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Old 16-12-18, 08:25 PM   #1
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Default Only seeing 12.7V at the battery with the engine running?

Hi guys,

First post on here so hope somebody might be able to help.

2012 2lt diesel, 80k and running well.

So my wife has this Expert van, the engine always starts but turns over slowly.

So I checked the battery voltage ~ 12.35 without the engine running, ~ 12.7 with it running. I assumed alternator failure and took it to be replaced, also replaced battery - woman driver and didn't want to take chances. This was # 9 months ago.

It has not let us down but starting has remained sluggish, this weekend I checked the voltage again and it is the same as before. The alternator is still under warranty so will be going back there for checks.

With the engine running and 12.7V at the battery terminals, I can switch lights, heater, hazards and radio on - still only 12.6 / 7, use the wipers and the voltage rises to 14.2.


When I originally took the van to have the alternator changed, the mechanic told me the ECU controls the alternator output - is this correct?

In addition, I forgot to say there are no warning lights on the dash. As a knowledgeable DIY mechanic I am worried that 12.7V isn't good especially in cold weather.

During starting the battery voltage drops to ~10V but has not let us down. We needed to go for a long journey last week and I took the decision to see what would happen (we have breakdown cover), still only 12.7 but hasn't let us down.

Could this be ECU troubles?

Has anyone experienced the same problem and identified the fix?

Thanks in anticipation.

Andy
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Old 17-12-18, 12:10 PM   #2
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You might have already done this, but have you tried another voltage tester?
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Old 17-12-18, 07:01 PM   #3
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Hi, thanks for your reply.
You do raise a good point but I have tried to measure the voltage across the battery terminals a few times with the same output. By turning on the wipers with the engine running; the voltage went up to 14.3 which is more like the value I would be expecting.

So far I have been unable to find owners on this and other sites suffering with the same problem.


I am hoping it is not ECU related but at the moment there is something seriously wrong with the alternator output - we can't drive with the wipers on on a dry day!!!


Looking at the battery cables, do these have diodes in line just before the terminals? The cables have a plastic block in line with only 2 connections I can only assume they are diodes to prevent voltage drain - anybody shine some light on this?

Cheers guys.
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Old 17-12-18, 08:34 PM   #4
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Very odd circumstances - turn something on to REDUCE the load?

I suspect the self-parking circuit of the wiper motor is causing a partial short circuit, drastically reducing the voltage. When the wipers are running, the self-parking is, of course, inoperative.

You could prove this by disconnecting the wiper motor and measuring again. If you then get 14.3V, that's conclusive.
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Old 18-12-18, 12:56 PM   #5
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Or it could be that turning something on is making another circuit to one with a better earth?

But I like Ian's hypothesis better.
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Old 18-12-18, 08:04 PM   #6
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Its not the ecu that controls the alternator its the BSI and your issues sound normal to me the BSI does indeed control the alternator based on load so the more you load it the higher the output will be 12.7 is its normal state under light load your old alternator was probably fine but your slow starting may well be the starter getting weak.
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Old 18-12-18, 08:37 PM   #7
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I'm having a similar dilemma
with my 2010 3008 2.0hdi.
Symptom was: Battery was struggling to start the engine, on checking the terminal charging voltage it was 12.6 at the battery.,which I thought was low but at this stage thinking the voltage was being controlled by ECU or BCI so possibly acceptable. I then purchased a USB plugin phone charger with integral volt indication to monitor the situation. By coincidence or not that immediatly read 14.5v. so that confirmed to me the Alternator was OK, so I went ahead and replaced the battery. Taking note of the advice given on our forum it was fairly straight forward. Powered it up and hey !! the usb voltage stick is now showing 12.7 volts...loading the system is not affecting the charge voltage.

suggestions or solutions... cheers
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Old 18-12-18, 08:40 PM   #8
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Thanks all 3 of you, your suggestions and info are really helpful. I will try the wiper motor - to prove the point - I am interested in this - should have picked it up earlier.
The reason this intrigues me is that with the engine running with headlights on full, heater fan on 4, hazards flashing and the radio on (not much more to switch on); the alternator output is still only 12.7V - I would expect rightly or wrongly to see 13.5 + V to support the energy draw from the battery. It strikes me odd that with all of the above switched off and just the wipers on the voltage jumps to 14.3V (this could of course be a large current drain by the wiper motor).
Because the alternator is only running at 12.7, with the engine off the battery is 12.3 which seems to equate to around 65% charged.

I have taken the plunge and await delivery of a Haynes Manual -I am sure all will be revealed - not!!!

Please keep hitting me with ideas - my wife drives the van (I cycle) and am ken to ensure the battery does not let hr down this winter.

Cheers guys.

Andy
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Old 18-12-18, 08:50 PM   #9
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Ref the starter motor, thanks for mentioning that - I had read elsewhere that they can become sluggish which I guess will drag the battery down further still. During cranking it drops to ~ 10V which seems pretty low.
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Old 18-12-18, 08:50 PM   #10
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Loading the system within design limits should not affect the voltage. Control is exercised by an automatic voltage regulator, which maintains the same voltage over a wide range of loads. When the demand is increased by additional loading, the AVR increases the excitation in the rotor coils (drives more current through them) to maintain that same voltage. In doing that, the current generated in the stator coils increases to meet the higher load.

I have four cars, and all the alternators behave in this fashion. All show in excess of 14.2V, whether the load is minimal or high.

If the voltage generated is markedly lower than 14.x, there is either a fault in the alternator, or there is an abnormally high load, such as a partial short circuit to earth.
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Last edited by IanML; 18-12-18 at 08:53 PM.
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