low voltage while driving

86SS86LS

Member
While driving to work this morning I had noticed that my scanmaster voltage reading was around 10. A few miles down the road it was reading around 8. What is the most likely cause of this? And what's the best way to get it 10 miles home....can I drop in a new battery and get home?
 
While driving to work this morning I had noticed that my scanmaster voltage reading was around 10. A few miles down the road it was reading around 8. What is the most likely cause of this? And what's the best way to get it 10 miles home....can I drop in a new battery and get home?

You might make it. Sounds like your alternator has taken a dump.
 
Check it's voltage output at the main lug on the back of the alternator while at idle. It should read 14.0-14.4 volts.
 
The multimeter reads 12.25 Volts at the bolt behind the alternator. Does this mean the alternator is shot and where is the best place to get a new or remanufactured unit?
 
Yes, it's bad. As to where you get one, is entirely up to you and your budget. Many of our vendors carry good ones. If you go "local" then make sure to specify at least a 120 amp unit.
 
I just exchanged my alternator from Autozone and it didn't cost me a dime. 124 amps with a lifetime warranty. No complaints here.
 
Another thing to consider

The alternator might be still OK because the alternator electronics is designed stop charging if the sense voltage goes too low. The voltage you measure with the scanmaster happends to be the voltage the alternator uses as it's sense voltage. A voltage of 10V will stop alternator from charging and it might make you think the alternator is toast.

The electronics in the alternator does this to prevent it from burning out with a mostly dead battery. The alternator is not meant to charge a dead battery. It is only meant to keep a good battery charged.

I'm not saying that there is not a possibility that your alternator isnt't toast, however you should do a little troubleshooting before you right it off.

Charge the battery with an external charger until the scanmaster voltage is above 12V. Then measure the voltage at the alternator again with the engine running. It will start to charge the battery and indicate this by the voltage going above 12V.

D.
 
With a new charged battery I am getting 12.3 volts at the alternator and at the battery. The scanmaster shows mid 11's at idle. I took it out for a drive today and the scanmaster displayed between 11.0 and 11.5 volts, didn't really increase or decrease during the drive. Looks like I will be buying a new alternator.
 
With a new battery and still only seeing 12v at the alternator, that's pretty definative that the alternator's bad, but there's the slim possibility that the volt light is bad and not allowing the alternator to be excited. You could have saved the cost of a new batter and just had your local auto parts store test the alt.
 
Bad Alternator.

Yes, TuboDave is correct. A bad low volt light in the dash can prevent the alternator from charging (as crazy as it sounds:confused: ).

D.
 
Turbodave, the battery was 4-5 years old anyway and in need of a replacement soon anyway, I'll use the old one on a less critical application.

I purchased and installed a remanufactured alternator from Advance. I get around a 14.2 Volt reading at both the battery and alternator. And the scanmaster is reading 13.5 while driving. The only problem is that the volt light on the dash does not light up with the key in the "on" position. As long as I am seeing good voltage readings is this anything to worry about or should I replace the bulb anyway?
 
Don't necessarily agree

Yes, it's bad. As to where you get one, is entirely up to you and your budget. Many of our vendors carry good ones. If you go "local" then make sure to specify at least a 120 amp unit.

You may want take your alternator to and have it checked out. The regulator will make adjustments based on the cars electrical output needs. So to see lower voltage especially at idle is not abnormal. If you don't feel like taking it out and having it reviewed by Autozone. Start the car, check the voltage. Turn on all accessories including the lights and then check the voltage. You should see the voltage increase as the system tries to supply the correct amperage to all the accessories.

Additionally if you don't see what you want to see in voltage increase, look to see if the battery light comes on when you turn the car to "on". If you don't get this from your dash then I would start by replacing this bulb.

Just my .02 cents!
 
You may want take your alternator to and have it checked out. The regulator will make adjustments based on the cars electrical output needs. So to see lower voltage especially at idle is not abnormal. If you don't feel like taking it out and having it reviewed by Autozone. Start the car, check the voltage. Turn on all accessories including the lights and then check the voltage. You should see the voltage increase as the system tries to supply the correct amperage to all the accessories.

Additionally if you don't see what you want to see in voltage increase, look to see if the battery light comes on when you turn the car to "on". If you don't get this from your dash then I would start by replacing this bulb.

Just my .02 cents!


Well actually I'd have to disagree. It is abnormal to see low voltage at idle, no matter what the demands of the system are. The alternator should be putting out 14+ volts at idle even with little to no demand.
The demand you speak of is the additional current required by more systms being activated. Additional electrical deviced don't change the voltage demand, they change the current demand.
A very heavy load could pull the voltage down on a weak charging system/wiring, but with a low demand, if anything will keep the voltage up. Ohms law.
The demand on the system at idle (with no lights on, or A/C, etc) should be very low.
I repeat, IMHO it is abnormal to have low voltage at idle also. It should never drop below 14v (at the alternator).
Something is causing it. Bad alternator, bad charge circuit, bad battery, etc.
 
Now I'm seeing the volt light when the key is in the "ON" position. I'm getting over 14 volts at the battery and the alternator and the scanmaster is reading 13.5. All is good.
 
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