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Duramax Second Generation: 2004.5-2005 (LLY) Discuss the second generation (2004.5-2005 LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion belongs in Third Generation forum due to the differences of the 2006 LLY and the 2004.5-2005 LLY. Engine related discussion ONLY.

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Old 11-26-2004, 09:15 PM   #1 (permalink)
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6.2 liter overcharging

Hi Folks,

I have an 84 Chevy Blazer military CUCV with the 6.2 liter diesel. As most of you know it has the 24 volt system. Lately, if the truck did not start right away and I had to keep cranking, I would only get about 25 cranks before I was dead in the water and have to wait an hour or so before I was back up to speed. I have noticed for years, yes years, that the volt gauge on the dash has always been just a slight touch in the overcharging range. A very good mechanic told me it sounded to him like one of my batteries may be a little tired. So, instead of messing around figuring out which one it is, I bought two 700amp batteries. So now, when she is tempermental and needs a few cranks to rev up, I have plenty, but...the voltmeter still is just a touch in the overcharging range; not alot, but is definately there. Is this going to wear the life of my precious new batteries, and, if this is a problem, where should I be looking? Of course, it has two alternators and I do not know if it has two voltage regulators or where they are even hidden. Any help would be greatly appreciated.


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Old 11-27-2004, 04:10 PM   #2 (permalink)
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GM Delcotron alternators are fully capable of charging at a rate of 15+ volts when cold. The normal charging voltage can be as high as 14.2 to 14.6 volts. These figures are from years of observing shop test equipment (VAT 40) while hooked up to various GM charging systems. The calcium separators in the GM Delco batteries are designed to handle a higher charging voltage than the other manufacturers batteries. Most batteries use antimony in the separators, and are designed to operate at a lower voltage.

Not knowing exactly which alternator(s) your vehicle posesses, it's a little hard to help diagnose what voltage level is "to high". Start by putting a volt meter across each battery and checking charging voltage. If you see anything above 14.8 volts, you may have an alternator/volt regulator problem.

GM CSI type alternators have the regulator built into the main case of the alternator. "CSI" stands for "Charging Systems Integral". Some manuals shorten the wording to SI and preceed it with the series size. 10SI being the small alternator found on cars and light trucks since the 1960's. They were available up 63 amps but were soon superceded by the larger 27SI series. It was rated at 80 amps and was used on the early GM big cars requiring lots of current. Big cars with high electrical loads like Cadillac, Olds 98, Buick Electra, GM 5.7 Diesel cars, etc. with lots of accessories and high cranking loads had them installed from the factory.

Newer vehicles have the new style, small frame, high putput alternators. These have changes in the windings allowing higher amperage at lower engine speeds to accommodate the overdriven transmissions and higher rear axle ratios in todays fuel efficient vehicles. The Government's CAFE ratings drive some of these changes.

Check your alternator voltage output with a good quality meter and proceed from there. If voltage is actually too high, the alternator can be removed for testing on a bench to verify the problem. Check all external wiring as well. It's possible to "short out" the regulator and cause it to go to maximum output without anything actually being wrong with the regulator.

Good luck!

GM Master Truck Technicians Guild Charter Member
'82 GMC 6.2 N/A, 3/4T 4X4, THM 400HD, 4:10's, 240,000 Miles, 15.5 MPG highway, 13 City. Sold and performing daily duty in British Columbia.
'84 Chev Suburban 6.2 N/A, 3/4T 4X4, 700R4HD, 4:10's, 210,000 Miles, 21 MPG highway, 16 City. Sold and performing daily duty in Indiana.
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Old 11-27-2004, 06:21 PM   #3 (permalink)
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Hey Fred,

Thanks for taking the time to explain all that. I have an appt. this Wednesday to have it checked out. I appreciate your time and wisdom...
If you ever need to know about not knowing anything, I would be glad to help!

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