Basic volt drop test

[Ricko1966]

Many people keep stressing about chasing electrical gremlins. A volt drop test is a quick easy way to find lots of issues. This one's for a fuel pump,but substitute the word starter,head light, blower motor,etc for the word fuel pump. Another little tip when troubleshooting with a 12v test light unless your dealing with ecm or something delicate use an incandescent bulb test light not an LED or better yet use a sealed beam headlamp as a test light. Many times the circuit can light an LED but not be sufficient to run a fan, a headilight (see what I did there?)etc.

 

[Hunter79764]

I'll add the next step, when you see a voltage drop, move one of your two connections to see when it changes.
If you see 2V drop from the fuel pump to the battery, try the fuel pump to the frame near the battery to rule out a bad battery ground. Then check between the fuel pump terminal and the frame closer to the ground point.
If that's good, try the terminal to the head of the ground screw, then maybe to the terminal ring on the wire. If your voltage isn't dropping anymore, it's the connection between the ground screw and the chassis, remove it, clean it, and reinstall, then check again between the terminal and the chassis (the last known "bad" spot). You should be able to isolate it to at worst two terminals and a length of wire and usually a single connection or component, rather than chasing your tail forever and throwing parts at it.
Find a point where you know it's good or you know it's bad, then follow along the path until it changes.

You can also extend this to flow issues, like bad fuel pressure, power steering, coolant, exhaust, etc.
For flow systems, PSI is basically the same thing as voltage, and gallons/minute is basically the same thing as amps, with hose or pipe being the wire, and electrical corrosion or cut wire similar to "crud" or other blockage in a line. And a pump is basically like a battery.
Use whatever units make sense based on the system, but the concept is the same. Find where it works and/or doesn't work, and find the spot where it changes.

 

[DoubleDingo]

Excellent info here

 

[Camar068]

Many people keep stressing about chasing electrical gremlins. A volt drop test is a quick easy way to find lots of issues. This one's for a fuel pump,but substitute the word starter,head light, blower motor,etc for the word fuel pump. Another little tip when troubleshooting with a 12v test light unless your dealing with ecm or something delicate use an incandescent bulb test light not an LED or better yet use a sealed beam headlamp as a test light. Many times the circuit can light an LED but not be sufficient to run a fan, a headilight (see what I did there?)etc.

Correct. When wires get old corroded or just have a loose connection, you will have voltage drop across a wire/connection. Keep your reference (negative wire to the meter) at the battery (known good) and follow the ckt to the load and back to the positive. If you see a significant drop anywhere, you've found an issue.

 

[Shawn Watson]

Timely posts; thanks. I've probably still got a drain to chase down and I'm not sure how to do it. I got frustrated and left it in the garage last August, lol. Any advice.

 

[Ricko1966]

Take off your negative battery hook 1 lead of a light to the negative battery terminal,the other end to the other side of the light any significant draw will light the light. Start pulling fuses 1 at a time if the light goes out you found your problem circuit. Lights still on disconnect the alternator,light still on disconnect everything at the starter except the hot lead to the post. You should have found your problem circuit by now,if not you could a short in the starter but I bet you found the problem way before you got this far.if a test light won't light you will have to use a multimeter instead of a light bulb because your draw is too small. Just rereading this should have proof read it. First line should have read take off your negative battery cable,hook 1 end of a test light to the battery,they other side of the test light attach to the removed cable, do not reattach the cable. So the light is the connection between the battery and the cable.

 

[gmbellew]

I've always just checked for parasitic draws by measuring volts or amps across the individual fuse by putting the meter on the test points on the back of each fuse.

 

[Buck69]

I've always just checked for parasitic draws by measuring volts or amps across the individual fuse by putting the meter on the test points on the back of each fuse.

Unless I am misunderstanding what you are saying, measuring across the fuse won't tell you anything unless it is blown.

 

[Ricko1966]

He's saying pull your fuse 1 at a time and replace them with something like this pic. If bulb lights there's a draw if it doesn't light move to the next one. I think he said use a meter. I made this just for this purpose,the male spades plug right on where the fuse was. After I've done post 6 and know which circuit has the draw I plug this in place of that fuse,then I disconnect components until the light goes out.

 

[Buck69]

He's saying pull your fuse 1 at a time and replace them with something like this pic. If bulb lights there's a draw if it doesn't light move to the next one. I think he said use a meter. I made this just for this purpose,the male spades plug right on where the fuse was. Now I can disconnect components until the light goes out.

That makes sense. I didn't read it that way and don't see any pics for some reason.

 

[gmbellew]

Unless I am misunderstanding what you are saying, measuring across the fuse won't tell you anything unless it is blown.

if you have current flowing through the circuit from a parasitic draw, you will read a voltage across the fuse. you can also measure amps directly, i think. but if you have a big draw it can blow the fuse in the meter.

 

[Buck69]

if you have current flowing through the circuit from a parasitic draw, you will read a voltage across the fuse. you can also measure amps directly, i think. but if you have a big draw it can blow the fuse in the meter.

OK. Sorry if I have derailed this a bit. I know electrical well and have been performing drop tests for years. It's a great test and easy to perform. Even for the DIY guys with little electrical background.
I was politely trying to point out that the terms "across the fuse" and "test points on the back of each fuse" in your posts are misleading and may give a beginner an inaccurate test to perform, leading to false securities.
Removing the fuse should be included and measuring across the fuse holder may be a better way to explain what is being said?
I mean no disrespect and don't want to come across as a know it all. Just trying to help others that may be reading this thread down the road.

 

[Shawn Watson]

If the key is off, both doors closed and nothing on anywhere, should your voltmeter read 12v from the negative post to the negative battery cable?

 

[Ricko1966]

Something is still on could be as minor as a radio,or glove box light. Pull fuses 1 at a time until the draw goes away then see what that Fuze supplies power to. If the doesn't work disconnect the alternator wires.

 

[Shawn Watson]

I've got to be doing this wrong and I'm an electrickery rookie. With the key off and making sure the doors were closed each time, I pulled each fuse and every wire from the fuse box, one at a time, and each time I got 12.?v from the negative post to the negative cable on my el cheapo voltmeter.

In this pic, the red arrow shows the 50a circuit breaker and that's the only thing feeding the cab of the truck. When I pull it, I still get 12v from the negative post to the negative cable. The blue arrow shows the fuses for the amps and they aren't installed. The green arrow points to the fuse feeding the headlight relays. When I pull it, I still get 12v from the negative post to the negative cable.

I can't be doing this right, can I? Oh, and that set-up looked much cooler in my head than it turned out, lol