Cranks, but wont start - No CEL, No fuel pump.

[mickfromperth]

Howdy Folks,

I've spent the past few days reading through this forum assisting the troubleshooting of my sons 2005 Audi S4 (Unsure if its a B6 or a B7). Its a 4.2L V8 petrol engine.

He's been having issues starting the car, replaced a fuel pump thinking that was the issue. 3 weeks prior the starter motor stopped so that was replaced. I dont think they are related, but I'm mentioning it just in case it might be...

After replacing the fuel pump, he's been needing to use a portable jump starter to start the car. We have had his car battery tested though and it is fine (although after this weekends troubsleshooting the voltage is low - its at 12.1V now and my battery charger just died on us.. Ugh).

Having to use a jump starter, makes me think he's got an electrical issue and not a mechanical issue.

Today I pulled the ECU out - it has power on pin 62, but the resistance is high (my multimeter reads 1.9 - is that Ohms?). The earth seems strong - its measuring 0.003 resistance. So I suspect the issue we have is not enough current getting to the ECU due to high resistance on the positive line.

On pin 62 - When the key is off power reads at 11.11V, when the key is on, Power reads at 11.5V. Remember the battery is currently at 12.1V - so we have voltage drop of .6V at least - I'm not sure why there is a change when the key turns on?

When the key turns on, all the dash lights cycle except the Check Engine Light. Which further makes me suspect the ECU over the other items I've learnt about from this and other forums.

Going by the pointers in this thread:
https://www.audizine.com/forum/showt...t=check+engine

Fuses are all good. I too can force the fuel if I jumper the relay..

I dont have a scanner - so I'm wondering where to go from here (other than to replace my broken batter charger). Is it logical to jump a 12V+ directly from the battery to pin 62 for the ECU? Is there a logical point in the wiring to investigate this high resistance?

Or, is it that a non issue and its likely the the crank sensors could be impacting the CEL?

Or is now the time to replace the ECU?

Or is this the kind of job that just needs a scanner - and if thats the case - what is the best scanner to go for? Is an ELM 327 reasonable?

Sorry tfor the shotgun approach & thanks in advance for guidance.

[Rol-co]

Jeeees, your car is not running and first thing to do is looking at the ecu? And even think about replacing it. 🙄

You need a scanner, vcds or autophix will do
Check ground cables, the are often broken at the firewall, block corner bank 1 and next to the starter motor a big one towards the chassis.

Maybe you forgot the camshaft sensor, the camshaft or crankshaft sensor died on you but its all guessing, scan it is first thing to do.


Without scanner first thing to check is if the car sparks during start and if it has something to eat ( petrol) 😄

[emilio]

needing a jump-starter may be a separate issue. a starter doesn't do anything special, it just emulates a better battery, so you should replace the battery either way.

absolutely do not dump voltage into ECU pins, as you'd be bypassing the main fuse.

i don't generally suggest throwing a bunch of parts at something without a decent diagnosis, but i would try replacing the fuel pump relay as it's inexpensive and pretty easy to get to. merely jumping the relay may not be a good test, as the car could be expecting other conditions before starting the car. there's also no code that indicates a fuel pump relay failure. this was my experience with it:
https://www.audizine.com/forum/showt...ts-off-(B7-S4)

[xpl0sive]

I actually just bought a parts car that had the same symptoms. Relay was good, but it would not prime the fuel pump when the ignition was turned on. Fuse 28 would only get power when cranking. I checked on my other car and fuse 28 would also get power when ignition was first turned on. I didn’t really mess with it any further, as I was using that car for parts, but I just assumed the fuel pump had died

[mickfromperth]

Jeeees, your car is not running and first thing to do is looking at the ecu? And even think about replacing it. 🙄

Erm - Yes it would appear that way - but in my defence:

You need a scanner, vcds or autophix will do
Check ground cables, the are often broken at the firewall, block corner bank 1 and next to the starter motor a big one towards the chassis.

Maybe you forgot the camshaft sensor, the camshaft or crankshaft sensor died on you but its all guessing, scan it is first thing to do.

Without scanner first thing to check is if the car sparks during start and if it has something to eat ( petrol) 😄

1) I've jumpered around the fuel pump relay - so I'm getting fuel. I can both hear the pump, and also check for flow at the engine and plenty of fuel is getting in.
2) There is no CEL coming on - so I was chasing down why that might be and thinking it was probably the root cause. I was hoping it would be a camshaft or crankshaft sensor - but as there was no CEL.. well...
3) I wrote earth cable - but I think that is ground cable yes? I checked these cables to the ECU and they are solid. But on the +ve pin (62) I have high resistance. Are you saying to check ground cables elsewhere?

Regarding a scanner - is this autophix a reasonable option - it is a bluetooth dongle - so I assume I also need software:
https://www.amazon.com.au/AUTOPHIX-O..._source=1&th=1

Thanks again!

[mickfromperth]

needing a jump-starter may be a separate issue. a starter doesn't do anything special, it just emulates a better battery, so you should replace the battery either way.

absolutely do not dump voltage into ECU pins, as you'd be bypassing the main fuse.

i don't generally suggest throwing a bunch of parts at something without a decent diagnosis, but i would try replacing the fuel pump relay as it's inexpensive and pretty easy to get to. merely jumping the relay may not be a good test, as the car could be expecting other conditions before starting the car. there's also no code that indicates a fuel pump relay failure. this was my experience with it:
https://www.audizine.com/forum/showt...ts-off-(B7-S4)

Thanks for sharing Emelio, some good info there for reference. It presents a strong business case for changing that relay before going for an ODB scanner!

[emilio]

Thanks for sharing Emelio, some good info there for reference. It presents a strong business case for changing that relay before going for an ODB scanner!

don't get me wrong: an inexpensive OBD2 scanner (US$10-20) and a phone diagnostics app (e.g. Torque) will always be handy, and should definitely be in your toolkit if you're willing to go as deep as pulling ECUs!

[mickfromperth]

I gained access to a scanner for a short while. We have the a tonne of codes on this old girl. The ones that I think matter are:

16706.004 - Ign./Distributor Eng Speed Inp Circ - No Signal - No Signal / Communication
17072.004 - EMC/PCM Power Relay - Sense Circuit Open - No Signal Communication

So this is either a relay that is busted (EMC/PMC Power Relay) or a sensor that is busted.

[M0ve0ver]

I had a similar issue years and years ago.

It was a bad power relay, so it sounds like you're on the right track.

[Rol-co]

Erm - Yes it would appear that way - but in my defence:



1) I've jumpered around the fuel pump relay - so I'm getting fuel. I can both hear the pump, and also check for flow at the engine and plenty of fuel is getting in.
2) There is no CEL coming on - so I was chasing down why that might be and thinking it was probably the root cause. I was hoping it would be a camshaft or crankshaft sensor - but as there was no CEL.. well...
3) I wrote earth cable - but I think that is ground cable yes? I checked these cables to the ECU and they are solid. But on the +ve pin (62) I have high resistance. Are you saying to check ground cables elsewhere?

Regarding a scanner - is this autophix a reasonable option - it is a bluetooth dongle - so I assume I also need software:
https://www.amazon.com.au/AUTOPHIX-O..._source=1&th=1

Thanks again!

Look for autophix 3310, the software is free to download, working great, have it in obd permanent.

[mickfromperth]

I had a similar issue years and years ago.

It was a bad power relay, so it sounds like you're on the right track.

Can you help me to find this power relay? Its not the Fuel Pump relay is it?

[Zdog]

I had a no start issue similar (maybe) to this, but my issue did turn out to be the fuel pump itself. Before I replaced the pump I put in new fuses, a new fuel pump relay, fuel filter and a new battery. Have you tried a battery from another car or a new battery?