Alternator Issues I Believe - Diesel Bombers

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Old 01-15-2009, 01:19 AM
fastfreddie's Avatar
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Location: edmonton alberta canada
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Default Alternator Issues I Believe

hey guys i need some advice. ok its winter up here in canada . after i start my truck it the alternator needle gos up and down quite a bit for about 5 min after start up even if it is warm. its the grid heaters i know that but what is bothering me is that after its running for a while once in a while the battery needle gos down for a while then jumps up to 14 again, sparatic at times . so i went thru the whole system disconnected every ground wire including battery's, brushed them reinstalled them seemed to work better as check gauge light gos out right away as before it stayed on for a while, what i noticed is that the battery hold down brackets stock made of steel dipped in rubber , the long bolts on the sides are covered in this white crud its the same stuff that is on the posts , i mean totaly covered from top to bottom, is this my problem should i change these out and use a rubber tie down whats going on to cause this , is this a indicator to my battery gauge going retarded whats going on . help
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Old 01-15-2009, 02:05 AM
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Welcome - cold enough for you?

How old are your batteries?

Do you have a multimeter? If so, when the truck is warm (meaning after the grid heaters have cycling), check the voltage at the battery terminals to see what the alternator is putting out. It should be around 14V or so - any lower and the alternator may be the culprit. You could just have a bad ground - you could remove and clean all grounds. At the very least check the grounds and make sure they are tight. Also, check the connections at the grid heater plate.

If the batteries are more than a few years old, I think Id take them to partsource or Canadian tire and have a load test done. Also, if needed, they can test your alternator as well. I think at minimum, you are going to need to change the batteries.

I replaced my batteries this fall and I had some of the same oxidation that you are describing. I switched to Optima red tops (Group 34/78) and because these batteries are AGM material, they dont corrode and you never have to clean the posts or clamps.

Again, welcome to the site

One last thing - dont ever use a rubber tie down or bungee strap on a battery

Last edited by Dr. Evil; 01-15-2009 at 03:43 AM.
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Old 01-15-2009, 03:11 AM
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You can test your alternator on the vehicle with a digital volt meter. Battery voltage with the engine off needs to be 12.5 to 12.6 volts (fully charged) when starting the test. Start the truck and let it idle and check the voltage at the batteries. It should be 13.5 to 14.5 volts. Start turning on loads such as the lights, blower motor, wipers, etc and it should stay at 13.5 to 14.5 volts. If it drops to less than 13.5 volts, you most likely have a bad alternator... To be sure though you can increase the engine RPM and see if it comes back, some alternators will not provide enough amperage at idle speed for the number of electrical loads you have turned on. If the battery voltage reads less than 13.5 volts, the alternator is bad.

X2 on rubber straps for battery hold downs
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Old 01-15-2009, 05:42 AM
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Keep in mind that you can have voltage and NOT have enough amps!!

Don't ask how I know!

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Old 01-15-2009, 10:14 AM
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My 96 would still fire the grid heaters periodically depending on how cold it was. I remember sitting in traffic a couple mornings last winter about 4 miles from my house and the grids were still firing. That may be what you're seeing.

Either way you have some corrosion from the way it sounds. I'd clean up those posts.
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Old 01-15-2009, 10:17 AM
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clean/replace the batteries. also keep in mind that when the grid heater cycles after start up itll affect the reading as well. my alternator is brand new from dodge, batteries are optima red tops, brand new, right after starting esepcialyl below 20 degrees the gauge will drop a little go up to 14 drop again and continue like that for about 2-5 minutes depending on air temps. This is normal according to cummins factory service manual and your owners manual.
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Old 01-15-2009, 11:54 AM
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Location: New Meadows, Idaho
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also remember the grid heaters take 190 Amps to run both heaters... The Alternator only puts out 120 Amps...
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Old 01-18-2009, 05:27 PM
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thanks guys very helpful,
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Old 01-18-2009, 05:54 PM
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Location: Shoemakersville PA
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I just went through the same thing here in PA when we had -10 degree nights and mine was just the grid heaters. Everything was fine just need the amps to run them that the alt was not making so the guage would drop.
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Old 01-18-2009, 07:58 PM
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Don't see how many miles you have,but it could be the brushes in the alternator too,they are easy to change and last ones I changed cost me $8.00,so it's fairly cheap.
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