Measuring voltage VERSUS Voltage Drop

[Wade Nelson]

Every Tom **** and Harry knows how to use a voltmeter (DVOM) to measure voltage.

Yet there are even a lot of auto MECHANICS (techs) today who don't fully understand the concept of voltage drop.

Take 20 minutes to read and understand this, ask me any questions you need to completely "get" it and you will be light-years ahead of 99% of people diagnosing auto electric problems.

Voltage DROP has to do with a circuit's ability to supply adequate voltage/current UNDER LOAD. It's more a "quality of power" type issue than "just voltage."

Imagine you have a fully charged battery, 12.6 volts or so, when you put your meter across the battery terminals themselves.

Imagine you now go and stick your voltmeter leads on your headlight terminals, with the headlight itself unplugged, and you measure 12.6 volts. Everything's great, the wiring's fine, right?

Wrong.

Turn the headlights ON, pull 4-6 amps, and you may only measure 12.2 volts.

You've got a voltage DROP of .4 volts. That's the simplest part. Identifying a loss.

The question is, "Where did that .4 volts go?"

Every length of wire, every connector, every light bulb socket adds SOME resistance to a circuit. Its when they add TOO MUCH that you get a voltage drop.

Remeber V=IR? voltage equals current times resistance.

So the MORE CURRENT a circuit uses, the MORE voltage drop will be created by an undesired resistance!

NO circuit is more affected by voltage DROP than your 60A or so starter circuit. But more on that later!

30 year old corroded connectors, wire gauges that were too thin to begin with, worn or corroded contacts on switches and relays, corroded bulb sockets, paint underneath body grounds, corroded battery terminals, all that resistance adds up, and STEALS some of the voltage you INTENDED the bulb to receive.

How much voltage drop is acceptable?

It depends on the circuit!

For a computer, like the ECM, you want .05 volt drop OR LESS!

For headlights and taillights, .3V might be acceptable, .2V desirable

For things like temperature sensors, etc. === .1V!

For your car stereo? Well, how loud do you want it?

So back to our headlamp example. We've "lost" .4 volts. How do we "find" it?

First, we have to determine which "leg" of the circuit lost the voltage --- the power side, or the ground side.

(or a little on both!)

In other words, WHICH side has an undesired resistance that we need to correct!

This is where you need six foot leads for your DVOM.

You put the black lead on your battery minus (B-) terminal, and the read lead on the power wire RIGHT AT THE HEADLIGHT, with the headlight on.

Let's say you read 12.55 volts. Dang, that's pretty good! you've only got a .5 drop on the POWER side of the circuit.

Now reverse the leads. Red on B+. Black on the negative side of the headlamp circuit.

You've only got 12.25 volts. You've got a .35 volt drop!

Somewhere between that headlamp connector and the B- terminal there is UNDESIRED resistance.

You've most likely got a bad body ground. Usually the headlights share a ground wire, going to a screw against the body somewhere. And instead of shiny metal underneath that screw, and a star washer, all too often there's paint, ESPECIALLY if the car has ever been repainted!

Body shops are the absolute WORST about cleaning off paint beneath body grounds!

So you fix the ground, and all of a sudden your headlights are white instead of yellow. You're getting 12.5 instead of 12.2, and THAT ALONE will make them 40% brighter.

(Actually, at night, with the alternator running, it'll be 13.8 or so at the battery terminal, and only 13.4 or whatever @ the headlights themselves BEFORE you fix the bad ground.)

I just PICKED a bad ground for an example. It could have been too-thin wire going to the headlights, corroded headlamp connectors, a worn out headlight switch, bright/dim switch, headlight relay, any of those could have been STEALING your voltage.

(And sometimes you can't fix all of them, so you simply wire in a relay and use the old circuit simply to turn the brand new relay on!)

Now, let's go to example #2, which happened today, and is why I'm writing this.

Neighbor asks to borrow a 10mm socket, to replace the starter in his sistah's Honda Civic.

I lend him the socket then get back to playing online poker.

An hour later I motorcycle past where he's working on it, stop and ask how he's doing.

The starter clicks, but doesn't crank. Fairly new battery, terminals don't look bad, and he has pulled the starter out of the car.

Lying beside the car, I show him how to test the starter with a pair of jumper cables and an alligator clip test lead.

Hook the black cable to B-, clamp it to the flange on the starter.

Hook the red cable to B+, then touch it directly to the motor lead. The starter motor spins. So the motor itself is good. What about the solenoid.

Hook the red cable to the place it normally connects on the solenoid, and use the test lead to jump into the starter switch input on the solenoid.

The solenoid slams, the motor spins. By all tests, there's nothing wrong with this starter.

Put the starter back in the car, turn the key, and all it does is click. It doesn't spin.

So is his keyswitch bad? Putting a test light on the keyswitch output (where it plugs into the solenoid, it lights up. Now that's not a GOOD test, but it'll do, for now. We really SHOULD voltage drop the keyswitch, too!

Next I do a VOLTAGE DROP test on the main power lead, the one that goes from B+ right to the starter.

(Black lead on B-, red lead right on the solenoid where power comes in from the battery)

I have him crank it.

Five volts. He's only getting FIVE VOLTS to the starter, UNDER LOAD.

If I had just put a voltmeter right on there, without the starter connected, I would have seen a full 12.6 volts. Why? Because, without a load, there is virtually NO CURRENT FLOWING

Remember, V=IR? If current is near zero, the VOLTAGE DROP will be also.

He's got one of those bolt-on battery terminals, and when I open it up, sure enough, it's full of white and green oxide. Looked fine from the outside.

But that oxide adds a LOT of resistance to the circuit. Let's calculate how much, ok?

Let's assume a Honda has a 60A starter. He has a voltage drop of 12.6 minus 5.0 volts, or 7.6 volts.

V/I = R, so 7.6V / 60A = .126 ohms

That's all, .126 ohms worth of corrosion-created resistance is enough to keep this car from cranking.

I cut off the ends of the cables, wire brush the terminal, re-assemble it, and it's good to go.

(I shortened this story for the blog's sake, he had ACTUALLY already gone and gotten a new starter, and it did the same exact thing the OLD one did.... and this was AFTER Autozone had already told him there was nothign wrong with the OLD starter!)

so what are the key points here.

A voltage drop is created whenever current passes through an UNDESIRED resistance.

The greater the current, the greater the voltage drop through the same resistance.

A bad ground, or dirty battery terminal offering as little as .1 ohm of resistance can make your headlights dim or even prevent your car from starting.

You can only measure voltage DROP when the headlight is on, motor is cranking, whatever is pulling a LOAD.

A voltage drop can exist on the positive side, OR the ground side of a circuit, or BOTH.

Voltage drops are primarily created by corroded things, connectors, contacts, switches, terminals, and paint beneath body grounds instead of shiny metal.

by measuring voltage DROPS, rather than just VOLTAGE, you can quickly identify WHERE the problem is --- a corroded old connector, a bad ground, dirty battery terminals, etc.

If you've read this far, congratulations. You will never go to the parts store and by a starter you don't need. Your headlights will always shine a little brighter. And you will quickly become the person everyone ELSE turns to when they can't figure out why some particular circuit doesn't work, "Even though it's got 12.6 volts! and lights up a test light!"

[EScanlon]

Wade:

EXCELLENT!!

While it does take a bit of careful reading, and consideration, it is an excellent primer on getting the "juice" flowing.

Thanks for the good descriptions.

E

[240260280z]

Here is what actually happens when connectors corrode:

This is a fictional circuit that has a fuel pump connected to a battery and a connector in the circuit

Notes:

• ignore alternator
  
• 12.8V battery V is rounded up to 13V for simplicity
  

• The battery's job is to maintain +13V.
  
• The amount of current drawn is determined by the load on the battery, in this case the only load is the resistance of the pump (1.86ohms). (pretend the wires and connectors are perfect conductors and have no resistance)
  
• So the resistance of the pump is what "draws" current from the battery. In this example 1.86 ohms draws 7Amps at +13V (Ohm's Law I =V/R = 13 / 1.86)
  

Now a poor connection is introduced in the connector and it's resistance increases by only 1ohm.

• The battery's job is still to maintain +13V.
  
• The amount of current drawn is still determined by the load on the battery (which has now increased by 1ohm) (1.86ohms + 1ohm= 2.86ohms). In this case the load is the resistance of the pump AND the resistance of the connector corrosion. You can also see that this results in lower voltage applied directly to the pump. It has dropped from 13V to 8.5V!!
  
• So the resistance of the pump AND the connector is what "draws" current from the battery. In this second case it is 4.5A (Ohm's Law I =V/R = 13 / 2.86). The corrosion causes the current through the pump to drop from 7A to 4.5A.
  

So if you look at the specs for a typical fuel pump.....

• For 7 Amps ~ 70 gallons per hour @ 45psi
  
• For 4.5 Amps ~ 70 gallons per hour @ 15psi
  

Bottom line: Fuel pressure and/or flow decreases because of a corroded connector!

Edited September 19, 2012 by Blue

[Wade Nelson]

Excellent addition. Pictures tell a story the way words NEVER can. Thanks.

[Wade Nelson]

Here it is stated yet another way, to help any of you still trying to "get" it.

Voltage dropping a circuit tests its ability to supply FULL battery (or alternator) voltage, (or very nearly so) while UNDER actual load.

Simply measuring voltage, or using a test light, only indicates that voltage is present, or that continuity exists, but it gives no indication of how WELL the wires/switches/connectors are able to handle the needed amount of electrical current.

In older cars, with "tired" wiring, connectors, switches, etc, the ability to located corroded contacts, connectors, grounds, etc. is key to repairing them. And you do that with voltage drop testing.

Edited September 19, 2012 by Wade Nelson