Hey All,
New to forum.
I bought a "no crank" Audi Q7. The guy I bought it off lied and said that one day it just wouldn't crank, and it has been faultless since new otherwise. I later found out after buying it that it broke down and was towed back to his house (talking to his wife a few days later as I had to pick up some stuff).

In any case, with new battery, it would crank (although starter sounded a bit tired). With jumper cables, it would crank faster, but appeared I was having fuel delivery issues.
Car also had a long list on fault codes and multiple warnings on dash, a lot of my testing was physical bench testing. It turned out that the tank pump(s) were not giving much pressure on the physical pressure test (less than 5psi)
I replaced fuel pumps, and then the starter pretty much crapped itself, barely turning over (and putting out a bad electric burning smell)

I painstackingly replaced the starter motor. It cranked much faster, that was, for about a few cranks while I manually went to bleed the rails as I couldnt get the pump bleed function to work in VCDS.
It looked like I just got a good bleed (I had glow plugs out while it would crank/ bleed and finally getting a mist out of plug holes) and the crank came to almost a complete stop. Thnking I maybe sapped the battery a bit (only gave it a few cranks) I connected a running host car via jumper cables. But that didnt help. I ran another earth cable from earth point down to engine mount near the starter, that didnt help either.
My immediate fear was the new starter motor kicked the bucket, but on touching it - it wasn't hot, and no smell. That, or the motor is seizing.

The battery is fully charged (new battery), and with a jumper from a running car, I am testing 14.4+v all round, at all grounds and at starter motor, but the car can barely crank even with the glow plugs out. I have checked all fuses and relays.
I then went to connect VCDS, and it cant connect to engine module. I plugged in my other OBD that I run through the phone, it wont connect to ECU either. I also cant remove the key from ignition now.

Anyone got any clue as to what might be going on, before I go and push this thing off a cliff or call the wreckers?

 

It's an '07 with presumably a lot of miles on it. These Q7s are well known for water drain blockage in the plenum below windshield, where the ECU resides, etc., and there's also critters/rodents that like to get in there and make a nest out of the wiring harnesses, etc.

You already know there's no communication to ECU via scan tools you tried, so that ought to be a rather large clue. Also, another thing that comes up in some cars living in rustbelt/snowbelt areas, is the shielded cable to the starter motor gets corrosion inside it, and high resistance in cable builds up heat as you keep trying it, and then the starter don't spin so good at that point.

IMO, probably couple issues going on, but since you already know the ECU isn't communicating, start with that, as it controls just about everything through feedback loops/sensors.. It's located on passenger side right beneath the plenum cover. They also came out years later with a black plastic cover for business end of the ECU to help keep water off the interface plug, so if you don't have that it's just another possible reason for ECU failure related to water incursion.

The good news is it sounds like it's fixable? A lot of owners just give up on these cars when it does this, and to be sure, some have nearly gone mad chasing this rabbit as DIY. It does occur all-of-the-sudden, so yeah, most Q7 owners who have encountered same issue type have to get the car towed home or to a mechanic shop. That's no cause for alarm. I wouldn't even say he lied to you; tried to start the car and it wouldn't start, and prior it had been doing fine...that's just how this scenario goes unfortunately. Hopefully you got a heck of a deal on it, and it's been well maintained otherwise.

There are several archival threads on here related to this specific 'Crank, No Start' scenario.

Use the internet search to find threads/articles/videos that match your specific symptoms and/or fault codes.

 

Bonus: If the car's dash is actually 'blank' except for the odometer lit up when you turn key on to accessory position; so not seeing the normal warning lamps all lit for their function check, nothing in driver's info screen, etc., then that normally is related to the steering wheel lock/security authorization module located on steering column; another very common issue as these Q7s age. When that occurs the scan tool cannot connect to the car until you perform a multi-step sequence via the car's switches, etc., so be sure of exactly what the car is showing you when the key is 'on'.

 

Thanks heaps for the reply.
Not that many miles considering (less than 120k miles/ 200k km).

While the ECU casing looks like its had a bit of corrosion, the internals look fine. When I was having issues to start it early on I inspected the ECU cirtcuit board and everything looks good there. Some marks suggest its not the original ECU/ or its been removed before.

Im in Australia, so dont deal with snow/ salt - all the cable wires look good, but I will double check the starter motor cables. The motor itself was getting full power without cranking, not sure what its like under draw yet - will test soon.

I have spent so many days going around and checking everything, replacing fuel fuel pumps, starter motor and now with next to no crank and no ECU communication its driving me crazy haha. Hopefully I can find the host of issues soon, as im sick of looking at it haha.

 

That makes sense, I had a similar issue in a jeep.
Unfortunately though, the dash all lights up like a christmas tree and multiple alarms.

 

Every part on this car has a production date code stamped/stenciled/labeled on the part, so that's a really easy way to check if it's original factory installed part or not. If you are seeing corrosion indicators and aren't an actual electronics repair wizard yourself, then it's most likely the ECU is the problem. The Q7 is a car filled with control modules (computers), and besides those plenum drains clogging up, there's plenty of interior water leaks and HVAC condensate drain clogs to go around as well, so check the carpets all around for water, and also the under the cargo area floor where sparte tire goes, as rear sunroof drains leak both sides, front sunroof drains down A-pillars can leak to the front footwells, HVAC condensate drain likes to clog, and those plenum drains backing up will likewise result in water spilling over into front floorboards area, above & beyond the damage it does to the electronics modules located in the plenum.

Bit of a learning curve a used Q7 has on it. Never discount the creeping damage of corrosion from water leaks. MMI circuit board will go out from the drink prior owner spilled on it years ago...it's a very common issue. You get the idea yet? Do the wiring checks and all, but don't ignore the myriad known issues on this platform. To do so is to add to your own peril, and you'll waste a ton of money throwing parts at this car that won't fix the issue.