Battery not charging properly

U

ukrainian

Member
Hi all, the latest entertaining little problem from the A2 is this:
- barely enough charge in battery to start the car from cold, a couple of very short journeys leave it completely drained
- sluggish turning over, sometimes fails to start first/second time
- EML comes and goes, VCDS-Lite returns this
17588 - Linear O2 Sensor; Pump Current: Short to Plus
P1180 - 35-10 - - - Intermittent
(clearly this is a secondary symptom though, caused by voltage dipping during cranking)
- FM radio reception poor when rear screen heater is ON

This last one made me think it's the main earth point for the starter motor behind passenger headlight, as it was one of the gremlins that went away after cleaning this earth point up a couple of years ago. I repeated the procedure but that wasn't it.

Some more info:
- the battery is only a couple of years old but was a cheap brand (Platin)
- after charging the battery with a mains charger (charging voltage is 14.1V), the engine cranks vigorously and starts first time, everything is back to normal for a couple of days
- with the engine running, alternator output is 14.26V wrt battery -VE terminal, but at battery +VE terminals I measure 14.05V (expect these should be the same?)
- voltage drop between alternator +VE terminal and battery +VE terminal (engine running) is 0.11V (measured at another time from above)
- voltage between starter motor earth point and battery -VE terminal (engine running) is 0.005V
- I have ruled out the battery fuse at +VE terminal, just a few mV is dropped across this
- In ny wife's Mazda, the voltage at alternator and at battery is the same at 14.5V. The battery in the Mazda is a stop-start type though.

Is it normal to drop 0.1-0.2V between alternator +VE and battery +VE terminal, can anyone comment? Am I looking at a poor connection somewhere between the alternator and the battery, or is it more likely the battery is shot, or the alternator needs seeing to? The alternator was replaced 4 years ago with some after-market type, not OEM type.
 
A secondary earth cable would be a cheap check
The earth cable itself can fail thru time
Not only where it connects with chassis
 
Although the radio reception problem is the least of your problems it is quite widespread in our A2s. Google 'a2oc FM radio reception poor when rear screen heater is ON' and you will get lots of links and posts to read that may be applicable to you.

The issue seems to revolve round the aerials(s) earth point in the tailgate and if you have a diversity unit check its connectors for corrosion (happens a lot if the boot has previously got wet) as well as the unit itself.

Andy
 
Thanks both for suggestions. In this case it seems clear that the problem with wiring - if there is one, is in the +VE side somewhere between alternator +VE and battery +VE. I was wondering if anyone knows what this current path physically looks like and where the likely problem locations are. The ETKA battery wiring diagram seems to show both +ve and -ve connections pass through a conduit to the engine bay where the +ve cable connects to the starter motor and a separate cable runs from there to the alternator +ve terminal. Can anyone confirm this? The starter motor was replaced a couple of years ago when the garage missed the obvious bad starter earth symptoms and relieved me of £400 for fitting a new starter motor. Perhaps I need to check that the cables were fitted properly.

Regarding the FM reception, I was discussing the possible mechanisms for this with a colleague. We came to the conclusion that either this is radio frequency noise from the alternator brushed connection injected into the aerial (rear screen heater) via a weak earth connection, or that the (potentially multiple) bad connection(s) may be generating noise on their own. The amount of noise will be proportional to current and FM receivers are tolerant of noise below some threshold which would explain why the problem manifests only under increased load such as the rear heater switching on.
 
So. it was a faulty battery, confirmed. Buy cheap - buy twice. The battery brand is Platin if you want to know which one to avoid.
Replaced with a Bosh type 110 85Ah/800A and I now only have to blip the ignition on to start the car.

Learned something new whilst diagnosing. The ETKA diagram does not show this clearly, but alternator +VE terminal connects to the starter motor +VE terminal. The battery +VE cable (thicker of the two, the one that is not fused at the battery) also connects to the same +VE starter motor terminal. Cleaning up the crimped ends of these two cables reduced the voltage drop between alternator and battery to 100mV but didn't solve the problem of poor starting from cold / low battery charge. This is the diagram I am referring to btw:

is negative ground from battery connected to the body ?

Hello Can anyone tell me if the negative ground from the battery is connected to the body ? or is it wired around and not connected to the body ? 2004 model A2 TDI 1.4 ATL
www.a2oc.net

So I bought a cheap (comparatively, £50) DC clamp meter on Amazon. Was surprised to learn that with the engine idling the alternator to battery cable carried 30A! The 100mV drop between alternator and battery +VE is reasonable in light of this indicating 0.003 Ohm resistance. The return cable (battery to car electronics/electrics, the one with the fuse) carried -28A, leaving a healthy 2A charging current for the battery. The 28A electrics consumption is not far off expected according to google but I didn't think at the time to experiment with turning off radio/climate/heated seat. Checked also current draw with engine off and car locked. It took a few seconds to settle from 0.5A to <100 mA (clamp meter not very precise at mA level). This is also as expected, according to google. So that left only the battery as the cause of trouble. New battery has been in place for a couple of weeks now and no symptoms have recurred. Except poor FM reception when rear screen heater is on which never went away - another red herring.
 
ukrainian said:
...
Except poor FM reception when rear screen heater is on which never went away - another red herring.
Click to expand...
Okay red herring to the issue in question but a common fault caused by corroded connector pins in the aerial connectors in the the boot lid/diversity unit if you have one or poor earthing of the boot lid.

Andy
 
ukrainian said:
.....

Learned something new whilst diagnosing. The ETKA diagram does not show this clearly, but alternator +VE terminal connects to the starter motor +VE terminal.
Click to expand...
Agreed, but if I have this right the wiring diagram does.

B is the Starter and C the Alternator.

1668738245122.png


Andy
 
Thanks Andrew. I have found these automotive current flow diagrams hard to follow, they are somewhat different from standard electronics schematics. Obviously 131 should be the direct connection to battery +ve but in my copy of the service manual this is not clear at all, the label for battery +ve terminal is P2. Seeing was believing for me.

The FM reception being affected by the rear screen heater, I meant to say that a couple of years ago this problem went away completely after a major clean up of the starter motor earth connection (to make the car start again). So when I discovered this time that it was back, I jumped to the conclusion that the bad starter earth was my problem (can cause poor battery charging also). Fell for a red herring. This time it is probably the result as you say of poor connection in the boot lid.
 
I can do a bit of guide on these diagrams if it would help people? They take some getting used to but are okay once you know what everything means. 131 means column 131, with the columns numbers at the bottom. So on column 131 you'll have a wire with a box with the number 36 in it. Do you have a picture of the diagram that shows "P2"?
 
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