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DiscoGeorge



Member Since: 20 Nov 2010
Location: Pretoria, South Africa
Posts: 193
South Africa 
Incorrect Voltage Signals with 80%+ discharged battery

Hi guys

had a nasty one happen to me recently. One morning, the Disco1 refused to start, was cranking very slow.

So I set out to find the fault. Took my trusted Fluke multimeter and checked the voltage. The Fluke was giving me 21V and later after we kickstarted it gave 28V. Now at that voltage it would have cooked every single ECU on the car.

So I had a good look at the old Fluke and the "low battery" icon was showing. So after fitting a new 9V battery everything was back to normal.

Why am I telling you this story. Simple because exactly that is what is happening to our electronically controlled cars, when the battery is on its way out or is less than 25% charged. Incorrect voltage signal from the various sendor units are sent to ECU' and incorrectly interpreted.

So now you know, why it is important to charge batteries at regular intervals.

BTW on the starter motor (new one) the two long bolts came loose, as they were obviously never fully tightened at the factory. After fitting two thin weave washers I torqued the bolts to 10Nm (6mm bolt) and the problem was solved. Those bolts will never come loose again.with kind regards With kind regards
DiscoGeorge
1998 Disco1 ES 300Tdi with twin TrueTracs
2005 Disco3 TDV6 S
And some more serious stuff
Post #93304 25th Feb 2011 8:53 pm
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alex_pescaru



Member Since: 12 Mar 2009
Location: RO
Posts: 4640
Re: Incorrect Voltage Signals with 80%+ discharged battery

DiscoGeorge wrote:
when the battery is on its way out or is less than 25% charged. Incorrect voltage signal from the various sendor units are sent to ECU' and incorrectly interpreted.


We've talked about this before (http://www.freel2.com/forum/post83136.html#83136) and I've told you that all sensors are powered at 5V (TTL/CMOS standard voltage), because all electronics are digital and this is the reference voltage for digital systems.
The regulators that are used for generating 5V are functioning properly with a difference of minimum 2V so a battery voltage as low as 7V will suffice for the system to work properly.

Now on your Fluke instrument, this is happening because of the incorrect voltage supplied to the reference input(s) of the A/D converters. This is totally different matter. To accomodate the multitude of units measured and for autoscalling purposes, the reference voltage is generated outside the measuring/metering chip.

In the case of automotive sensors, all metering processes are done inside the sensors, which are also digital.
Of course we are talking here about the real important sensors like airflow meter, fuel rail pressure sensor, etc.

But of course, a low battery voltage could be problematic for the system as a whole because of the improper functioning of the execution devices/elements as stepper motors, relays and so on, which indeed are functioning properly only on 10V on more.
But not because of the sensors...

Just my 2 cents... I could be wrong...
Post #93323 25th Feb 2011 10:53 pm
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DiscoGeorge



Member Since: 20 Nov 2010
Location: Pretoria, South Africa
Posts: 193
South Africa 

Alex

in theory your argumentation is naturally 100% CORRECT. However for it to hold true, incidents like cars locking out (or in) their occupants because of erratic ECU performance due to heavily discharged batteries and resulting from that incorrect system voltage should never happen.

BUT THEY DO! Rolling Eyes

So unfortunately for us suckers out there, until such time that the motor manufacturers REALLY get on top of this we have to live with erratic behaviour, weird error and fault codes etc etc etc and so forth.

I have seen this too many times in my time with MB and BMW. It does happen and will continue so for the foreseeable future. Big Cry With kind regards
DiscoGeorge
1998 Disco1 ES 300Tdi with twin TrueTracs
2005 Disco3 TDV6 S
And some more serious stuff
Post #93337 25th Feb 2011 11:43 pm
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druand



Member Since: 07 Sep 2009
Location: south ayrshire
Posts: 825
Scotland 2013 Freelander 2 SD4 GS Auto Fuji White

I dont know if this applies to FL2 but certainly on some Ford and Subaru models.
When you switch on all gauges go to maximum and back 2 or 3 times then settle to true readings, car starts and all seems OK.
This is an early sign of low battery voltage when instruments do a self test. All FL2's (3) Gone
2011 Mercedes C180 CGI Gone
FL2 GS Auto SD with heated leather, factory tints, alloy spare and a few other bits.
Post #93339 26th Feb 2011 12:52 am
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