Starter motor was getting very tired/ slow on a car that im working on/ trying to get going (Audi Q7, 3.0TDI).
Replaced with a new starter motor, which turned over fast at first, then started to get tired like the battery was going flat - thing is the battery is new (fully charged) and even with jump cables connected to another car started to barely crank the car. I removed the glow plugs (to make sure it wasn't hydrolocking with fuel) and barely made a difference, even with glow plugs out.
Now, It barely cranks the car, even though you can turn the car over by hand (with a socket and breaker bar). It also used to crank for ages, but now only does about 2 slow revolutions and gives up.

I put a multimeter on battery, and the crank voltage drops to 10-11v.
I then put multimeter on engine ground and starter solenoid contact, and another test to the main positive cable, and volt drops to the same, 10-11v when cranking, which should be more than enough?

Which tells me it has to be the starter motor right?
I have also lost contact with ECU through OBD and the car seems to have a multitude of electrical issues, but if the start motor is getting full voltage, the crank issue has to be with the starter, right?

Thanks in advance for any replies

 

A lot depends on if you sourced the OE/OEM starter motor for the TDI. Aftermarket is a no-go for the starter motor (and alternators too) on the Q7s, and especially since it is such a PITA to R&R that starter motor (and the alternator) you don't want it failing. If you bought the OE/OEM starter correct part #, then I'd doubt it is bad, and would look for other source. You can always apply direct voltage to the starter to see how fast it spins.

You say you got lots of electrical gremlins and fault codes on non-running Q7 TDI you bought, but do you understand how this car actively limits electrical drains at specific voltage and low amp hours available set-points via the BEM system, which is constantly monitoring the state of the battery and charging system?

The older Q7, and especially those from snow-belt/rust belt regions at some prior point in their history, is known for developing high resistance from corrosion getting started inside the power and grounding cables. Often times there is a visible signs of heat damage (melted outer sheathing, etc.) related to internal corrosion, but often it is difficult to figure out you've got damaged supply voltage or ground cables. As the cable heats up... What you are interested in is the resistance in the cable, and not the test voltage you can measure or pass through it via a multimeter.

On a non-running TDI, I'd definitely post up the full VAGCOM scan details using the 'spoiler' symbol in the edit bar (+ symbol inside a box), to make that imported data collapsible. Often times there is a lot the experienced DIYers on here can read from fault data in a full scan that someone less familiar with the Q7 will overlook as inconsequential.

You've been posting about this for a bit now, so change your approach to troubleshooting this car is my advice; the learning curve is fairly steep on a Q7, and more so for the TDI Q7, where the engine may not run simply due to an exhaust sensor fault in the SCR system; run out of reductant or just a bad NOx sensor...the car won't restart. Lots of Easter eggs on the TDI to discover. And then there's the more traditional stuff like a bad/water damaged ECU, and that's fairly common too due to clogged up plenum water drains allow water to back up, etc.

Good luck!

 

If it sounds like the battery is going flat, the first thing to eliminate is a flat battery. A new battery connected to a car that needed a new battery due to parasitic losses will quickly become a flat battery. Forget the age of the battery. Just prove that it’s capable of giving sufficient kick.

If the starter motor is also recent, the odds are leaning towards misdiagnosis. As mentioned above, the learning curve with a Q7 is pretty steep. What makes a steep curve unassailable is some frankly dubious advice that comes from this forum. If a suggested course of action seems illogical, it probably is.

That said, if you’re chasing down a non-starting issue, go easy on the starter motor. They’re not designed for sustained cranking.

Most of the reasons for non-starting given on here are nonsense. Dead glow plugs? I disconnected mine and it starts but runs a bit rough. Injectors? They don’t die simultaneously and they deteriorate gradually with some pretty obvious symptoms. Fuel pump(s)? Relatively easy to test for pressure and flow. Fuel contamination? Replace fuel filter. Sit old filter in a bowl for a couple of days to drain and then inspect between the pleats for metallic debris.

If you’re inclined to throw money at replacing parts, start with relays (anything to do with glow plugs or ignition) and then consider sending the engine ecu and the start-auth modules for inspection/testing.

In that water-ingress is the big module-killer on a Q7, it’s wise to make sure any water-damage repairs are worthwhile. Tip a pint of water into the plenums front left and right and check for a corresponding flow under the car. Use something like a weed-sprayer to direct some water to each corner of the sunroof gutter and check for water under the car. Also check for rust staining and water behind the dashboard end fusebox covers and in the spare wheel well. Nothing is 100% watertight but if it’s not freely draining water onto the road then it’s got to be going somewhere.