bad connections,digital meters,and test lamps

[lostforawhile]

I've seen a lot of frustration on here lately with members testing circuits,finding voltage,then having electrical issues with items not working. I'm going to clear up a myth about digital meters and testing voltage in an automotive circuit.

First, all the circuits in a car,especially those in areas subject to engine heat and vibration, water etc, operate in one of the worst environments you can imagine, they often corrode, get dirt of water in the connections, and sometimes even melt or burn, causing things to stop working..In a situation like this, you grab the ole' meter and start checking, hmmm it's not working, I have voltage, the item in question was working, what gives? :confused:

A little info about your meter,especially a digital meter, your meter is a high impedance device, to simplify, it causes such a small voltage drop,for all intents and purposes the circuit doesn't know it's there,

In order for a circuit to work, we need both voltage and current, Think of voltage as the water pressure in a hose, think of current as how much water can flow through that hose, imagine the hose is kinked, if you measured pressure at the kinked hose, you would still indicate the pressure in your water line, but no water would be able to flow through the hose to do any work,

This is the same thing that's happening in a circuit with a bad connection, your meter is showing the circuit is good, because almost no current is required by the meter when it's reading voltage, yet not enough current can flow in the circuit to operate anything,

To test this you need both a test lamp, and a meter, It's always good to have both, you will connect the alligator clip on the test lamp to ground, then connect the test lamp to the circuit in question, If the lamp doesn't come on,or is very dim, the answer is obvious, but if it does, you will also connect the meter across the same circuit, alligator clips are your friend here as people don't have three hands, with the test lamp on, and the meter connected, read the voltage in the circuit, in a good circuit, the voltage will be close to battery voltage, a large voltage drop indicated a bad connection.

Often a fuse will go bad yet look good, and the circuit will indicate voltage, this is another use of the test lamp, this is why blade fuses have the exposed areas at the tops of the terminals, with the test lamp connected to ground, you would probe each one of these, on a good fuse the lamp should light brightly on both sides, a blown or bad fuse, will only light on one side, or the lamp may be dim

[gp02a0083]

excellent write up Tim.

It's very handy to know how to test for resistance, voltage drops and continuity.

Our cars do not have a very complex electrical system, the worst is tracing down a short


i vote to have Tim's post stickied, I would like to see for our fellow members more of these posts, like tracing down shorts and a separate sticky for testing relays

[Oldblueaccord]

A good ground is hard to find on a car esp inside the car. I have found on every car I have ever owned the ignition key in the ignition switch makes a great ground. Just hook your test lead ground there and your good to go.


wp

[lostforawhile]

A good ground is hard to find on a car esp inside the car. I have found on every car I have ever owned the ignition key in the ignition switch makes a great ground. Just hook your test lead ground there and your good to go.


wp

this one most of the brackets under the dash are good, Honda uses a number of them for factory grounds

[Dr_Snooz]

Good info. Thanks Lost.

[dieselgus]

One of the handiest tools I use occasionally (occasionally because you will not pry my Fluke out of my cold dead hands. Ever) is the Power Probe. Quick, simple, down dirty easy to use DMM that connects directly to the battery, has enough cable to reach the trunk of anything, and allows you to both meter a circuit AND put either battery power or ground onto the circuit with the push of a button.

In the case of finding opens or shorts, knowing how to properly use a DMM goes a long way (as does owning a quality one. Yes, there is a difference in a Fluke versus a cheapo Harbour Freight meter). A lot of problems on many vehicles are ground related. Knowing how to check a circuits voltage drop and weather or not something as simple as a loose or corroded ground is what is causing the issue. Here is a nice starter link to a solid little tutorial:

http://assets.fluke.com/appnotes/automotive/beatbook.pdf

For something like a fuse that looks good but is suspect, use a meter. A good fuse will not show any voltage when metering across it. Nor will it have any resistance to it when tested outside of the circuit.

I know these are not electrically sensitive cars, but learning to use measurement devices properly now is a good trouble shooting skill to have in the long run. Pecking around with a test light (even the computer safe kind) on some of these newer vehicles is not a generally good idea or accepted practice in vehicle repair.

Just the other view from the other side of the coin.

[ecogabriel]

Nice writeup; it should go sticky.

[dieselgus]

Let's check a relay:

There are usually 4 terminals on many of the relays in our cars (not going to go into 5 pin Bosch style or some of the DPDT stuff. A lot of info can be found here though: http://www.the12volt.com/relays/relays.asp )

The terminals may be numbered (85, 86, 87, 87a, and 30 on the typical Bosch 5 pin perhaps. see the link, finks). Just what do these numbers correspond to? The two lowest (85, 86 for demonstration purposes) are usually the coil. Why is there a coil? The coil is part of an electromagnet. The other portion is the core. WHen current passes through the coil, a magnetic field is created, and it attracts the armature. When the armature pulls in, it connects a pair of contacts, and completes a circuit. This is the simple version. But how do you check a relay?

Well, the first thing to identify is that the circuit is getting power. You may have a coil that triggers with a ground, so there will be voltage on one of the coil pins (if it is battery, it is there all the time. If it is ignition, it is only there when the key is in the on position), and a ground will be applied to the other pin from whatever is controlling the relay (a switch, output from the ECU, a sensor, etc). Verify that there is both the control signal and the opposite voltage on the other terminal (in the case of positive trigger, one side is always grounded, and a positive voltage is applied to the other side to complete the circuit). Now that you have verified that both things appear, yet your relay does not make a click, chances are the coil is "open" or not conducting electricity through the fine wire. This is always a replace type of situation as the coil is not worth repairing.

So.......say your relay makes a click, yet still does not turn on whatever it is supposed to??????

Now on to the other two terminals. This is where you measure the one marked 30. It will be supplied through a fuse or some other source (hey....may be a ground switching relay. Never know). Verify that you are seeing the correct supply on the supply terminal. If you do not, your issue may be a blown fuse, corroded terminal, broken wire, etc etc. But say you are seeing the correct voltage on there.......yet it is still not passing it through to the device. Chances are the little silvered conductors at the ends of the relay contacts are either arced and damaged or are oxidised and are not conducting. An easy quick fix is to use some very fine sandpaper (500grit) to gently shine up and flatten out the contactor tips. This may or may not work. Or the coil is weak and not pulling in the contactor tips closely enough to make proper contact. Either way, for the price of it, may as well change it out.

It is a 50-50 shot as to what makes a relay stop functioning. Sometimes the relay itself is the culprit. Sometimes (more often than not) it is the circuit that is controlling the relay (or the supply voltage side) that is at fault. Before you start swapping in parts, always make sure that everything that the relay needs to operate is present first. Verify. Check the load side as well (maybe something is shorted slightly down stream and is taking the fuse out. Or the load is just on the upper side of the capacity of the relay and it is slowly killing the contact tips.

As with any troubleshooting, always divide and conquer. Pull the relay out of its socket and check component itself (DMM on continuity for the coil is the best bet). THen check the turn on circuit. Then check the supply and the load.

[Dr_Snooz]

"Situational awareness" also helps. If the wiring is clean and orderly and every connector you open looks clean and well greased, then you likely do not have to worry too much about corroded and dirty connections. If, on the other hand, the car is rusted out and plagued with lots of little electrical gremlins, then you probably do.

[lostforawhile]

One of the handiest tools I use occasionally (occasionally because you will not pry my Fluke out of my cold dead hands. Ever) is the Power Probe. Quick, simple, down dirty easy to use DMM that connects directly to the battery, has enough cable to reach the trunk of anything, and allows you to both meter a circuit AND put either battery power or ground onto the circuit with the push of a button.

In the case of finding opens or shorts, knowing how to properly use a DMM goes a long way (as does owning a quality one. Yes, there is a difference in a Fluke versus a cheapo Harbour Freight meter). A lot of problems on many vehicles are ground related. Knowing how to check a circuits voltage drop and weather or not something as simple as a loose or corroded ground is what is causing the issue. Here is a nice starter link to a solid little tutorial:

http://assets.fluke.com/appnotes/automotive/beatbook.pdf

For something like a fuse that looks good but is suspect, use a meter. A good fuse will not show any voltage when metering across it. Nor will it have any resistance to it when tested outside of the circuit.

I know these are not electrically sensitive cars, but learning to use measurement devices properly now is a good trouble shooting skill to have in the long run. Pecking around with a test light (even the computer safe kind) on some of these newer vehicles is not a generally good idea or accepted practice in vehicle repair.

Just the other view from the other side of the coin.this particular thread isn't about using a test light to check a computer, or other such nonsense, it's about showing how a modern digital meter can be fooled by a having a bad connection in a harness, and how to do a voltage drop test on a hot wire to verify there isn't a high resistance connection, Even on a connector plug that looks clean, there can be enough oxidation between a pin and it's mating connector to cause an issue in a circuit, this is one of the reasons that you are supposed to pack the connection with silicone dielectric grease, this helps keep moisture out of the connection, yet won't block current flow, I'm sure you've seen that white grease in a connector, The reason you have these issues,especially as a wiring harness ages, is simply the environment it's operating in, Be glad Honda usually does a good job and uses quality wire and connectors, The wire they use is so good, I often buy salvage yard harnesses,dismantle them, and reuse the wire out of the loom, it almost always looks brand new. Compare this to a Ford or a GM harness of the same age, the wires are often corroding back into the insulation, the plugs often have water in them etc, They have gotten better, but lousy was the standard for years, I'm tracing electrical gremlins in my 90 towncar all the time, the factory wiring is just crap

[ecogabriel]

I can testify that even slight corrosion may cause big problems. A guy at school had his car that would not crank, only made the solenoid clanking sound and nothing else. I saw a little bit of white crap on one of the battery terminals, but the inside looked OK. To be sure, after cleaning up that terminal, the car started right up. Even if not evident, oxidation may be present.

[dieselgus]

I spend all day doing automotive electrical, and yes, I can agree to modern domestics having atrocious electrical issues. Especially GM C-K series platforms (Taho, Suburban, Truck). Last good one involved going through every ECU, TCU, BCM and fusebox connector and cleaning them with contact cleaner, then a good coating of dielectric, then reassembly and test to clear up slight oxidation issues to cure an intermittant no start condition. Vehicle in question was a 2010 Suburban. Does the police no good if they can't start the truck to head to a scene at all. That was a day of chasing things that I will never get back.

But it is still good practise to be very familiar with good DMM techniques. In 20 years of automotive electrical, I have yet to have anything fool a DMM when used properly. Different strokes for different folks, bit if I backprobe a connection and am only getting 10 V instead of proper 12.6, (or 14.4V if running), and I meter across the connection and see a 2+V drop, then I sure can see the poor connection, and know that there is a high resistance in there that can be verified instantly in resistance mode. Gotta remember, these may not be the only cars members on here will be fixing themselves over time. May as well get used to good diagnostic procedure now using modern proper test equipment so that if by chance in 4 or 5 years they do get something modern and want to go probing for something that no inadvertant damage or frustration comes to them. Just a thought.

[lostforawhile]

I spend all day doing automotive electrical, and yes, I can agree to modern domestics having atrocious electrical issues. Especially GM C-K series platforms (Taho, Suburban, Truck). Last good one involved going through every ECU, TCU, BCM and fusebox connector and cleaning them with contact cleaner, then a good coating of dielectric, then reassembly and test to clear up slight oxidation issues to cure an intermittant no start condition. Vehicle in question was a 2010 Suburban. Does the police no good if they can't start the truck to head to a scene at all. That was a day of chasing things that I will never get back.

But it is still good practise to be very familiar with good DMM techniques. In 20 years of automotive electrical, I have yet to have anything fool a DMM when used properly. Different strokes for different folks, bit if I backprobe a connection and am only getting 10 V instead of proper 12.6, (or 14.4V if running), and I meter across the connection and see a 2+V drop, then I sure can see the poor connection, and know that there is a high resistance in there that can be verified instantly in resistance mode. Gotta remember, these may not be the only cars members on here will be fixing themselves over time. May as well get used to good diagnostic procedure now using modern proper test equipment so that if by chance in 4 or 5 years they do get something modern and want to go probing for something that no inadvertant damage or frustration comes to them. Just a thought.
please stop trying to derail my thread, it's good information but it's putting this thread off topic, I've been doing this stuff most of my life, you are not the only person on this site who knows electrical, if you don't understand how a circuit can show voltage, yet not be able to flow enough current to operate the circuit, then you need to go back to basic ohms law and also read up on your DMM again, You will hear this same thing from nearly every guy who works on 12 volt systems, if the circuit shows voltage and still isn't functioning, put the circuit under a load and check voltage again, if you get a good voltage drop, the circuit can't flow enough current and you need to start tracing backwards

[Oldblueaccord]

Lost its a public forum you'll get input. Start a blog if you want a soap box. I use a meter everyday in my work your not the only guy in the world that does electrical work.

Just to add,god forbid, a megger works well for a load test on a wire,motor etc.


wp

[lostforawhile]

Lost its a public forum you'll get input. Start a blog if you want a soap box. I use a meter everyday in my work your not the only guy in the world that does electrical work.

Just to add,god forbid, a megger works well for a load test on a wire,motor etc.


wp
I don't mind the input I frequently get in headbutting contests with him is all

[2oodoor]

Lost its a public forum you'll get input. Start a blog if you want a soap box. I use a meter everyday in my work your not the only guy in the world that does electrical work.

Just to add,god forbid, a megger works well for a load test on a wire,motor etc.


wp

What's a megger?

[Vinny]

What's a megger?

megohmmeter used to check high voltage

[lostforawhile]

What's a megger?


http://wiki.answers.com/Q/What_is_a_megger_test


I used to have one until some of my expensive tools came up missing a few years ago, it vanished along with my really nice portable scope