Test light results at fuse panel - 2017 Duramax - voltage drop/short to ground?

Hello - I just watched scannerdanners video on voltage drop testing with a test light  and it’s caused me to question some of my previous conclusions about some electrical issues I’m having with my 2017 Silverado 2500 with the Duramax.

at the moment I believe I have poor grounds somewhere, which I’m in the process of cleaning connections and replacing passenger side battery fuse block and negative ground wires. I also believe I have a short to ground in some wiring or a shorted out component/module as well .ill explain why I believe that as briefly as I can but will ask my main question first as it’s possible no further  info is needed to help me out ….

so one of my issues is regarding emissions and I’m in 5mph limp mode . I saw error codes relating the reductant tank heaters so I decided to check for power at the fuse to make sure I was at least getting power to the reductant control module .. with the clip of the test light on negative battery post,  I touched both sides of the fuse with the test light and sure enough it lit up both times ..

the error code as related to a short circuit in the reductant heater  heater so I decided to put the alligator clip on the positive post of the battery and then touched the test light back onto the same fuse (the one that powers reductant module) and it lit up again on both sides of the fuse but it was noticeably dimmer then when I had the alligator clip of test light on the negative battery terminal …

I thought (I’m no expert by any means! )  this result told me I had a short to ground in the wiring headed from the fuse down to the reductant module. To confirm that suspicious i had the thought that if I disconnect the reductant module wiring from the module and the test light still lights up when connected to positive battery post and touching the fuse that powers the resultant module , that I could be more sure I had a short to ground .. so I left the test light on positive terminal and the other end touching the same fuse so I could see the light on. Well the second I disconnected the wiring feeding into the reductant module relight turned off.. I then double check the light still turned on with alligator clip on negative terminal and the other end of test light touching the reudctant fuse and it did turn on then.

those results made me believe that my resultant module must be internally shorted as once removed it stopped the ground current from flowing through the positive power wires and making my test light turn on .

after watching his video I’m unsure there are any conclusions I can draw from my test results other than maybe I can rule out short to ground , but it still could either be bad grounds or a shorted resultant module ?? 

I hope all of that makes sense and that I don’t come off as a complete electrical moron here but I think I did find a problem. I’m just not sure exactly what problem I found any longer. 

any input or thoughts would be appreciated

If you can touch the same fuse with the point of the test light and the test light will light with the alligator clip connected to both the positive and negative battery posts. Then you have voltage on that fuse, but it is not battery voltage. It is much lower than battery voltage. If you want to know how much voltage, you will have to use a volt meter.

The test light ( and all other electrical loads ) works because of a "difference in potential". The potential at the positive battery post should be 12.6 volts. The potential at the negative battery post is 0 volts. The difference in potential is 12.6 volts and the test-light shines, brightly. If you put both ends of the test light on the positive battery post, 12.6 volts, there is no difference in potential and the light does not shine. Same thing if both ends of the test light where where put on the negative battery post, 0 volts. Again, no difference in potential and the light does not shine.

Now, suppose the fuse had 6 volts.

Difference in potential between the fuse and the Negative battery post and would be ... 6 volts - 0 volts = 6 volts... and the light would shine, dimly.
The difference in potential between the Positive battery post and the fuse would, also, be 6 volts ( 12 volts - 6 volts = 6 volts)... and the test light would, again, shine dimly.

If the fuse had 8 volts:
The difference in potential between the Negative battery post and the fuse would be 8 volts and the light would shine a little brighter than it would with a potential of 6 volts.
The difference in potential between the Positive battery post and the fuse would be 4 volt and the test light would light but, be very dim. Dimmer than if the potential were 6 volts.

What you need to do is figure out why the voltage of the fuse is low.

 

Thanks for taking the time to explain that me , it makes a lot of sense to me finally and I wasn’t having much luck figuring it out searching the web .. I have a multimeter and will figure out the voltage of the batteries, the fuse, the voltage at the wiring harness that connects to the reductant module both with and without the module connected and then I think I’ll have a more precise idea what I am actually searching for sure …


if I follow you correctly then if that fuse is actually for a 5v reference componant ( I’m not saying it is but using that for examples sake) , then everything theoretically could be working perfectly fine on that fuse and the test light would light up while touching positive terminal and the fuse with the test light because the difference in potential would be 7 . It would light up as well of course from negative post to fuse too .

if I’m correct in those last statements then I’ve learned something for sure .

ill do some more work/testing , the finish cleaning all the grounds and replacing a few suspect negative cables, then test again and where I’m at from there 

I know I have some electrical problems as my batteries drain very quickly (even since they were brand new) and often time truck won’t crank over after sitting unused a day or two , I have flickering in my dash lights , my backup camera screen turns on when truck is off and sitting there if if just tap the break pedal , sometimes my scanner says it can’t communicate with BCM, other times when it does not say that it will kick out 20+ error codes just related to the bcm, the dash dimmer sometimes with dim the lights then seconds later they are bright again or sometimes the dimmer won’t even change the dash lights, a couple times while truck was off I heard the wiper washer motor just running and running for no reason at all , sometimes “service trailer brake system “ pops up on dash when I start it … lol .. there’s probably more issues I could list but those are just off the top of my
head.. normally I would just buy some parts and throw them in to see if it fixes a problem but this issue/issues involve so many expensive parts that I can’t afford to replace everything on the list so I’m trying to pinpoint actual problems that need repaired and resolve them one by one until get this rig back on the road  

thanks again for your help ! 

if I follow you correctly then if that fuse is actually for a 5v reference componant ( I’m not saying it is but using that for examples sake) , then everything theoretically could be working perfectly fine on that fuse and the test light would light up while touching positive terminal and the fuse with the test light because the difference in potential would be 7 . It would light up as well of course from negative post to fuse too .

You understand, correctly. However, 5v reference circuits are, rarely, if ever, fused. Also, the chances of a heater circuit being 5v are slim to none.

The best way to check for a bad ground is to measure the voltage on the ground while the circuit is loaded, or energized. As you would expect, the voltage should be very close to 0v. If the ground is bad, the voltage will be elevated. Less than 100mV, or 0.1 volts on a sensor ground is the accepted rule-of-thumb. Circuits that carry higher current, such as a heater circuit, may be a little higher. However, it should, still, be very close to zero.