Voltage Drop Ground Side
Ok, so I've got to confess, the "voltage drop test" is not my first diagnostic technique I use when facing circuit tests, especially ground side. I'm more of a traditional volt/ohm man, and that ended up costing me. I have been fighting a intermittent misfire/stall on a '04 Silverado 6.0L for a couple months now. Let me assure you that the diagnostic steps were several and so were the hours watching/scoping live data on my verus. I did confirm spark loss when misfires happened, and no DTCs in any module anytime. I performed the "no spark" diagnostic flow chart. The ground side for the primary coil circuits passed an ohms test with flying colors. I performed several tests after that, following other diagnostic flow charts, and all lead me nowhere. Going on a tip I found in ProDemand, I removed and inspected the ground wire for the primary coil circuit. At first the ground appeared healthy and without fault. I decided to cut the insulation back from the crimp end. Sure enough, most of the wire strands were broken. Only one or two strands still remained intact. A voltage drop test of the ground side would have indicated this circuit was failing under load. I guess it's true, never test an unloaded circuit! Adios Ohms.
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Last edited by Matt; 04-15-2016, 05:31 AM. -
A resistance test is like checking your garden hose for leaks with the water turned off. -
I could never understand why Asian manufacturers always wanted circuits tested with an ohms check....They've been doing that for....forever....
Did the GM diagnostics say to check for ohms-?? I find that a little surprising. Typically they would check voltages and voltage drops but I haven't looked this vehicle up at all.
Testing a circuit ACTIVE is always the best practice.
You sure found something definitive. Good work.Comment
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Actually, my flow chart for "no start/no spark" has you focus on verifying RPM is registering in live scan data and confirming spark plug wire is ok. There is no mention of testing primary circuits of the coil. I studied a schematic and had to create my own diagnostic plan.Comment
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I was on a training course recently and the Master Tech didn't like it when I said that I isolate electrical circuits and load test the wiring using a power probe, he went on to say that load testing any wiring on any vehicle first used on or after 2004 model year and damage would be done to on board electronics because he said, even though you think you have isolated the circuit to be tested, it is not guaranteed.
The only test he recommended for checking vehicle electrical wiring was the ohm test, he went on to say that after the ohm meter was calibrated when the wiring was tested, if the wiring was fault free then a reading of 0.00 ohms should be achieved.
To prove or disprove his theory, preferably before I go on the next course on the 20th of this month, I need some evidence to prove if the ohm test is valid for both good wiring and a bad wire example.Comment
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Do you mean like taking a piece of multi strand wire and hacking away all the strands except one, then ohms test it?Comment
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Well first, never load test a circuit at a higher amperage than the component would normally draw. Second, most load tests just involve using a low amp probe while the component is supposed to be activated or performing a voltage drop test. If it's a ground control component, just manually ground it (assuming it would be normally grounded at 100%) while checking amp draw or voltage drop at various points. I've seen a few controllers and components "fried" by people using power probes. I just don't think that they (power probes) are necessary. A look at the schematic and you should be able to figure out where there should be voltage and where there shouldn't be any.Last edited by greasybob; 07-09-2016, 01:24 PM.Comment
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I do, but I can't see until I actually do a test how or if a ohm meter would show a difference between any length of wire say with 10 strands at 0.00 ohms and 9 strands broken, would 1 strand still show 0.00 ohms, and if so then a load test is required to find the faulty wire.Comment
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I'm only referring to load testing actual disconnected wiring Bob, no components attached, the Master Tech was implying that even though I think I have disconnected the wiring he said in his opinion there was still a chance due to a short circuit that current leakage was a possibility and could fry an ECM.
I understood what he was saying and I felt to some degree he was taking the safe option, but to date I'm not sure an ohm test alone is good enough to establish electrical circuit wiring faults, I stand to be corrected.
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If the Chassis ground is clean and has good continuity, to the negative battery terminal and the engine, and the logistics of accessing the harness is insane then I sayI'm only referring to load testing actual disconnected wiring Bob, no components attached, the Master Tech was implying that even though I think I have disconnected the wiring he said in his opinion there was still a chance due to a short circuit that current leakage was a possibility and could fry an ECM.
I understood what he was saying and I felt to some degree he was taking the safe option, but to date I'm not sure an ohm test alone is good enough to establish electrical circuit wiring faults, I stand to be corrected.
"When in doubt, Re-route"
Coronary Bypass $200,000. A damaged wire bypass $200. Get turd-done!
Last edited by Witsend; 07-11-2016, 04:20 AM.Comment
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