[V6-12v] Fwd: Motor won't start - G28 - engine speed sensor

Andrew Muff andrew at muffster.com
Thu Jan 3 13:04:05 PST 2013

Argh! Ok, red herring. After a day and a half of tracing down this speed sensor, turns out that's not it. Turns out the speed sensor is a white plug and 1 of the 2 plug wires from the ignition coil is a white plug. They were reversed. 

But problem not solved. Now ECU throws no codes. And engine won't start. Just tested for spark and both #1 and #4 cyl are dead, so the whole ignition coil has no power. I've traced back the wires and confirmed these are plugged into the right harness by confirming wire colors with the schematic. 

On Jan 3, 2013, at 12:02 PM, "Andrew Muff" <andrew at muffster.com> wrote:

>> Guys, I need a little help with a diagnosis. Those of you familiar with the speed sensor on the AFC engine, in this case from a 1995 A6 Quattro, will initially think this an easy fix but I'm not having any luck.
>> The background, some of you might remember my last post about running on 5 cylinders. Well the short answer to the story is I sorted it to an overheated and unsprung piston ring. Rather than rebuild the bottom end too or even just 1 piston I decided to swap out the engine with a good used one. The old engine had 157k and the new one only 80k. So I've just finished with the engine swap and have all wires and hoses plugged back in.
>> The engine won't start and throws a code saying there is no signal coming from the G28 engine speed sensor which is mounted on the side of the motor and counts the teeth on the toothed ring gear that the starter engages into. So first I did the obvious thing and swapped with the known working sensor from the old engine. Did I mention I had the car running on 5 cylinders before the engine swap?
>> Anyway, when that didn't clear the code, I swapped the crankshaft position sensor with the engine speed sensor. The code fault remained with the speed sensor. So I'm 100% certain that the sensor is not to blame. But just to triple check I pulled the sensor and checked it with an ohm meter. It checks out fine.
>> Reading through the forums I've found suggestions that a poor ground could also cause this fault. So I've checked all of the ground wires and they are all back in place and connected properly. There are 3 that I can see. 1 main near the alternator, 1 at the top of the intake manifold, and 1 by the ABS pump. Are there more to check?
>> The other obvious areas of concern are 1) where the sensor connects to the main wiring harness all the way back to the ECU. No changes were made there and highly unlikely that we could have damaged that. We did check all relevant fuses too. All good. 2) the other end of the system is where the sensor plugs into the hole in the bell housing. We can see the teeth through the opening. No way to really check that the sensor is precisely the right distance from the teeth to register, but I assume that are all stock parts and nothing has changed. The ring tooth gear is the same one from the old engine and it only mounts to the motor in 1 direction and in 1 position on the six bolts that attach it.
>> I'm out of ideas on things to check to test.  Thoughts?
>> One more thing, I have a VAG-COM and not afraid to use it. ;-) Seriously, I've tested other output blocks that check the RPM values. As you might expect it shows zero since no signal coming from the speed sensor. But are there other things I can check with the VAG-COM that I should try?
>> Happy New Year!
>> Andrew
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