I have a 2012 GS manual coupe. 17,000 miles. Very clean.

I have an intermittent issue. The voltmeter gauge isometimes drops to around 12V (from its usual 14V). When this happens, the DIC display and a multimeter at the physical battery will concur, reading between 11.9 and 12.1 volts. However....a multimeter at the alternator itself will read 14.6V.

Thank you in advance to anyone who could suggest what the issue might be and/or the next troubleshooting steps. Additional observations:
- the battery appears to be good - with the car off, it's always at 12.6V;
- alternator casing to ground is solid;
- during the fault condition, if I shut the car off, the battery voltage will jump back up to its usual 12.6V;
- I've had no functional issues of any kind (note that I do keep the car on a tender);
- the fault condition is intermittent, and will only occur durig the first five or ten minutes of a drive;
- the indicated voltages are not affected by RPM.

 

Put an electrical load on your electrical system then at 2000 RPM place one lead of multimeter on Alternator B+ and other lead on battery positive…record your voltage drop…next do the same thing from alternator case and other lead on battery negative…write both numbers down…max voltage drop on either should be no more than 0.5 volts…if excessive on the positive side check your starter solenoid where the battery cable and alternator cable are attached there….may be loose connection or oxidation…looks like you have a 2.5 volt drop !!

 

I have 0.32 voltage drop on positive side and 0.10 on negative side.

 

Thanks very much for the reply.
I'll follow your instructions and post the results.

 

C5 Diag - the fault has not occurred since I posted this thread. When it happens again I'll do the voltage drop tests, and In the meantime I'm going to inspect all my connections.

So to summarize - with a known good battery and the engine running, if I have low voltage at the battery but 14V at the alternator - it's NOT the alternator. It could be a bad connection, on ground or positive side.

However, just so I know, could the fault condition that we're discussing also be caused by an electronic control issue of some kind?

Thanks again for helping me out here.

 

Yes, I would clean the battery cable connection on the engine block and also remove the alternator B+ and battery B+ connections at the starter solenoid and clean everything with some emery cloth.…make sure your terminals are clean at the battery too…this will at least take this off the table…you must clean them and not just look at them…you can start here…there are some “B” or Body codes that can let you know if you have any charging system issues…the check engine light will NOT illuminate for these..you will need a factory scan tool or equivalent aftermarket one...does your 2012 have a battery current sensor on the battery negative post ??...it mentions it in Service Info but I don't see it for an R&R.

 

The fault occured today so I did the tests. It's pretty definitive - I have a 2.5V drop on the positive side; the negative side is fine (.01V).
The connections at the alternator and at the battery are good. The connections at the solenoid are going to be a bitch on my car because of the dry sump lines. Not being on a high lift doesn't make it any easier (I've got the car sitting on the engine cradle). The solenoid area looks very clean, no exposure to the elements, looks like new - I know it's not definitive, but odds are there is no corrosion.

Q1 - Do you have any suggestions for working on the starter on dry sump cars?
Q2 - Does the voltage drop being only on the positive side definitively rule out ground connections as potential culprits? Ie - it could only be a positive connection issue?
Q3 - Can you please recommend a scan tool? I looked at millions of them online a while back but could never decide on one (I guess I didn't really have a specific objective at the time LOL)
Q4 - In answer to your question, there is no sensor at the battery, but what did you mean by "Service Info" and "an R&R"?

 

Q1- No I do not…maybe someone with a dry sump setup can chime in.
Q2- Yes, the testing that was done from the alternator B+ to the battery B+ narrows it down to just the positive side…that is why we check negative side from battery negative to the alternator case.
Q3- What’s your budget ??…do you own another car or cars that are not GM ??…if more than one manufacturer an aftermarket bidirectional scan tool by Launch or Autel are good choices…I have one of both and a Snap On but they are all well over $2,000.00 but you can get one for a little less than $500.00…if just GM you can get a Chinese “clone” Tech 2 with the Candi module for $300.00 or so these days if they are still selling them and covers up to model year 2013 or a computer based but software glitchy VCX Nano that you can buy on Amazon for $135.00 or so.
Q4-Service Info is like a factory service manual that you can subscribe to online for a month, a year, or 3 years…I use Alldata DIY but have used Mitchell but Alldata is much better…you get factory repair info, OEM and non OEM color wiring diagrams and a bunch more…if you only work on your car occasionally the 1 month plan for $19.99 can’t be beat…R&R is Remove and Replace.

Now getting back to the voltage drop you can have corrosion say between the battery post and battery terminal so to find that drop each lead of the DVOM is placed on those connections…now you can have corrosion underneath the lugs on the battery or alternator cables so to find that I would place one lead on the lug and the other lead on the cable itself by piercing the cable maybe an inch downstream of the lug…you can try this test just to rule that out…all the multimeter does is measure difference in voltage potential between each on the leads…some members are puzzled when you tell them to place each lead of the voltmeter on both the positive sides of a connection…you are just measuring the “difference” !!…same thing measuring between battery negative and the engine block with each lead…there are many YouTube videos on electrical systems and even today I’m still doing online or in person training just to increase my knowledge where I can find it…last week I did a 4 hour online class on misfire diagnostics by the best instructor in the country…not cheap but well worth it.

 

Look at the 4 minute mark and you can see where we test the battery negative from the post to the battery cable itself…there can be a bad crimp or lug that might have high resistance underneath it.

 

 

Thanks for the answers to my specific questions. And thank you for the additonal info and video on the voltage drop testing idea. I do understand that you can apply the voltage drop testing concept virtually anywhere - a great example was between the actual battery post and the cable itself just a few inches away. Good stuff, I'm always learning too. Thanks.

So back to my specific situation right now, at this point I'm looking for an intermittent failure point in the positive side of the charging circuit.

Q - Do you know all of the relevant positive connections on my car? I can think of only three: alternator B+ / battery B+ / starter solenoid B+ (to battery and alternator). If those three connections (including the connectors and cables themselves) are solid, what else could it be?

Thanks...

 

Thats all !!…what you can do if you want to see these issues go away is if you have some thick jumper cables connect one end to the alternator B+ and the other end to battery B+….just a simple bypass test it’s called…and remember you may have some high resistance underneath any of the factory swaged cable lugs that can’t be seen…if you do pierce the cable you can use some liquid electrical tape or even nail polish to seal the cable back up…BTW, I’ve seen many seasoned technicians during some hands on training seminars I’ve attended have no clue about this voltage drop testing…most are just wrench turners and all they think is that red lead goes on positive and black on negative and that’s it…an analogy I learned a long time ago concerning high resistance on a ground circuit is to think of that high resistance as a traffic jam and your voltmeter is another road for the current to take to get back to battery negative so it’s easier for current to go through the voltmeter and that voltage is what we are measuring say from the engine block to battery negative.

 

Yeah, the jumper cable in the "bypass test" creates a path that replaces the faulty (higher resistance) path - it confirms the voltage drop test result.

So I just need to confirm all the actual connection points on the positive side of this circuit....
Q - is there a single uninterrupted cable connecting alternator B+ directly to the starter solenoid B+?
Q - is there a single uninterrupted cable connecting alternator B+ directly to the battery at the very back of the car?

BTW - thanks for the screencaps of the scan tools / I appreciate it /

 

Since you have a dry sump (missed that) and your battery is in the trunk so the positive lead is different from a battery mounted up front…looks like the positive battery terminal cable goes to the fuse box where another shorter cable goes down to the starter…guessing it could be a bad connection at the fuse box…there is just one cable from the alternator to the starter.

 

 

Ok thanks for the schematics. I can see the LS7 (dry sump car) circuit goes to the fuse-box - one more potential culprit for voltage drop.

Thanks again for your time and help on this.

 

Yes !!…I’d check there first since it’s easily accessible.

 

I just received the "LAUNCH X431" scan tool from Amazon for $160, it has a good bit of features, bi directional. I'm sure they have better ones on the market, but for the money, I'm pleased with it. I just got my first corvette recently, so I'm still learning. 2013 vert base model

 

Hey James - thanks for the scan tool recommendation. I'll check it out. Sounds like it might suit me well - it's a bang for the buck sort of choice, you can never get all the features. l like the bi-directional ability.

 

So I found the problem definitively and I figured I would post the results.

Unbelievably, the nut was missing entirely from the solenoid B+ stud. The two ring terminals were just held in place by the thread groove - the cables are very stiff so that helps. I've owned the car since July 2023 (second owner) so I'm pretty sure there's been no torque on those terminals for many years - the nut would take a long time to spin off completely. Apparently, solenoid nuts coming loose is not uncommon on these cars so it's worth checking it once in a while. To look at it and work on it, I removed the #4 and #6 spark plug wires. To tighten the nut you need to assemble a bunch of extensions and a U-joint (I like to tape them together so you don't end up fishing for tool parts). There are varying opinions on lock washers (split, tooth, no washer etc) - I chose a split washer because I could get enough torque on it to have nice wide flat contact surfaces. So no more voltage drop.