100 series turbo diesel low idle voltage

SimonM

New Member
Sep 2, 2015
12
23
3
Perth
#1
My turbo diesel couple of weeks ago delivered a couple of flat batteries. I replaced these and the installer said that the charge rate when at idle is sitting on about 12.9v when loaded with lights and aircon. That isn’t right I was advised.
My mechanic said alternator probably on way out. ( the car has done 127k) so I replaced the alternator.
Problem is still there. At idle the voltage drops from 14ish Volts to about 12.9 when lights on etc.
Does this sound right , I would have thought Brand new alternator would be sitting at about 13.5-14 at idle.
Or have a replaced the alternator for nothing as it’s a diode or regulator that’s failed.
Anyone come across similar problem?.
 

Drover

Well-Known Member
Nov 7, 2013
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Cooloola Coast, QLD
www.expandasdownunder.com
#2
That sounds about right really not being a smart alternator with an ECU hanging off it, at idle you will have a lower charge rate than at a faster engine RPM, with a charged battery, one start and its showing its putting out 14 at idle without a load and 12.9 with lights on that should be about right increase the revs and charge rate should increase, read voltage from alt and battery.............. at idle with lights on you are only keeping the status quo basically, go for a drive is when the battery gets a decent charge and runs all the gear ............. thats why just idling an engine trying to charge aux and van batteries is a dumb idea............... a workshop manual will give you rates and check sequence which is more reliable source of info............... Do you have 2 batteries hanging off the alternator ????
 

SimonM

New Member
Sep 2, 2015
12
23
3
Perth
#3
I have the factory standard two batteries and a third hooked up for aux power( by arb years ago) that has not had any issues with.
Thanks drover
I’m starting to think this is what has always been , I’ve just not paid attention at start up and idle.
 

Bellbirdweb

Well-Known Member
Jan 24, 2014
1,913
2,736
113
Sydney
#4
That sounds normal but to me.

I went through 3 alternators on my 100 series, the last one I installed back in March just before trading up to a 200z

It was sitting around 12.9-13v at idle

If the batteries are let run too low, it kills the regulators in the alternator.
 

Drover

Well-Known Member
Nov 7, 2013
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Cooloola Coast, QLD
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#5
Forgot about the 2 batteries, of course, swapping the batteries around every 12mths can give them a longer life as I seem to recall one does harder work than the other, glow plug etc but both crank, clean terminals and cable runs the same length essential, never followed up an old mate who was modding his to just use one decent size battery for crank and the other to power his fridge as spotties.
 

Boots in Action

Well-Known Member
Mar 13, 2017
854
779
93
Ferny Grove, Queensland
#6
That sounds about right really not being a smart alternator with an ECU hanging off it, at idle you will have a lower charge rate than at a faster engine RPM, with a charged battery, one start and its showing its putting out 14 at idle without a load and 12.9 with lights on that should be about right increase the revs and charge rate should increase, read voltage from alt and battery.............. at idle with lights on you are only keeping the status quo basically, go for a drive is when the battery gets a decent charge and runs all the gear ............. thats why just idling an engine trying to charge aux and van batteries is a dumb idea............... a workshop manual will give you rates and check sequence which is more reliable source of info............... Do you have 2 batteries hanging off the alternator ????
Spot on @Drover . At idle and with NO loads, one can expect a higher battery voltage only IF the battery/ies are in GOOD condition. If batteries are low or on the way out, the alternator will see the flat battery/ies as a load and try to charge them. Max charge voltage will be at approx 1200rpm and no load and good batteries. Once ANY load is placed on electrical system, there will be a voltage drop overall. The alternator has to provide the extra current for the loads plus try to keep the battery/ies fully charged. This is usually only possible at the higher revs.
The attached may be of use to you as a lot depends on your vehicle and the current use of "smart alternators".
 

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