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Customer Cars Provide Real-Life Teaching ScenariosPosted 12/10/2001
By Brian Manley
The '39 Ford that Didn't Charge
My friend and fellow instructor, Tony, brought in his newly purchased 1939 Ford with a no-charging concern, telling me it only reads just over 12 volts on the dash-mounted voltmeter with no accessories on, and just above 11 volts with the electric cooling fan and headlights operating. This car showed up at a perfect time to help me illustrate several concepts to my advanced auto students:
My student, James, was leading the charge and he began diagnosing by verifying what Tony had told us. Sure enough, the voltmeter on the dash never really climbed over 12 volts. This was a good time to point out that the voltmeter on the dash could be lying to us, or there could be a myriad of splicing issues related to the six decades of roads over which this car had rambled.
At this point, when we have a live vehicle repair in the shop, I always harp to my students that the next step is to pull the proper diagnostic procedure for the vehicle they're working on. But, because the power train in this Ford looked like it came from a General Motors vehicle when the Bee Gees still played on the radio, we had no way of finding a procedure - or did we? Because many GM cars had the same charging system, I suggested pulling a procedure for a mid-'70s Camaro with a three-wire alternator.
Back at the car, James hooked up the starting and charging system tester to perform an alternator output test. Due to the fact that the battery was in the trunk, and the alternator wasn't, he had to hook the current probe around the battery cable. With the rpm raised and the load control knob twisted, we saw 32 amps, and with no load, read just 12.5 volts - not good enough. This is a 60-amp alternator, and should read closer to 14 volts. Next, I instructed James to perform the same test right at the alternator. Sure enough - we read 14.1 volts and 56 amps. Talk about a teachable moment. I asked James, What should we do next?
Happy Voltmeter, Sad Voltmeter
James knows what voltage drop testing is, so we hooked up a DVOM from the negative battery terminal to the case of the alternator. Figure 1 shows the voltage drop of 0.01 volts - no resistance here. He then performed the same test on the insulated side of the alternator circuit, where we found 1.74 volts dropped with all accessories on! Could this be a problem? I asked. Without hesitation, he lifted the Ford up on the rack and rolled under the car to trace the B+ wire from the battery to the next junction, which turned out to be at the starter solenoid. First, we hooked the voltmeter from B+ at the battery to the starter solenoid B+ lug.
Figure 2 shows the high-heat environment that the wires and the starter solenoid must survive in. Notice the heat wrap on the solenoid - we thought we'd surely find some voltage drop right here, but our DVOM only read 0.06 volts - a testament to the low resistance of a continuous, zero gauge battery cable! Next, we tested from the same starter lug to the output wire from the alternator, and, voila - 1.69 volts dropped between these two terminals with all vehicle accessories on. No wonder the dash-mounted voltmeter sits at 12 volts!
The Hot Rod that Melts Conduit
Figure 3 shows an illustration by Master Trainer Jim Linder, from a manual he penned, titled Facts About Volts, Amps and Ohms. I show this picture to my students when I describe unwanted resistance and voltage drop testing. It's easy to relate our sad voltmeter analogy to our findings on the Ford, but where could the resistance be? I was letting James continue his investigation when he noticed some slightly melted red, plastic conduit coming off of the alternator wiring. After snipping some zip ties, and peeling loose the red plastic, a badly burned and corroded butt connector was revealed (Figure 4).
James thought it would be fun to test the voltage drop across this two-inch span, so we did, and found 1.10 volts dropped across one splice! (Figure 5).
Since we knew we had more lost voltage still unaccounted for, he kept stripping away the conduit, revealing (Figure 6) - another butt connector! Based on the overwhelming evidence of poor workmanship staring him in the face, James proceeded to strip away the entire two-foot section of conduit coming off the alternator where he found, you guessed it, another poor connection. This time it was a splice in the wire heading back to the dash, which had simply been twisted together and taped over (Figure 7). The final total: three butt connectors and one twisted pair of wires causing the low voltage condition at the battery and at the dash-mounted voltmeter.
Repairing these wires allowed 14 volts and 55 amps back to the battery, and a corresponding increase registered at the dash meter. This vehicle illustrated the absolute need to perform methodical voltage drop testing on any circuit that performs at a substandard level.
The Olds that Had No Resistance
Our Olds arrived with a customer concern of power door locks inoperative. So, as a class, we walked through the diagnostic procedure. The overwhelming consensus of where to start was at the fuse block. I had one student pull the wiring diagram for this car while another located the fuse block. Checking each fuse with a test light revealed no fuse was blown, but a circuit breaker looked crooked in its seat. Grabbing the circuit breaker made my student holler out, It's hot! Sure enough, this 30-amp circuit breaker had more than 30 amps flowing through it, and pulling it out from the fuse block separated it into two pieces. As an instructor, I asked the class what could cause this breaker to become so hot. Too much current said one of my students. A proud moment for me, for sure. OK, I said, What could cause too much current to flow through this circuit? I didn't get the response I wanted, probably because we hadn't discussed short circuits in detail, so I pulled out our short detector and explained how it functions to my class.
I plugged the detector into the circuit breaker's socket and turned it on (Figure 8). Immediately, it began cycling its internal 10-amp circuit breaker. I held the gauss gauge over one of the tester wires to show my students how the lines of flux make the needle jump when the tester cycles. Then I explained how moving the tester along the circuit and past the point of the short circuit will cause the needle to stop moving due to the absence of current flow at that point. I also pointed out that many of the wires leaving our circuit breaker are covered by body panels and carpet, so the first step in finding the cause of the short, in my opinion, was to scrutinize the wiring schematic for this circuit breaker and try disconnecting portions of the circuit, one at a time, until the tester stops cycling (Figure 9).
The 30-amp breaker feeds the following circuits: rear window defogger control, rear wiper, power door lock relay, power seats and seat recliner. Where do we start? asked one student. At the easiest point, I said. Because the power door lock relay was easily accessible near the fuse block, we started by unplugging it (Figure 10). The tester still cycled, so we moved on to the plug at the rear of the defogger control - the tester still cycled with this unplugged. Next, I fished under the driver's seat for the connector to the power seat and - after disconnecting it - the tester stopped cycling!
We pulled the seat from the vehicle and found that the ORG/BLK wire that powers the seat assembly had been rubbed through by a seat spring, causing a direct short to ground (Figure 11).
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