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5B.5
Section 5B
EFI Fuel System
5B
Three different styles of ECU’s have been utilized in
EFI production. The first style is easily identified by its
metal case with large 35 pin connector block, and also
as MA 1.7. See Figure 5B-1. The second and third
styles have plastic cases, but are smaller in overall
size. These have either a 24 pin or 32 pin connector
block and identified as MSE 1.0 or MSE 1.1
respectively. See Figures 5B-2 and 5B-3. Basic
function and operating control remains the same
between the three, however, due to differences in the
internal circuitry as well as the wiring harness, none of
the ECU’s are interchangeable. Certain individual
service/troubleshooting procedures also apply, where
applicable, they are covered individually as: “35 Pin”
(MA 1.7) Metal-Cased ECU, “24 Pin” (MSE 1.0)
Plastic-Cased ECU, or “32 Pin” (MSE 1.1) Plastic-
Cased ECU.
General
The ECU is the brain or central processing computer
of the entire EFI fuel/ignition management system.
During operation, sensors continuously gather data
which is relayed through the wiring harness to input
circuits within the ECU. Signals to the ECU include:
ignition (on/off), crankshaft position and speed (RPM),
throttle position, oil temperature, exhaust oxygen
levels, and battery voltage. The ECU compares the
input signals to the programmed maps in its memory
to determine the appropriate fuel and spark
requirements for the immediate operating conditions.
The ECU then sends output signals to set the injector
duration and ignition timing.
The ECU continually performs a diagnostic check of
itself, each of the sensors, and the system
performance. If a fault is detected, the ECU turns on
the Malfunction Indicator Light (MIL) on the equipment
control panel, stores the fault code in its fault memory,
and goes into a default operating mode. Depending on
the significance or severity of the fault, normal
operation may continue, or “limp home” operation
(slowed speed, richer running) may be initiated. A
technician can access the stored fault code using a
“blink code” diagnosis flashed out through the MIL. An
optional computer software diagnostic program is also
available, order Kohler Part No. 25 761 23-S.
The ECU requires a minimum of 7.0 volts to operate.
The adaptive memory in the ECU is operational the
moment the battery cables are connected, however
the adapted values are lost if the battery becomes
disconnected for any reason. The ECU will “relearn”
the adapted values if the engine is operated for 10-15
minutes at varying speeds and loads after the oil
temperature exceeds 55°C (130°F).
To prevent engine over-speed and possible failure, a
“rev-limiting” feature is programmed into the ECU. If
the maximum RPM limit (4125 RPM on MA 1.7, 4500
RPM on MSE 1.0 & MSE 1.1) is exceeded, the ECU
suppresses the injection signals, cutting off the fuel
flow. This process repeats itself in rapid succession,
limiting operation to the preset maximum.
Service
Never attempt to disassemble the ECU. It is sealed to
prevent damage to internal components. Warranty is
void if the case is opened or tampered with in any way.
All operating and control functions within the ECU are
preset. No internal servicing or readjustment may be
performed. If a problem is encountered, and you
determine the ECU to be faulty, contact your source of
supply. Do not replace the ECU without factory
authorization.
The relationship between the ECU and the throttle
position sensor (TPS) is very critical to proper system
operation. If the TPS or ECU is changed, or the
mounting position of the TPS is altered, the applicable
“TPS Initialization Procedure” (see pages 5B.8 or
5B.9) must be performed to restore the
synchronization.
Engine Speed Sensor
Figure 5B-4. Engine Speed Sensor.