Glad you're trying to tackle it. Electrical is the last frontier for many Chevelle enthusiasts. It's not that hard - really.
VENDORS: I totally agree with the other guys, American AutoWire is an excellent vendor. Extremely helpful folks and a true "better than original" quality parts made in USA. M&H makes great parts as well.
THOUGHTS BEFORE YOU START
Engine harness is the easier of the two. Do it first - then decide if you want/need/are able to tackle replacing the dash harness - or whether you should just freshen and repair.
Be aware that it is a fair amount of work to replace a dash harness, including the removal of the dash. Make darned sure your dash harness is really in need of replacement before you rip it out.
If you replace the engine harness, that's a good time to upgrade your wiring for a modern ignition system and internal regulator alternator. Decide your route BEFORE you order the harness. They're different, depending on the ignition and regulator style.
Some Tips / things to consider as you work at this.
I'm a Chevelle hobbyist for fun but an electrical engineer for a living, and the following is garnered from 30+ years of pretty heavy solder sniffing. :-)
This is pretty long, but I promise you it is worth the read if you are serious about automotive electrical system restoration on your Chevelle.
PRESUMING YOU ALREADY KNOW SOME STUFF - HAVE A SCHEMATIC - ARE NOT AN IDIOT - ETC. :-)
If you find your existing dash harness worthy of salvation, the invest the time and money in the right tools.
- Correct pencil type 30w (ish) soldering iron and fine gauge electronic rosin core solder. NOT RADIO SHACK. Get a reputable brand. Get "eutectic solder" if you can. 63/37 lead/tin.
- You'll want an DVM meter AND a test light. (Both are useful, depending on the situation) Inexpensive DVM is o.k. for this
- An LED hiker's headlamp is essential.
- a supply of shrink tube, (correct color is stealth and nice, but black will do)
- Harness wrap tape (cold shrink tape will work)
- IMPORTANT that you have the correct crimper tool for the contacts. About CRIMPERS: I do NOT mean a GENERIC RADIO SHACK MULTI CRIMPER...O.K? If you don't have a real crimping tool DESIGNED FOR THESE STYLE CONNECTORS, that's o.k. Simply plan on soldering any new connector terminals to the wire as well. forget crimping them with pliers or other improvised means. They'll eventually fail. Correctly soldered, they will exceed OEM connection quality.
- A set of hard picks. Use these to "unlock" the connector contacts from the connector housings as you disassemble the connectors during restoration
- quality miniature needle nose pliers will be highly useful as well
- Quality aerosol contact cleaner. TIP: You can purchase Mass Air Flow sensor aerosol cleaner at most box auto stores. It's an exceptionally good "stealth" contact cleaner and won't melt your plastic. DO NOT USE WD-40, DO NOT USE Brake Cleaner, DO NOT Carb Cleaner etc. or any other aerosol cleaner containing aliphatic solvents. Aliphatic solvents will dissolve nylon parts in switches as well as many other plastic parts. This is bad. Get the correct cleaners, one with cleaner only, and one with cleaner/lube. CRC makes a good ELECTRONIC spray cleaner (not their "electric motor cleaner", but the ELECTRONIC cleaner). Forget the spray crap sold for contact cleaner at Radio Shack. It's just plain rubbing alcohol. That is worse than worthless in this application. If you troll estate sales, look for a product called "ElectroWash", in spray cans. An exceptional old school contact cleaner - no longer available.
- A Dremel tool with a buffing tip can help you clean up hard to fix corrosion problems under the dash and in tight spaces.
- Also, you'll want dielectric grease (for repacking the bulkhead connectors and other water-exposed connections under the hood like regulator and headlamps).
- CONTACT ENHANCERS: I recommend using a quality contact enhancer. I use and recommend Caig Deoxit products. Properly used, I find them to be beneficial. You might also keep your eye out for bottles of "Cramolin" at estate sales. It is a fabulous old school contact enhancer/cleaner no longer available in the US. I see it every now and then.
- scotch brite pads, which you can cut up into small shapes to buff up dull contacts in places like fuse blocks, etc.
- a small brass wire bristle brush, hand operated, and one for your dremel.
- Phosphoric acid is an excellent contact brightener for heavy connections. Neutralize it after use with baking soda and blow dry, followed with a cleaner/lube to wash away any moisture and residue.
- Purple Power in a plastic bin is an excellent way to clean greasy engine harnesses.
- Lacquer thinner is an excellent way to soften brittle harness wires and wrap tape, allowing you to bring them back to life for routing corrections, etc. Once the OEM wrap tape is clean and soft from the lacquer thinner, you can rewrap your harness with it. Note with photographs where and how it is tied off before you unwrap it.
For connector repairs on your existing dash harness, (if you go that route) buy NEW connectors from AAW. They will sell you just the blank connectors if you need to fix a few places (like a cut radio power harness or a glove box light wire that's been chopped, etc.)
Order a few spare male and female connectors when you get your parts. Practice your wiring assembly technique on a new SPARE - BEFORE you start on a real harness repair. Understanding how to correctly crimp or crimp/solder wire to the contact is essential to success.
Once you remove your old engine harness, take the time to disassemble some of the left over plugs from that junk harness. Learn how to take them apart down to the last component without
destroying them. It is deceptively simple. Learn how the male and female plugs are assembled and how they make contact. Most Chevelle connectors can be successfully disassembled down to the component level without damaging them. Know how will greatly assist you when restoring a good used harness. You can very often solve "mysterious" electrical problems by disassembling, cleaning and retightening, and/or replacing just the internal contacts in your original harness connectors. Keeps the original plugs, looks totally stock, and gives you "like new" performance. In my experience, the connectors are the vast majority of the failures, after amateur chop jobs and idiot work.
HOW TO START:
STEP 1: Get your roll of black electrical tape.
STEP 2: Put it away. Proper automobile wiring harness repairs do not use adhesive backed electrical tape ANYWHERE.
Now that we have that out of the way, Inspect the under dash wiring visually, using your headlamp. Make a spread sheet of all observed physical changes/chops, and in the same spreadsheet test and list function or not of every single electrical function. every light, every switch, every blower speed, turn signal...everything.
Then, methodically go through the electrical functions of your car and test for 100% functionality. A correctly repaired OEM wiring harness with fresh or freshened up connectors will work perfectly and without complaint for EVERY SINGLE feature.
WHERE TO REPAIR:
Find your problems by first looking where others have chopped. Idiot "repairs" and chops are the most likely source of problems.
Do not ASSUME the fuse block and bulkhead connector "is fine". Disconnect, clean and polish any contact showing any signs of oxidation. Retension, as appropriate. polish/clean all fuse contacts. Test all fuses electrically - not just visually inspect. Re-grease the bulkhead connector with dielectric grease.
Do not overlook any non-factory repair. Anything with tape on it will be a do-over. If enough wire is present to allow proper routing, I cut away stubs and idiot repair remnants. I then replace the internal contact points on chopped up wires RIGHT AT THE CONNECTOR. If they've been cut and taped in an unfortunate place (far from the connector), then I restrip to fresh wire, solder and shrink tube all such connections. It takes only a moment and the connection is much more sure. (If you know how to solder) Do not attempt to solder any wire that has black tape glue residue. This is a worthless waste of time and will result in compromised solder connection. In the RARE case that you must do this due to inadequate wire length, clean the wire strands individually, as well as the wire jacket with lacquer thinner until it is perfectly clean and shiny. Brush the wire strands individually with your brass brush, if they are not bright. Then solder.
Grounding is absolutely critical to your electrical system function. It is not optional. There are many ground points supplying sub harnesses in your car. ALWAYS remove, clean to a bright shine all grounds. "All" means every single battery, engine, body, and service ground... wherever you find them. This single step will often solve MANY mystery problems on old electrical systems. Find, inspect, and clean every single one.
Anyway, just some tips from an old solder sniffer. Now you see why guys say "buy a new harness", right? You'll say that too...right up to the point where you are pulling your dash to get your old harness out.