Car loses a volt as the engine warms up..., weird one Options

[anthony]

Ok, here's the situation. When I start the car cold I get 13.2 volts across the battery terminals. Perfect, right?

As the temperature gauge moves towards the middle of the gauge I slowly loose more voltage until I'm down to 12.2 volts.

Is this normal? I'm thinking that at full operating temperature with no accessores (lights, brake lights, etc) I should have full charging voltage. It seems that something is kicking in as the engine warms up and draws a full volt.

Are there any D-Jet components that would draw voltage as the car warms up.

The voltage drop is gradual. As the temperature gauge goes up the voltage gauge drops in sync. And this is not a faulty voltage gauge issue. I've verified this with a volt meter across the battery terminals.

[SirAndy]

hot wires (and other electical components) have a higher resistance ...

same goes for the battery. it heats up back there too,
Andy

[davep]

Something is very wrong. A fully charged battery will give 12.8 volts across the posts. Your alternator is not charging the battery properly. You will have to use a voltmeter to find the various voltages around the system. Even check such things as the voltage difference between the negative post and the alternator housing ( the alternator is grounded through its case to the engine case, to the tranny, the tranny ground strap, the car body and the ground lead between the body and the battery post. A quick check may be to ground the battery post directly to the fan shroud to see if that makes any difference. You need to get close to 14 volts at the battery posts to have proper charging. The heat of the engine can cause resistance to increase along the current path. You will have to find out where. What state is the battery in? Does it have water?

[anthony]

A fully charged battery will give 12.8 volts across the post

I'm talking about the voltage while the car is running.

Optima actually recommends that the alternator put out 13.8 to 15 volts.

[davep]

I'm talking about the voltage while the car is running.

Optima actually recommends that the alternator put out 13.8 to 15 volts.

That is the point I'm trying to make. If the running voltage is lower than the not-running voltage, the alternator is not doing its job. When you turn on the ignition does the alternator light come on at all? Does it go out when the engine is running? There is a fundamental problem here. What is the battery post voltage with the engine not running? Can you charge the battery out of the car and get it up to 13.8 volts when it comes off of the charger?

[Joe Bob]

What gauge are you looking at? The one in the dash? Or a multimeter across the terminals????

Dash gauges usually run thru the fused hot when ignition is on circuit(s)....this can give a low reading due to heat and impedence due to the resistance of dirty grounds or corroded fuses.....

Best read is across the terminals when the car is hot with a GOOD multimeter.

If you are STILL getting low reads, could be a alternator, regulator or BOTH taking a dump. Most FLAPS will do an in car load test and pinpoint the problem.

[lapuwali]

Check your alternator belt. I've had belts slip w/o making any sound, esp. if they've gotten any oil on them.

[anthony]

When you turn on the ignition does the alternator light come on at all?

The light comes on and then goes out. The car runs and drives normally. The battery always seems to have plenty of charge when I go to start the car.

The only "problem" I've had is that my last Optima only last two years. I got that replaced.

So my first questions is what voltage should I be seeing across the terminals with the car running?

What gauge are you looking at? The one in the dash? Or a multimeter across the terminals????

I'm looking at both the console gauge and at my multimeter. The console gauge reads 1/2 volt less but other than that it's consistant.

I have an extra regulator I can swap with. Are there replaceable brushes in our alternators?

[davep]

Engine not running, the battery should read 12.8 Volts. Running, about 13.8 volts.
It is a good sign that the idiot light appears to work normally.
Still, you will need to find out what the alternator output is at the alternator. You could have high voltage drops on both the positive and ground circuits between the alternator and the battery. I believe you can unplug the regulator from the board as well as the harness from the battery and just plug them together; in effect you bypass the relay board and that could be a soure of the problem. Try your spare VR as well. Essentially the car runs off of the alternator when the engine runs; the battery serves as a voltage reference and gets recharged while the engine runs.

I don't fool with my alternators. I get a local shop to rebuild them to 75 Amps output from the 50 amp original. That way it is rebuilt like new.

[anthony]

Anyone know if our alternators have brushes that are replaceable?

[bob174]

Check your voltage regulator. If it's never been replaced since you've had the car, that may very well be the root of your problem. You have a spare and it's super easy to swap...

[davep]

I think I have a new set of brushes, not for sale BTW.
However, what is the result of testing for voltage drops?
What happened when you changed the VR?

[anthony]

I haven't got to it yet. I will have time on Saturday.

[anthony]

Problem solved! It was the voltage regulator. I swaped it out and now I'm getting 13.8 volts across the battery terminals when the car is running. And the voltage no longer drops as the car warms up.