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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1686
Experience:  GM Grand Master Technician 2007. 14 year experience.
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I have an 03 GMC 2500Hd with the 6.6 duramax.....Sunday I left

Customer Question

I have an 03 GMC 2500Hd with the 6.6 duramax.....Sunday I left the house and went fishing ...when I returned to my truck both batteries were completely dead...I purchased 2 new batteries. I drove home and on the way home I noticed the voltage gauge on the dash was staying around 12 volts. On monday I went to crank my truck and again both batteries were completely dead. I removed both batteries and charged them over night. On tuesday I bought a new alternator and put in on. I reinstalled both batteries that were fully charged. I cranked the truck and the voltage gauge was reading right at 14 volts. so i turned off the truck thinking my problem was fixed. Two hours later both batteries are dead again. Any suggestions?
JA: Battery issues can be tough to diagnose - lots of things to consider. Luckily, the mechanic I'm going to connect you with knows all the tricks and shortcuts. Are you hoping to fix this yourself?
Customer: I need to after purchasing 2 new batteries and a new high output alternator.
JA: Is there anything else the GMC Mechanic should be aware of about your GMC?
Customer: No its beena good truck
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the GMC Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: GMC
Expert:  Jared D replied 1 year ago.
Hi,Do you have a multi-meter to do some draw testing?What brand of alternator did you purchase?
Customer: replied 1 year ago.
I have a Fluke 25....a delphi alternator
Expert:  Jared D replied 1 year ago.
Sorry, got pulled away to put the kids to bed.What does your meter read as the output of the alternator?Are the batteries dead now?
Customer: replied 1 year ago.
Batteries were good.....the amp draw is above 10 amps because it blew my fuse in the meter....I will have to get another fuse tomorrow.....but I will also have to get a DC amp clamp (to get above the 10 amp draw)....I am charging my batteries for the night and will resume this test when I get off work tomorrow evening around 4CDT
Customer: replied 1 year ago.
Sorry I never read the output of the alternator....It took you so long to reply so I went ahead and started the amp draw test. The alternator was new and rated at 145 amps.
Expert:  Jared D replied 1 year ago.
The two most common causes of a draw on this system are the instrument panel and the radio. Once you get your meter going I would suggest pulling each of those fuses and then watching the draw reading. If it drops to acceptable levels for either one then there is your draw. Another potential cause is the module underneath the driver's seat. Sometimes they will short out internally and make a faint clicking noise. It's usually not a 10 amp draw, but it's enough to kill the batteries after sitting about a day. If it's none of those items you might disconnect the main alternator wire and watch the draw reading. I've seen a couple of out of box failure alternators from Delphi and Delco both over the last few years.
Expert:  Jared D replied 1 year ago.
Also, does your truck have onstar? Those modules will sometimes fail and cause a major draw.
Customer: replied 1 year ago.
The instrument panel was completely rebuild about a year ago....the truck does have on-star(I have never used it and its not active now)....I installed an aftermarket radio (Pioneer) but that was about 2 years ago...I will have to check the module under the seat....and the alternator would be an easy check as well.
Customer: replied 1 year ago.
One more thing....with the batteries fully charged....the truck will drain both in about 2 hours
Expert:  Jared D replied 1 year ago.
That is a serious draw. It's unlikely that it would be coming from anything with less than a 10 amp fuse, otherwise the draw would be popping that fuse. I would probably start with the onstar fuse when you start pulling them. I would then look at the possibility of a defective shorted alternator.
Expert:  Jared D replied 1 year ago.
That kind of draw could almost be found by just touching modules with the engine off. Anything causing a high enough draw to run down two batteries in 2 hours is going to be making some heat.
Customer: replied 1 year ago.
I agree...with the truck off for the 2 hours the batteries are warm to the touch when I remove them...I forgot to mention...I do have aftermarket trailer brakes on the truck (I just thought about that could be going bad)
Customer: replied 1 year ago.
I was also told that bad grounding sometimes causes a significant draw (but I dont know about that one)
Expert:  Jared D replied 1 year ago.
Bad grounds can cause poor charging, but not a draw. The trailer brake should have a 20 amp fuse or so. It could be the culprit.
Customer: replied 1 year ago.
I hooked up one battery and the amp draw was 64 amps....I unhooked the trailer brakes and still 64 amps.....I disconnected the alternator and still 64 amps....I have removed all Fuses above 30 amps and all relays and still 64 amps
Expert:  Jared D replied 1 year ago.
What if you isolate each battery? If on of the batteries has an internal short it would cause a major draw like that.