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Electrical Diagnosis Not Always SimplePosted 11/16/2005By Brian Manley
Our first customer owned a 1974 four-wheel-drive Ford pickup. The concern: "My dash lights will work for awhile - sometimes a few hours, sometimes a few days - before the fuse blows. I just keep putting new fuses in." I quizzed the owner about any electrical "enhancements" that may have been added to the truck such as a radio, driving lights, alarm system, etc. All of these had been added or modified over the past 30 years. The area under the dash looked like multicolored spaghetti with all of the splicing and rerouting of the original equipment manufacturer harness. At this point many of you may have done the same thing as I did: grab a creeper and a flashlight to roll under the rear bumper. I found it immediately, pushed up against the back side of the bumper - a red wire with a burnt end. A quick check with a test light confirmed that this wire was "hot" with the parking lights on. I ended my diagnosis right there on the service drive before the customer left my shop. This truck was an example of a short-to-ground caused by the addition of the trailer hitch and substandard connections at the pigtail harness. Quizzing my customer about add-ons helped jog my memory about prior issues with trailer connections in the past, which led to a fast and easy diagnosis. The customer was excited that the fix was fast and simple, and that the charge was minimal for the repair. Wouldn't it be great if all electrical diagnosis was this simple? My next customer had a 1985 Dodge Ram Charger with a no-start concern. This was a straightforward case of a dead battery that failed a load test after proper charging, and it apparently died from old age (it was five years old). However, during a routine test of charging system output, we discovered that the AC generator was charging at 16.7 volts!
I directed the students to the wiring diagram, asking them to carefully make certain that every wire was in place for the charging system. It took them awhile, but they eventually found a ground wire tucked down next to the radiator; the ground that connects the battery negative cable to the fender, and ultimately, the voltage regulator. We supplied our own ground to the case of the regulator, and the voltage dropped to 14.2 volts. Repairing the fender ground corrected the overcharging condition. This open circuit condition did not render the charging system inoperative. It let the AC generator work too well, which would have boiled the water out of our new battery. Our third customer came to us with a 2001 Nissan Altima, with a concern of inoperative dome light. Our "room" light - as the troubleshooting charts like to call it - did not function when the doors were opened, but did work when switched to the "on" position with the ceiling switch.
Our circuit is controlled by a Smart Entrance Control Unit located under the dashboard. Our SMART module is in control of the following circuits:
Thankfully, we had several different ways to investigate the inputs and outputs for this module. We had the good 'ole trouble tree, complete with boxes and arrows; we had a good description of how the circuit operated, including which inputs happened in what situation; and we had a pin-out chart with a description for each wire, and what it did. First, we checked all power and ground circuits, just to make sure we didn't overlook an easy fix. This possibility was highly improbable due to the fact that all other functions of the module were working. Power locks, warning chime, remote key fob - all worked normally. Next, we moved our attention to the door input switches, making sure that each switch was grounded when the doors were opened. They all did. We then moved our attention to the actual 12-volt signal that leaves the module and heads up to the room light. Guess what? We had a signal from the module, but no light at the bulb. Testing the input at the switch in the ceiling revealed no power arriving at that junction. Where could our open circuit be?
My first thought was, "How did that wire get ripped apart with no screws near the harness and no indication that the cover had ever been off on this '01 Nissan?" Further inspection showed little shards of glass inside the ribs of the cover, and more on the edge of the dashboard beneath the cover. I also saw a few short strands of copper wire laying below the trim piece. The mystery was solved. I'll bet the glass installation company played a part in this failure, accidentally letting a pick or an awl slip down between the trim and the windshield. A call to the customer confirmed that "I just had my windshield replaced two months ago." This is a fact that slipped his mind when I quizzed him about prior repairs to the car. But, in his defense, I would have never guessed that a windshield installation could have caused a room lamp to go dark.
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We followed the wire into the headliner, then removed the door plate at the edge of the carpet next to the driver's door, but no red wire with a white tracer was in that wire loom. We then pulled the screws and the plastic cover from the "A" pillar on the driver's side, and bingo - there were two mangled wires, with a red/white wire broken in two!
