'78 F250
300

Sorry if this is in a "sticky", but I checked and didn't find it.

I am really, really new to auto mechanics. One thing I don't understand is "ground."

What I don't get is how to find one. If I have a wire that the instructions says is the ground wire, what am I attaching it to? Is it just any metal piece of my truck (that doesn't have paint on it)?

For example, I bought a circuit tester recently. I use the clamp that goes to ground on any thing metal (like instructions say). Is it that easy? What if I'm grounding a wire? How do I know where to ground a wire so that I don't burn up my truck?

Sorry if you're rolling your eyes...you guys seem to really know what you're doing. I don't. I bought this truck to learn.

Thanks!
Jon

 

No rolling of eyes at all. These are great questions. You're learning the right way.

All electrical loads essentially have two "terminals." One terminal to accept current from the source (battery in this case), and one to return current to the source. If both "terminals" had to be wired back to the battery, it would be a lot of wiring, considering all the bulbs and accessories throughout the truck.

Instead, the entire structural metal of the truck, from bumper to bumper, is the "return." The structural metal of the truck is electrically connected to the negative terminal of the battery, so we set a "rule" to say that all of the structural metal on the truck is this return terminal, called "ground." That's where the term "negative ground" comes from. Doing it this way cuts the amount of wiring required in half, because now only the power wires have to be connected. The structural metal acts as the other "wire" - one big one!

The negative terminal of the battery always connects to the engine block through a large cable - so we say that the engine block is the true master ground. Things like the frame, firewall, support brackets behind the dash, etc. ultimately make it back to this point. Typically, the engine block makes a connection to a paint-free point on the firewall through a ground strap, which is a thick, strong, braided length of wire. Now the cab is grounded, and all the electrical loads in the cab can have a return path to the battery by simply tying to some metal in the cab.

The frame itself is grounded by another ground strap to the firewall, and the bed grounds by bolting to the frame. As such, the ground points of all the rear lighting simply screw to the bed sheetmetal. Engine mounts and body bushings are not ground paths. As you mention, when finding a ground connection, it needs to be absent of paint. If you are wiring something up and you mean to connect it to ground, you want to verify that there is a proper electrical connection with a multimeter. However, from a safety perspective, assume any metal is grounded. This is important if you're installing something with an exposed power terminal near a bracket, etc. Don't assume something won't conduct because it has paint. This is why good workmanship is important when wiring a vehicle - proper use of grommets, securing runs of wire, preventing melted or pinched insulation, etc.

Here's a good example: I had to ground a terminal of a relay when I installed an electric fuel pump. The relay happened to be mounted to the firewall. So, I simply ran a small length of wire from the terminal that needed to be grounded, to the mounting screw of the relay. As you can see, I buffed off some paint to create a guaranteed electrical connection.

 

As FMC400 has stated you are learning the right way and welcome to the world of auto mechanics. To add a little; what I do is cap seal the ground once the wire is secure to the attaching point with some sealant to prevent the metal from oxidizing and degrading the ground.

 

Thanks a lot guys! I am trying to install a tach and a voltmeter and I'm getting lost. The youtube videos are terrible in showing where to ground the black wire, so that's why I was asking.
Jon

 

Never be afraid to ask a question! That's how you learn.
YouTube can be helpful....or not. I've seen a lot of good videos and a lot that I just walk away thinking "geeze, what a moron".

 

Ground, ground, ground...can never have too many grounds!....battery to engine, engine to frame, frame to cab, cab to frame.....etc. etc. this eliminates almost 90% of all most electrical issues......

 

That's a great tip. I've just started doing that myself recently.

 

Good plan! It definitely pays to go over ground points and terminals, SAE connectors &c because they are subject to pollution and road salt. My favorite is NO-OX-ID "special", a thick conductive grease long used in outdoor telecom apps, buss bars, and other critical connections. Great for battery terminals, cable ends, starter cable connections, grounding straps. Cheap too. Worked so good on the 64 have taken to pre-empting any troubles on the 09

 

Here ya go...Im planning on this for my truck...

 

Disregard the GM alternator and the order of which I posted. Then you can ground your block and heads to the same stud as the rest....same for the cab and bed.....NEVER can have too many grounds.