Just wondering if any of you knowledgeable guys have a suspicion of what problem my old faithful MK3 gti 8v is currently suffering from please? Stood for a week with battery disconnected, on reconnection would crank but not fire. All symptoms were that of a poorly crank sensor (no spark, fuel or tach bounce) so replaced sensor but still no start. Checked fuses and although getting a click with ignition key one turn, replaced ECU relay & fuel pump relay (fuel pump primes OK). On testing, found that whilst cranking engine, there's no ignition pulse to coil. Connecting VCDS-Lite throws up there's a 00546 Date wiring fault (27-00 implausible signal) problem with the immobiliser so I've checked all the wiring from the ECU, immobiliser, ignition halo and OBD socket and all look sound. A mechanic I know wired it up to a full Vag-Com diagnostic but it couldn't link with the ECU, (neither could the VCDS-Lite I initially used). Testing the OBD socket, I get 12v from pin 16 to pin 4 but from the K-line (pin 7) to pin 4 it's reading nearly 9v (too high?). Does anyone reckon the ECU is fried? Any suggestions would be gratefully received as I've been driving the MK3 for nearly 20 years and have no intention of scrapping it because of an electrical problem!
I'm not a Mk3 man myself but could it be the immobilizer itself, I know you said checked the wiring but I'm wondering if could it be the electronics within? I'm presuming prior to disconnecting the battery all was good so to me it might be that there was a surge of some kind when reconnecting the battery and the immobilizer may have not liked. The diagnostic seems to be pointing to the immobilizer and I would start there - is there any way of bypassing/removing it - I would nearly bet money that's where the issue is rather than the ECU. You could see if you could find a second-hand ECU at a scrappers/e-bay possibly as well.
Your logic certainly seems sound, eliminating/bypassing the immobiliser would certainly flag up if that was the problem but unfortunately bypassing an immo is way beyond my knowledge. However though, if the immobiliser box was replaced, would that need to be coded to the car's current keys and ECU or would the ECU/keys have to be coded to the new immo? Thanks for the response by the way, much appreciated.
I'm not 100% sure on your queries regarding coding there would most likely be some kind of coding required (keys I would think) but I don't know to what degree. Another possibility would be getting a second-hand (functioning ECU) and trying it. I don't believe you would have to code the ECU - the ECU should just see a signal or not as the case may be from the immobilizer and it will decide if it's happy to allow ignition or hold it back.
I reckon that's a plan; a second hand ECU isn't much (not withstanding the risk of getting one that itself is already fried), the same for the immo box. Yes, I'll give that a try, Thanks very much again for your input.
to rule out internal immo fault preventing comms to ecu unplug it and loop a wire between the 2 grey wires on the connector, this will allow direct comms from port to ecu. does the throttle do anything with ignition on? check you see 12v on the coil between the black and brown both with ignition on and while cranking, pluse check fuse 15 as well
Sorry rubjonny, you'll have to forgive my ignorance: loop between two grey wires that are both on one of the connectors or connect together grey wires that are on each connector? (sorry again, I'm not actually able to look at the immo at the moment so don't know what colour the connector wires are).
yeah so first find the box, which is a challenge as its hidden up under the knee bar. follow the key reader loop wire till you find it. then disconnect the bigger plug and get a bit of wire, paperclip or whatever and carefully poke it into the 2 grey wire pins. this will join the diagnostics port side to the ecu diagnostics side, bypassing the immobiliser. It wont stop the immobiliser cutting in, just allows you to talk to the ecu if the box is playing up which does sometimes happen. however since the car doesnt start at all as GVK says this suggests the main issue isnt the immo box but this will rule out a problem with the box preventing you from talking to the ecu with vagcom. if you still cant connect then this tells you either the ecu is dead, theres damage on the diagnostics wire between immo box and ecu, or ecu isnt powering up for some reason. if the throttle is making noise and/or the fuel pump primes with ignition then we know the ecu is trying to power up, if the fuel pump runs while cranking we know ecu is seeing crank sensor signal. coil power check tells us if theres an ignition switch issue, sometimes they power up on the first click OK but then the ignition power goes dead when turned to cranking position. its also possible the battery is in poor condition so when the starter is cranking it drains too much for everythign else to work, but then that doesnt explain why you cant connect to the ecu with vagcom
Thanks very much for your advice, you've raised something there; first click there's a single spark from the coil but then on full ignition, nothing. There's 12v from the live of the 3 pin lead to the coil but no pulse from the middle pin when cranking. Guess I've a busy weekend ahead when next I can get to the car. Thanks very much again for your opinion, it's appreciated.
as long as you see voltage ignition on and cranking thats fine, the lack of pulse from middle pin along with no comms points to ecu not powering up or failed I recon but get some more tests done and work from there
Sorry, one other thing: Testing the OBD socket, from the K-line (pin 7) to earth (pin 4) it's reading nearly 9v, Is this too high, I've read on other blogs that it should be nearer 2 to 3v?
Check voltage on the crank sensor pin 1-3, autodata says to check sensor put led on pin 2 and ground slowly turn engine by hand with ign on. Led should pulse. Same symptom as my AGG, after i washed the engine. But i could check rpm on vagcom, did not find the fault though!
well we know the k-line is working fine up to immo box since you can scan the immobiliser itself, so you can rule that out
Bypassed the immo via your two grey wire advice rubjonny, Vag-Com was unable to link/detect anything, even the previous immo 00546 data wiring fault. Plugged back immo connector and Vag-com is at least back to reading the 00546 again although still no link to ECU. Guess swapping in a replacement ECU is next on the list. Thanks again.
If the ecu has power but no connection to vcds, could be dead. Measured on mine and got 11.5v on k-line. 2-3v usualy on newer can-bus cars.
Ah, there's the thing unfortunately; I'm ignorant of how to check if there's power to the ECU.What are the checks please? I read blogs on checking pin such and such but looking at my ECU connector, there are sixty odd of them! I've searched for a diagram of what is which for my car's ECU (1995 8v GTI, ADY engine) but have had no luck. I now reckon I've actually two separate unrelated problems: 1) whatever it is that's preventing the car from starting and 2) the ECU not talking to the OBD. If I can solve No. 2 then hopefully it'll point me in the direction of what No. 1 is. I've read on a couple of blogs that an unprofessionally fitted after market radio was sometimes a source of the ECU not linking so along with a substitute ECU that I'm awaiting, I'll investigate that. That's for your advice.
Crank sensor has power from ecu, so if the sensor has power, ecu power should be fine. The same with fuel pump, if it primes ign on, ecu has power. Anyway, autodata says power on pins 38 from f15, pin 23 from ecu relay. Ground on pin 1.
