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Automotive/trailblazer voltage problem/idle & A/C

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Expert: - 10/6/2008


Question
Sir,
We own a 2006 trailblazer xlt 2wl.drive w/an I6 engine. it has 48Kmiles. Recently, when at a stop or redlight, the voltage would drop from 14 to near the redmark of 10volts, the a/c starts to blow hot, and the lights would dim slightly. If you increased rpms, while at a stop, the voltage would rise accordingly and the a/c would "kick back in". The check engine light is on. after running above 1K rpms, either driving or just in park, the voltage would return to 14 and the a/c cold. Note that the rpm idle speed never fell below 500 rpms unless I pressed the accelerator the entire time(s). The battery was original. I checked to see if the battery had corrosion or needed water, but it was maintainence free. I believe disconnecting the battery cleared the check engine lite. We brought the trailblzer to a mechanic. He connected it to the diagnostic and put in a new battery. He said it had no codes, but ran fine with the new battery. I started the tblzer and the rpms varied from 400-600, voltage holds at 12, and the a/c & lites work fine. When you increase the rpms, the voltage rises to 14 (which I know the manual says it should when you increase rpms). The check engine lite is back on, the rpms vary at idle speed, and the voltage falls off from 14 to 12 when the rpms decrease as you come to a stop.
Shouldnt the voltage always be at 14? I own a '96 1500 4.3 v6, have a chevy work truck w/a v8, and drive other chevy vehicles at work and w/friends. What am I missing as I believe the vehicle is still "broken"?
Thanks,
Chris in new orleans

Answer
Modern alternator regulators will initially (on engine start-up) provide 14.5 volts plus to the battery in order to equalise its cells and to provide a fast bulk charge to make up for the starting motor draw.  Thereafter, the voltage will taper off to 13.8 to 14.2 volts, a voltage below the battery's gassing point, in order to take on the electrical system load while providing a nominal charge to the battery.

The regulator is there to provide voltage regulation and does this by modulating the alternator's field excitation.  The alternator supplies power as a function of the mechanical power it takes in from the engine, a product of torque and speed.  At idle speed (500 rpm or so), most alternators will develop full voltage under very low load, but the voltage will often drop below the regulation setpoint if the connected load draws more power than the alternator can produce at idle speed.  Note that most gasoline engines will initially run at a "fast" idle during the warm-up stage, and alternator output can be greater during that time than it would be once the engine has settled to normal idle.

It ids possible that the alternator itself may be faulty, perhaps an open rectifier diode, or it may just be that its mechanical drive - the belt - is just loose.  These are the things that you might want to check out.  

Shop manual information would be good to have to speed diagnosis and to avoid assumptions.

Good luck ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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