Technical Battery light won’t turn off

[Pjrowe95]

Hi all,

Had van in to garage to have the alternator replaced, they’ve told me everything’s working as it should be and they’ve checked everything but the battery light won’t turn off has anyone experienced this before and know what the issue could be?
Fiat ducato swb multijet 2.2 2009
Thank you

 

[Communicator]

Hi,

1. Have you checked the voltage across the battery terminals with the engine running? Voltage should be about 14.4V. Battery is located under LH cab floor.

2. If the thinner wire connecting to the M5, field or D+ terminal is shorting to chassis, then the warning light will not go out, but also the alternator will not work. If the same wire has not been reconnected to the field terminal, and the wire only is shorting to chassis, then the light will stay on, but it is possible that the alternator may start generating which may require high engine rpm.

 

[mharris5]

I have exactly the same problem after having a new alternator fitted, did you ever get this sorted?

 

[koalar]

As a first test I would measure the resistance between D+ and the case. Should be around 1.5K.

Some Hella are too high and do pull the body computer pin down to around 1v at ignition on

This is flagged as an error and even if the alternator is pumping out over 13v down the D+ The light will never go out.

Seems to be a common problem with some alternator alternatives

Of course if it’s around 1.5K it a matter of testing voltages and wiring, but it is where I would start

 

[mharris5]

As a first test I would measure the resistance between D+ and the case. Should be around 1.5K.

Some Hella are too high and do pull the body computer pin down to around 1v at ignition on

This is flagged as an error and even if the alternator is pumping out over 13v down the D+ The light will never go out.

Seems to be a common problem with some alternator alternatives

Of course if it’s around 1.5K it a matter of testing voltages and wiring, but it is where I would start

Pin 25 at the bcm measures 14.02v with the engine running. I'll check the resistance tomorrow. But after testing this signal wire for continuity and earth's, and its clear, I think they have fitted the wrong alternator.

 

[varesecrazy]

@Pjrowe95

 

[koalar]

Pin 25 at the bcm measures 14.02v with the engine running. I'll check the resistance tomorrow. But after testing this signal wire for continuity and earth's, and its clear, I think they have fitted the wrong alternator.

Voltage at ignition on is the important one

Should be around 1V

Here’s mine

Some alternators just can’t pull it down

If it’s too high your not going to get anywhere

If it’s around 1V the resistance of the alternator is correct and the D+ Wire is probably correct all the way to the BCM

 

[Communicator]

As a first test I would measure the resistance between D+ and the case. Should be around 1.5K.

Some Hella are too high and do pull the body computer pin down to around 1v at ignition on

This is flagged as an error and even if the alternator is pumping out over 13v down the D+ The light will never go out.

Seems to be a common problem with some alternator alternatives

Of course if it’s around 1.5K it a matter of testing voltages and wiring, but it is where I would start

I accept the general idea that when the ignition is switched on, the alternator field circuit could draw too much current from the BCM. and it makes sense for the BCM to flag this as a fault. However I cannot see why too high a resistance would cause this?

When trying to measure resistance between the D+ terminal and ground you have semiconductors involved, either the half 3 phase bridge that supplies the regulator, or via the regulator and rotor winding to earth. The results will be polarity sensitive with semiconductor forward voltage drops involved, I cannot see a meaningful reading being obtained.

 

[koalar]

I accept the general idea that when the ignition is switched on, the alternator field circuit could draw too much current from the BCM. and it makes sense for the BCM to flag this as a fault. However I cannot see why too high a resistance would cause this?

When trying to measure resistance between the D+ terminal and ground you have semiconductors involved, either the half 3 phase bridge that supplies the regulator, or via the regulator and rotor winding to earth. The results will be polarity sensitive with semiconductor forward voltage drops involved, I cannot see a meaningful reading being obtained.

Let’s no over think it.

The D+ works as a pull down resistor (https://www.google.co.uk/search?q=pull-down+resistor&client=safari&hl=en-gb&sxsrf=AJOqlzVcxIp7UgZrdoh7fWfRSeYH8kwntw:1678996420635&ei=xHMTZLSxJsGskdUPoISj8AI&oq=pull-down+resistor&gs_lcp=ChNtb2JpbGUtZ3dzLXdpei1zZXJwEAMyBAgjECcyBAgAEEMyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgQIABAeMgQIABAeOgoIABBHENYEELADOgcIABCwAxBDSgQIQRgAUJtGWJtGYOtcaAFwAHgAgAFbiAFbkgEBMZgBAKABAcgBCcABAQ&sclient=mobile-gws-wiz-serp) when the ignition is on

Which drop the floating battery voltage at the BCM down to 1 volt

D+ to chassis ground should be around 1.5K as tested in the real world

The pin at the BCM should be around 1V as per picture above

If it not it will never work

It’s a simple test.

from elearn

“When the alternator is not generating voltage and thus not recharging the battery, it sends an earth signal from pin D+“

Here’s seven pages

Battery warning light and no power steering | FIAT Bravo 2 (2007+)

Hi all, I have been searching through the forum and read what I could find on the subject - but I just can't find any good leads on how to fix the issue. I recently bought a cheap 2009 Bravo 1.9 Mjet to have as an all round daily driver. It came with battery warning light and no power steering...

www.fiatforum.com

But it all boils down that in this case to the replacement alternator could never work because the resistance is 410K instead of 1.5K which leave the BCM at 9.5V instead of 1V at ignition on

Simple test. Either it pulls down to 1v ish or not. If it does move onto more tests. If not it’s the wiring D+ or alternator

 

[Communicator]

Thank you, I fully understand the principle and use of a pull down resistor.

Perhaps I misunderstood your interesting post.

I will look into that link.

The last sentence quoted from eLearn is eminently clear.

Edit. I now realise that I had overlooked your explanatory post #7, while I was typing my query.

So to summerise the BCM needs to see the D+ connection pulled down to about 1V, or it flags an error.

"We learn something new every day"