Confused about reverse polarity - Page 1
Forum section: electronics / testgear

author: Joe Dillman
author_rank: Newbie
timestamp: September 27, 2024, 02:42:43 am
content: As an automotive technician I run into certain signal or command circuits that read zero volts when troubleshooting. Some circuits will get shut down when a fault is detected. I want to know if the circuit is at least connected without having to get to the other end of it. (I'll worry about the integrity of the circuit after this test)I often test ohms to ground and see the meter auto range around or read some high ohms on these computer circuits. Not always but mostly. OL has me looking at the harness closely. Some kind of ohm reading makes me look at pin fit on the connector before going any further.Now to my example and question. I once had an ignition coil driver circuit that read zero volts. Next step was to see of it had any connection to ground through the computer circuit board. I did this same method and had OL. The meter topped out at 40 MOhm. Reversed the leads and switched to voltage now got -10v.So why didn't the ohm meter register something when it clearly had at least some path to ground as indicated by the Reverse polarity voltage?That circuit would normally show a square wave 5v pattern BTW

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: September 27, 2024, 04:38:32 am
content: Do not double-post.

author: Halcyon
author_rank: Unknown
timestamp: September 27, 2024, 07:05:12 am
content: Duplicate post deleted.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 27, 2024, 08:46:03 am
content: Quote from: Joe Dillman on September 27, 2024, 02:42:43 amYou need to understand how a multimeter measures resistance. A DMM uses its internal battery to try to make a known current flow through the DUT. It then measures the voltage across the DUT and applies Ohms Law to calculate the resistance. When you do that in a powered circuit the measured voltage is incorrect and makes the displayed resistance value incorrect. Depending on polarity it will either be too low or too high. In the high case it can be "off the scale", ie OL.

author: ptluis
author_rank: Frequent Contributor
timestamp: September 27, 2024, 10:17:32 am
content: Quote from: Joe Dillman on September 27, 2024, 02:42:43 amNormally sensors in a car are always positive feed by ecu and what varies is the connection to digital ground with pwm signals from ecu. For eg. Egr valve is always positive connected and it operates by varying the duty cycle. Some ecu also use back emf to operate certainly sensors. The best way to detect the quality of the signal sent to a certain sensor is by using an oscilloscope not a multimeter. Also OBD scanners won't detect realtime glitches effectively like an oscilloscope. For modern car automotive use you need at least 3 devices multimeter, obd scanner, oscilloscope. and of course programming software and respective interfaces for each brand to execute certain operations. Keep in mind that most obd scanners won't detect fast random problems and only using an oscilloscope you are able to detect it for example bad connection to chassis ground due to a oxidized connection point. Remember that you have several connections to chassis ground and each related to a specific function of the Car eg left rear lights with rear wiper, right side with a different ground point. each ground point varies with make and model of car manufacturer.