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Weight Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 12 Nov 2002 11:52:23
In Reply to: Electrical question..., Jeff Cunningham, Tue, 12 Nov 2002 09:22:27 Members do not see ads below this line. - Help Keep This Site Online - Signup |
In general, the reason to ground to the chassis is weight and cost. Running a wire back to the battery post requires, well, a wire. Depending on the current flow, that wire has to be pretty thick. That wire has a cost, you need to find a place to run it, and it adds weight. Believe it or not, every ounce counts, and an electrical harness can add 50-150 pounds to a car.
Instead of running potentially hundreds of extra wires, just tie it to the chassis. As long as the chassis is a good electrical contact to the battery ground, it acts as one big wire.
The key is the 'good electrical contact.' That's where 'ground loop' comes in. A basic in electricity is that current through a resistance produces a voltage. In an ideal world, Ground is ground. But we don't live in an ideal world. There is always some resistance between places. Let's say there is a slight resistance - 1 ohm - between the fuel pump ground point at the rear of the car and the battery ground. Say the fuel pump pulls 2 amps. Two amps through 1 ohm is 2 volts (amps x ohms = volts). So if you put a volt meter between the battery ground and the fuel pump ground, you'd measure 2 volts. That's called a 'ground loop'.
The reason a ground loop is bad is that is steals voltage. The supply to the fuel pump is still 12 volts (OK, more like 13.5 when the engine is running, but who's counting?) but that 12 volts is referenced to the battery ground. If the ground at the pump is actually 2 volts above the battery ground, the fuel pump only sees 10 volts. It won't run as well on 10 as on 12.
If you think about a piece of wire, you'll see that fatter wire has a lower resistance than thin wire. It's the cross-section of the wire that counts. (this is true for low frequency and DC signals - anybody that's going to talk about the skin effect gets a smack!). If you think about the chassis of the car, it's a lot of thin, wide pieces of wire all connected to the battery. The effective cross section of the chassis is pretty big - that makes for a very low resistance. As long as the welds at the connections are good, the chassis is an excellent ground point.
posted by 192.249....
, Wed, 13 Nov 2002 14:45:53
, Wed, 13 Nov 2002 10:56:06, Tue, 12 Nov 2002 11:52:23 <-- Viewing This Message, Wed, 13 Nov 2002 09:04:12
, Tue, 12 Nov 2002 13:18:20
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