The actual PRACTICE of VOLTAGE DROP testing
In Part One I covered the THEORY of voltage drop testing, which is, that unexpected/undesired resistance in a circuit, adding as little as 1/10th ohm, can cause dim headlights, improper operation of starters, etc. The most common CAUSES of voltage drops are bad grounds, corroded connectors, burned contacts in switches, and wires with multiple strands internally broken -- but STILL a few making connection.
I outlined how, instead of attempting to use an OHMMETER to chase down voltage drops, we INSTEAD "look for" the voltage drop these undesired resistances CAUSE when electrical current flows THROUGH them.
Now how do we put it in practice, and actually LOCATE a bad ground, burned contact, corroded connector?
The circuit we are diagnosing HAS to be operating, current flowing, in order to DETECT a voltage drop. If it's a lamp, it has to be lit. If it's a motor, it has to be spinning, And so on.
So we begin, as always, by testing the battery (and our meter!) by checking battery voltage. Should be 12.6 or above for a fully charged battery, or 13.8 or thereabouts if the engine is running and the alternator is working properly.
Next we go to the malfunctioning device --- the dim bulb, the rear window defroster, whatever, and COMPARE the voltage actually available to it.
So let's say you own an old Subaru wagon, and the rear defroster can't get the job done in under an hour. You put the meter on it and it's getting a whopping 9.2 volts while the battery shows 12.6. You've got a voltage drop of 3.4 volts.
So there can be unwanted/undesired resistances on BOTH legs of the circuit supplying the rear defroster, both the positive leg, and the negative (or ground) leg.
Let's start (by constructing some 15' test leads, and then) put the positive lead of our DMM on the battery positive terminal, and the negative lead on the negative connector to the defroster grid.
Ah ha! We only measure 11.6 volts! With full 12.6 at the battery terminal, that means we've got a 1.0 volt drop on the ground connection. (Which means we've STILL got a 2.4 V drop on the positive leg) And THIS leg will be easy to fix.
I locate the ground connection for the defroster grid, a ring terminal screwed to the body somewhere near the tailgate, pull it off, sand both the ring terminal and the painted part of the body it was screwed to till both are shiny, install a 'star" washer (if you've got one) and re-measure. 12.55 volts. GOOD ENOUGH, although it wouldn't hurt to check our main battery (or engine) to chassis ground, see if we can bring it up even further!
We have 'fixed" the voltage drop on the negative leg.
JUST A REMINDER: The heater grid has to be powered up and TRYING to work while we do this testing! If it's an ignition feed rather than a battery feed, the ignition must be on, OR, the engine running!
Now we put the negative lead from our meter on the battery negative terminal ...pause...
(obviously, you can reverse the leads and just ignore the minus sign on the meter instead of actually changing leads)
and the positive lead on the feed to the heater grid. Instead of 12.6 we see 10.2, JUST AS WE WOULD EXPECT. We "got rid" of one volt of voltage drop out of 3.4, and 2.4 is what remains.
Now THIS side is going to be a lot harder to diagnose. Let me make up some numbers.
Imagine we probe at ALL these points along the positive "feed" all the way back to the battery:
At the connection to the defrsoter grid: 10.2 volts
At the defroster switch output 10.9 volts
At the defroster switch input 11.6 volts
At the ignition switch output 11.7 volts
At the ignition switch input 12.1 volts
At the fusebox output to ignition 12.3 volts
At the battery + terminal 12.6 volts.
So what have we got.
We have .7 volt "voltage drop" in the wire, the wire alone, from the defroster switch to the rear defroster. Subaru simply used too small an AWG wire! We install a 10AWG wire from the switch back to the defroster!
(aka "detour wiring!")
We have a.7 volt "voltage drop" in the defroster switch itself! Unacceptable, should be .2 volts OR LESS! And replacement switches are prohibitively priced or totally unavailable. What to do!
We have .1V drop in the feed from the ignition switch to the defroster switch: That's ridiculous for a 2' piece of wire. Pull the connectors off of both ends, clean 'em, and the .1v drop completely disappears!
We have a .4V drop in the ignition switch itself! It's tired! contacts worn or burned! Replace it with a new one for $17, Rock Auto!
We have .3V of voltage loss in the fusebox itself! Pull that old fuse out, clean the "wiper arms" with a steel brush, steel wool, sandpaper, whatever, install a nice, new uncorroded fuse and the drop drops to .1V or less.
So this begs the question. How much voltage drop IS acceptable! Well, for headlights, I personally want .2V drop or less because I want 'em as bright as I can get 'em!
For engine controls, computers, sensors, etc. you ABSOLUTELY need less than a .1V drop or they'll misread sensors, deduce incorrect temperatures and flows, and inject the WRONG amount of fuel, etc.
For a rear window defroster.... .5V or less is fine, the less drop, the faster it will defrost.
Now mind you EVERY wire has resistance. The skinnier the wire (higher the AWG) the greater the resistance, and the LONGER the wire, the greater the resistance.
So personally I wire headlights with 10AWG wire. Most folks use 12AWG, and hell, Subaru even used 14AWG wire. Unacceptable, IMHO!
This is why cables to starters are so FAT! You want .2V or less voltage drop in the WIRE ITSELF going to a starter!
(This is ALSO why buying "Monster Cable" for your stereo system is a complete waste of $$$ --- there ain't enough amps to justify it!)
(BTW, there are online voltage drop calculators where you put in the wire gauge, amperage, and length, and it will CALCULATE the expected voltage drop. Useful, if, say, you're mounting batteries in the trunk...)
So let's summarize. We hunt down a voltage drop by checking both legs of a circuit WHILE IT IS ENERGIZED. We first determine the TOTAL drop we are dealing with, and then we backtrack, all the way back to each battery post, to determine what wires, connectors, switches, etc. along the way are CAUSING voltage drops.
So how did I actually FIX the Subaru rear defroster, given that I wasn't gonna give Subaru $200 for a NOS defroster switch on a 4WD wagon I only PAID $1200 for? I wired in a relay, so all the OLD defroster switch was doing was ENERGIZING the relay stuffed into the tailgate that took power from the new 'detour' 10AWG wire and fed it directly to the rear defroster. There's more than one way to skin a cat, the relay was a whopping $10 or so.
Once you grasp voltage dropping a circuit, it seems INCREDIBLY simple, a head-slapping "Duh" why couldn't I see that Before!
Don't be hard on yourself! I spent 4 years getting an electrical engineering degree and it was a high school dropout auto tech who had to teach ME how to voltage drop circuits. Fortunately I have a bad writing addiction, and hopefully these two tomes have helped YOU understand it.
And if not, I'm more than glad to take calls from ANYONE who realizes just how critical a diagnostic procedure this is, but ...can't QUITE figure it out from just a written description.
Again, I voltage drop dim headlights, blower motors that don't spin fast enough, radiator fans, defroster grids that don't heat adequately, any circuit that is UNDERPERFORMING, and that INCLUDES starters that only "click" when a NEW, FULLY CHARGED battery is hooked up.
FWIW, I estimate that only ONE out of FIVE "ASE tested" (or equivalent) auto/motorcycle techs working at dealerships understand and actually perform voltage drop testing. So they sling new batteries, new starters, new solenoids etc. at "no cranks" instead of actually DIAGNOSING the bad connections, worn switches, bad grounds, etc. CAUSING a "no crank" condition. Pity.
Common Sense: Today it's considered a superpower.
Last edited by wadenelson; 02-10-2015 at 08:51 PM.