Maintaining electron pipelines

Jan. 1, 2020
There are hundreds of color-coded wires on every vehicle, some of them carrying loads, but many more conductors these days are carrying delicate signals that require almost no current at all through tiny little wires.

There are hundreds of color-coded wires on every vehicle, some of them carrying loads, but many more conductors these days are carrying delicate signals that require almost no current at all through tiny little wires, some of them feeding terminals practically as thin as the shaft of a straight pin. 

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With vehicle weight and material cost such important factors, manufacturers are meticulous when it comes to using the smallest wire they can get by with, and when vehicles are serviced or modified by people without understanding, even the simplest modification can lead to disaster. Case in point – a nearly new Thunderbird came to me once when I was at the Ford dealer, and it had a bevy of problems. The park lights were always on and you couldn’t switch the engine off – it would keep running even if you removed the key entirely. 

The source of that problem was simple to find. The owner had added a simple trailer tow pigtail using Scotch Locks, and then he pulled his boat trailer a on a very long trip at night. The tail light wires on that T-bird are 22-gauge leads, and they just don’t have the carrying capacity to tote a bevy of trailer clearance and taillights all night. The heating and oxidizing started at the Scotch Lock and began to melt the insulation off that wire like a slow burning dynamite fuse all the way back to the power source.

While there was no dynamite on the end of that fuse, there was a lot of damage along the path of the burn. The process happened very slowly, but in the end, the wire harness was compromised.  From the back of the vehicle all the way through the harness that travels under the instrument panel, that one brown wire had melted its way into the insulation of first one circuit and then another, creating hundreds of short circuits. 

That “cheap” trailer plug install led to a $3,000 repair.  I had to strip the trim panels out of that car (even the door panels) and replace the ruined body wire harness. It was either that or split the harnesses from one end to the other and fix it while charging clock hours, and nobody wanted to pay for that. In the course of my electrical classes, I teach my guys to use relays when adding a load and while it’s a little more trouble, it makes for a better job. 

I’ve seen more than a few problems on trucks where people have added running board lights and even cab clearance lamps.  Sometimes those extra loads just cook the headlight switch and its connector, but current will always find the weak link, and there are times when oxidation and melting can happen at component connectors or relays even if no aftermarket loads have been added.                                                                                                                                                                                                                                                             

The point is that some people know just enough about wiring to be dangerous, and even in shops there are some nuts-and-bolts leaning technicians who just see wiring harnesses as a necessary evil. How many screws have you seen driven into a wire harness while aftermarket accessories were being installed? How many harnesses have you seen pinched to ruination during an extensive mechanical repair? What about the harnesses that are routed wrong so that they chafe on sharp edges or rub against a dusty A/C suction line? And the thicker and heavier wire harnesses are, the more likely they are to have insulation compromised by metal.  Harness anchors and tie straps are there for a reason, and when I was at the Ford dealer we could get paid a few tenths warranty time for re-taping and re-attaching a wire harness anchor. 

But to be fair to the DIY folks, there obviously are owners who understand how to avoid damaging their wire harnesses. They know when to use a relay on an added load instead of a direct feed, and they know how to properly route and protect their harnesses when adding accessories. Some folks just have aptitude. Then there are some guys at some shops that don’t.

An almost new Wrangler came into my service bay one day with the concern that it stalled when hitting bumps. This vehicle wasn’t a month old. I found a cheap looking black module under the dash connected to a crumpled up wad of wires shoved up under the dash that would choke a rhino. As a part of this anti-theft system installation, the accessory shop had cut one of the main wires leaving the ignition switch and had used a dandy yellow butt splice connector (frankly, I hate those things) to insert their accessory circuit, which allowed the anti-theft system to prevent the vehicle from starting if somebody tried to steal it.  When the butt splice was crimped, it just squished the wires and shoved them out of its barrel to the point that they were barely touching.  Warranty didn’t pay for that repair. Solder and heat shrink is obviously a better fix than butt splices or tie-and-tape.

Whenever I install accessories, I like for them to look as factory as possible, and I match wire lengths, then solder and heat shrink my connections.  It takes a little longer, but every job is a picture of the person who did it. Do it right and whatever you add to the car will likely be trouble-free for years to come. On more than a few occasions I’ve received the request to remove aftermarket accessories that were sloppily installed when the car was newer, and that needs to be done neatly as well.

Now That You’ve Found It…
One of the most important pieces of information I teach when launching a wiring repair in the course of my teaching is to treat connectors and terminals with the gentleness and respect they deserve. This can be challenging, because there are times when simply disengaging a connector turns into a problem solving exercise.

The Nissan connectors with that spring loaded green release wedge are interesting – you push that green wedge down into the connector and latch it, which releases the connector, then when you reseat the connector, the green wedge should pop back up.  That’s peachy when those connectors are new, but plastic tends to change shape under age and hostile conditions.

That kind of connector is just one example among dozens, and all of us know how unfriendly some of them can be. Of course, I know I’m the only one who has broken stubborn wire harness connectors out of ignorance and frustration, right? Toyota likes to run their harnesses all over the car with a minimal number of in-line connectors, and there’s a reason for that.

Next there are the problems that come with the terminals themselves.  A shop owner friend of mine attended a GM school on wiring a while back and he was told by that instructor that GM connectors can begin to give pin fit trouble if they’re disconnected and reconnected more than about five times. It’s not to say that they will give trouble at that point, but there’s a reason why GM emphasizes pin fit in their diagnostics.  There are entire sets of tools on the market for circuit testing and for checking pin fit.

Usually you can see with your naked eye if a female pin has been spread, and a bent male pin will obviously prevent the connector from seating. If you’re getting old like me, put on your second set of eyes and use a really bright light or photograph the connector with a good digital camera and then zoom in on it and you’ll be surprised what you’ll see that way that you missed seeing the old fashioned way.  Digital cameras are also good when you want to get a good look at the cavity ID numbers that are molded on some connectors.

When you repair or replace a wire harness in a high mobility area, make sure you arrange things so that the wires don’t bend to sharply in a small area to the point that the copper gets brittle right where it feeds the connector. One example of this would be the shift interlock solenoid wires on some cars, where the shifter (and the solenoid move back and forth a lot. It’s good to put a curl in harnesses like that one so that movement doesn’t over-stress the wires with tight bending.

Repair With Care
It goes without saying that tapered test light leads and meter probes are terrible for spreading female connectors to the point that they have no contact with their male counterpart even after the connector is fully seated. With that in mind, it pays to be careful.

When working with blade type terminals I like to take one of those narrow stainless steel wiper blade stiffeners, we usually throw away with the old wiper blades and clip off a piece of it for use as a probe. Sometimes I’ll have to gently reshape it that it is a gentle fit in the terminal. The narrower ones work well for back-probing as well.  For round female terminals, a small set of numbered drill bits works well, because you can always find one the same size as your male pin. These handy pieces of stainless steel wiper blade stiffener can also be modified to make dandy terminal de-latching tools for many types of terminals. There are sets of terminal de-latching tools available from tool suppliers for anywhere from $10 to $100 a set.

Some terminals are available at the dealer for repairing connectors and some aren’t. I used to keep a box of male and female GM Weatherpack connectors in my toolbox for wire harness repairs.  And those colorful flip charts from Motorcraft and Delco have a myriad of OE replacement pigtails, but be prepared to pay heavy money for them. I paid $90 for a 12 pin connector a while back, but if you’re in a real bind and you don’t have a match in your box or bucket of clipped off connectors, the big bucks will have to flow. Saving old connectors from junk wire harnesses has saved my bacon (and the customers’) on many an occasion, and I have a big bucket full of them.

Now you’ve determined that the connectors aren’t the problem, but the short or open is buried somewhere deep and you loathe the prospect of jerking the instrument panel. If the wire doesn’t branch off and go several places (like courtesy light circuits do), then you simply can start at the place where the circuit is complete and bypass the short or open to get power to the dark part of the circuit, but make sure you run your bypass wire in split loom and use tie straps to secure it so that it stays out of trouble. If, however, it is a short, the offending wire should be clipped out so as to isolate it. This kind of repair is permanent, works very well and is more sensible than ripping all the trim out of the car.

Dog, rat, and squirrel damage are other concerns wire harnesses have to endure.  I fixed a thick wire harness under a Ford pickup that had been chewed by a neighbor’s dog, and even though I routed and tied it factory firm during the repair, that same dog dragged the harness back out a few days later and trashed my repair by literally chewing the harness in two.  I’ve seen dogs return to the scene of their crime more than a few times, and so it’s a good idea when fixing those to take extra measures. I actually have added metal shields to keep the dogs at bay.

Finally, when we’re working on RFI sensitive circuits that are twisted and shielded need to be just as twisted and shielded when we’re done with our repair, and with the same number of twists per inch.

Just about every seasoned technician could probably add volumes to the pointers we’ve outlined here. And whether an internal combustion engine or electric motors drives our cars, wires are here to stay.  Without them, everything eventually stops.

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