V40 1.6 diesel 2015,went to start it this morning dash completely dead,had to put jump leads to start,checked battery and is 100% charged,this is the third time this has happened,been told it could be immobser problem,can anyone help please??

 

If you can put jumpers on the car and the problem goes away, I'd have the battery tested. How do you know the battery is 100% charged or even capable of holding a charge?

 

So, I am no expert so take my words with a grain of salt, but:

When you jump start the car, the alternator kicks in and is supplying electricity for you car and charging the battery. At that point, if you test the battery it would say it's 13-14v, but at that point you cannot measure the true battery voltage. Furthermore a Digital Multimeter has an input impedance in the order of 1-10 or more megaohms. At that range, it can pick up potential energy, but it could be in the order of microamps or less. And as soon as a higher load is put on it, any circuit really, it would not be able to deliver the current and the voltage would topple.
Consider how static electricity works. It can be very high voltage, kilovolts but you only feel a zap, because a truly small amount of current passed through your body. And as you probably know, 50-100 milliamps are enough to affect the heart.0, even stopping it

 

full disclosure - I am an electrical engineer with 40+ years of auto repair as a hobby.

As you know current and voltage are related (V=IR) - so when you hit your starter motor, the battery has the challenge of delivering substantial current to spin up the starter motor. As a battery ages it loses its ability to produce that current (imagine all those electrons getting confused by the impurities creating resistance within the battery cells). As a result the voltage drops and your starter either spins slowly or not at all. when you put your VOM on the posts, you are drawing picoamps so you read a full 12.6 V. try to start and boom, the voltage drops and car no work. So now you put your jumpers on and the current is drawn from source #2, system voltage is maintained and the car starts and the alternator takes over providing the system voltage. At this point the battery is a current sink not a source - in fact on old school cars you could run the car without a battery at all once the engine was running (modern cars detect battery voltage and as a result may behave differently)

As to the question regarding the possibility of the immobilizer being the cause - its kind of illogical to think that a jump start would effectively bypass the immobilizer (who's job it is to keep cars from being hot wired...). Considering your car's security system includes a programmable key and sensors, it makes sense that a system voltage issues (like voltage dropping during start up due to a weak battery) could trip the immobilizer setting off warning messages to the dash.

So the simple next step is to verify whether you have a good battery or run additional tests to confirm how good your starter is, the battery cables/terminals etc or you can assume your car has a very complex computer issue which may require a trip to the dealer with Volvo factory reps on call to help diagnose your no start condition.