Where to use a logic probe and where to use a test light?

[signcrafter]

I figured I would start a different post on logic probes since another post went from noid lights to logic probes. So what particular systems do you have to use a logic probe on and what can you use a test light on? I would think if a test light might not be safe for systems with computers that you should be using a logic probe almost everywhere since almost everything is controlled by computers in the newer cars?

So are test lights almost a thing of the past and just use logic probes everywhere to be safe?

 

[OHMS LAW]

I wouldn't use a test light but a logic probe is good to see pulses and fast switching voltages I.e. injector pulse.

 

[signcrafter]

You wouldn't use a test light anywhere?

 

[Brownsfan]

I would not use a test light on a vehicle newer than say 1997. Too many circuits that can be damaged if probing the wrong wire. A lot of modern vehicles have circuits that are on the can networks or are low voltage and have rest resistance ands require a different resistance value to operate the circuit. Probe the wrong wire with a conventional test light and watch the magic smoke be let out. Incadecent test lights really have no place in today's vehicles. I would even be VERY careful probing wires with a logic probe. I don't even own a test light anymore. Case in point I probed a wire in a kick panel looking for a park light wire and when I did the doors locked. So be careful with a probe because they have voltage coming off of them

 

[FiendFX]

Or you can get a power probe 3. But I prefer a test light over a logic probe.

 

[signcrafter]

Or you can get a power probe 3. But I prefer a test light over a logic probe.

Isn't there risk in using a test light on newer vehicles?

 

[Brownsfan]

Yes there is a risk. I would never use one. The safest tool for testing in a modern vehicle is a DMM. Logic probes are ok but you have to know what you are testing first. I use it to see pulses that are too fast to see with a meter.

 

[richfinn]

I,ve got an open mind about test lights on modern vehicles, I,ve seen them used to prove circuits by people who can read a schematic. I say keep it.

 

[srmofo]

I use a lot of different test equipment depending on the situation.

Have no brake lights? First thing I do is hook up a test light to the connector. You would not believe how many people will drive around with a burned out taillight long enough for the other one to also burn out. Most logic probes wont reach that far and I can see a test light from the drivers door, not so much with DVOM

For those that say they are worthless because you damage things if you test the wrong stuff.....Do you make it a habit to just randomly stick test equipment into unknown wires? Thats the habit that needs corrected. Test lights are fine for certain things, like checking for voltage at lights, starters, motors, etc.

 

[srmofo]

I would not use a test light on a vehicle newer than say 1997. Too many circuits that can be damaged if probing the wrong wire. A lot of modern vehicles have circuits that are on the can networks or are low voltage and have rest resistance ands require a different resistance value to operate the circuit. Probe the wrong wire with a conventional test light and watch the magic smoke be let out. Incadecent test lights really have no place in today's vehicles. I would even be VERY careful probing wires with a logic probe. I don't even own a test light anymore. Case in point I probed a wire in a kick panel looking for a park light wire and when I did the doors locked. So be careful with a probe because they have voltage coming off of them

Yes there is a risk. I would never use one. The safest tool for testing in a modern vehicle is a DMM. Logic probes are ok but you have to know what you are testing first. I use it to see pulses that are too fast to see with a meter.

This is my point. Randomly jamming test probes looking for a certain wire is a bad habit. Get up off your **** and go get the wiring diagram.

 

[Brownsfan]

Not always available. And multiples of the same color in the kick panel. So how in the hell would a diagram help me when it is just a bundle of wire and you have to test and FIND the correct wire. So please don't just say someone is lazy when you do not know the situation. That's why I said a meter is the only safe tool. And that was when I was new and used a meter from that day forward. Doing what I do you HAVE to probe random wires to find the correct one. In a kick panel on some vehicles it is just a wire bundle and no plugs so a diagram won't help because there is no pin positions

 

[richfinn]

This clip shows a test light being used on an engine management system.

Its a valid test no matter what any manufacturer tells you.

 

[loosewingnut]

I just took an auto electrical class to freshen up some of my skills and prepare for the computer classes on cars. They told us to just quit using test lights, and especially logic probes because of the danger to sensitive electronic equipment. It is too easy to supply power where it shouldn't be, especially since you are theoretically testing a car with electrical problems. These include shorts which can redirect power to unintended places. THE ELECTRIC TOOL OF CHOICE IS THE DIGITAL MULTI-METER. My teacher used all caps when he lectured us, so I am just passing on his info.

 

[richfinn]

I just took an auto electrical class to freshen up some of my skills and prepare for the computer classes on cars. They told us to just quit using test lights, and especially logic probes because of the danger to sensitive electronic equipment. It is too easy to supply power where it shouldn't be, especially since you are theoretically testing a car with electrical problems. These include shorts which can redirect power to unintended places. THE ELECTRIC TOOL OF CHOICE IS THE DIGITAL MULTI-METER. My teacher used all caps when he lectured us, so I am just passing on his info.

Thats what they want.......There are other forms of study available.

Never take someone elses word for it, make your own mind up.

Thats how new stuff gets discovered

 

[Brownsfan]

I just took an auto electrical class to freshen up some of my skills and prepare for the computer classes on cars. They told us to just quit using test lights, and especially logic probes because of the danger to sensitive electronic equipment. It is too easy to supply power where it shouldn't be, especially since you are theoretically testing a car with electrical problems. These include shorts which can redirect power to unintended places. THE ELECTRIC TOOL OF CHOICE IS THE DIGITAL MULTI-METER. My teacher used all caps when he lectured us, so I am just passing on his info.

Same here. But a meter can't see fast pulses that are needed to see. So probes do have there place but it should not be the only thing used to test circuits

 

[srmofo]

Not always available. And multiples of the same color in the kick panel. So how in the hell would a diagram help me when it is just a bundle of wire and you have to test and FIND the correct wire. So please don't just say someone is lazy when you do not know the situation. That's why I said a meter is the only safe tool. And that was when I was new and used a meter from that day forward. Doing what I do you HAVE to probe random wires to find the correct one. In a kick panel on some vehicles it is just a wire bundle and no plugs so a diagram won't help because there is no pin positions

Youre missing the point. You didn't know what you were testing and still choose to use the test light. So if you weren't lazy for getting a diagram, you were certainly lazy for not getting the correct tool. I'm not trying to insult you. I'm just trying to show others that test lights are not these evil contraptions that will destroy computers. You made the choice to jam that test light into an unknown wire. They are not the correct tool for every job, and they certainly are not the best tool. They are however very useful in the right circumstances. You don't agree with that, and its fine. I'm allowed to have my opinion as are you.

I'm not sure what you are working on, but i have never seen an automotive harness that had all the same color wire. Very rarely do you see the same color more than once.

You should also consider that all you may have done was complete the circuitfor the for locks. Without further information,i really have n no idea.

 

[Brownsfan]

Your are missing what I said. Lesson learned and never used it in that situation again. I have said multiple times a DMM is the only tool you should use if you are not EXACTLY sure what you are probing. Yes I was lazy for not getting the meter and never did it again. I will say it again logic probes do have a place in today's cars but conventional test lights do not. They should never even be pulled out of the tool box. Unless your working on a car from the 70s. And it was a logic probe not a test light that I used. I have never owned a test light

 

[nathan_454]

this:

I can plug it into a brake light circuit at the back of a 53' trailer and go back to the fuse panel at the tractor and read V+ if need be.

but i do have a test light, and a logic probe..... both see very very little use.

 

[srmofo]

Your are missing what I said. Lesson learned and never used it in that situation again. I have said multiple times a DMM is the only tool you should use if you are not EXACTLY sure what you are probing. Yes I was lazy for not getting the meter and never did it again. I will say it again logic probes do have a place in today's cars but conventional test lights do not. They should never even be pulled out of the tool box. Unless your working on a car from the 70s. And it was a logic probe not a test light that I used. I have never owned a test light

You're killing me with your blanket statements, but it sounds like we are mostly on the same page. Explain to me how you can damage something testing a motor circuit with a conventional test light?

How do you test a circuit that shows voltage on a meter, but fails when a load is applied? Ive only encountered it a few times, but it does happen. Usually the result of a bad connection or corroded wire.

 

[jamesc]

There is a time and place for everything, test lights have there place and in the real world you most likely aren't going to damage anything with a test light, but there is a chance of it. I use a power probe 3 for the majority of electrical work, I love the light on the end and the ability to power relays and quickly test bulbs and other actuators is very convient. I found the power probe to be quite accurate for voltage readings and It has really reduced my need to pull out the multimeter.

I even have a headlight test light for checking power wires to fuel pumps and abs motors, things like that. occasionally you have 12 volts and a ground at something but it still doesn't work. If the the wires won't power a headlight then they its a wiring problem. this works well for corrored wires/connections. you need to think before using it though as it draws more amps then a test light.

 

[Brownsfan]

You're killing me with your blanket statements, but it sounds like we are mostly on the same page. Explain to me how you can damage something testing a motor circuit with a conventional test light?

How do you test a circuit that shows voltage on a meter, but fails when a load is applied? Ive only encountered it a few times, but it does happen. Usually the result of a bad connection or corroded wire.[/QUOTE
You won't. I use the load Pro or the waekon hi vis volt pro. It will tell you voltage and with a press of a button you can load the circuit and see exactly how much voltage is getting to the circuit. Yes it may be a blanket statement but I am willing to bet the op is not a pro so a test light would not be the best option. The safest option is always a meter

 

[richfinn]

Your are missing what I said. Lesson learned and never used it in that situation again. I have said multiple times a DMM is the only tool you should use if you are not EXACTLY sure what you are probing. Yes I was lazy for not getting the meter and never did it again. I will say it again logic probes do have a place in today's cars but conventional test lights do not. They should never even be pulled out of the tool box. Unless your working on a car from the 70s. And it was a logic probe not a test light that I used. I have never owned a test light

So if your EXACTLY sure what you are doing and can read a schematic its OK to use a test light??

I,ve never damaged a car with a test light in 25 years, so I will stick to the open mind.

 

[Brownsfan]

Yes it is. If you know it's something your not going to damage. But I do not even take that risk. Let alone a DIY type of person. I don't use one and if you do that's fine. But when you do I bet you know exactly what you are testing

 

[richfinn]

Yes it is. If you know it's something your not going to damage. But I do not even take that risk. Let alone a DIY type of person. I don't use one and if you do that's fine. But when you do I bet you know exactly what you are testing

I kind of know, generically

 

[richfinn]

I dont know if its manufacturer training that is erring on the side of caution or what with regards to circuit testing, but I stopped listening to what GM said in 2001 when they told me I couldn,t claim any time for diagnostics

I stick to my methods now

Sending 12v into a 5v digital signal circuit with a test light, no ill effects

 

[signcrafter]

So is a power probe III considered a logic probe and safe for computers?

 

[richfinn]

So is a power probe III considered a logic probe and safe for computers?

Its perfectly safe as long as you understand the tool and the circuit.

As Brownsfan has suggested Load-pro leads are more useful if you have a good meter.

Keep the test light as well

 

[Skin]

So is a power probe III considered a logic probe and safe for computers?

of any of the tools mentioned the pp is the most dangerous. You need wiring diagrams.

 

[FiendFX]

of any of the tools mentioned the pp is the most dangerous. You need wiring diagrams.

As long as you don't apply power to where you are not suppose to you are safe. If you are just taking simple readings, it's computer safe. It even states it on the website.

 

[Skin]

As long as you don't apply power to where you are not suppose to you are safe. If you are just taking simple readings, it's computer safe. It even states it on the website.

applying ground is equally as dangerous if you don't know what you're doing. As I said its the most dangerous of anything asked about. I think the PP has its place, great tool for testing motors and 12v light functionality, but I wouldn't use it in an engine bay, that's just my personal opinion.

 

[JASTECH]

I still have the PP3 on my Amazon wish list incase I decide it will be of use per it's cost. Yes, I am old school and have a probe. But probes should be used at contact points. If a probe "needs" to peirce the insulation on a wire then I put a dab of silicone on it to seal/protect the wire. It was mentioned that the DMM was'nt fast enough to catch voltage changes? I don't agree with that, my Fluke 45 and Keithley 2000 are fast. If not then use your scope, if you don't have a DMM or Oscilloscope for the higher tech/diagnostic problems then don't accept the car/truck job and pass it on to another mechanic that has the proper equipment. If no one in your shop poseses the tool or talent then be honest with the customer and send them to another shop that does.
>
Thanks, JASTECH

 

[Toolhorder]

I just took an auto electrical class to freshen up some of my skills and prepare for the computer classes on cars. They told us to just quit using test lights, and especially logic probes because of the danger to sensitive electronic equipment. It is too easy to supply power where it shouldn't be, especially since you are theoretically testing a car with electrical problems. These include shorts which can redirect power to unintended places. THE ELECTRIC TOOL OF CHOICE IS THE DIGITAL MULTI-METER. My teacher used all caps when he lectured us, so I am just passing on his info.

Those that can't teach for a living. Don't listen to one guy make your own mind up after you learn other ways. I use a power probe and have been told for years by Honda that it's not acceptable but it sure finds all the problems I see and makes me a lot of money. A DVOM is still in my box but it's getting used less and less.

 

[Toolhorder]

applying ground is equally as dangerous if you don't know what you're doing. As I said its the most dangerous of anything asked about. I think the PP has its place, great tool for testing motors and 12v light functionality, but I wouldn't use it in an engine bay, that's just my personal opinion.

Wow, are you wrenching for a living?

 

[Skin]

Wow, are you wrenching for a living?

yep, but I don't work on one make so I don't have the luxury of knowing what everything is and what common problems are on everything that comes through the door like you.

Now, are you really serious that you'd judge someone by their willingness to use a PP around unknown sensitive electronics?

 

[Toolhorder]

yep, but I don't work on one make so I don't have the luxury of knowing what everything is and what common problems are on everything that comes through the door like you.

luxury really? Come on a car is a car. When I worked at a friends shop for awhile I worked on everything and didn't have a problem using a power probe on anything. I can read a Honda wiring diagram just as well as a Ford wiring diagram (even if it's inferior) and can apply the same knowledge I learned working on a Honda on anything else I encounter. It's really not that complex.
I just can't believe people do things with such a closed mind sometimes when it could help them fix a car easier. I have no problems using a power probe on anything electrical. You just have to know what the circuit does, what it's suppose to be doing, etc.. so you can test it properly.

 

[Stick]

I just can't believe people do things with such a closed mind sometimes when it could help them fix a car easier. I have no problems using a power probe on anything electrical. You just have to know what the circuit does, what it's suppose to be doing, etc.. so you can test it properly.

I can't ++ this enough. Always have an open mind when working on a vehicle. Consider the thread that spawned this one, about noid lights, which degenerated into a discussion about why they are mostly useless. I've got a set and use them quite a bit, just not as intended. If you've got an open mind and understand how things are supposed to work, sometimes uses for tools are right in front of you. So what if it's not a "manufacturer approved" method of doing something, as long as it gets the job done...

 

[loosewingnut]

I wonder what could have happened if instead of just bypassing a bad wire or connection, the electrical problem was a short that turned that circuit into a parallel circuit with multiple loads. Then the test light would have just jumped the amps up to possibly unsafe levels for the computer or sensors. I am not saying that you can't use a test light. I have used them for years and like them. I am just saying that with today's sensitive electronic components, you run the risk of burning something up. Who knows, the time savings might be worth it to you. I was just saying that it is possible to be bitten by using a test light.

I dont know if its manufacturer training that is erring on the side of caution or what with regards to circuit testing, but I stopped listening to what GM said in 2001 when they told me I couldn,t claim any time for diagnostics

I stick to my methods now

Sending 12v into a 5v digital signal circuit with a test light, no ill effects

 

[Danglerb]

If you need to ask the question, then you might be a little safer using a logic probe and/or a DMM.

 

[Toolhorder]

I can't ++ this enough. Always have an open mind when working on a vehicle. Consider the thread that spawned this one, about noid lights, which degenerated into a discussion about why they are mostly useless. I've got a set and use them quite a bit, just not as intended. If you've got an open mind and understand how things are supposed to work, sometimes uses for tools are right in front of you. So what if it's not a "manufacturer approved" method of doing something, as long as it gets the job done...

That funny you mention noid lights. I went to an interview for a techline job for a manufacturer and was told noid lights were useless by the head of the techline dept.
I thought the statement was a little out of line but I didn't say anything.

 

[richfinn]

I wonder what could have happened if instead of just bypassing a bad wire or connection, the electrical problem was a short that turned that circuit into a parallel circuit with multiple loads. Then the test light would have just jumped the amps up to possibly unsafe levels for the computer or sensors. I am not saying that you can't use a test light. I have used them for years and like them. I am just saying that with today's sensitive electronic components, you run the risk of burning something up. Who knows, the time savings might be worth it to you. I was just saying that it is possible to be bitten by using a test light.

Have you bothered to watch the video? Its a simulation test to see if an ECM will react to a false input. I posted it to prove that test lights aren,t the evil contraptions vehicle manufacturers would have you believe.

I dont think vehicle electronics are that sensitive, I think they are fairly tough to be honest