OBD Diagnostics oddity

[Stitch626]

Hi everyone,

A couple of times this week I have had the Charge Fault light go on for a short time on my ZT CDTi. The longest period was when it lasted for about 5 miles at motorway speeds on Wednesday morning on my way to work. My commute journey is about 35 miles. Since then I have not seen it at all.

My immediate thought was the alternator was playing up although this was replaced 2 years ago. The battery is less than 6 months old.

Today I decided to check the voltages. Using a multimeter I measured the voltage across the battery before starting (12.3V) and once started (14.5). I turned all electrics on that I could, and the voltage was still 14.2V. All looked good or so I believed.

I decided that I could keep an eye on it by using the OBD diagnostics mode. However the values seen using this method are much lower. Once started I was seeing 13.2V and under load 12.9V. Note that multimeter is still reading the higher values.

I tried the same thing on my 190 and both the OBD diagnostics and multimeter readings are very close, and look good.

So where does the OBD diagnostics measure the voltage? Where does it get its values from?

Does anyone know how could they be lower than the multimeter readings?

The real question though is whether the alternator is okay or is it all some sort of strange reporting issue with the instrument panel? Does any one know what triggers the Charge Fault light?

Thanks in advance,

Mark

[Union Wheels]

Hi Mark,
before you get too involved, I would check the plug on the rear of the alternator for poor/high resistance connections.

Pete.

[HarryM1BYT]

The real question though is whether the alternator is okay or is it all some sort of strange reporting issue with the instrument panel? Does any one know what triggers the Charge Fault light?

Thanks in advance,

Mark

The dash and OBD voltmeter measures the voltage via the loom and the loom is under some load, so there is bound to be some voltage drop. The only reliable way to measure the voltage across the battery is with a meter across the actual battery.

There have been instances of the charge light appearing, with no actual fault being present, due to damaged wiring where it passes through the wing, rubbing on the metalwork - on the CDT(i). It feeds into the engine ECU and the ECU puts the charge light on.

[Lancpudn]

I had the same kind of readings, I was seeing 13.6 in TOAF & 14.4 with my multimeter.
I thought my alternator was on it's way out a few weeks ago, red battery warning on the dash, 11.5 volts showing on the multimeter so I took the alternator off & into an auto electricians shop I've been using for decades to rebuild alternator/starters & when he bench tested it there was nothing wrong with it:shrug:
To cut a long story short it was a Short in wiring some previous owner had done that was causing the problem.

[Stitch626]

There have been instances of the charge light appearing, with no actual fault being present, due to damaged wiring where it passes through the wing, rubbing on the metalwork - on the CDT(i). It feeds into the engine ECU and the ECU puts the charge light on.

To cut a long story short it was a Short in wiring some previous owner had done that was causing the problem.

This may be my problem. A few weeks back one of my sidelights appeared to fail yet the bulb was fine. On measuring the voltage at the bulb it was 9.6V. The cause was damaged wiring where it passes through the wing!

Sounds like I may have missed some other damage in that area

Guess I'll be taking the front off the car again :cool:

Better than changing the alternator anyway ....... :eek:

Thanks for the help,

Mark