In reply to a message from sbobev sent Thu 29 Aug 2013:
It is not clear from the OEM jag manual that the ECU actually does
indeed require BOTH primary inputs to function. It certainly leads
us to believe that they are required for function, but it does not
say that, for example, if the crank sensor fails, the engine won’t
run, or, if the flywheel sensor fails, you will only have ignition
It simply says both are considered primary inputs, and things won’t
work if they are faulty or absent.
If I were designing the firmware for this ECU, I’d definitely make
sure that if one of these two primary inputs faulted, that the
ignition firing outputs would be inhibited. There’s too much
consequence if they’re not, as we’ve seen, and so such a
circumstance would be an additional candidate scenario for a
Marelli fire (and not only due to a rotor burn through, as is
Thing is, I’ve only ever seen posts in this list that refer to
crank or flywheel sensor failures as leading to the engine dying
out, and many of those are thermally-drive sensor failures (sensor
gets hot and the element goes open, only to return to continuity
when it cools down).
And, the trigger point of the front crank is referenced to a point
ATDC…surely one would want to compute advance for the A bank,
also, wouldn’t one? If so, one would need an engine speed signal,
so that the trigger on the crank merely starts the game off, and
the ECU, counting the teeth on the flywheel, now knows when to fire
A, and, again, knowing speed, can compute B as well.
Point is, A bank is not directly fired off the crank sensor, it
NEEDS the speed as well, or else the advance can’t be computed.
OK, I’ve come back out the same door I went in on this.
It would be a good exercise to verify this, and it is easy enough
When I look at my car this afternoon, I will start it, and then
pull the flywheel sensor connector and see if I lose the B bank, or
if the engine simply dies. I run TT pipes so I am not terribly
concerned about a CAT fire under these conditions, and in any case,
I won’t run it long that way.
I will also check the ECU that Gene has sent.
It happens that I have a crank sensor harness that I replaced
during the diagnosis of my own car’s ignition, and I since believe
that sensor to be good (certainly measures good on the DVM). There
were other problems at play that were responsible.
Both the crank and flywheel sensors share the same P/N, and so I
will send one of these down with the ECU when I return it to Gene,
and we can see what the outcome will be.
The original message included these comments:
I’m with Alan on this one. We rarely hear about Marelli ECU
failures, but quite often about no-start issues due to
The results from the test of the ECU will be telling, Mike
please keep us updated on your progress.
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
// please trim quoted text to context only