Diagnosis by Exclusion Was Good
1997 Pontiac Grand Prix 3800 supercharged.
I ask students to bring vehicles into the shop that pertain to the unit on which we're currently working. For example, we are in the middle of our engine performance unit, and we recently finished our electrical/electronics unit. I look for cars that have driveability issues, starting concerns, charging complaints or illuminated lamps of any kind on the dash. I need to be careful what I wish for.
David, one of my advanced auto students, mentioned to me recently that his Grand Prix had a charging system problem. His antilock brake system (ABS), generator (GEN) and other assorted lamps were lit while driving, and his system voltage had steadily dropped. He had installed a rebuilt AC generator from a local jobber, but it still didn't charge. At this point, I asked him to bring the car in for diagnosis.
This 1997 Grand Prix GTP had been "tweaked" by David. He had installed a smaller drive pulley for the supercharger, replaced the factory air filter box with - what else - an aftermarket "high flow" filter, and upgraded the programmable read-only memory (PROM) in his power control module (PCM). When I pop the hood on this car, I keep all of these performance enhancements in mind.
I approach this charging system issue by performing a thorough visual inspection: I look to see that cable connections are clean and tight, the drive belt is intact and the replacement alternator is installed correctly. A quick test of generator output reveals none; the battery is losing electrons quickly. I ask David to locate and print the procedure for testing his generator.
I have heard the expression that trouble trees are great for growing trouble. I approach their use with a bit of trepidation, mostly due to the fact that the last line usually reads something like "If xxx tests correctly and circuit 456 has no voltage drop, then replace the expensive, dealer-only, hard-to-find electronic control module (ECM)."
The first step in our trusty Battery is Overcharged or Undercharged tree is to unplug the generator connector, install a scan tool, command the "L" terminal "ON," and then test the "L" wire at the generator connector for B+. We toggle the generator "L" wire off and on, and the voltage at the connector switches between 0 volts and 11.1 volts. Is this B+? The battery voltage is 12.6 volts, but I assume the generator control circuit is capable of varying this control voltage, so I say "yes."
The next step in our trouble tree is to test for battery voltage at the large, red B+ terminal of the generator to ground. This terminal reads 12.4 volts. The next step in our tree has us replacing - surprise - the generator. OK, I can understand this. The generator has what it needs to function, but it doesn't; a black-and-white situation. David takes the rebuilt unit back to the store for a replacement, but this time, he talks them into giving him credit and moves up to the new replacement unit.
After the new unit is installed, we confidently fire up the engine and voila - 18.1 volts and 96 amps of output! Of course, I yell for him to shut the engine down, lest we let the smoke out of some expensive, dealer-only, hard-to-find ECM.
The damaged wire loom.
The next step, assuming that 11.1 volts is not B+, the end of the branch we are now on - I'm sure you're way ahead of me on this - is to replace the expensive, dealer-only hard-to-find powertrain control module. All about this module is true except for the expensive, dealer-only and hard-to-find part. David has a number of Web sites that sell high-performance, "upgraded" PCMs that he can get for around $200. Now, I am not ready to jump to this conclusion yet, but David immediately says that he was going to "upgrade" for Christmas anyway. I get the impression he is pleased to have the opportunity to enhance his horsepower that much sooner than expected.
Corrosion has displaced the copper wire strands.
Here comes the part where I need to kick myself. In the connector at the generator there is a second wire - an orange wire - that has 1 volt on it when unplugged. All along I was assuming that this value was acceptable, because the short description I'd read of this wire called it a "voltage sense wire." The entire statement reads: "The 'S' terminal may be used to sense electrical system voltage somewhere else on the vehicle for voltage control." I assumed that while running, the generator sent output to this wire. Since the trouble tree for this charging system had eliminated any mention of this wire, I follow it on the schematic from the generator connector back to an "ALT SENSE" fuse in the under-hood electrical center. When I voltage dropped this wire with the key on, engine off, I found 11 volts of loss across the circuit! The schematic showed no other detour between the fuse and the generator, so I went searching for a break.
David strips the wires to prepare for soldering.
I remove the battery and cut the black electrical tape away from the ends of the plastic conduit that engulfs this section of loom; then I peel it all back. The exposed section shown and a closer inspection finds that two wires - one orange and one green - are now completely broken in half, exposing more corrosion than strands of copper (see photo below). We jump the orange wire to restore 12 volts to the generator "S" terminal, and the charging system stabilizes at a nice 14.1 volts and 30-40 amps, never jumping high again!
Evidence at the right-front of the Grand Prix points to a past accident that probably broke the battery case, which in turn soaked the wiring harness with acid. The acid soaked into the harness and actually corroded the wires without any visible breach in any of the wire insulation. A clue was the discoloration of the harness for a short section in between two untouched sections, with affected wires swelling from the reaction to the sulfuric acid.
Critical Thinking Gone Bad
The wires are soldered and heat-shrunk.
My attempt to correctly diagnose this vehicle by excluding all things I knew to be in working order was right on track except for that one wire that I initially overlooked. Unfortunately, this probably won't be the last time I let other distractions or "assumptions" keep me from a thorough diagnosis of a system before trusting a "tree."
|Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at firstname.lastname@example.org.|
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