Mechatronics Zone
My Mercury Wiring Harness Nightmare

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The wiring harness controlling the windows and door locks in the author's 1996 Mercury.
The wiring harness controlling the windows and door locks in the author's 1996 Mercury.

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Alexander Wolfe
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It's not the harness, it's the installation
Alexander Wolfe   9/13/2011 10:38:50 AM
You rightly note the wiring inefficiency issue in the older harnesses. Even if new ones are better, the problem when fixing stuff isn't so much the harness as it is access to the harness. Taking all the crap off the door to get the window and lock assembly off. Those snap on caps which cover screws are notoriously difficult to remove; half the time they break, or their prongs get messed up and you can forget about reinstalling them. Forget it if you have to access some part of the harness that's really hard to get it. It's sorta like changing a dash cluster bulb. Unless something's really broke (i.e., it makes the vehicle undriveable), it ain't worth fixing.

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ferd   9/14/2011 9:30:36 AM

I agree.  I had the same problems with the driver's window in my 1999 Grand Marquis.  It has the same wiring shown in your pictures.  The window mechanism was designed so I could not even manually close the window - the broken regulator locked it part-way open.  The regulator assembly is very difficult to access, and is riveted in place, requiring special tools.  In my case the car has low mileage so I grudgingly paid the $300 for the dealer to fix it.  But the design IS unnecessarily complex and difficult to repair, and the least reliable I’ve experienced amongst many cars I’ve owned.  My next car will have manual window cranks, if I can find one like that.  Not only are monkeys designing this stuff, their greedy management forces you to buy it by not providing alternatives.


Charles Murray
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LIN bus
Charles Murray   9/13/2011 10:42:00 AM
The LIN bus, commonly used for door locks and window lifts, was launched around 2000, and it probably didn't start to appear in vehicles until about three years later. That helped alleviate some of the wirning bundle messes. Unfortunately, it was too late for your '96 Lincoln, Jon.

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12 volt vs. 24 volt
DougM21   9/14/2011 9:34:29 AM
There was a push some years back to convert to 42 volt automotive electrical systems (36 volt lead-acid battery, charging voltage is 42 V).  I think the purpose was to reduce the current draw for mild-hybrid, stop-start operation (no engine idling).

Turns out this isn't so easy to do, two of the problems I remember hearing about were: arcing/welding inside switches-42V is high enough to sustain an arc, and, life of bulb filaments-thinner/longer filaments failed from vibration.  The next step was a split system, with 12V kept for traditional functions and 36/42V for new technology--the cost problems of this are obvious.

24/28V might be a better choice, it's used in aircraft, but the resistance to change will be immense.  Just think of every autoparts section doubling the size of their light bulb and battery displays...

I'm no expert on this topic, it would be interesting to hear from someone with more direct experience.


David McCollum
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manufacturing ease vs. repairs
David McCollum   9/14/2011 11:40:16 AM
I have had the same issue with the windows on my Aerostar. . .a mighty vehicle that does van things, truck things, and towing equally well. Yet, everytime I work on the beast (often, at 240,000 miles) I see where something is made for a quick and easy assembly with no regard for repair. After all, the repair center is usually the auto dealer's greatest profic center.

The window motor seems riveted into the door, so the rivets must be ground away and replace using nuts and bolts. Even then it is an event to remove and replace the motor and to make any other needed repairs. Add to that the non-availability of parts. . .everything comes as an "assembly" with most individual parts not even listed. Thus, you replace the whole unit or ride with the window secured with duct tape.

Unfortunately, the throw-away society is here to stay. . .except for a few of us dinosaurs who enjoy the challenge. . . and the victory that follows.

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Vehicle wiring
dougspair   9/14/2011 10:00:08 PM
..Well...I once had 2 Cadillacs, a 1961 and a 1963...both had more wiring in the drivers door arm-rest than you show in this picture....I just bought the service manual at auto parts store...no big deal...

  Of course I was used to working (back then, in 1972-77) on Machine Tools with many thousands of wires, switchers, relays, motor starters, and a control cabinet with 40-50 large printed circuit boards with 125-150 TTL '7400' logic chips on each one........this was before even the 8080 micro-processor was available...the 4004 was considered 'advanced' at that time....SCR drives were the newest thing for running DC motors...PWM was still on the horizon....and TV and radio still had a few Vacuum Tubes in them.

William K.
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Wiring harnesses, why so large?
William K.   9/16/2011 9:41:31 PM
I could tell you some true horror stories about the engineers working with window regulator motors at one large auto company, mostly about what they did not know and didn't realize that they didn't know. None of them had ever even touched a wrench, I don't think. That has a lot to do with it.

I recall all of those "wondeful" predictions about how cars would soon have much less wiring, mostly a single power wire and a pair of skinny wires for the controls data bus, and how the vehicle would be controlled by a single controller module. But I also recall some of the problems that arose in even creating the listing of commands, with the associated address and command word. One other problem was that single centralized control module. How would you like to spend $1500 for a new module when it was a 15 cent power transistor that failed, and the rest of the module had to be replaced with it because the dealer was unable or unwilling to repair the module? Your next car might be a different brand is my guess.  Then, just a few years later I saw an article on how the auto companies" would be reducing the complexity of the vehicle by going to distributed controls", instead of one monster module. It seems that it is cheaper to have many different modules, so that each can be designed for the tasks that it needs to control, and also it makes repairs simpler and a lot less costly.

As for the 42 volt (36volt) electrical system, the part that is seldom mentioned is the (for now) un-mandated catalytic preheater that some fool proposed a few years back. The problem wound up being that it takes a whole lot of power to heat the catalyst up to operating temperature prior to starting the engine. The power equated to cranking a cold engine for almost 5 minutes. That might work with a new battery, but how many of us could crank thge engine for 5 minutes and then have any hope of being able to have the power to start it? So the problems that would come with the 36 volt system are what saved us from a cruel trick, the ctatlyst heater caper. Most of the original information about this came from Design News, many years ago, if you wish to research it.

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Re: Wiring harnesses, why so large?
sglobe   3/12/2012 9:56:16 PM
yes i agree. the mercury wiring harness is a nightmare. me too i've experience the repair of the harness of mercury parts and its a toxic work.

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