Rich’s Repair Ramblings #7: Electrical 103, the Ground

I may have been fixing cars for a long time, but that doesn’t mean that I still don’t make mistakes! I certainly do, and one of my most common mistakes is following that diagnostic path called “jumping to conclusions”, also known as “starting at step 4 while skipping steps 1, 2, & 3”. (Professionals who are honest with me admit to still making this mistake at times.)

In electrical repair work, it is so easy to conclude that “the component” is bad, without first diagnosing the wires and connections for that component. So what happens? You purchase a new replacement component, plug it in, and when the dang thing still doesn’t work, you end up performing diagnostic steps 1, 2, & 3 anyway.

In our previous brief overview of the basic electrical circuit, we stated that electrical flow starts at battery positive, flows through a wire to the “load” (bulb, motor, or other electrical device), and then back to battery negative through a common ground (metal chassis, body, or engine block). The test light can verify the presence of 12V on the hot side of this circuit, up to the device itself. What about the ground side? That is our topic this time.

Let me state something I was taught 40 years ago: the number one cause of auto electrical problems is loose, dirty, or corroded connections. Our AACA beauties are at least 25 years old. Many of them are two, three, or four times that old. Temperature, moisture, dirt, rust, and vibrations all wreak havoc with these connections. If you’re trying to fault-trace an electrical problem, with or without a test light, you could do worse than first checking that all connections are clean and tight.

If you are using a test light as we described last time to fault-trace a circuit, you might run into this theoretical dead-end: let’s say it’s an inoperative headlight bulb. You tried a new bulb (easy enough to do), replaced the fuse (even though the old one looked ok), and confirmed voltage at the bulb connection for both low & high beam (it’s a 7” sealed beam unit). What now? It’s not the headlight switch, because you have voltage at the bulb. Could it be the ground connection?

While there is a way to use a test light to check for ground, I find it easier to take apart the ground connection to verify that it’s clean and tight. There are several common ways that components are grounded: a separate ground wire attached to the body with a screw or bolt; a mounting screw for the device itself passing into the bare body or chassis; or a wire in a harness running to a “gang” ground some distance from the component. To check a device’s ground, you must find it.

Look at this illustration of a wiring diagram. Along the top we see the left turn indicator light and both headlights. At the bottom left we see the flasher unit. For each of these four components, I’ve drawn a red circle around “ground”; the schematic uses a triangle drawn as a series of lines. Remember, as this diagram verifies, that every electrical device must have a ground. What the diagram does not show (because of space limitations) is exactly how each component is grounded.

Ground is usually not hard to find. Invariably, if the ground is bad, removing the offending screw or bolt will reveal that it was loose, greasy, or covered with rust. Cleaning and tightening the ground screw (and sometimes replacing it with a fresh one) renews ground and brings back functionality. In the field, I’ve used a flat-blade screwdriver or a pocketknife to scrape away rust. At home, I may use sandpaper, a file, or a Dremel tool. If the mounting hole has become enlarged from rust, try a larger bolt. Do not paint the area, thinking you are protecting it from further rust! You want the ground attachment to be tightly secured to bare metal.

The photos show various grounding methods used on my own cars. Another shortcut, if you can’t find ground or want to verify that a good ground connection will make the device work, is to use a test wire to temporarily run your own ground. Beware of this potential dead-end: on some vehicles, a large metal component like an engine block is grounded to the chassis via its own separate ground strap. If that ground strap is loose or broken, your real problem isn’t the ground wire into the block; it’s the ground strap disconnecting the entire block from the car’s ground circuit. It bears repeating: the number one cause of auto electrical problems is loose, dirty, or corroded connections.

All photographs copyright © 2023 Richard A. Reina. Photos may not be copied or reproduced without express written permission.