My jeep's stock wiring has two ground cabled coming from the negative terminal. I recently upgraded all of my positive cables, and now I'm on to the negative. Is it necessary to have one cable going to the engine, and one going to the frame - or would one large wire going to the engine, since the engine is connected to the frame, work?
The frame ground is needed. Although the engine is bolted to the frame all the connection points are isolated hence insulated by rubber. The reason for the rubber mounts is to isolate the engine vibration and the resulting stress from the frame. Some older vehicles did use a single ground to the block. This required a second ground wire from the block to the frame or chassis. A possible reason for the switch to a frame/chassis to battery ground connection is the increase in electronics that require a more secure ground.
You should maintain both ground wires. If you removed battery to chassis you would have to add engine to chassis wiring. The battery to engine wire is there to ground the alternator.
Battery to chassis wiring is probably a safer, shorter, and easier run than engine to chassis anyways so why bother.
You probably want 2gauge wire all the way around. You could probably "get away with" 4gauge but I by no means recommend it since I am not an electrician.
Yes, you need both grounds. I had a 1963 Oldsmobile giving to me. The person could not fix it. Every time it made a right hand turn, the car would die. He took the dash apart looking for the cause and did not find anything. I got the car and first thing I saw was the ground strap to the engine was broke. Just by making a turn the strap came apart and the car would die. I put a jumper cable on frame and engine. I drove it back to owner's house in 30 min. He was not happy. I got it working. Nice car. It was a 2 door Jet Star.
There should be 3 grounds from the battery negative.
When I build or restore a car, I run a heavy ground cable from the battery negative to the engine block, from that ground point on the engine I run the other 2 smaller grounds to the Body and then the frame, this way I only have one connection to the battery negative. I call it daisy chaining and cuts down on the amount of wire used and makes for clean looking battery terminals. I spray all the ground points with spray grease to prevent corrosion.
I purchased a '92 Jeep Cherokee that the owner couldn't get running. After chasing down electrical problem after electrical problem and fixing a few mechanical from the "tune up" he had a shop do, I discovered the alternator was unable to reliably output 14.4vdc at throttle, and was unable to output more than 12.3vdc at idle.
I replaced the crappy 2 piece battery cables that one of the shops placed on with the "tuneup" they did after soldering the screw down connections. I replaced the batt cables to the starter/block with 2/0 cable and ran 2awg from the block to the frame, and a 4awg from the neg of the batt to the body (less than 6"). The alternator died a short time later due to bad regulator, replaced the alt with an OEM Remanufactured unit and added a length of 4ga wire from the alt to the pos of the batt and added an additional 12awg wire from the alt bracket to the frame.
At idle I saw 14.1 and at throttle 14.6 on an aftermarket gauge I installed, huge difference in cranking/charging, base point being, multiple small grounds with a big primary ground helps quite a bit.