Help please! Voltage Dropping Issue

[MPA]

Hi Everyone,

Bit desperate here, first time posting.
This will be a bit of a long post just so I can get as much information out there to help diagnose the issue.

Car: HOLDEN COMMODORE VX (S2?) 2002 4 Door Sedan 3.8 litre, LN3 (L36) V6

So my issue is my voltage starts off at 13.5V (Dash Readout) and gradually drops to <11V over time.

There is no issue getting it started and it always starts with what I believe to be reasonable voltages.

First sign there is a problem is the Tacko comes and goes and then drops out all-together. Second the music system goes in and out, just the sound over the speakers at first and then the whole head unit starts turning on and off. The ABS light comes on next and headlights get dull. Push it any further and all the dash lights starts coming and going. Eventually the car stalls.

The Battery Light does not come on until all the light start playing up.

The voltage drops faster the faster I drive. So 10mins at 60km it's fine, I can get to the shops before I run into major issues. But if I try to drive 10mins at 80km plus it drops much faster.

What I have done so far.
1) I replaced the Alternator, this had no effect at all. (Advice from RACQ and Family)
2) I replaced the Battery. This gave me about 3 weeks with no problems. But they have now returned identical to before.

I checked with my own multimeter and also had RACQ check and the Alternator and Battery seem to be fine on start-up.

I have had family suggest the ECU as the next part to replace however reading other posts online, including this forum. It sounds like the issues are too far spread to be the ECU or at least JUST the ECU.

Any help or advice is much appreciated!
I'm a bit stressed out at this point.
I'm worried taking it to a shop is going to be more than I can afford and put me in a bad financial situation but if I get desperate enough it'll happen.

 

[lout]

when you see the dash readout being below 13v with engine running
measure the voltage at battery with multimeter and see if they are close
symptoms are classic alternator, what alternator did you fit, new, reco, good from wreckers
is this car electrically stock
is this car in good condition
have you checked your ground leads

 

[shane_3800]

I could diag it for you if you were close to me as it sounds like it's an issue on the battery side. The BCM is likely playing up due to the voltage drop So all the cables need to be checked also could be the alt cable. Is the alternator getting real hot? if so its the alt cable.

 

[Immortality]

AS above, I'd be looking at cables and connections including all earths (engine block, battery and chassis etc). Make sure all connections are tight and clean. Measure the voltage at the alternator from +ve output post to alternator body and then start doing checks between the alternator and battery looking for voltage drops.

The alternator senses battery voltage via a separate wire that has a fusible link at the battery end, check the link (it may have partially failed leading to high resistance or open circuit when hot).

The PCM/BCM have no authority over battery/alternator systems (unlike the VE) but they will not be happy with a dropping voltage.

 

[gtrboyy]

Splurge on some new battery terminals then see if still happens.

 

[MPA]

Hi!

Sorry for the delayed reply.
I think it will be Saturday before I get a real chance to tinker and test but I will try to answer what I can now.

The Alternator was ordered online from a wrecking yard. They seemed reputable, said it had been tested before sending it. I have a returns and exchange policy I think I might just still be within it if I needed to return it for another.

Electrically stock. Not really. There is LED lighting strips in the foot wells, a non-stock head unit and an electronic rust protector that may or may not have come with the car originally.

Car is in reasonable condition, not fantastic but not run down. Previous owners really looked after it.

I DID test if the Alternator was getting hot. I burned myself touching it after only a 10min drive if that gives any indication? I will check the Alternator plug first and then all the wiring, I can see/get to and checking grounding first thing on Saturday.

Thanks for all the replies, suggestions and info request so far. Already feeling a bit better about the whole mess.

 

[JohnW]

Don't know for sure what the charging rate for a VX should be, but the genuine Holden service manual seems to imply it is the same as a series 1 VT, which is 14.5 +-0.2 volts @20C.

Buy a $20 Chinese multi meter sold at just about ever auto parts store, even Bunnings. These things are accurate enough for checking battery voltage. Check battery voltage before starting the car, and after starting. If it is 13.5 volts at the battery with the engine running, to me that's a problem.

If you do decide to replace the alternator, consider you can buy a brand new genuine Bosch alternator (same as original) for around $250?? if you shop around on the internet.

 

[Immortality]

Anything pre VE the alternators are rated at 14.4 volts (it's stamped on the alternator).

(using a random interwebs pic)

You can see on the sticker this particular unit is rated at 28 volts and puts out 10amps at it's minimum rpm and a max of 55 amps.

You can't just say 13.5 volts is an issue as it depends on engine speed and electrical load. At idle with high load the alternator will struggle to keep up.

 

[shane_3800]

Battery initial voltage will affect the the alternator too. If the battery is at like 12.1 it's low on charge so the whole system will have a voltage drop because of the battery.

 

[hiker]

A large number of auto electrical faults relate to poor earth/earth return .Perhaps it would be prudent to check the mechanical side of the earth connections - battery to engine,battery to body,body to accessories etc. This can be achieved with the el cheapo multimeter on OHMS scale and the positive side of the battery disconnected. The reading should be 0.3 ohms or less. Maybe temporarily disconnect the rust prevention system. Hope this helps Hiker.