Erichamilton

Hi, my 2010 Q5 3.0TDI has an issue when it is hot, when you press the start button, the dash lights up and everything look ok, except the motor does not crank.
I recently searched the forums about this issue and found a few posts that say to start with replacing the starter motor, the starter has now been replaced with a new unit, however the problems still there when hot.
If i start the car and only go for a short drive, it starts ok, even though the motor is fully up to temperature, it only plays up after a longer drive.
It does always start if you let it sit for a while, but i need to sort the problem before it fails completely.
Does anyone know the sequence that it goes through to start, obviously there are relays and a control unit somewhere that control the start sequence, I am thinking that the problem electrical component must be in the engine bay area and doesn't get hot enough on short trips but the heat soaks in on longer trips. I am thinking a control module may be faulty, are there any modules in the engine bay area that are used in the start sequence??
Any suggestions anyone?

rdA4WtQ5

Start by scanning for codes. The relay that powers the ECU goes bad but usually gives warning by working intermittently and setting lots of codes. Could be the brake switch since it is acting like you did not step on the brake to start. See if your brake lights are lighting when it does not start. Good Luck.

Bob Petruska

Redd

FWIW, I had an intermittent "won't start when hot on hot days" problem on my last car. Drove me nuts, I couldn't find it, dealer couldn't find it. Finally I was checking the fuse panel and found the EFI system main fuse was melting. Not blown, but the plastic fuse body was melting. Somehow moisture had gotten into the fuse panel (under the hood) and the seat that the ATC fuse plugged into had corrosion on it. Don't ask how hard it was to get in there and clean it out. But apparently that increased the resistance of the fuse enough to overheat it and start slagging down the fuse body, and also reducing the current flow to the point where the EFI system wouldn't allow the engine to start up.

Gremlins.

There could be something subtle like that, anywhere in the cranking circuits. Or even in the starter safety circuits that tell the car the brake pedal is down and the car is in park. That's why scanning the car, shelling out for VCDS if you don't already have it, is a good place to start. At least that confirms or eliminates a number of potential issues. At $200, that's about what the fist 90 minutes of dealer "sniffing around" time, or doing a scan, would cost anyway.

canon_mutant

Maybe it is just my luck but I have lost MORE batteries in the summer heat than the winter cold.

MurrayA4

No canon mutant.That is in fact the stat.More batteries fail in the summer.Used to be in the Biz and maintained an inventory of batteries, Napa and Interstate.
Heat stresses batteries more than cold.I'm in Quebec so I sorta understand cold LOL.

Erichamilton

Thanks for the reply's everyone, I have found a relay under the drivers side that had a high resistance, that has been replaced, will have to see if the problem still persists on a long drive.
I am fairly sure it is not a battery as the voltage is good when the problem occurs, I am going to dig a little deeper today and check all fuse and earth connections

Redd

Murray-
You may be happy to know that this month's issue of Consumer Reports confirms, as oyu say, that more battery replacements happen in the hot summer months. And that batteries tend to last 3-4 years, rather than 4-5, in our south as opposed to our northern states.

I'm not sure of the logic behind this but suspect that a large part of it has to do with battery care. That is, long summer rides are more likely to overcharge the battery. Summer heat is more likely to help electrolyte evaporate. Combinations like that would leave people (who we know always maintain electrolyte scrupulously) with dried out batteries, just waiting to die. If they broke down the stats and wet lead were dying in summer more than AGMs were...that would say something too.

Either way, compared to modern solar and marine chargers (using computer profiling and lookup tables and battery programming which, unlike Audi, they actually USE to set the charge profile) the chargers in the finest of cars are still designed to be cheap, not effective. Once cars start to use $1000-2000 battery banks, maybe that will change.

Erichamilton

The car failed to start again today after an hour and half drive, I have done a scan but I am new to the forum and to the software, several communication faults with the ECU, but i notice later on it says that the unit is powered up? Does this indicate that I have a problem with a communication module rater than a lack of power to the module?