OT - Maybe? - Alternator replacement

mkirsch

Well-known Member
For many years, I've read and been told that you can't just disconnect the battery and replace the alternator on a modern vehicle...

"Oh no, you can't do that. You'll wreck something."

Nobody ever knew what you actually needed to do. Nobody ever could define what would get wrecked. I could not find any reference to a proper procedure online, just "Unhook the battery, take the old one out and put the new one in."

The voltage on my 2003 Chevy Silverado with 215,000 miles on it has been diving at stop signs for a while now, so I figured the regulator was going. It took me a whopping 20 minutes to replace the alternator. Nothing bad happened.

Worst part about it is the alternator cost about 10% of what the truck is worth...

Does anybody know what bad could have or should have gone wrong?
 
Nothing bad is going to happen. I disconnect batteries all the time. In fact, on trucks like yours I leave it hooked up because I hate resetting clocks and such. I think people are getting confused on an old alternator test procedure. Old timers would disconnect the battery while running to test the charging system. This should not be done on modern vehicles because the battery stabilizes the voltage in the system. Some modules are easily damaged by voltage spikes over 15 volts, or by the loss of the isolated ground. When a battery is disconnected while running, the ground circuit can become "noisy".
 
You can invoke the anti theft in the radio on some models..(volkwagon, audi ) meaning that you will have to put a special code back in to get it to operate. Simple but,, problem is that most folks have lost the piece of paper from the factory that had the code. And now your radio no longer works. Also loose all memory settings for various devices, and have ecu have to recalculate driving and engine parameters... Not such a big deal there. memory for seats, for mirrors, for gps, radio stations, garage doors, etc
 
I have disconnected batteries, and swapped
alternators many times including modern vehicles
with no issues short of losing radio/clock
presets. There are a lot of electrical experts
(much like computer experts), but many times you
will find out they base their information on old
wives tales, or hearsay, or shops that simply want
you to pay them to do what you can do in your
driveway with a little research and common sense.
My son had a BMW (I tried to talk him out of that
mistake, but he had to learn the hard way), and a
dealership told him that in order to replace the
battery, he needed to have a dealership reprogram
the ECM upon battery replacement, and was asking
over $400 to replace the special trunk mounted
battery on it (good time to trade in my mind).
Strange that Napa sold and stocked that special
battery, and I believe would also replace free of
charge and would do just fine without any
reprogramming of the ECM. Not sure if this is
still the case, but I used to take the alternators
in for testing, and replace just the diodes or
regulators in them. It has bee a few years since
I have had an alternator issue (knock wood). I do
know the last time I tried to disassemble an
alternator (92 GM truck with low mileage at the
time, but weak front bearings) the bearings were
swaged in somehow so that I was unable to take it
apart and repair. In that case there was nothing
electrically wrong with the alternator short of a
bad front bearing, but I ended up having to
purchase a rebuilt alternator due to one bad
bearing that should have been easily replaced
otherwise. I have also found it best to shop
around. I have seen large price differences
between parts chains, and online parts houses.
There are also a few local reputable rebuilders,
but no experience with those personally. You
likely made a good move by doing yourself, and
even if you overpaid a bit for the alternator, you
probably came out ahead by not paying shop rates
for it.
 
replaced alternator on Ford Windstar van a while back, only problem was getting the dang serpentine belt back on, hands just too big to get in far enough to route it around everything, used $9 HF belt tool to remove tension.
 

You can buy a device that plugs into the 12V Power Port to keep all of the memories "alive" while the main battery is disconnected.
 
clock, radio presets and anti theft, normal stuff.
on my F 250 it does shift hard the first few times
after disconnecting the batts.
 
If you have a new vehicle and need to replace the
battery , take a small 12 volt battery with clips
wired to the battery, hook the battery to the battery
cables before removing the old battery , put in the
new battery , then remove the clips from the cables,
this will keep the radio and computer powered so as
not to have to reprogram the computer or radio.
HtH
Red
 
Last few I have done I didn't even unhook the battery. I took off
the wires (they weren't multi-connector type) and shoved each
one in a piece of rubber fuel line. Put on the new one and
hooked it up.
 
mkirsch,

I have a 2002 Silverado which the battery light comes on in the message window. Now this isn't something that is happening all the time. When it does happen, I look at the volt meter and it shows 14 volts. Had it to a shop and they scanned it and a code showed up. Something about the alternator. They reset the code and the problem didn't happen again for 6-7 months. Then it came on again.

Anyway I replaced the battery without any problem except the radio.

I also fixed the battery light, put a post-a-note over it :)
 
I was told to disconnect the battery on my 13 Silverado
if it's going to sit very long. (more than a month)
Did it last winter with no problems whatsoever.
 
On some cars, strange things do happen when the
battery is disconnected. I replaced the battery on
a friends high milage year 2000 VW New Beetle. It
ran like crap after that, and finally a indy VW
shop looked at it. The TPS was out of limits. The
computer had, with the battery hooked up, knew and
corrected for this. When the battery was
disconnected, it lost its memory of this and with
the TPS being out of limits, it could not re-learn
it. It was a rather expensive fix, as the TPS is
an integral part of the Throttle body and the two
must be replaced as a unit. Doesn't surprise me,
as the blower switch for the heat-A/C went south
and you cannot replace the switch, it is molded
into the plastic of the heat-A/C control head and
the entire head must be replaced, $136 a number of
years ago when I did this.

Charles
 
(quoted from post at 12:07:57 11/26/14) I have disconnected batteries, and swapped
alternators many times including modern vehicles
with no issues short of losing radio/clock
presets. ........... It has bee a few years since
I have had an alternator issue (knock wood). I do
know the last time I tried to disassemble an
alternator (92 GM truck with low mileage at the
time, but weak front bearings) the bearings were
swaged in somehow so that I was unable to take it
apart and repair. In that case there was nothing
electrically wrong with the alternator short of a
bad front bearing, but I ended up having to
purchase a rebuilt alternator due to one bad
bearing that should have been easily replaced
otherwise. I have also found it best to shop
around. I have seen large price differences
between parts chains, and online parts houses.
There are also a few local reputable rebuilders,
but no experience with those personally. .......

Most likely a CS130 alternator. The bearing is installed and then a thin lip in the front housing is rolled over the bearing. When you remove the bearing this lip is broken off. You can get a clip that hold the replacement bearing in place or I have also seen quite a few where the bearing is just help in place with Loctite. I rebuilt a lot of those alternators for the trucks I was servicing back then. By the mid 90s the price of after market replacement units had gotten so cheap I couldn't afford to fix them. At that time I could but a new alternator with pulley for $42. NAPA wanted about $125 exchange and GM was $140. I still have about 20 old cores.

The next series of alternators had the brushes accessible from the back without disassembling the alternator. I would get a call that the truck had died and the battery was dead. And Oh yeah, the battery light has been on all day. I would go out to the truck and connect jumper cables. Removing the alternator, replacing the brushes, and reinstalling the alternator would take about 15 minutes. I would remove the battery lead from the alternator and make sure the boot covered the end so it wouldn't short to ground while I was doing the R&R on the alternator. By the time the alternator was reinstalled the batter was charged enough to send them on their way.
 
I was told that when you remove the battery from new VW's the dealer has to reset the computer. Same with replacing the auto tranny. Dealer only. Terre Haute has no VW dealer. I think the closest dealer is Indy. Best reason I don't own a VW. My transmission man who has been in business for 40 years. HE says there are some VW and Audi trannys that cost more to repair than the car is worth, plus a ride to Indy to reset computer.
 
(quoted from post at 12:37:09 11/26/14)
You can buy a device that plugs into the 12V Power Port to keep all of the memories "alive" while the main battery is disconnected.

But that will not help if you hit the positive cable to ground while using the device.
 
On almost all electrical replacements (alternator included) the battery needs to be disconnected. You always disconnect the ground side and isolate it. This is done, in the example of the alternator, to prevent the power lead to the alternator from grounding against the case and ruining the diodes. If the power lead is "hot" and it touches ground, it becomes a large fuse. When the battery is replaced "keep alive" memory savers save headache by maintaining memory in components. The main concern in battery replacement is to have the ignition off when replacing the battery. Leaving it in the run position when replacing the battery can result in a no start on some vehicles due to vehicle theft security systems. Hope this helps.
 

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