My alternator wasn't charging yesterday, so I replaced it today. I expected to see it charging at 14.2 v, but it's at about 13.6 (at idle). Yesterday, I was in stop-n-go traffic for about 45 minutes and when I got where I was going, I had to start the car again... it wouldn't start. I waited about 10 minutes and it started. But, the voltage was 11.8 v at idle.

After a few hours, it started OK again and it seemed to be running at proper voltage (13.6v). But, after being on the road for about a half hour and coming to a stop, I noticed the voltage dropped below 12 v at idle again. I had the alternator tested at the parts store before replacing it... it tested fine. I wasn't surprised, as I'm thinking it's only misbehaving when it gets hot.

I'm about to find out if I've solved the problem, but wonder if others have run into this somewhat intermittent problem. If it doesn't charge properly after it gets warmed up, I'll suspect the voltage regulator, or perhaps the battery is dragging it down. Both of those options seem unlikely to me, but I've run into stranger electrical problems.

 

Clean the connections at the alt and at the battery and then check voltage at both at the alt terminal and at the battery.

 

13.6 hot is ok. It is normal to see 14.3 cold and a drop to 13.3 hot. What isn't normal is to see 12.0 or lower at idle. If the alternator isn't charging, you should see the red battery light come on. Measure the resistance between the positive battery cable and the output terminal on the alternator. It should read the same as the ohmeter probes touching together. You may have a damaged fusible link on the alternator output wire.
Take 65Z01's suggestion and remove your battery cables (neg first) and clean the cable lugs and the battery posts and replace (neg last).

When the 12v appears during engine idling, measure the voltage on the alternator output terminal. If hot, the voltage should be about 13.3 volts.

 

I'll get out the meter tomorrow and do some measuring, because the behavior hasn't really changed much with the new alternator. After the car is warmed up (on the road for a half-hour, or so), at stop/idle, the voltage will slowly decline from 13.5 to 12.5, or so, over about 30 seconds.

The "battery" warning light never has come on.

When I got home to the garage, in idle, I experimented with different loads... the voltage seemed to drop with any load. Applying the brakes would cause it to drop a few tenths, with the A/C, stereo, headlights, brake lights, all on... it got down to below 12 v again.

The battery is probably 4 years old. Could it be that a battery on its last legs could be the cause of this? I've heard of stranger things.

Thanks for the help...

 

Your battery could be at the end of its life and have very low amphour capacity. You can determine the state of charge by measuring the no load battery terminal voltage (hood lights are ok). 12.0v and lower, discharged. 12.9v and higher, charged. Also, measure the battery terminal voltage when cranking, it should not fall below 9.0 volts or the battery is discharged, cable connections are dirty, or the battery is at the end of its life.

 

Thanks for following up, guys.

I got the meter out and took some measurements. First thing, the battery voltage is 12.7 v (key off, no load, not even hood lights; they just happen to already be disconnected :-) Same 12.7 at the alternator; and the resistance between batt and alt is 1 ohm. I removed the cables from the battery... clean as a whistle. So, all that seems good.

With key on, but not running, the dash reads 11.4 v, but the meter reads 12.2 v. So, it seems the dash voltage is a bit "off".

Running, at idle, the dash reads 13.6, the meter reads 14.6 at the alternator, 14.4 across the battery. In gear with A/C, fan and stereo on, the dash reads 13.5, the meter reads 14.5 at the alternator and 14.3 across the battery. The readings were the same after I removed and checked the cables.

So, it looks like the alternator is doing its job at this point. It looks like the dash voltage is a little off (low by as much as 1.0 v). The battery seems to be accepting some charge, or I wouldn't be able to start it each time (I've only had starting trouble that one time).

But, something's not right after I've been driving it for a while. I'll take the meter with me to see if I can "catch" it in the act!

I think it's time to head down and get a charging system test while the car is good and warmed up and has been driven for a while. Actually, there really doesn't seem to be a big problem unless I get stuck in stop-n-go traffic for a half-hour or more.

But, I'd like to get this figured out since it's probably just a matter of time before I'm stranded somewhere, unable to start the car and need of a jump, at least.

I'll consider the alternator a "preventive maintenance" purchase :-)

 

The only problem you have is the low dash voltmeter reading. Your battery is almost fully charged and the alternator is doing exactly what it should. I would not bother with checking the alternator or the battery!!!

 

I replaced the battery and it didn't solve the problem!

Well, jfb, I wish I could agree with you, but it seems I still have a problem. I took the car to the auto parts store to test the charging system (the only place open on a Sunday and I'm going to have trouble finding time during the week). They used their little hand-held tester and it said the battery was bad; they were willing to give me a few bucks in warranty for my old battery, so I just replaced it.

It was all good until I noticed, on the drive home, while stopped for a light, that the dash voltage was down to 11.8v. When I got home, I checked it with the meter. The dash showed 11.4v, the meter showed 12.4v at the alternator. When I turned off the A/C and the stereo, the alternator went up to 13.2v. Something is dragging the system down when it's warmed up and at idle. I'll be damned if I know what it is, or even might be at this point! This is

I guess my next step is to take it to a real shop with real equipment and real expertise and see if they can tell me WTF is going on.

Of course, any advice on things I can check at home are welcome, as well. I'm into it for an alternator and a battery at this point, but I'm OK with that because those parts were old and tend to wear out anyway... however, I'd like to find out what the problem is before I throw any more parts at this problem!

 

Please clarify something for me regarding your no-start. When you say the car "won't start", what happens when you turn the key? Does it crank slowly or click or does nothing happen at all? I'm wondering if you're having a problem with your ignition switch/starter circuit that is heat related. From the other information presented, it seems as if the Charging system is OK. Let us know. Ahh, troubleshooting.....

 

Thanks for asking... it was cranking, but too slowly to starty (you know, very, verry, verrry slow ;-) It has started fine, since then, so I don't suspect the ignition or starter. It just seemed to have a hard time starting when it was being charged properly (in stop-n-go traffic for a long time).

I picked up an idea from a mechanic friend today... it could be the belt tensioner is getting weak and permitting the alternator to slip. I had the tensioner on my list, anyway, as I thought I'd heard it making noise (but haven't been able to confirm it). So, I might just replace the tensioner and see if that changes anything.

 

I stopped by Napa last night, but they didn't have the complete tensioner assembly and pointed me toward the dealer. I just called the dealer and was told the tensioner assy was about $120!!! Holy cow!

I think others had said they've replaced the tensioner (spring and all)... what did it cost?

Thanks!

 

Sounds to me like the battery's at the end of it's life.

 

I recommend getting a Fluke (or another high quality) DMM and measuring the cranking current and voltage. A Fluke 88 costs about $400.00 but after you've resolved a couple of problems, it's paid for itself. Without knowing what's happening while the engine's cranking, I wouldn't want to "guess". Good luck.

 

Well, I agree with the "guessing" part... I've already done plenty of that. But, I've only had a starting problem once, and that was after driving for a long time in stop-n-go traffic where I was at idle a lot and the battery got discharged.

Since then (and before then), I never had a starting problem. But, I've been watching the voltage (normally I have it set to show the coolant temp.) since this problem started and after the car is warmed up, I see low system voltage at idle. And the voltage is further affected by the various power draws... lights, A/C, cooling fan, etc.

The only thing that makes sense to me right now is that the belt is slipping and the alternator isn't turning fast enough to charge properly. And that is only happening at idle.

It seems a likely culprit is the belt tensioner. I tried to check the marks on the tensioner, but the problem is I don't know what's "good" and what's "bad".