Won't go....560 SEC

[jasonS]

On my way home the other night , cruising along in the old 560 SEC she suddenly died.I free wheeled to the closest layby.I attempted to start her but all she does is turn over as though there is no spark.If I keep cranking the key , the engine stays alive but the minute I stop turning the key , she dies a sudden death.
Anyone have any ideas where to start looking and if there are any related relays or control modules worth replacing..?

 

[Richard Moakes]

Fuel pump relay is a likely culprit.

Or a sensor feeding the relay with a signal that the engine has started.

On LE jetronic the air flow meter flap has contacts which make when air forces the flap open, these trigger the fuel pump relay.

Not so sure about K or KE, but I suspect the relay needs some sort of signal that the engine is turning to remain energised.

On the old Senators, you could sometimes get them running by giving the relay a bit of percussive maintenance, i.e. Hit it! But not too hard.

Alternatively you can bridge the contacts in the relay socket to test or get you home.

Richard
1989 W124 300E

 

[Malcolm Brown]

You can check fuel pump relay easily by listening for whether the pump activates briefly when you turn the ignition on. If not, then remove the relay and bridge pins 7 & 8 (ISO terminal nos 87 & 30) in the socket. If I remember right a red male bullet connector on each end of a 3 inch length of wire is ideal for this. This should run the pump constantly when the ignition is on. If the pump runs, then the relay is broken. If the pump doesn't run then either the wiring is interrupted between the pump and the relay, or the pump itself is broken.

The relay is easy to find in the W126 - to the right of the back end of the fuse box (looking from the front of the car), plugged vertically into a 12 pin socket. [Note: that is true of 6cyl models, 8cyl varieties might be different.]

If driving with the relay removed, don't thrash the revs because it has an additional function of limiting max revs.

Interruption to the wiring could be by an alarm or immobiliser. I had a problem with a faulty alarm wired into the fuel pump relay circuit - which is how I found out about the above.

 

[Andy]

Hi Jason,

If you havn't a spark, then I would find a mate with a ignition pack and try it.


Regards

Andy @www.mercedesservicing.com

 

[Rog_E]

Don't know if this is relevant for your car but........

Back in the days when I used to be a mechanic, long time ago now, this symptom was 99% of the time due to a failed ballast resistor.

Typically the engine would fire whilst the starter was engaged and stop when the key is released.

The Ballast resistor reduces the ignition coil voltage to 9V during normal running.
During starting it is bypassed giving the coil 12v, they have been known to fail (the resistance is via windings around a ceramic core).

This may now be integrated in the electronic ignition pack (it was a long time ago!! )