The THEORY behind VOLTAGE DROP testing - Motorcycle Forum
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post #1 of 5 (permalink) Old 02-09-2015, 08:45 AM Thread Starter
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The THEORY behind VOLTAGE DROP testing

You've got a motorcycle that starts and runs fine, but your headlight is dim. While the bike is running and the headlamp is on, you backprobe (stick your test leads, needles, or T-pins in the back of the headlamp connector) and see that you've only got 11.8 volts instead of the 13.8 you measure, engine running, at the battery.

You've got a 2V voltage drop.

In the case of a headlight circuit, a 2V voltage drop may reduce it's light output by 50% or more. In other circuits, it may prevent it from working at all. Like a starter that won't crank.

What are the CAUSES of voltage drops?

Physical causes are corroded battery terminals, corroded bullet connectors, worn or dirty contacts inside switches, paint or corrosion underneath battery-to-frame ground connections, and....

A wire with 13 strands of which half or more are broken!

A test light (or meter) will show you you've got continuity, but that single strand of wire cannot carry enough CURRENT for the circuit to operate properly!

But let's back up. All the way to Ohms law.

V = I * R

Volts equals current times resistance.

On a motorcycle (or car) the voltage we're USUALLY dealing with is either 12.6 (a fully charged battery) or 13.8 (typical alternator output). 12.3 is a 50% discharged battery, 12.1 is a 90% discharged battery.

A VOLTAGE DROP occurs when there is an UNEXPECTED resistance in a circuit. One that shouldn't be there!

Let's say that bad battery to frame ground, where some numbnuts bolted the battery negative cable to the frame of your bike WITHOUT scraping the paint off the frame, adds .1 ohms of resistance to that circuit.

.1 ohms of UNEXPECTED resistance.

Doesn't sound like much, does it. But it's more than enough to hinder PROPER operation of a circuit.

So why don't we just take our ohmmeter (DVOM, DMM....) and measure the resistance of all our wires, ground connections, etc.

Quite simply, ohmmeters do a LOUSY job of measuring anything below 10 ohms or less. The resistance of the test leads comes into play, how hard you press the leads against what you're measuring, etc.

So we locate voltage drops ANOTHER WAY!

Remember ohms law?

V = I * R?

Well if you've got a starter that pulls 30 amps, and a .1 ohm resistance,

V = 30 * .1

V = 3 Volts!

A .1 ohm voltage drop at your MAIN GROUND connection will result in your starter ONLY seeing 9.6V, instead of 12.6 V.

It may not even crank!

So by measuring VOLTAGE DROPS we can LOCATE bad grounds, wires with multiple strands broken, switches with dirty or burned contacts, dirty fusebox connections, etc.

Key thing to remember: The circuit (headlamp, starter, radio...) has to be OPERATING to measure a voltage drop. If you unplug that dim headlamp and just stick your test leads in the headlamp connector, you'll see a full 12.6 (or 13.8) volts!

Why? There's no current flowing! V= IR, and I equals ZERO! No current flowing, no voltage drop occurs!

So lets go back to the headlight example, and DO THE MATH!

Let's ASSUME there is a .2 ohm voltage drop in the dim/bright selector switch, and SEE how many volts the headlamp actually gets.

Ok, lets ASSUME we've got a Sylvania 9004 (55 watt) headlamp, ok? What's the resistance of the filament in that bulb?

Watts = Volts * amps

55 watts / 13.8 volts = 4 amps (3.98...)

V/I = R

13.8 / 4 = 3.45 ohms

But we've got 3.65 ohms (3.45 + .2)

13.8 / 3.65 = 3.78 amps

Watts = Volts * Amps

13.8 * 3.78 = 52.1 Watts

So we're only getting 52.1 watts of light instead of 55 watts of light out of our bulb!

That may not sound like much, but a .5 volt drop can reduce actual LIGHT output by as much as 50%.

So what is the most COMMON use of voltage drop testing? No-crank situations. Either the starter itself, or the starter solenoid is typically NOT getting enough voltage because of a voltage drop SOMEWHERE in the starting circuit. Common causes? Loose battery connections, bad grounds, worn out starter switches, tired batteries.

So, to summarize: A voltage drop occurs when there is an UNEXPECTED resistance in a circuit. Since ohmmeters are LOUSY at measuring really low resistances, we instead measure the VOLTAGE DROP that occurs when CURRENT FLOWS through the unexpected resistance to LOCATE that resistance.

A voltage drop can occur on either leg --- positive, OR ground --- of a circuit. You can have a bad ground connection, OR corrosion in the fusebox between the fuse and the lugs that adds undesired resistance.

Final, final note.

Loose or corroded BATTERY terminals/posts are the # cause of charging problems, etc. Especially on a car, remove, wire brush, clean, and re-install all battery cables, and FULLY charge the battery (12.6 or above) before you start ANY diagnostic work.

You simply cannot TRUST any measurements you make unless you're working with a fully charged battery.

Good luck, HTH.

Common Sense: Today it's considered a superpower.

Last edited by wadenelson; 02-09-2015 at 08:49 AM.
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post #2 of 5 (permalink) Old 02-10-2015, 08:21 PM Thread Starter
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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.


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Last edited by wadenelson; 02-10-2015 at 08:51 PM.
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post #3 of 5 (permalink) Old 02-15-2015, 12:07 AM
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that was the best lesson ...

the key is to connect the voltmeter inline. what pros call in series....
K what math is the greatest brain of engineering?
iso. perp, or linear?

I would say yes,,, that was a great educational lecture... thank you sir.

so that it reads the voltage flowing in the line, not what is going to the ground...

wrong way +________+meter lead________-
-meter lead_______ground

hard to say how you missed the point, but that was it!
+________ +meter lead_____meter_____- meter lead________-

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post #4 of 5 (permalink) Old 02-15-2015, 02:06 AM
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Voltage drop testing is putting the meter in parallel with the circuit, from the source to the load. so

Source -------- wiring, switches, etc. ---------- load (lamp, coil, etc.) --- ground
|_ (+) meter lead - meter - (-) meter lead _|

This is to test whether the wiring, switches, etc. have too much voltage loss. If you put the meter in series, all you determine is that the load goes to ground, and the source is present.

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'68-7? CB450 and '98 Valkyrie Tourer, too

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post #5 of 5 (permalink) Old 08-07-2015, 05:22 AM
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In practice

I did a voltage drop on the CB360. It had a drop of about ten volts. What you are seeing is the six pin connector to the fuse box. Yes, it tested 12.6 volts until drop tested. Then it tested about 2 volts. There is supposed to be six pins there, but one of them was corroded to dust.

Hondas try like heck to keep running, but this one would run and then shut off after it lost spark.

Pull off the Band-Aid. Eat our peas. Just do it.
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