Alfa 75 TS fuel cut off

[bteoh]

Hi,
Was hoping someone here could help diagnose my problem. My 75 TS starts and idles, but after a few seconds, the fuel pump shuts down. I checked the pink/white wire connector under the rear seat that connects to the fuel pump - it supplies 12v but after a few seconds , the power turns off.
It will start again without problems, but dies after a few seconds. I swapped the fuel pump relay in the engine bay but that didn't help. Would a failing crank angle sensor cause these symptoms? Other areas that I may need to check are the TPS , airflow meter and possibly ECU? But was hoping someone may point me in the right direction with the above symptoms.
Thanks

[Citroënbender]

Is it an early Motronic or similar, with ignition amplifier module?

The easiest fix if you're handy and originality isn't a big concern, is to power the fuel pump relay using a Peel brand LPG relay. You'll need to splice in a tach feed from the coil low tension negative terminal.

[Colin Edwards]

Hi bteoh,

The symptoms you describe suggests three possibiliities:
# The air flow meter contact that ultimately controls the fuel pump relay is not functional
# The combination relay control signal from the air flow is not reaching the pump relay
# The combination relay is not processing the signal from the air flow meter

A contact in the AFM closes when the AFM detects air flow consistent with an idling engine.  Once the engine starts and idles, this contact must close.  This signal confirms to the fuel pump relay the engine has started and idling.  If the engine does not start or does not idle their is no need for the fuel pump to operate.  The air flow dependant contact ensures a stalled engine disables the fuel pump - very important!!

With the ignition on (NOT in the "start" position) and the engine not running, you should see 12v at terminal 88A of the combination relay and 12V at the AFM contact - terminal 39.  The AFM contact can be closed by simulating air flow - move the vane slightly.  The AFM contact should close and 12v be seen at terminal 36.  12v should now also be seen at terminal 86B on the combination relay.  This signal should energise the "pump run" half of the combination relay and the fuel pump should then power up.

If your not seeing 12V at terminal 36 with the AFM contact closed the AFM the contact is worn or dirty and needs attention.  It can be cleaned by wetting thin carboard with contact cleaner and wiping it between the two contacts - tread gently!  DO NOT USE AN ABRASIVE OF ANY SORT!!!  The wiper arm that actuates the AFM contact prevents these contacts from closing unless air flow reaches a critical level.  The contacts can be "adjusted".  Again tread very gently!  The AFM typically has 7 connections.  The two closest to the outer edge of the case are numbered 36 and 39.  If your AFM does NOT feature terminals 36 and 39 please ignore this post!!

If you are seeing 12V at terminal 36 on the AFM but not at terminal 86B on the combination relay you have a busted wire or dodgy connection. 

If you are seeing 12V at terminal 86B on the combination relay but not at terminal 86D you have a dodgy combination relay.

This part of the EFI system is 100% binary.  The signals are either high or low / 12V or 0V / on or off.  No messy analogue voltages to worry about. 

The above assumes the system is the Bosch L Jetronic, although identical principals apply to similar systems. 
   

[bazzbazz]

Just for my own curiousity, would this mean if the AFM was unplugged then the same problem would occur?

[Colin Edwards]

Hi bazzbazz,

Possibly.  However, hopefully Bosch built in a "sanity check" where if the analogue / potentiometer area of the AFM went open circuit or out of spec, creation of igniter pulses was disabled.  Simple to implement and would prevent the engine from trying to run on a damaging overly rich or lean mixture.  A theory i'm not going to explore!

[bteoh]

Hi Colin,
Thanks for the detailed reply. The 75 TS runs a Motronic 4.1 and doesn't have the combination relays like the L Jetronic. I was wondering why the ECU would cut fuel and the usual suspects were the AFM , TPS or Crank Angle sensor. However, if the CAS was to fail, that would mean the car wouldn't start? However, in my case, it always starts but fuel cuts off after a few seconds. So, I think you might be right in suggesting the AFM being the possible culprit? As BazzBazz mentioned, would it run ok or longer if I was to disconnect the AFM connector?

[GTVeloce]

The 75 TS uses Motronic not L Jet and as such is quite different. There is no combination relay like the V6 cars have and the AFM has 5 terminals not 7. The car will start and run even with the AFM disconnected albeit fairly rough.

A stalled engine is controlled via the CAS that send the signal to the ECU.

I am not quite sure why your car is starting and suddenly stalling but I think you are on the right path. If you don't already have the attached diagram you will find it very useful to diagnose problems.

The second attachment is the pin out list for the ECU. I have never had an ECU fail yet but have had almost everything else fail at some point. The last one that had me stumped was the pulley key was damaged and meant the pulley was out of alignment in relation to the crank. The CAS was working correctly but the timing was still off.

Hopefully yours is something easier and best to start from the beginning and work your way through the list. I'd start with the full fuel electrical system first just to ensure there isn't an issue there even though I feel it is more likely a sensor that is sending a poor signal to the ECU resulting in the ECU killing the fuel. Again, you could test that by seeing if the lambda probe heater (also an output from the fuel pump relay) is also losing power when the fuel pump does. I would also disconnect the lambda probe heater and test to see if it is shorting and resulting in the relay shorting.

Good luck!

If you need other pages let me know and I can attach them.

[GTVeloce]

Second page of the wiring diagram.

[Colin Edwards]

Hi bteoh,
Maybe time to invest in a Fault Code reader.  However likely culprit will be the CAS.  As well as being responsible for ignition the CAS also provides crankshaft rotation speed - rpm.  Does the tacho reveal idle rpm at any stage during starting / momentary idling?

The CAS maybe only just be ok to provide an ignition reference point but not up to providing a higher frequency output.  At an idle speed of say 800 rpm the CAS should be producing something like 800 pulses per second.  The high frequency signal is used by the ECU to generate an engine speed / rpm value. 

A couple of missing teeth on the CAS disk produce a defined loss of CAS output.  This regular loss of output every engine revolution is the ignition "window".  If the CAS is ok at these lower frequencies but not ok at higher frequencies the ECU may not "know" the engine has reached a particular rpm - started!

I'm not sure however if the ECU needs air flow info AND engine rpm info to decide if the engine has in fact started.  A Fault Code Reader should confirm this.

[bazzbazz]

Errr, just what would he plug the Fault Code Reader into? Unless I am mistaken 75s don't have diagnostic ports? 

At least not any that you can plug a generic code reader into?

[Citroënbender]

Motronics usually have a flasher "code" output ability.

[bazzbazz]

And where would one get a suitable reader from to use on it?

[bteoh]

Thanks guys,
I have a multiecuscan software that I will plug it into and check tomorrow. It doesn't have a listing for my 75 TS but has one for the 164 TS ( Same engine ). Hopefully it may throw up a fault code that I could check. The 75TS has the same diagnostic plug as the 164, located under the passenger dash.
Will keep you updated on what I find tomorrow.
Cheers

[bteoh]

GTVeloce,
Thanks for that 😀👍🏻

The 75 TS uses Motronic not L Jet and as such is quite different. There is no combination relay like the V6 cars have and the AFM has 5 terminals not 7. The car will start and run even with the AFM disconnected albeit fairly rough.

A stalled engine is controlled via the CAS that send the signal to the ECU.

I am not quite sure why your car is starting and suddenly stalling but I think you are on the right path. If you don't already have the attached diagram you will find it very useful to diagnose problems.

The second attachment is the pin out list for the ECU. I have never had an ECU fail yet but have had almost everything else fail at some point. The last one that had me stumped was the pulley key was damaged and meant the pulley was out of alignment in relation to the crank. The CAS was working correctly but the timing was still off.

Hopefully yours is something easier and best to start from the beginning and work your way through the list. I'd start with the full fuel electrical system first just to ensure there isn't an issue there even though I feel it is more likely a sensor that is sending a poor signal to the ECU resulting in the ECU killing the fuel. Again, you could test that by seeing if the lambda probe heater (also an output from the fuel pump relay) is also losing power when the fuel pump does. I would also disconnect the lambda probe heater and test to see if it is shorting and resulting in the relay shorting.

Good luck!

If you need other pages let me know and I can attach them.

[simmi1983]

My CAS was buggered and was doing similar things.

The fuel pump will only operate if it sees a signal from CAS.

We are you located? Maybe someone here could help you swapping some parts around. Ive got all soare EFI stuff but am located in Brisbane.

Luke