipment maintenance and control. We have many strange instruments that most electronic engineers and technicians rarely have occasion to use for testing high currents (up to 80KA), high Voltage (up to 150 KV), high resistance (TeraOhms), and Low Resistance (MicroOhms). We generally were looking for extremes in whatever direction was the best for the application or test we were performing.
I also owned a VW Beetle for 35 years. The ONLY connection to the battery on the grounded post is the uninsulated braided grounding strap that goes straight to the belly pan. There are NO other wires on that post. VW did that starting in 1937, and never saw fit to change it even until the air-cooled VW was no longer manufactured. It works, if a clean connection is established and maintained. Every light fixture or electric device on the car had a short ground (usually brown insulation) just long enough to get to the body somewhere. I probably held at least one end of EVERY wire in that car by the time I sold it. Before 1965, VW's with 6V systems used a Positive Ground (put the battery in backward). Many an aftermarket car stereo was smoked by folks that didn't know that. The voltage regulator didn't fare well either if the ignition was turned on with the wrong connections. I just pushed our '64 to start it more often than I started it with the starter.
