Test Lights and Bad Grounds

Test Lights and Bad Grounds

Test Light – A probe-type device that lights up, under certain circumstances.  Sold in automotive and other novelty stores, it is used to troubleshoot simple electrical circuits, and other witchcraft.

Test lights are great for troubleshooting tail light circuits on your boat trailer.  They have NO business inside a combi-box.  Telebelt combi-boxes have circuits carrying 24v, 12v, 5v, variable voltage and signal circuits that are read in milliamps.  You can complete circuits, and cause components to fry.  In other words, you can “let the smoke out.”

Basic Rule of Electricity #1 – Smoke makes all electrical things work.  Smoke makes computer chips work. Every time you let the smoke out of a computer chip, it stops working.  Electric motors have to be large to handle all that smoke. A lot of smoke leaks out when they quit working.

Use a multi-meter.  Get one that measures DC voltage, amperage and resistance.  You don’t have to spend hundreds of dollars for a top-of-the-line Fluke meter.  You can get one for less than $10, and it will do what you need.  Keep it in your combi-box.  It will live longer there.

Sooner or later, you might end up calling in with an electrical problem.  You should have a copy of your schematic and a meter.  You are our “eyes and hands” when we troubleshoot over the phone.  We’re not much help if you can’t furnish voltage readings for us.

Bad Grounds

Some of the most difficult calls we get are traced to bad grounds.

I recently had a call from an owner that could not get his TB-105 e-stop to reset.  After going through the steps to figure it out, I determined his e-stop relay was toast.  This was an older TB-105 with a negative Mack battery disconnect.  These were notorious for high resistance at the wire contacts of the Mack switch.  Electrical components that are not properly grounded, can fail.  E-stop relays were particularly sensitive.

I wanted him to check his ground, but he didn’t have a meter.  He did have a test light that lit when he touched the positive and negative terminals of the e-stop relay.  That really doesn’t tell me all I need to know. 

He went and got a $5 meter.  When he connected the meter to the positive and negative e-stop terminals, he read 13.8 volts, since the truck was running.  Then I had him connect the negative lead of the meter to a frame ground and touch the positive lead to the ground side of the e-stop relay.  He read 2.3 volts.  I had him touch the positive lead to the M12 screws (combi-box ground) and he still read 2.3 volts.  In both cases, he should have read zero.

The problem will be traced to the connections on the Mack disconnect switch.  NOTE:  When loosening or tightening nuts on the back side of the switch, use 2 wrenches.  There is a wrench flat at the base of the studs.  Use one wrench on those flats to keep the studs from turning and twisting the wires off internally.

Another way to check the ground is by resistance.  Set your meter to OHMS.  Checking between M12 and a frame ground should read nearly zero ohms.  There is resistance in the wire.  Some meters have a continuity tester in the ohms scale.  They should “beep” when checking this way.

Oh, and meters can check diodes, too.  This way we can help you find the smoked diode in the 25 AMP fan circuit that somebody put a 40 AMP fuse in.