Technical Painless Wiring Warning

So out of curiosity how are factory harnessess, specifically 80-2000 models surviving?

Ive pulled several harnesses, and these are from ECM and fuel injected vehicles running significantly higher currents. Yet I don't recall seeing a single wire in them over 10 guage???

 

Copper is expensive and heavy. Today they use the smallest wires they can get away with. Most of the time it does the job.

 

The factory harnesses work because they're designed by electrical engineers and tailored for the specific application. No one-size-fits-all harnesses there. If you're willing to do the work of gathering all the individual load data (accurately) and calculating voltage drop in each individual wire and adjusting wire lengths to reduce the drop to an acceptable minimum, you could duplicate their results, although that will affect where you mount things. If paying someone to do that, the harness design shouldn't cost you more than several thousand $$.... That still won't give you a 100% 'safe' harness, but you'll be close. Even the factories screw up...

Automotive electrical has evolved over the years. Prior to the adoption of unified fuse panels in the early '60s, fuse protection was minimal. Lighting generally had fuses located on the light switch and the various accessories such as radios, heaters, lighters and wipers were fused at either their switch or at the device itself. Ignition, starting, gauges and charging wiring was unfused. And all the wires between the battery and what fuses there are is also unfused. Even after the unified panels came in, ignition, starting and charging circuits remained unfused usually.

In the late '60s/70s fusible links started showing up. Detroit realized that some corners cut were a bit too sharp so the links were added as additional protection against meltdowns under 'temporary' overloads. When EFI and all the other electrical loads started showing up in the '80s and beyond, the unified panel was gone. In their place was 'power distribution centers', usually located as close to the battery as practical, the fuse panel was still under the dash but limited to smaller branch circuits only. This was to reduce circuit lengths to their bare minimum, all to reduce voltage drop.

I'm a bit surprised that Detroit has stuck with 12V. When they switched from 6 to 12 in the mid-'50s, that effectively doubled the electrical system capacity at very little cost. Another doubling to 24 volts would do the same now. 24 volt stuff exists, it's common on severe-duty vehicles and expensive, but economy of scale would reduce the price sharply.

 

And they use the same gauge wire as a non-relay system. The point of a relay is to switch the load to a heavier wire but no one adds the heavier gauge wires

 

Or to switch the load to a shorter, more direct wire. The only time it makes sense to have the relay right next to the switch is when you have cause to use a particular switch, and that isn't up to the load.

 

This may be the route I go....I already have the painless kit, but am not a fan.

Im thinking one 12 fuse panel for the continuous feeds, and a second fed by a relay as you mentioned for the switched loads.

If Im thinking right any high amp relays could be fed off of the contiuous fuse panel as well, or a seperate ditribution block.

Im also debating dedicated battery grounds instead of chassis. I hate trying to find bad grounds and running a bit of extra wire seems like not much more work than connect ground straps.

One thing Idid notic on the painless get is it says it cannot support an ammeter. I guess thats due to the no. 10s they use.

 

Anyone got a "fixed" schematic to post?

 

Painless is correct and you don't want an ammeter anyway. this si a god article about why not. https://www.motortrend.com/news/hot-rodelectrical-system-2/

 

I always find it interesting to see a hot rod with basically no circuitry other than start, ignition, charging, and lights; but still has a fuse panel with 20 some fuses.

 

Bringing this thread back up because it’s a rainy day and I’m not at Glory Days in Byron today. I’m actually working on something else, but recalled this thread and dug back in to it to understand it.

I have the Painless 20103 harness, with the wiring diagram Crazy Steve originally posted, wired as diagrammed.

Summing up @Crazy Steve objections:

1. The Maxi Fuse does not protect the entire harness under all possible failure conditions.

2. Parallel undersized wires used instead of a single properly sized wire to handle anticipated loads.

For #1, the Maxi Fuse does protect the harness, when the car is not running. It’s only once the car is running, the alternator comes in to play, and part of the harness is now unprotected.

Re-drawing the Painless diagram shows this:



Battery to Starter is always unfused, normally a big heavy gauge cable. Grounding this cable is always going to be bad.

The 8 gauge Red from Starter to Maxi Fuse is unprotected. This is mitigated, somewhat, by making it as short as possible and routing and securing it to minimize the chances of shorting it to Ground. Shorting this to Ground will melt the wire pretty close to instantly as several hundred amps of battery power cannot be handled by #8 wire. In my case, this wire is about 16” long, with the Maxi Fuse mounted to the firewall directly above the starter.

The three 10 gauge wires (915, 916, 960) are the concern. There is an undocumented jumper between 915 and 916 in the fuse block. Without that in the original diagram, it’s less clear how the wiring is actually working.

With the car off, the only source of power is the battery. Short any of 915, 916, or 960 to Ground, and the Maxi Fuse blows. I have actually done this, twice. Once by letting a wrench touch the Alternator terminal and Ground. Once by letting the disconnected terminal of one wire touch the alternator case. 70A Maxi Fuses are not commonly available at auto parts stores, so order a spare one before you need it.

With the car running, and the Alternator now supplying a second source of power, shorting any of 915, 916, or 960 to Ground will not be protected by the Maxi Fuse, so something is going to melt down. No matter which wire you short, the other two have more capacity and will supply power to melt the shorted wire.

The argument for #2, using undersized wires, I think is going to come down to most people not having the tools or ability to work with wire heavier than 8 gauge. As a generic consumer product, Painless have to assume the customer will have what they can buy at the local auto parts store. Crimping ends on a #6 or #4 wire isn’t as easy as a #10 or smaller wire. So parallel #10 wires makes some sense.

@Blues4U suggests this modified circuit:



This leaves A (our former #8 Red) and C (former 960 Red) wires unprotected in all cases. Leaving A unprotected in no real change from before. Leaving C unprotected though seems like a potential problem. Maybe a fuse or fusible link would be a good idea here. Careful routing and securing of this new C wire is important.

The new B, former 916 Red, is now protected in all cases, as is the rest of the harness behind the fuse block. This new B requires changing how the power supply is connected to the back of the fuse block. I haven’t pulled mine out to see how easy (or not) that might be.

All of A, B, and C now need to be heavier than #10. I don’t yet know how much heavier. #8 maybe? Or #6? This depends on load anticipated on the devices connected to the harness behind Fuse Block, both fused and unfused.

If I have this right, so far, then @swade41 failure starts to make sense to me. He had the harness as originally described, and his failure point was in grounding one of the terminals that should have been connected to the alternator while the engine was running, like:



He doesn’t say which wire it was, but I think it was 915. That one is wrapped in to the harness and goes to the fuse box, which his pictures show melted down. But it doesn’t matter, shorting either of the wires connected to the alternator is going to melt the unprotected wires between Maxi Fuse and fuse panel.

Swade41’s new harness diagram shows a similar potential failure.



I think the changes shown here may help, but I’m more interested in what wire gauge they used for “cutoff 915”, and the new 916. I’m guessing that the two wires from “factory splice” to the fuse block are the same two #10 wires as before, bonded in the panel, so that each carries approximately half of the load.

The modification shown to 915 also leaves it unprotected, just as with Blues4U’s modified circuit.

The choice of fuse, Maxi Fuse, MIDI fuse, or fusible link does not matter. They’re all going to do the same thing. They are a designed weak point, intentionally failing first, in a controlled manner, to protect the rest of the wiring harness.

Have I missed anything here?