DME conundrum

[chud]

Evening,

So the time came to try and fire up the 3.2 but unfortunately it was an anti climax, the wife put the party poppers away and everything. So the fault finding began. I’ll start at the beginning....

So she cranks, and the following does happen

Fuel pump fires up
We have spark
And then apart from some impressive flames and pops nothing else happens.

So far I’ve tested the following.

Distributor timing is good
Firing order is good
Flywheel sensors are showing resistance in spec
Fuel pressure is spot on
Injectors are all showing similar resistance

And then we come to my head scratcher. There is 12V on the common side of the injectors - according to the wiring diagram this is fine and what I’d expect anyway, but there is also 12V on the ground pin. I tested across both banks and they are the same. Little to no resistance between the two separate ground cables from the DME when it’s plugged in. I’ve ruled out the harness as it’s all clear when the DME is unplugged.

Is this how they should be in ignition on but engine not running state or is there an issue as logically in my head it isn’t right.

Any thoughts appreciated.

[Landie]

Hi hope this helps a bit. The injectors get a permanent 12v on one side but the DME (Motronic) box pulses the other side of the injector low (i.e. ground) when cranking or running. This ground pulse comes from pins 14 and 15 on the DME box. This can be seen as brief flashes if using an LED test probe on an injector harness connector. Note that pin 14 and 15 are internally connected together in the DME so all injectors are driven together, so all should pulse exactly the same.

If you are not getting any injection pulses then could be the wiring between DME box (pin 14 & 15) and injectors or a possible injection circuit (eg driver transistor) failure in the DME.

David

[chud]

Thanks David, well bit of an update. I bought a noid led kit to test for pulse - which proved we do indeed have a pulse from the DME but very weak - barely lit the noid up. I’ve sent the DME off to a company that can bench test it to see if this weak signal is the fault of the DME itself or something down stream. Early next week we will have the results.

Meanwhile I’ll just fart about with a tractor.