Alternator - Voltage in gear vs. parked (1 Viewer)

[BamaOverland]

Newbie here: I have searched the forums and cannot really find anything similar so I figured I would post here.

Last week I noticed that my voltage gauge would dip slightly at lights and stop signs, occasionally going as low as 2/3 before rising back to just below the 3/4 mark. my first thought was to put a multimeter on it (several days later) which read 12.5 off and 14.5 at idle, a little lower with a/c blowing (14.05) from a cold start. put the meter on it again today (3 days later) after driving for a while in the heat and it reads at about 13.95 at idle. Why would the gauge read more consistently in park with the engine running then when I am in gear stopping at a long traffic light? I have been told that there isn't anything to be concerned about, and that the problem likely ranges from the A/C coming on, the extreme heat, and things like the radio having an effect. Driving does not seem to be affected and I have not noticed any electrical issues or dimming lights as well as no issues starting it. Battery was replaced 10 months ago. In the end, it may be nothing. Maybe the gauge is a little funky since it is 23 years old and that is part of owning an old car.

2001 LX 470

 

[chrisbruck]

Mine had a similar behavior... seemed like under load the engine RPM was just a tad lower than idling in PARK and the volt gauge would drop. Mine ended up being a bad alternator. Keep and eye on yours or get it tested before it gets worse or leaves you stranded.

 

[BamaOverland]

Mine had a similar behavior... seemed like under load the engine RPM was just a tad lower than idling in PARK and the volt gauge would drop. Mine ended up being a bad alternator. Keep and eye on yours or get it tested before it gets worse or leaves you stranded.

Is there a way to test the alternator that’s different than checking the voltage on the battery?

 

[chrisbruck]

Most of your FLAPS can do a quick check in the parking lot to determine if it's a battery issue or charging issue... mine was also whining so I knew it was going bad.

 

[Mike NXP]

You may want to invest in a cheap OBDII scanner that you can connect and monitor while operating the vehicle.

The voltage shouldn't fluctuate with engine RPM or load. If it does, the most likely culprit is that the alternator is getting weak, next would be weak battery. Sometimes in extreme cold or heat you may see it dip but under normal situations it should remain steadily above 13V.

My rig fires up and pushes closer to 14V and then slowly levels down to 13.4 ish. In extreme cold I see it dip lower at times. New battery and new alternator.


>> Check the battery, you mentioned it was replaced. Are the connections clean and tight? How about connections at the alternator?

 

[BamaOverland]

Most of your FLAPS can do a quick check in the parking lot to determine if it's a battery issue or charging issue... mine was also whining so I knew it was going bad.

Thank you! I’ll get it checked today

You may want to invest in a cheap OBDII scanner that you can connect and monitor while operating the vehicle.

The voltage shouldn't fluctuate with engine RPM or load. If it does, the most likely culprit is that the alternator is getting weak, next would be weak battery. Sometimes in extreme cold or heat you may see it dip but under normal situations it should remain steadily above 13V.

My rig fires up and pushes closer to 14V and then slowly levels down to 13.4 ish. In extreme cold I see it dip lower at times. New battery and new alternator.


>> Check the battery, you mentioned it was replaced. Are the connections clean and tight? How about connections at the alternator?

the battery terminals were greased a few weeks ago, but I did go on a pretty rough off road trip the week after. I’ll see if I can check the alternator connections. The temp has been steadily in the 90’s and up to 100 over the past couple of weeks as well.

 

[e9999]

there was a thread on this a few months ago. There seemed to have been a range of observations.
Anything you do on the truck in real time will have some effect on the instantaneous battery voltage, just having your foot on the brake at a red light will likely result in a small but noticeable voltage drop (well, at least if you measure it with a voltmeter). The alternator can't maintain a perfectly even voltage and the battery charge level will affect what the alternator does too. For instance, from what I saw, as the battery charges, the alternator voltage will drop down (which suggests some sort of smart algorithm in play).
Get a digital voltmeter and plug it in the cig lighter, much more telling than the dash tiny gauge.
If your battery is at a healthy level, the alternator is likely fine.

 

[SteveH]

Here's how I test an alternator. In this order:

1. Test battery voltage - engine off - should be 12.62 or higher
2. Test battery voltage - engine on/idle - should be high 13s or low 14s - my '99 was 14.2-ish.
3. Test battery voltage - engine on/idle - but fan on/high beams on/seat heaters on, etc. - should be a few hundredths of a volt less than the #2 test above.

If the voltage sags badly on the 'loaded test' (#3) your alternator is on its way out. Dying alternators can keep up with light loads, but not heavy loads.

There are other, much more elaborate and computerized alternator tests, but this one has never let me down.

 

[vipergrhd]

Here's how I test an alternator. In this order:

1. Test battery voltage - engine off - should be 12.62 or higher
2. Test battery voltage - engine on/idle - should be high 13s or low 14s - my '99 was 14.2-ish.
3. Test battery voltage - engine on/idle - but fan on/high beams on/seat heaters on, etc. - should be a few hundredths of a volt less than the #2 test above.

If the voltage sags badly on the 'loaded test' (#3) your alternator is on its way out. Dying alternators can keep up with light loads, but not heavy loads.

There are other, much more elaborate and computerized alternator tests, but this one has never let me down.

^^^ This is a good test but I would add is find a shop that has a carbon pile load tester it will tell you exactly what is your current output under load and max output. I don't trust the small electronic gadgets.

 

[BamaOverland]

Here's how I test an alternator. In this order:

1. Test battery voltage - engine off - should be 12.62 or higher
2. Test battery voltage - engine on/idle - should be high 13s or low 14s - my '99 was 14.2-ish.
3. Test battery voltage - engine on/idle - but fan on/high beams on/seat heaters on, etc. - should be a few hundredths of a volt less than the #2 test above.

If the voltage sags badly on the 'loaded test' (#3) your alternator is on its way out. Dying alternators can keep up with light loads, but not heavy loads.

There are other, much more elaborate and computerized alternator tests, but this one has never let me d

does this test need to be from a cold start?

 

[BamaOverland]

there was a thread on this a few months ago. There seemed to have been a range of observations.
Anything you do on the truck in real time will have some effect on the instantaneous battery voltage, just having your foot on the brake at a red light will likely result in a small but noticeable voltage drop (well, at least if you measure it with a voltmeter). The alternator can't maintain a perfectly even voltage and the battery charge level will affect what the alternator does too. For instance, from what I saw, as the battery charges, the alternator voltage will drop down (which suggests some sort of smart algorithm in play).
Get a digital voltmeter and plug it in the cig lighter, much more telling than the dash tiny gauge.
If your battery is at a healthy level, the alternator is likely fine.

thank you! I just ordered the meter and will pick it up this afternoon. My mechanic said something to similar effect as to the reasons why voltage may fluctuate.

 

[BamaOverland]

Here's how I test an alternator. In this order:

1. Test battery voltage - engine off - should be 12.62 or higher
2. Test battery voltage - engine on/idle - should be high 13s or low 14s - my '99 was 14.2-ish.
3. Test battery voltage - engine on/idle - but fan on/high beams on/seat heaters on, etc. - should be a few hundredths of a volt less than the #2 test above.

If the voltage sags badly on the 'loaded test' (#3) your alternator is on its way out. Dying alternators can keep up with light loads, but not heavy loads.

There are other, much more elaborate and computerized alternator tests, but this one has never let me down.

Voltage is dropping to about 12.5 on test 3. I guess it’s time to order a new one! Bummer

 

[BamaOverland]

there was a thread on this a few months ago. There seemed to have been a range of observations.
Anything you do on the truck in real time will have some effect on the instantaneous battery voltage, just having your foot on the brake at a red light will likely result in a small but noticeable voltage drop (well, at least if you measure it with a voltmeter). The alternator can't maintain a perfectly even voltage and the battery charge level will affect what the alternator does too. For instance, from what I saw, as the battery charges, the alternator voltage will drop down (which suggests some sort of smart algorithm in play).
Get a digital voltmeter and plug it in the cig lighter, much more telling than the dash tiny gauge.
If your battery is at a healthy level, the alternator is likely fine.

My Napa cigarette voltmeter reads at 12.5 under a heavy load, but around 13.9-14 at normal idle with everything off.

 

[e9999]

^ yup, that is a big drop. Much more than I've seen on mine, from memory.

 

[vipergrhd]

Get a Toyota or Denso unit, if the alternator is still original and not making any noise there is a very good chance it just needs new brushes.
On my 93 truck at 290k alternator died found out brushes was so worn out $25 fixed the problem and I get to keep the original alternator.