[928uk] Non Start issue

Tue Sep 22 09:56:32 UTC 2015

I would also recommend sending the LH ECU to John for testing. BUT

Thinking further afield..

The EZK ECU (Spark timing brain) recieves a signal from the Crankshaft
Position Sensor (CPS) that the engine is turning over (it uses this to time
the spark). From a valid signal from the CPS it then triggers the LH
(fueling ECU) to start injecting fuel and more pertinently also to energise
the fuel pump relay turning the fuel pump on. 

If it is doing this while the starter motor is turning but then the fuel
pump relay stops being energised once the starter is not turning this
suggests to me that the LH is doing what it is supposed to and that the
fault may lie with the electrical portion of the ignition switch itself. The
ign switch has two parts - the lock part and a separate electrical "switch"
part clipped on the back end of the lock replacable separately from the
lock. These are known to fail electrically creating all sorts of wierd
electrical symptoms due to the number of separate switches inside the
"switch" and the possibility to back feed current from one circuit to
another depending on the position the key is turned to.

As a first step I would bridge the fuel pump relay to ensure it is operating
all the time irrespective of the ign switch position and then see if the car
will start and run.

Probably worthwhile changingthe ign switch anyway as the genuine part is not
that expensive and would eliminate a known potential issue.

If the LH has failed or is failing, then if the car will run with a bridged
fuel pump relay , and if the injectors can be heard to fire (clicking noise)
with the ignition switched on but the engine not turning you may find that
the engine will start and run but only at about 1500 rpm with a gently
feathered throttle. In this cas the LH ECU has failed. 

Jon in Sydney - BTDT

  _____  

From: 928uk [mailto:928uk-bounces at lists.928.org.uk] On Behalf Of Richard
Armstrong via 928uk
Sent: 22 September 2015 19:21
To: 928uk at lists.928.org.uk
Subject: Re: [928uk] Non Start issue

Send the LH to JDS for testing
Richard

On 22/09/2015 10:00, Darren Norman via 928uk wrote:

Hi Guys

Would anyone know an auto electrician who could run a diagnostics test on my
1991 S4. I've had a very frustrating few months trying to get my 928 back on
the road. 

I understand a Dutch company has developed a bespoke diagnostic system for
the 928 which is able to study, monitor and Troubleshoot the car, does
anyone have any experience using this piece of kit?

Alternatively the KTS300 Bosch hammer should hopefully do the job. 

I did send out an SOS back in the summer and you all kindly responded with
useful tips and methods for solutions to the problem. The car splutters into
life on start up and then dies.

To date I've changed fuel pump and filter, removed and cleaned out the fuel
tank.

Cleaned all the earth points.

Replaced both ignition amplifiers.

Replaced one fuel damper.

Checked fuel pressure to the rails.

Removed an aftermarket alarm immobiliser.

Cleaned and tested all fuses and relays.

I've had 2 auto electricians on the case but both have drawn a blank,
although they did establish that the fuel pump electrical supply is cut once
the car has turned over which points to a sensor malfunction?

I'm based near Woking in Surrey so if you could suggest anyone local who is
mobile Id be grateful.

Darren

1991 S4

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