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Matt, Mechanical Engineer
Category: Volvo
Satisfied Customers: 22750
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I've replaced the alternator on my Volvo S80 2008 2.4 diesel.

Customer Question

Hi I've replaced the alternator on my Volvo S80 2008 2.4 diesel. The alternator is now charging at 13.7 volts but the battery light and battery voltage warning system is still on?
Submitted: 1 year ago.
Category: Volvo
Expert:  Shantal-Mod replied 1 year ago.
I'm Shantal and I'm a moderator for this topic.
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Customer: replied 1 year ago.
Yeah I still need an answer
Expert:  Matt replied 1 year ago.


1st thing to ask is how old is the battery ? as if the alternator was damaged then this can have a knock on effect with damaging the battery

so if you haven't already then I'd consider replacing the battery

if this is OK then check the state of the battery wiring for clean and tight terminals and if thats OK then I'd do a current draw test

I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine / gearbox.Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.

A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.

Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

down the system voltage too far even with jump cables

one other thing to consider is that the immobiliser is faulty and preventing the starter from being enabled - however if the dash / ignition lights are on then this usually means that its off

If this is all OK then its possible the car needs a new starter motor

Customer: replied 1 year ago.
All connections are ok, there's no draw and the battery has been tested and showing as being ok. On this alternator there's a Lin terminal on the regulater which is controlled by the engine control unit, and is linked on a data line. The alternator is self exciting then the charge of the alternator is controlled by the data link, depending on variables on the engine, what I need to know is how I can test the wave form to see if the ecu is sending the correct commands,
Customer: replied 1 year ago.
The car deffo does not need a starter motor and just to be sure I've removed the ignition feed to the starter motor once the car has been started. I've checked the terminal for the Lin on the regulater and that is showing 5 volts, disconnected from alternator, and 12v once attached, I need to know the parameters and what it should be reading, I have gotten a lend of an oscilloscope and can read the wave forms I just need to know what I should be looking for, voltage and wave form
Customer: replied 1 year ago.
For what ever reason the ecu is not recognising the alternator in a charging state, and is not enducing the alternator into a higher charge rate. This is not a voltage set point regulater again it is controlled by the engine control unit on a data line
Expert:  Matt replied 1 year ago.


understood, thanks for all extra info

in that case I'd suggest to check the condition of the main battery earth connection as it must be clean and tight and do the same for the small earth wires that run out of the engine bay loom and are secured to the engine with ring terminals as these are the ECU specfiic earth connections

the excitation wave from the ECU to the alternator should be a square wave

this page here has an excellent how to for diagnosing smart alternators

this way you can check with your scope to see if the alternator is being sent the correct signal from the ECU

Expert:  Matt replied 1 year ago.


do you still need help?

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