fdv101

Hi everyone, hoping to get some help on this. Has anyone ever seen completely different test results from testing the battery at the battery versus the terminals under the hood?

I'm suspecting this might be part of my problem. Over the past 3 months, my battery has died multiple times. The battery is a NAPA battery that is 3 years old. I had it tested while in the car and it tested as a good battery at NAPA.

I wasn't 100% convinced that it wasn't a bad battery so I borrowed a friend's tester that we know works. When I tested it at the battery it also tested as a good battery (see attached "good battery" picture).

I then decided to hook the tester up under the hood at the terminals. I should also note that while attempting to charge the battery using the under the hood terminals, I have never been able to get the charger to get to the full charge state using the 10 Amp setting on the charger. Back to the tester, under the hood terminal test shows completely different results saying to replace the battery (see "replace battery" picture). Is this normal?

I had always thought that the under the hood terminals should provide similar if not exact results as directly on the battery.

I also tested the charging system and that showed up with a charging system issue (see "charging low" picture). I know there's a good chance that the alternator or generator as GM calls it is bad and needs to be replaced. But since the battery test was done without the car running, therefor the alternator not being a factor, could I have another issue happening along with the alternator?

Am I completely off base with thinking there could be an issue with the cables running from the battery toward the front of the car? I don't know if this is related, but back in February the CEL did come on with a P0742 code that had the car going haywire until I restarted it. It never had that problem since.

Also, recently I've had both passenger side door speakers malfunction (they were working fine prior to the battery issues and are now acting as if they are blown out) as well as losing AC, air blows, but just not cold. Only mentioning those things in the event there may be a connection. Thanks

***Post title should have been "Battery Test Results Difference"

Conax

On my '09 I compared volt readings from the power socket and the battery itself and they were identical.
An easy fix to try would be to clean all terminals involved, the battery terminals, the 50 A fuse and its socket in the ground cable at the battery, and the (+) cable that goes to the main fusebox hot terminal.
If that doesn't do it you could repeat the process on the ground side. It could be a loose or corroded chassis ground somewhere.
I'm sure there will be more ideas coming, these guys know the system pretty well.
Good luck to you!

RJ_RS_SS_350

Conax has given you some good leads to check out. The 50 amp maxifuse is mounted near the negative, but it is positive. However, it goes to the BCM and would not be a factor here.

The positive cable goes from the battery post, to the jumpstart terminal, to the starter, then to the alternator. Between the starter and the alternator is a fusible link. I'm wondering that if the fusible link were partially damaged, would that limit the charging system? (a real longshot). But, once you clean up connections, that may allow current to flow better, and charge better.

donbrew

What did you use for ground under the hood?

I have had trouble getting a jumpstart under the hood, but worked fine from the battery.

The engine hoist point is a bad ground.

Oldblue

Always test the battery at the two battery terminals

Conax

For me the best jumper cables are heavy gauge jumper cables. That positive wire from under the hood isn't very large gauge. It's about 13 feet of wire to get from that post to the battery post. The impedance could hinder good jump starting since quite a few amps are needed to crank it. My little Battery Tender (old style) smart charger didn't work well from under the hood. It charged but got pretty warm so I used my dumb charger to charge the battery instead.
(This could have been due to the parasitic drain I was getting from the OnStar module, not sure, just throwing it out there.)
According to a diagram I got here (thanks!) the wire runs to the battery post, then another lead connects the battery positive to the starter and the alternator, therefore the line length to the starter is pretty long from the jumping terminal to battery back to the starter, raising the impedance.

Oldblue

Exactly why , I always test the battery at the battery.
A battery tender should be connected at the battery, I know many have connections at the jumper post and grounded to the strut tower, but to me that also effectively sends current to other systems like the BCM , fuse box and others

firemangeorge

Hmm.
Charging up the battery you want to connect the charger as close to the battery as possible to avoid any resistance or loss from that long battery cable.
Now, jump starting should be a different story. When jumping, you're trying to get the power to the starter. Using the jump location noted from GM eliminates that long battery cable and any loss you would get trying to jump start at the battery.
Just have to be sure you have a good, clean connection if trying to jump start under the hood.

Conax

I can't post the charging diagram, but on it you will see two lines in red from the battery. One to the Power distribution System (the fuseblock under the hood) and a separate one from the battery to the starter, then on to the alternator.
This makes the path from post to starter longer, from post to battery, then battery to starter. It will jump from here (obviously) but that long path has its consequences.
Or am I not reading it right?

RJ_RS_SS_350

Nope, FMG has it right. Power distribution goes through the maxifuse near the battery, to the BCM. Starting circuit goes from positive post, to jumpstart terminal, to starter, to generator.