Engine speed sensor advise please

[UrS4boy]

I am not sure the Cam Position Sensor stops the fuel pump on 3B / ABY. I thought it just controlled the timing of the ignition.

I would check your Cam Position Sensor is working, and have you tried the water down the back of the cambelt trick? This sensor is often one to let go and not allow firing of the ignition system. It is very hard to get to and test mind you. It can be tempermental too...Steve.

According to Electrical Diagrams B73-02 and B73-04 at Paul's S2.net website (under Technical), the J17 FP relay is energized by pin 3/55 of the ECU just like the AAN (follow tracks 20 and 42). Therefore, I conclude that an ABY cam sensor is involved with the FP via the ECU just like the AAN.

In addition to the water down the back of the cam cover trick, Marc S. (a4quattro of twin turbo 3.8L V8 cq and VEMS fame) has figured out grounding the CPS pins 2 and 3 together can work in a pinch, as documented here:

http://forums.audiworld.com/s4s6/msgs/126842.phtml

[Fat Audi 80]

Fair enough.

[S2central.net]

@UrS4boy - you may well be right on that... I haven't scoped it out yet. The fuel pump activation could be ECU controlled based on any combination of G4, G28 and G40 signals. My gut feel is that its not G40 (CMP) alone - otherwise the fuel pump would die when CMP is unplugged - which it doesn't...

I think sparks are gated by G40 (CMP) and that fuel is gated by G28 (RPM)... That way the engine is tolerant to failure of G40 and G4 whilst running (with some timing safety margin of course) but when G28 fails you have no reliable rpm indication so you may as well shutdown and stop the fuel pump (also handy if you've just crashed into a wall and stopped the engine).

[iib3327]

brand new cam postion sensor fitted (kindly supplied by quattro corner) and still nothing

i have fuel as tested the new pump is doing its job.

whats next to check?

[iib3327]

ow yeah forgot to add its an ABY code engine..

[S2central.net]

Check you are getting 5V to the CMP

Check with an LED that the CMP is providing a pulse every cam rotation.

Check you are actually getting a sawtooth waveform from the RPM sensor.

Check you are getting a crank sync pulse every rotation. The sensor itself mught have failed, or the flywheel pin has broken off - rare but not impossible.

I've posted waveforms before - outside of changing all these sensors - you need a scope to see the waveforms.

Also check that ALL ECU related fuses are OK... and that all grounds are good at manifold and rear of cyl head.

[UrS4boy]

@UrS4boy - you may well be right on that... I haven't scoped it out yet. The fuel pump activation could be ECU controlled based on any combination of G4, G28 and G40 signals. My gut feel is that its not G40 (CMP) alone - otherwise the fuel pump would die when CMP is unplugged - which it doesn't...

I think sparks are gated by G40 (CMP) and that fuel is gated by G28 (RPM)... That way the engine is tolerant to failure of G40 and G4 whilst running (with some timing safety margin of course) but when G28 fails you have no reliable rpm indication so you may as well shutdown and stop the fuel pump (also handy if you've just crashed into a wall and stopped the engine).

I am not disagreeing about the potential "interlock" between the G40, the G4 and the G28 with providing the ECU with the info it needs to fire the J17 FP relay through Pin 3/55. Here is some G4 and G28 info:

http://forums.audiworld.com/s4s6/msgs/116988.phtml

Based on the fact that the G40 cam sensor has now been replaced and there is still no start, I am liking your suggestion that the crank position sensor or the crank position sensor pin have gone on early retirement holiday. This has happened more than once on my side of the "pond". Here is a post that shows what the pin looks like:

http://forums.audiworld.com/s4s6/msgs/108072.phtml

The pin (if there) can be view through the flywheel timing port on the transmission. If necessary, it is *possible* (with skill and lots of luck) to weld the timing pin (if found loose somewhere) back onto the flywheel through the starter hole.

[iib3327]

right then.. (im sure i've been here befor lol)

all engine earths are fine, checked all fuses and relays again.

pulled the g28/g4 sensors out and can see the pin on the back of the flywheel (pic below)



so am i better off looking at gettin a new G28 and G4 sensors next?

cheers for all your help people.

[AUDIS2avant]

Ive just had almost the same problem, and the main fault was the cranck sensor, not speed sensor, badground and dirt in some connectors.
Would just buy a new cranck sensor, and if it´s not that, then just switch them over if it´s the speed sensor as they are the same sensors

[iib3327]

Ive just had almost the same problem, and the main fault was the cranck sensor, not speed sensor, badground and dirt in some connectors.
Would just buy a new cranck sensor, and if it´s not that, then just switch them over if it´s the speed sensor as they are the same sensors

have tested all the grounds and added new ones to double check, swopped G4 and G28 over still nothing..

whats next i feel im running out of options?.. anyone want a cheap s2 scrapperwagen!



every now and again it'll fire but dies straight away even with throttle etc..

[UrS4boy]

every now and again it'll fire but dies straight away even with throttle etc..

Okay so there is a glimmer of hope there. Not much but some.

I helped somebody with the above symptoms on an AAN last week. The cure was he is now borrowing my complete OE 20v Turbo coil pack. All his coils were weak and since I am not using mine since the 1.8t coil pack conversion in 2005, it was available. Prior to swapping coils, we checked his coil fuse (it was fine).

So, for you, I think you (we) have to start from the beginning and go back to basics:

1. With the ignition on and the engine cranking, do you have spark? (You need to remove the 20v Turbo coil pack plate and rest it on the cam cover and use a coil tester as shown in http://forums.audiworld.com/s4s6/msgs/115408.phtml. Have a friend turn the ignition on to start while you check the spark on EACH cylinder while the engine is cranking. Make sure that you pull the fuel pump fuse and ground the coils not being checked with old spark plugs grounded to the cam cover or something.

If you don't have spark, you need to check for power at the firewall. You might have to find and check the coil fuse. You might also want to check the POSs (swap in some know to be good ones). Same with the coil packs. If you have a friend nearby with an S2 or UrS4/S6 see if you can borrow his/her complete 20v Turbo coil pack for a short test (only takes minutes to swap).

2. With the ignition on and the engine cranking, do you have fuel in the fuel rail? Easiest way to test this would be to remove the fuel rail complete with attached injectors from the intake manifold. Place large glass jars over the ends of each injector to catch the fuel. Have a friend turn the ignition on to start while you check the fuel flow on each injector while the engine is cranking. If you don't have fuel, you need to look at the fuel injector fuse (S24), fuel pump (I know you say it is fine but), the fuel line (clogging), fuel filter (clogged), cam sensor (trigger for ECU and J17 FP relay), J17 fuel pump relay (swap in a known to be good one).

Something is wrong. You just need to start from the beginning and slowly and methodically check all the possibilities (in some cases, again).

Good luck.

[S2central.net]

Good advice there from Canada... Its easy to get all bamboozled by the technology - even on our 20yr old cars, but at the end of the day all you need is four working strokes, fuel and sparks !

[iib3327]

ITS ALIVE!!!!!!

thanks again for all your help guys. really appriciate the effort. and i've had a crash course in the aby engine so i have a much better understanding of these now..

and in case ur all wondering it was water in the ecu of all things

shocking as its a brand new ecu but wasnt mounted on its hanger just sat in the footwell letting any water trickle in.. stripped it all down and dried it out and she runs brilliant. got it taxed and out on the road last night and im more than impressed with these cars to say the least..

will get some pics up later when its all clean in the right section..

thanks again guys im sure i'll be seeing some of u about.

[S2central.net]

Jees - water inside the ECU - that'll explain lots of ills alright...

Bizarrely - its not the first time I've seen it though... I've had a couple of ECUs on the bench with tidemarks and was amazed they still work at all. One was from a scrapyard where the car had sat for a while without a windscreen - I could at least understand how so much water got into that one.

Its just not something you would suggest as a thing to check on a non-starter though. So well done for keeping the faith and following things thru to a successful conclusion.