This projects makes it simple load 100 ohms onto anything reading 12 volts. If the voltage reading changes when you switch this device on, you have something else causing a resistance.
The design of this couldn't be any easier. It is simply two pair of banana jacks. The reds are connected together and the blacks are connected together. With the switch off, the meter can be used normally.
Turning on the switch, puts a 100 ohm 100 watt resistor across the red and black jacks. I picked this up on eBay for under $6. While this is extreme overkill for 12 (or even 24) volt systems, this WILL NOT WORK FOR 120 VOLTS. At 100 volts, the resistor is dissipating 100 watts of heat and that is the limit.
Simply install the resistor after soldering on some leads.
Wire up the red terminals and switch.
Install the switch.
Finally, I tested it. When switched on, it puts the 100 ohm load across the leads.
How does this thing work?It sets up a voltage divider on the circuit you are measuring. With the current flowing (through our 100 ohm resistor), the same current is also flowing through any corrosion or loose connections. The voltage drop read at the meter is across our known 100 ohm resistor so if it changes, we know some of the voltage is lost to corrosion or a loose connection somewhere else.
Does this thing work?Here's a picture of the EXACT same setup of 12 volts being read at the meter. There is a 56 ohm resistor representing some minor corrosion. With the checker turned off, we see the 12 volts as expected.
If you do any electrical troubleshooting on a car or boat, build one of these. It will save you more time the first time you use it than it did to make it in the first place.