Technical Wiring from scratch: opinions/guidance welcome

In racing (drag and circle track) the master disconnect is always on the 'hot' or Plus side.
Make sure the alt. output circuit is interrupted properly too, or else it will continue to feed the rest of the car with the switch off.
As for a simplified wiring diagram, I'd start out with a similar pro one for something like a mid 60s Chevy or Ford truck (12V, alt, minimal accessories, common lights and gauges) and modify it to reflect your specific routing and colors. Auto diagrams try to have either components situated in basic locations (left front at bottom left, right rear at right top, cluster in the middle) or for the more modern complex stuff it's Battery positive at the top, ground (or earth) and the bottom.



For clarity, you could also break it down to systems like Lighting, Starting/Charging, Drivetrain, Switches/Gauges and Accessories. One thing that techs used to complain about with manuals is that Harness Routing and the Wiring Diagrams didn't show where each circuit was located in the car. With something as simple as a T bucket and breaking it down by system, you could do this.
Take a look at the bottom figure in post 6 of this thread for an example...
http://www.jalopyjournal.com/forum/threads/52-f1-parts-catalog.713114/

I'm also a big fan of modern harness connectors. Unless you have these visible and want a vintage look, I'd go with a weathertight, repairable, solid connect as needed.

 

My 17x21 double sided laminated wiring diagram just arrived from COACH CONTROLS, Inc. Christmas came early! Every nickle I need to spend on wiring will be going to these folks. If they can take the time to finally draw a wiring plan that the clueless can understand, then they deserve my business. Bob

 

If you are referring to the Oil Pressure Switch as the safety cutoff for the fuel pump circuit, here is how I understand it to work. Background: I was looking for a switch that would do what I wanted, as in kill pump power when the engine is shut down (on purpose or an accident).

The switch I am using is the part number for a 2.3L 1975 Chevy Vega. Sounds like an odd place to get one but it is the same thread as the oil pressure gauge fitting on the top of my SBC and it operates as Squirrel explained a couple years ago:

Below is a simplified diagram of how I have it laid out.


I have been doing some thinking on this circuit though. I would like to have the ability to momentarily power the fuel pump for priming, leak checks, setting the fuel pressure regulator and float bowls, etc with the engine off. I would like to add a (ON) OFF push button momentary switch to bypass the oil pressure switch. I believe the below diagram is correct. I would be happy to hear feedback if it is not.

(UPDATE)-This design will not work for a momentary switch to prime the fuel pump. As 12v is jumped from "I" to "P" to operate the pump, "P" will then supply "S" with 12v as it is the NC position of the switch in return triggering the starter solenoid. Thanks Squirrel for the heads up on that one. Snuck right by me.

Below is the oil pressure switch. Note the terminals are marked.

 

don't do that! it will crank the engine when you push the button.

how about you put the push button across the fuel pump relay contacts?

or use a single pole, double throw pushbutton, wired with the normally closed contacts in series with the P wire, so it disconnects the P terminal from the wire to the relay, and instead connects the I wire to the relay.

 

Someone asked about the safty switch on my clutch, it goes in the wire from the start pole of my ign switch and requires the petal to be down and the clutch disengaged in order to start the car. It's a good garage door wall protected.

 

I am wiring my yellow truck from scratch,one wire at a time, no kit. No fuses either.I am using circuit breakers for every circuit

 

Squirrel is right. This design will unwantingly feed the starter solenoid. See my above post for explanation.

Below is the updated design and I think it takes the input Squirrel provided to operate as desired. I removed the momentary push button altogether. Added a SPDT On-Off-(On) switch. The "P" pole feeds the "On" circuit of the switch, "I" pole feeds the momentary "(On)" circuit of the switch for prime, and "Off" position of the switch kills the pump if desired. Doing it this way separates the "I" and "P" circuits. This was no accomplished with the SPST push button design before.

 

Is this something you're going to do often enough to be worth dedicating wiring and a switch to? If not, just a jumper wire with a couple of alligator clips on the relay terminals would do the job. I'm not a fan of adding unnecessary complexity to a wiring harness.

 

Yeah, it's not something you'd use very often. But it is handy every now and again. See mine above the wiring mess, on the top edge of the firewall.

 

Wiring ethics/good practice question.

Is it wrong to insert two smaller wires into a terminal designed for a single larger wire. Example: two 18 gauge wires in a terminal designed for a 10 gauge wire. Or wiring could be split with a step down butt connector (one in-two out). This is assuming the wire mass is the same.

This could be the situation when two wires need to be connected to one male push on terminal, or when lighting feeds need to split, etc.

I've got some step downs like this I could use.

 

GM used to do it with the Packard 56 terminals, but they had one that was wide enough for two wires. I do it occasionally with normal modern crimp terminals. Haven't had any problems.

 

The way I have my diagrams drawn up, the Battery Cutoff Switch does not kill the Magneto. I have thought about it a while and the only way I can see to kill the magneto with the Battery Cutoff is to use a relay that grounds the mag when power to the relay is removed. Below is a diagram that shows how the mag relay is receives 12v from the "I" post on the Ignition Switch which receives its 12v through Battery after the Battery Cutoff Switch. There is still a Mag Kill Switch between the Ignition Switch and the relay. All switches must be closed to operate the mag, if one is opened then power to the relay is removed allowing the mag to ground circuit to be completed.

If I am incorrect or have mislabeled something please let me know and I'll correct it.

 

The difference in where the disconnect is put is to prevent the charging system from providing power to the ignition system as the alternator is usually grounded to the engine. Sanctioning bodies want the disconnect to shut a running engine down as well as preventing electrical fires in a crash.

Equipment use the disconnect to prevent the engine from starting while the machine is being worked on. It doesn't matter, electrically, which side of the battery the disconnect switch is put on to keep the engine from starting. Technically, since electrons are a negatively charged particle, they flow from negative to positive but any opening in the circuit prevents their flow.

There was a question about a Magneto. A true magneto creates it's own electricity for the spark and has to be grounded to stop it from creating a spark. A relay can be used to do this by connecting the normally closed contacts to connect it to ground and having the ignition switch power the relay coil to open the contacts.

 

Generally no problem as long as the total area of the wire is within the 'normal' range of the connector and it's fully crimped. Better wire charts will have the wire cross-sectional area in KCM (1000 circular mils), simply add up the total of the sizes needed and select a wire crimp that's equivalent. I've even 'shimmed' these by added an unused wire then cutting off the excess part after crimping. But I'll emphasize that you need the correct crimp tool for the connector if doing this. One other caveat; be very careful mixing disparate sizes if individual wire strand sizes vary a lot (i.e. like adding a #12 to a #2, or a #18 with a #10 in one connection) as the smaller wire can literally fall into the 'cracks' between the larger strands.

Do keep any smaller wire 'tails' down to minimum length if the 'main' wire is fused for the larger size to avoid overloading the smaller wire.

 

If the circular-mil area of the two wires is close to the larger one (intended use), not a problem. You can even twist the smaller ones together so they're stronger.

We had a huge flap at work, maybe close to 25 years ago. This was regarding a test set that had a 12 and 24 gauge wire in the same lug. Our Q.A. manager raised it up to the local DPRO office, who oversees almost anything with USAF contracts.

Nothing in MIL-T-7928 said we couldn't do it, nothing in our internal workmanship standards touched on it. So... I had to make up a number of pull-test samples with the 2 wires in one AMP lug, think I had 6 made up and crimped using the right tool, etc.

On the pull test, we only did 3, held the lug and pulled the wires. In all cases, the wires failed long before the crimps did. Funny that the job it was for, something we took on when we closed another facility and we inherited the work. The Q.A. manager, he was booted the next week.

 

Milspec wire almost always has fine strands, no matter the size. Not always the case with commercial wire, so you do need to pay attention.

 

Does anyone have a non insulated terminal crimper for 8 gauge that they would recommend. I've had good luck with the type pictured below but they are only good to 10 gauge. I tried 8 to see but the crimp is a no go. Would love to find the same type but for 8 gauge.

 

I looked for a while a few years ago, and didn't find any...now they seem to be around?

http://www.amazon.com/Pro-Crimp-Tool-Non-Insulated-Terminals/dp/B0002KR9MU

...but you can use this type, too? less money, you probably won't use it often

http://www.amazon.com/NOCO-D800-Hammer-Indent-Crimping/dp/B006VU5WX8

 

Spend a few bucks more and get this..... http://www.amazon.com/Hydraulic-Bat..._UL160_SR160,160_&refRID=0SN4XNKAZ0FHFKCR2YCP
HF has what I assume is the same unit also if you can buy locally.

Once you go above #10 there just aren't any hand-held crimpers. All you'll find is lever types like Jim linked to or hydraulics. The hydraulic units are much more versatile and will give a more consistent crimp. Those hammer types (his other link) can work, but they're tough to use with consistent results and you can over-crimp and get a damaged connection.

 

Here is a helpful post.. http://www.jalopyjournal.com/forum/threads/wiring-101.843579/page-6#post-11246871