Mechanical Air Flow Meter TTM3.0
What it does
It is an electro-mechanical unit that converts the
rate of airflow into an electrical reading. The ECU
uses this to calculate the engine ignition/fuel requirements.
It fits into the intake trunking between the air filter
and inlet manifold often incorporates a flange which
the air filter housing bolts to.
How it works:
Air flow through the unit deflects an internal, sprung
loaded vane. The vane is connected to a rotary potentiometer
which is responsible for the varying voltage reading
to the ECU. The tension of the internal spring dictates
the ratio of movement relative to the volume of the
incoming air, this is preset at manufacture. A small
amount of mixture “trimming” was possible
on pre-catalyst cars by the inclusion of an additional
potentiometer. The idle potentiometer was omitted
with the introduction of closed-loop engine management.
The unit often also housed the intake air temperature
sensor.
Reasons for failure/testing:
The unit is fairly robust but has some common failings.
Mechanical damage, the vane is prone to damage in
the event of backfires which results in internal vane
sticking. The resulting vehicle condition varies from
very bad starting and low speed engine running to
being a complete non-runner. Test for this by deflecting
the vane with your finger/screwdriver and check for
smooth operation throughout, when releasing the plate
it should return to its fully closed position with
no hesitation. If any signs of sticking are detected
then the air flow meter will need renewing.
The idle potentiometer is another common failure
caused by adjusting the unit past its limits. This
can result in either rich or lean idle conditions
depending upon which end of the scale the idle potentiometer
the failure occurred.
Caution;
The idle potentiometer has a limited adjustment range,
over adjustment will permanently damage the air flow
meter.
Ageing of the internal spring causes inaccurate fuel
control. The spring tension was always seen as a common
tuning/modification “tweak”, consequently
“bodged” units are often encountered.
An accurate voltmeter or oscilloscope can be used
to test the mechanical air flow meter. Measure the
input voltage and signal at the air flow meter, measure
the output voltage/signal at, or as close as possible
to the ECU to check the condition of all wires and
connectors.
The voltage output should increase in direct relationship
to an engine speed increase. If testing with an oscilloscope
check for noise/harsh signals which would indicate
resistor track wear. The readings can be compared
to specific data from a know data source.
Due to the age of the air flow meters, water and/or
corrosion damage to the connector is often encountered.
Clean the connector pins as required. Replacement
connector plugs are available from Fuel Parts UK Ltd.
Note:
This type of air flow meter is no longer used, all
manufacturers have adopted the electronic air mass
type meters, see separate Tech Tips for information
regarding these. These units are available as a new
or reconditioned unit.
For further technical information
concerning Mechanical Air Flow Meters please phone
our technical help line 01527 839307
