CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Alternator

I have a 2002 Dodge dakota with a 3.9.
The charging meter on the dash reads that the system is charging, but after driving a few miles the needle slowly drops to the far left. It takes a few seconds to drop, not a sudden crash of the needle. Would this be a sign that the brushes are bad. I already cleaned the battery terminals good, and made sure they were tight when I reconnected them. I get 13 volts at the battery with the engine off, and 14 volts with the engine running.
When this first happened I noticed that the bolt to tighten the negative bat terminal was missing, so I replaced the bolt thinking this would solve the problem.
I know a guy that works rebuilding alternators and I can get the new brush set from him if need be, but right now he’s on vacation and I’d like to know if this would be my problem, since he’s not around to ask… So when he returns I can have him order the parts and have him ship the parts to me right away.

Yosemite

By the way, I just put a new Serpentine belt on and the belt tenmtioner is working fine.

Yosemite

Your voltage checks at the battery show good voltage levels. Check to see what the voltage is across the battery while the dash meter shows the voltage is low. There may be some other problem beyond the charging system that is causing the dash meter to drop in level. If you are driving the car this way and not having a problem starting it then it would seem the charging system is really ok and some other issue with the wiring is causing the dash meter to read low. Possibly the ignition switch contacts but you should be seeing other signs of trouble if that was the case. If no other issues are noted then perhaps the problem is limited to the meter itself or in the wiring to it. The trouble you describe doesn’t sound like brush problems to me.

Another note, when tighten the battery connections be sure not to over tighten them. That is a common mistake made and can damage the clamp by over stretching it. Making the connections snug enough so they won’t twist on the post is all you need to do.

What kind of charging meter is on the dash? Does it register volts or amps? What does dropping lower mean?

I would persume that the dash meter reads amps.
It is the factory needle guage that came with the car.
There are no numbers but if there were, it normally read (before this problem) just over half the range of the needle, or about 60%. Yesterday it held at about 50% for about 5 minutes then slowly dropped to 0% which it took a minute or so to do. I know that without running lights you may be able to drive quite a ways and it may take a day or two to drain the battery enough to notice that it’s getting weak on starting. Now today, I had it on the charger overnight, and drove 15 minutes to an appointment, spent two hours there and drove the 15 minutes home and I saw no problem and the needle stayed at the 60% range the whole time.
I only thought brushes because when they do wear to the end of their travel, they can work intermittently. I had one that if you tapped on the alternator case with a hammer handle they would make contact for awhile, but it was a sure sign that they were not going to last long.
I also know that if it was the armature, satator or almost any other part, the alternator is gone and will never put out any more power no matter what you do.

I didn’t overtighten the terminals as you mentioned, just enough that they couldn’t be turned by hand.

I guess I’ll take it over to my shop and do a little more examination this afternoon. Maybe I knocked something loose when I changed the belt, but I doubt that. I’m sure I got the belt on right.

Yosemite

It sounds like you have a current meter, but it should have a negative part of the scale to show a discharge.

It is normal for the meter to show a high charge right after starting and then drop off to near zero after the battery is fully recharged. If it doesn’t do that, then you have a battery that is going bad. I doubt that you need a new brush for the alternator.

I do have a volt meter, but it’s not digital, so in the 0-50 range 13 and 14 volts are not much of a difference. Sorry the next lower range is 0-10 volts.

If this guage was a gas guage it would have the marks for
Empty…1/4 tank…3/4 tank…Full
No 1/2 full mark so it does not show a - discharge to the left and + charge to the right.
No numbers… only a icon of a battery in the middle.

There is also a light and chime that tells you to “Check Guages”. This comes on when the needle drops below the 1/4 mark. this always happens when the needle is on it’s way down to zero.
So this is not just that the battery is fully charged and I’m misreading the guage. It drops to the point where it would sit if you turned on the ignition, but didn’t start the engine.

This is what confounds me. I’ve never seen a guage that just…over a 10 second period, slowly dropped. I’ve always seen them just drop to zero in a flash.

I cleaned the posts and the clamps again with a wire brush and used a wire brush type post and clamp brush to clean everything. There was little if any corrosion …even the other day on the first try. I checked every connection I could find and all were clean and tight.
I removed the connections from the alternator and cleaned them real good and made sure those connections were tight. I couldn’t get the needle to drop while at my shop, but on the way home again I had the same problem after about 5 miles.
I happened to have the heater fan on and just a few seconds after the needle dropped to empty
(as if the engine was not started) and the warning light and chime came on…a few seconds later and I could hear the fan speed up a little as if the charging system kicked in, but no change in the needle. I stopped and tapped the post clamps and kept checking the guage, but it never went back up. It never goes back up durring the drive…like a connection was made.
When I got home again and it read Zero on the guage, I shut the truck off and waited about 15 seconds, started back up and the guage was back up at just past 1/2 if there was a mark for that.
I didn’t want to try going back to my shop because it was getting dark and with the headlights on I might not make it all the way back home from there.

FYI The battery is only a year old.

Yosemite

Just a guess but it could be your voltage regulator. That is probably built into the alternator so a new brush probably won’t help much, but you could try.

The dash meter sounds like it is a voltmeter to me. Few cars today use a amp meter in the dash to monitor the charging system. I think the problem you are having is due to a bad power connection somewhere past the power panel under the hood. If I understand your comments about the problem correctly the car will start up fine even though the dash meter shows no voltage. My guess is the bad connection is heating up and the voltage drop across the bad connection gets worse as it heats up. If the battery voltage is fine using your meter to check it and the dash meter shows low then there is a connection problem somewhere. It could be with the ignition switch possibly.

Cougar;
I remember the old Amp meters, back when I didn’t have so much hair growing out of my ears and back.
Your idea about the connection heating up and getting worse might be true. When I left yesterday morning it was only 28degrees and about the same on my way home. But when I left again the cab was warmed up a little and then allowed this connection to break only after a few minutes instead of the longer trip when everything was so cold.

Buuurrrrr, I just checked and it’s only 19* out!!!

I’m going to call my mechanic and see if he has time to, at least diagnose the problem. Maybe if I bring it in while the meter reads 0 and not shut the truck off.

I’m sure he’ll find the problem, he’s pretty good at that stuff and has better equiptment than I do.

Thanks to all and I’ll post our results.

Yosemite

The one day that it doesn’t do it, is the day you take it to the mechanic. Since last friday it has happened every day…not today!!!
I drove the 15 miles there and took a longer route to warm whateven the problem is, up.
He tested the alternator and it is putting out 14.6 volts and 26 amps at an idle. He put a load on the battery and it ran up to 120 amps.
He checked codes and history values( I think that is what it was called ) and found everything to be in order. His $2000 scanner can read a lot more than my little $150 can, but even he is stumped.
He checked all the connections and found no problems. I then took it for a drive and put on another 60 miles on the backroads, but the volt meter never dropped.
Until I have the problem again and can bring it down to him while the meter is reading “no charge” , I guess i’ll just have to wait.

From all of your posts it sounds to me like the problem is with the gauge not the charging system. If you are reading 13VDC with the vehicle off the battery if fully charged, no matter what the gauge on the dashboard says.

Have you had any symptoms of low battery charge OTHER THAN the reading on this gauge? (i.e., vehicle won’t start after being driven, lights going dim…LOW voltage reading the battery after driving (under 12VDC)?

To see if the trouble is just with the meter or the circuit to it you can check that pretty easily. Have your mechanic let you know which fuse in the dash ties to the meter having trouble. Then use your voltmeter and check the voltage on that fuse with reference to ground. Use the small slits on top of the fuse to put your probe on and measure the voltage. If you have 12 volts at that point while the trouble is occurring then the wire connection problem is after the fuse. Chances are the connection problem is just with the connection to the meter. Just tapping on the meter might solve the problem.