Hey friends, so I have a 2007 Yukon Denali and a few weeks ago, the battery died. Truck wouldn’t start and the battery wouldn’t hold a charge on the trickle charger.
It was an old battery, no harm, no foul, I bought a new one and installed it. Everything seemed fine, 2 days later after driving on the highway for an hour, I stopped at a gas station and the truck woudn’t start, sure enough the new battery was almost dead.
I checked voltage coming out of the alternator and it seemed really low so I just went ahead and replaced the alternator. Its almost 20 years old, no surprise it decided its time to rest had come. I’d also note that the mega fuse connecting the alternator to the battery was also replaced about a year ago.
At the same time, I also bought a bluetooth battery monitor thing so that I could monitor voltage and charge while I drive.
The new alternator seems to be working, but I’m not sure if the behaviour is correct/expected, so I just wanted to see if this is normal or if I still have a problem.
When I first start the truck it sits just about 14v, both at the battery and at the alternator (tested both using my battery monitor and a voltmeter)
It will then slowly drop down to about 12.8-12.9 volts or so and just sit at that point. (Its takes about 90 seconds to drop from 14 to about 12.8) Regardless of RPM. Even if the engine is at 3000 rpm the whole time it still drops down. If I do a “charging system” test at this time with my battery monitor, it says I fail.
My battery monitor reads the battery at about 80-85% State of Charge
If I turn on all the accessories in the truck (A/C, Defrosters, light etc) the voltage will rise back up to around 14v, but as soon as I turn them off it drops down to the 12.8-12.9 again. If I do a “charging system” test at this time with my battery monitor, it says I pass.
I assume this is the voltage regulator only supplying the truck what it needs? Is it weird that the voltage doesn’t stay high until the battery reaches a higher percentage? I’ve never had a battery monitor on a vehicle before, so I am just not sure what is “normal” behaviour.
I hope I am just being a little paranoid? I have a 16 hour road trip next weekend so just want to be sure everything is ok, I can always take it to a shop if absolutely necessary.
Thanks so much for any advice! I really appreaciate it.
Check for a parasitic draw in the electrical system. If the current between the battery negative cable and negative pole is over 25 mA, start pulling fuses until the current goes to near zero.
Thanks, I have already checked for parasitic draw and there is none. (Though that shouldn’t have any impact on voltage coming out of the alternator, even if parasitc draw was present, no?)
I have owned several GM vehicles of that vintage and that is typical charging behavior in my experience. Be aware that the charging system is quite complex and has many states that it mayh enter based on measured state of charge, current draw and vehicle conditions (mode of operation, accessory states, etc).
Whether or not it engages charging and at what level are determined by these conditions. The battery state of charge is measured at the time the vehicle is started. The rate of charge/discharge is continuously read by the current sensor attached to the positive battery cable. It is quite sophisticated IMO and even has a sulfation mode.
So I wouldn’t put too much weight in the 80-85% charged reading by the external monitor. It may be perfectly normal and the truck will eventually enter a state to more fully charge the battery.
I’ll second the suggestion to eliminate the potential for a parasitic draw. Simple test to do with the right tools and know how.
Secondly, it takes a fair amount of driving for the on board charging system to replenish a discharged battery. An external trickle charger would be better for getting back to 100% charged and then gauging the truck’s behavior after that.
I did some work on motor and took out alternator and soon after had charging issues, turns out sense wire to alternator had broken under insulation. 90% sure it broke cuz I flexed old wires. You may have “touched” 1 wire in system that is causing your issues. If you remove a ground wire and let it sit for 1-2 days it can build up surface corrosion and now have intermittent grounding issues. Lots of things have been disturbed.
Thanks, this is what I suspected but just wanted to be sure. I don’t think there is any parasitic draw, the battery doesn’t deplete when the truck is off and I am not seeing any draw when using my mulitmeter.
I haven’t had a chance to drive enough (and may not before my trip) to fully test that it maintains charge while driving an extended distance. On the old alternator, it obviously did not but if it is normal to have the system not be at full voltage even if battery isn’t full then I will just keep an eye on it and assume my issue is fixed. When hooked up to my trickle charger, the battery does get to 100% SoC.
thanks so much, I really appreciate your response.
This wouldn’t surprise me at all, those wires are old, the plastic insualtion on them just crumbles if touched. Not sure how I would diagnose this though? It looks like that sense wire (assuming its the other wire that goes into the alternator) just goes into the main wire harness so if its the case that sounds like I’d need a complete new harness? That sounds like a big job. Hopefully its not the case.
There is a tip in the owner’s manual; use tow mode or switch on headlights to increase charging system voltage to 14 volts.
If you are charging a remote (non-vehicle) battery,
press the tow/haul mode button located at the
end of the shift lever. This will boost the vehicle
system voltage and properly charge the battery.
If the trailer is too light for tow/haul mode, you can
turn on the headlamps as a second way to
boost the vehicle system and charge the battery.
Hopefully its not the case.
