Clicks but doesn't crank --- Understanding Voltage Drop

[Wade Nelson]

Recently my 280ZX would click, but not crank. I took a screwdriver and shorted between the main starter post (connected directly to the battery B+) and the solenoid terminal , and it cranked over just fine. The diagnosis: A failing ignition switch, or something between the switch and the solenoid.

But how did I know that?

It all comes down to Voltage Drop. It's a concept that seems incredibly mysterious until you finally "get it," and then it seems so simple you're gonna *facepalm* or *forehead slap* yourself. Let me try, once again, in DIFFERENT terms and examples than the last time to explain this SO IMPORTANT concept.

If you don't understand Voltage Drop, a voltmeter is pretty much useless in your hands OTHER than for simple tasks like measuring battery voltage or checking continuity.

Voltage drop can make:

Headlights dim

Batteries not charge properly

ECM's misbehave

Sensors read incorrectly

Starters click but not crank

...

So here's the basic concept. Wires and switches and connectors ACTUALLY do have some resistance. (If I remember right 1000 feet of 14AWG wire has about 17 ohms)

A corroded connector or battery terminal can have lots of resistance.

A "bad ground" of a ring terminal screwed on painted body (versus bare metal) may only have .1 or .01 ohms of resistance, but that may be enough to prevent a circuit from working AS DESIGNED.

That means if you put 12.8 volts INTO a thousand feet of 14AWG wire, you're going to get something LESS than 12.8 volts OUT of it.

How MUCH less than 12.8 volts you'll get DEPENDS on how much current is flowing through how much resistance.

A voltage drop occurs when a current flows through an undesired resistance. MEMORIZE THIS!

A voltage drop occurs when a current flows through an undesired resistance.

Let's say I had taken my voltmeter, and put it on the solenoid wire going to my starter, then turned the ignition switch to crank.

It might show a full 12.8 volts.

Why? Because the meter has 1million ohms or more, and virtually ZERO current will flow into the meter itself.

Now if I put the solenoid wire BACK on the terminal, touch my meter lead to it, and THEN try and crank:

I would have seen 5, maybe 8 volts. NOT enough to fully engage the solenoid!

Why? Voltage drop! Either my contacts in my ignition switch are corroded, or the wire from the switch to the solenoid is in bad shape, passes through a corrded connector or relay with dirty contacts.

Since the solenoid isn't getting enough VOLTS, it can't engage the starter.

If your headlights are dim, you can REST ASSURED they're not getting the full 13.2 or whatever alternator output is, and are instead getting less than 12.0 volts. Why?

Voltage drop through too-thin wires, dirty connectors, relays with burned contacts, etc.

If you put one lead of your meter on the battery minus terminal, and trace along the path voltage takes from the other terminal, through the headlight switch, through the dimmer switch, through the headlight relay, through the bulb, and back to "ground" you can usually SPOT where your voltage drop is occuring.

More often than not it's a bad ground. A car gets repainted and the body shop bolts the headlight ground terminal right on top of fresh paint.

Solution: Use your die grinder or "wheel of death" to remove the paint, leave behind shiny metal, sand the ring terminal, and re-bolt it to the firewall.

Alright, assuming ANYONE is actually reading this, it's time for me to take questions about VOLTAGE DROP....

Edited June 27, 2013 by Wade Nelson

[Darrel]

I had a problem like that, only it wasn't dirty connections. It was more vibration disconnecting wires. On my way back home from seattle to denver I stopped for fuel in the middle of nevada. After paying I went out to start the car. All I got was the click. 10 mins of searching, I found the wire off the starter solenoid. Crimped it so it would stay on and started the car.

Who knows what I would have gotten charged in the little town and what parts they would have replaced "tracking" down the problem. Know your car, fix it yourself, best way to not get raped by others who know less than you do about your car.

[madkaw]

I read it-and very true. Especially true for you FI guys. 35 year old connections need some loving. Best part of this type tune-up is that it is free, just a little labor.

[Wade Nelson]

Madkaw perhaps said it best: Those 35 year old connections "NEED SOME LOVIN'"

Dirty corroded connectors, battery terminals, burned switch and relay contacts are the most frequent causes of voltage drop.

They may induce only .001 or .01 additional ohms of resistance into a circuit, but that can be enough to cause a failure of the circuit to operate properly.

so how much voltage drop IS acceptable.

Well, it depends on the circuit!

Let's use headlights as an example. If your alternator is putting out 13.2 volts, but your headlamps are only receiving 12.9, they'll be 25-35% less bright.

So on lighting circuits, a .3V drop is more than most of us care to tolerate.

On a computer circuit, say a temperature guage sensor that feeds the ECM, you want a .1V voltage drop or less.

Back to those headlights.

Subaru (especially), Nissan, and others used wires that were BARELY thick enough to carry the amount of current needed by the headlamps. And THAT was back when the connectors were all brand new and shiny metal and not contributing much voltage drop.

You're getting a voltage drop through the wires AS WELL as that poor ground connection, dirty or burned relay contacts, the tired headlamp switch, etc.

Remember me telling you a 1000 foot spool of 14AWG wire has a resistance of around 17 ohms?

So a 500 foot spool will have a resistance of about 8.5 ohms. And so on.

The longer and thinner the wire, the more voltage drop it will incur.

Auto manufacturers GENERALLY design for a 2-5% voltage drop in their circuits.

Below is a chart showing the current, the wire length, and the gauge.

[Wade Nelson]

Let's assume you have a "bad ground" on your vehicle that has .1 ohm of resistance between the wire and the actual ground, or negative battery terminal.

How much voltage drop will that create?

the answer is IT DEPENDS ON THE AMOUNT OF CURRENT FLOWING THROUGH IT!

Let's say you have 10 AMPS passing through that "bad ground" on the way to the headlights.

V = IR

Volts equals current (10amps) times resistance (.1 ohms)

So you'll incur a 1V voltage drop as a result of that "bad ground"

Now pretend it's the cable to your starter that is passing through that bad ground.

100Amps x .1ohms = 10Volts! Well hell, if your battery only has 12.6 volts fully chraged, and there's a 10V voltage drop, the starter is only "seeing" 2.6 volts!

It's gonna click, not crank!

And that's what corrosion between the battery terminal and post does, is "inject" resistance into a circuit where there should be none --- or very little.

A voltage drop occurs when current flows through an undesired, unwanted, or unexpected resistance in a circuit.

Memorize it!

Next: Determining if your voltage drop is on your positive or ground side of your circuit!

[71Nissan240Z]

This is why its a good idea to run a lot of your electronics through a relay system. My headlights and fog lights both get supplied power directly from the battery and use the light switch as a switch instead of supplied power.