1992 Marelli - Flywheel Sensor - Fuel ECU Engine Speed

(Geekay) #1

I’m trying to understand the impact of a fully or not fully functioning flywheel sensor. I’ve done some searches and have not seen a definitive answer.

Documentation on the 1992 Digital Ignition says:
“The crankshaft and engine speed inputs are necessary for the engine to start”. Then in BOLD.
NOTE: The sensors …are unique to the digital ignition system and are not used or shared by the EFI system"

Then in FUEL ECU section:
“The ECU senses engine load from Intake MAP and engine speed from the ignition pulses …The ECU receives Inputs from sensors…and the ignition system”.

It sounds as though the flywheel sensor input is being passed to the Fuel ecu from the the Ignition ecu, implying that if the sensor was flaky the car might start, but would also contribute to fueling issues.

Can anyone confirm that?

I’ve just found this thread which is an interesting/educational read.
Marelli Question


(Steve) #2

Again, taken from the description of the engine management for the 6.0L, but the ignition part should be applicable to your 1992 almost word to word:

FF77 is the code for misbehaving flywheel sensor.

The fueling of the 6.0L is slightly different as there are two fuel pumps (the second one coming online above 2800 rpm) and the pressure is 3.0 bar, but the rest should be the same:

In other words, yes, both fueling and ignition ECUs need the engine speed as primary input.

(Geekay) #3

Thanks Steve. I see similar diagrams in the '92 section. But could the speed sensor be “flaky”, the car run (albeit with not perfect ignition), with the fuel side being affected? Or lets say the air gap is not quite correct affecting the voltage as an example of “flaky”?


(JimD in Alabama) #4

Nuttin’ from nuttin’, but at idle
from Bywater’s A6 Engineering site ( http://www.jagweb.com/aj6eng/ignition.php)
“A switch attached to B bank throttle linkage signals the ECU when the throttles are closed. In this condition the timing is governed according to engine speed and coolant temperature alone, the vacuum signal being ignored in order to achieve the most stable running condition.”

But I can’t answer how much influence the MAP has, or how , to the ignition ECU once the throttle closed switches go to open.

IIRC Steve, the FF77 (and 67) codes only came with the change from 26CU to 36CU and the 6.0 liter engine? Thankfully I had Roger disable the FF67 when I got the SU chip from him. I had cleaned the secondary air rails and replaced the check valves, then replaced the HO2S, and had David Faircloth overhaul injectors and fuel rail. Still kept getting FF67s sometimes and got tired of clearing that one code.

(Steve) #5

That’s right. Apologies. I myself posted the codes the 26CU flags just a few days ago and FF77 isn’t one of them… :-((