i just want to make sure…..what is the “main fuse” on the positive battery cable? Is there a fuse in the cable itself (have not examined the entire length of the cable…..I have some air box screws on order, mine are borderline stripped and I need to remove the air box to look at the positive cable well.)

 

Another update. When I had the incident, it was very cold, had the heated seats on, the radio on, phone charging, lights on. Replicated that today and my voltage drops to 13.1 or 13.2 under those conditions.

Alternator has a hard time keeping up. The phone charging causes the big drop. Doing the charger alone drops it to 13.7.

 

Just curious, does your cooling fan run for a long time (or not shut off at all) when you start the car?

 

It did right after this incident the first time I drove it after, but has not since.

Just did voltage drop to alternator and it was 0.03 V off and 0.05V under load.

Also did a ripple test, getting 3 amps at idle and max 6 amps when held above 2000rpm for a while with everything on. 10 mV at idle and max 14mV under load. So I believe the alternator checks out.

To be honest, I’m actually suspecting the combination of my cig lighter inverter and my wireless phone charger to be the culprit. It causes a big voltage drop and gets very hot to the touch when in use.

 

First, The main fuse is that black plastic box about 3 inches from the positive terminal. If you unlock the tab and open it up, you will see a very large 100amp? fuse. Actually now that I think about it, never mind, that runs to the starter.

As for the question about the cooling fan is because it is an indicator that your alternator is going out. There a few threads on this forum that go into greater detail than I can right here, but basically when the alternator starts making noisy power (think of the ripples you mentioned before) the HVAC control head thinks that it is getting a high temp signal from the engine temp sensor and turns on the cooling fan in an effort to help lower temp. Weird huh?

I think you have one of 2 things going on
1. You have simply overloaded the alternator and experienced a voltage drop that several of the 19 ECU's did not like.
2. You alternator is starting to fail.

One thing I can say for certain is that after we installed a second batteries in our D3/LR3 we have not had any of these crazy dash lights associated with electrical issues.

Jeff

 

Thanks, this is where I am at as well. Also, I had just swapped in Xenon headlamps but had not configured them with the GAP tool when this happened.

I have another car due for a new H7 battery and I’m going to start by upgrading to an H8 Interstate AGM. It will increase my amp hours to 95 from 80.

I have driven it every day this past week and have tried to stress it to reproduce but have been unsuccessful so I’m hoping it was a one off in really cold weather. I did clean a slightly crusty negative terminal and replaced the alternator fuse which looked pretty discolored.

I like the idea of an auxiliary battery but will just monitor for now until I get some more definitive signs on whether the alternator is going bad.

 

Finally got to the engine ground cable. Had to take the fender cover off (quite the task). The connection to the frame rail was very rusted, cleaned it off but it didn’t change the voltage drop or amps under load. It was fine before and after. I also discovered my 12v voltmeter is inaccurate when it has something charging compared to GAP readings.

 

Well the GAP tool is not accurate when it comes to voltage. Not that its GAP's fault, ODBII ports in general are just not ideal locations for voltage readings. At the battery with a meter is best. My GAP shows 11.7v when meter at battery shows 12.5 at rest.

 

If you really want to get enough detail to get a clear picture of voltage drop, alternator output, ripple, noise etc at idle and varying load conditions, then you're going to need an automotive oscilloscope.
Even some of the cheap ones are capable enough to show you the full picture of what you're trying to see.

 

Yeah, thought about buying one instead of just doing the max function on my meter under different loads. All but the cheapest ones approach the price of a new alternator. Just going to sit on it a while. Driving fine.