Voltage reading low...

[Jon K]

So my S-belt is somewhat cracked (i used to be supercharged using a diff belt) because its old and what not but recently i noticed that my OBC volt meter shows like 13.86 - 13.94 MAX but will drop as low as like 13.4 or 13.5, and when cruising, it doesn't hit the 14.09 or 14.15 it used to... its a relatively new alternator... bosch 140A replaced ti in like 2002 or early 2003. I left the car all day at the train station and checked OBC with the IGN ON II and it read 11.8 volts which is fine for a 10 hr sit. Car starts fine (though the one time i started it was a little rough, but i think i let off the crank too quickly...). I am sort of paranoid - should i try another battery? Can I test the alternator directly to see if it is the culprit? Woudl some sort of electronic short eat my volts some how? Help guys.

Also - I tested with volt meter @ B+ terminal (jump start point) and strut bolt as ground and got about .2 volts higher than what OBC reads (13.9 or so). Argh.

[Kalevera]

What do the regulator brushes look like?

[Jon K]

I'll have to remove them wednesday sometime - the alternator was put in around 120k or so, now has 188k... so.. i hope they're ok? How hard are the brushes to remove with the alternator in the car?

[Kalevera]

Easy...a minute worth of work. Think Don Gale has a pic of what to look for on his web site. Also, is there any kind of perceptible voltage drop from the engine to the chassis? Maybe there's a weak ground from loosening/tightening things. About 8 months ago, I had a similar issue on my iX -- battery stopped charging completely, though. Turned out to be from my bead blasting and clear coating efforts after a timing belt job.

best, whit

[Jon K]

Easy...a minute worth of work. Think Don Gale has a pic of what to look for on his web site. Also, is there any kind of perceptible voltage drop from the engine to the chassis? Maybe there's a weak ground from loosening/tightening things. About 8 months ago, I had a similar issue on my iX -- battery stopped charging completely, though. Turned out to be from my bead blasting and clear coating efforts after a timing belt job.

best, whit


I will look around if you can find that pic that'd be great - as for volt drop, how can I measure it?

[shogun]

Last Saturday I checked the voltage on the battery pole in the engine room and on the OBC, with ignition on, and in idle and then at abt. 1.500 RPM.

The OBC showed always around 0.4-0.5 V less than measured directly at the pole. Even tested with a seconds DMM, same result.

[Anthony (M5 in Calgary)]

I will look around if you can find that pic that'd be great - as for volt drop, how can I measure it?

Measure voltage drops along the circuit by placing one DMM lead on a good chassis ground then the other lead to a local ground for the suspected circuit. Same for the positive - one DMM lead to battery lead and the other to load point. Significant voltage in either test indicates poor connection somewhere. You can test at various points along a given circuit to isolate the problem to connectors, switches or relays.

[Jon K]

Is this with the DMM in "voltage" mode? I know a fair bit about electornics but only on circuit board level where I am used to measuring.

[joshua43214]

to measure voltage drop, you simply attach the leads along a circuit, ensure the polarity is the same as ion flow. for instance, to test the voltage drop on a positive battery cable, attach the negative lead to the positive terminal at the battery. attach the positive lead to the stud at the starter. aply voltage. teh meter reading is the voltage drop along that circuit, or in other words, the amount of voltage "spent" passing through the wire.

acceptable voltage drop varies depending on the type of circuit. It is far better than a simple resistance check, since you actualy see a circuits performance under load rather than at rest.

It is normal on most cars when they are older for the dash voltage displays to be up to 1/2 volt or so lower than actual voltage due to voltage drop.

[Anthony (M5 in Calgary)]

Is this with the DMM in "voltage" mode? I know a fair bit about electornics but only on circuit board level where I am used to measuring.

Yes, in voltage mode - sorry. With a DMM, there's no need to worry about the polarity as they read either way but will give a negative number if leads are reversed.

[DanH]

The resistance in the circuitry between the alternator and OBC will cause the OBC voltage to be lower.

Also cheap voltage meters could give bad readings.

[gmannino]

Yeah, i wouldnt rely on the obc for an accurate voltage reading. Measure the output directly from the alternator. If you are not having starting problems or charging problems, i'm sure everything is ok.