Stephen VA stated that it's only an issue if you go and try to put the cable back on with the engine still running.

Yeah as you have learned, never listen to the AAA drivers.

I had to show a AAA driver how to get my car onto flat bed one time. It didn't have a front tow hook so he was just going to use a control arm

[QUOTE=Joshdub;1062615]Yeah as you have learned, never listen to the AAA drivers.


Yes I have learned my lesson haha. My mistake fully on that. But I did tell him multiple times just to tow it to my house haha he insisted on my driving it.

Looks like my tahoe trip is going to be postponed. But I'm glad it failed here not on a snow trip in the snow haha.

Second guy was all surprised that I already had my tow hook on the car ready to be towed haha

now I`m thinking its your alternator ^

[QUOTE=dabenthusiast;1062623]Sucks about the trip man but yeah you are right. Much better to have it fail there than out in the middle of no where

I would not go that far. The real issue are the spikes that are unknown, unforeseen, and mostly unmeasured when disconnecting and reconnecting an active charging system. Examples of this type of damaging spikes are found in loose connectors to modules. The intermittent on/offs FRY the modules pretty quickly. The whole system gets a shock each time there is a disconnect and reconnect of a intermittent Alt for example. This is why all kinds of bad things happen to our vehicles when the Alt starts goes bad besides the battery loosing a charge.
Bottom line: It is NEVER a good thing to disconnect the battery or alternator on a running vehicle. The consequences are always unknown and expensive, and will be hard to diagnosis why a electrical part failed later.

When it comes charging systems, connect all, then start the car, just as you would do with household 110V wiring and devices.

Now to answer a common question that used to come up in tech training classes: "Can you do it just once and not have a problem, just to check a Alternator or battery?" Sure and possibly NO. If it is your car, go ahead. If it is a customer's car, no as YOU will be paying for the repairs and be looking for a job if things go wrong. Techs would argue with the instructor as they "did it all the time" with no issues while the vehicle was in the shop. After being questioned by the instructors all admitted they did not track vehicle returning later for electrical issues. :rolleyes: Just an example of poor shop practices. There are too many tools/testers on the market to test a system without having to resort to that.

All of us agree that checking starting/charging systems is just a series of quick tests to pin point a failure/weakness.

Based on the last posting from the OP, it sounds just like his X5 has been running off the battery due to a Charging system failure to the point the voltage fell below 9.6 volts and things only got worse to complete shut down. Now the battery is in deep discharge and may not come back. Lets hope it does so he can get back to enjoying the Christmas holidays instead of busting knuckles pulling an alternator. :thumbup:

All this discussion about removing the battery cable while the engine is running really cracks me up and is really so stupid.

The only reason this was ever done back in the 60's & 70's is people did not own a multimeter and it was a backs woods way to test for charging.

Why this is even discussed or considered in 2015 when you can buy a multimeter for the price a a few cups of Starbucks and possibly risk damaging or even glitching a module is not even worth the discussion.

Especially when many of the BMW models have a built in Voltmeter and you do not even have to buy anything, open the hood or get out of the drivers seat.

A Voltmeter is really all that is required to get a very good idea as to what is going on with any battery or charging system. Yes, having more than a Voltmeter or tools that can verify AC ripple, measure Amerage and apply a load is all fine and good, but if the charging system or battery can meet some basic Voltage measurements, it is pretty much the case that things are working correctly and in good shape.

So anyone that wants to disconnect their battery cable, go ahead, I believe the more knowledgeable crowd would recommend that this not be done.

:iagree:

NO. The car starts off of the battery, it runs off of the alternator. This has been true since the beginning of time. Back in the olden days, Grandpa cranked the engine over by hand, then a magneto took over keeping the engine going -- just like a lawn mower. Then cars started getting electrical stuff -- lights and such -- and a generator was installed. Grandpa still cranked the car, but the electrical loads were sustained by the generator. Somebody got the bright idea that an engine could be started with an electric motor, but it took an electrical charge to make that happen. Enter the battery. The generator still provided for the electrical needs of the engine, and it also replenished the battery. Eventually the generator was replaced by an alternator -- the generator produces DC while the alternator produces AC that a diode pack changes into DC -- but the idea that the battery starts the engine while the alternator sustains it remains the same.

It is not a good idea in modern systems to unplug the battery while the engine is running, but in a car from before the days of computer controls it was common practice to start a car, then take the battery out and move it to another car so it could be started. Jumper cables make this much easier, but sometimes one might not have cables but for some reason was able to harvest a battery from another car.

The point is, the engine runs because of the alternator, it starts because of the battery. If the battery is gone, starting is lost, if the alternator is lost then engine operations will be lose when the battery goes flat. This can take varying amounts of time depending on the loads and the condition of the battery.

:thumbup: