The Toyota 22R-E engine is electronically fuel injected. As such they
lack a mechanical carburetor and instead split the function of the
carburetor into three parts, namely the Air Flow Meter in the air
cleaner box, the throttle body and the fuel injector. The air flow
meter uses a flapper vane and temperature sensor to detect the amount
and temperature of the incoming air flow. The throttle body controls
the air flow into the engine and the fuel injectors supply the proper
amount of fuel to each piston depending upon operating conditions.
While this information is based upon the AFM system in the 22R-E
engine, most of it applies to other Toyota EFI engines. For specific
information, be sure to consult the service manual for your model
engine. On the 22RE engine, the AFM is located atop the air filter box.
In a stock engine compartment, this is in the driver's side front
corner. In the image below, the AFM is dead center in the image:
So how exactly does the AFM work? It is basically an input sensor to
the ECU (Engine Control Unit). In the schematic diagram below, you can
see how the ECU and AFM are connected. Basically the ECU receives the
B+ battery voltage(about 14 volts with the engine running) from the
Main Relay. It passes this voltage on to the AFM via the VB terminal. The voltage at the VC terminal
is used by the ECU as a refererence voltage (it should be around 12
volts) and that voltage is basically the highest voltage that it would
see from the air flow part of the sensor. And it is the air flow that
is sensed as a voltage on the VS terminal.
As you can see in the voltage graph in the lower left corner, the VS voltage runs from around 0 volts up to around 12
volts depending on the air flowing through the AFM.
It is actually quite a simple circuit, as you just have the ECU's
internal resistor (R) and the VB to VC resistance (R1) and finally the VC to E2 resistance
(R2) in series between B+ and ground, so there is just a constant
current flowing through those 3 resistors (I = V/(R + R1 + R2) ). And
the VS terminal just pulls off some
portion of the voltage across R2 to send back to the ECU.
The two final portions of the AFM are the intake air temperature sensor THA and the fuel pump contacts FC
and E1. The THA
circuit is just a temperature sensitive resistor in series with a
fixed resistor (R) inside the ECU and the ECU "sees" the
voltage across THA to ground and uses this
to determine the temperature of the intake air, which affects its
density. And since the density of the air affects how far the AFM
sensing vane is pushed open for a given volume of air passing through
it, the ECU needs to know both the temperature and the volume or air in
order to estimate the mass (or weight) of air (or oxygen) coming into
the engine. And finally, the FC and E1 contacts, which you will note do not connect to
the ECU, are used to keep the Circuit
Opening Relay energized which in turn keeps the fuel pump running
as long as air is passing through the AFM. So this is a safety feature
or sorts. If the engine stops, say in a traffic accident, the fuel pump
is shut off to minimize the chance of high pressure fuel leaks and
fire. Also, if you were to run out of gas, the engine would of course
stop and thus the fuel pump would also be stopped and this can help
prevent damage to the pump as it is both cooled and lubricated by the
fuel it is pumping. If the pump were to keep running, it would likely
be damaged in short order. Note, the schematic diagram below is
somewhat generic in that the exact connections to the ECU may not match
any given wiring diagram. So you should go by the FSM wiring diagram
for your specific vehicle for exact connections.
So now that you know how the AFM is supposed to work, on to the actual
device testing procedure...
The AFM is not really designed to be adjusted, so it either works or
doesn't work (outside of some folks adjusting the tension of the
internal spring to vary the AFM calibration).
As viewed looking at the AFM:
(*) While not specified in the FSM, the value of the resistance between
the VB - VC
terminals is aprpox. 100 ohms. And on my spare AFM, I get a total of
277 ohms between E2 - VB
and I measure 178 ohms from E2 - VC and that leaves 99 ohms between VB - VC.
Two quick sanity checks to see if you are measuring correctly are:
And to see why this is so, look at the Theory of
Operation section above...
Then if the above tests are OK, then you can test the actual operation
of the air flow sensing portion of the AFM:
One critical item that is often overlooked as a cause of problems is
the intake plumbing between the AFM and the throttle body. In stock
form, there is usually 2 rubber bellows flexible sections and at least
one length of rigid plastic tubing. Any air leak in this part will lead
to a poor running engine at best and one that won't start at all at the
worst. The rubber bellows can crack down in the seams of the accordian
folds and then open up/leak air when the engine vacuum changes. The
only real way to look for leaks here is to pull the flexible section
off and hold it up to a light source and then fold/bend/flex all
sections of the part looking for light coming through a crack. Also,
inspect the plastic tubing for cracks and holes.
The reason air leaks are bad is that the engine ECU reads the incoming
air flow from the AFM. If extra air happens to leak in through a crack
(that intake can be under a partial vacuum when the engine is running)
then there is more air than the ECU "knows" about and the
mixture will be too lean and bang! - you can geta back fire as the too
lean mixture detonates too early.
Since the AFM has two basic functions, that of measuring air flow as
well as turning on the fuel pump, problems can arise in these two
areas. Without the fuel pump running, the engine will cut out and die
from lack of fuel. If this happens, check the E1-Fc connection. If the
engine runs overly rich or lean, especially at full throttle, the AFM
could be the cause, if its telling the ECU that extra air is present
(that isn't), the ECU (running on open loop mode at full throttle)
would increase the fuel injected into the cylinder, causing the rich
condition. Likewise, a too-low air flow reading my result in lean
operation. For testing both the above conditions, an in-cab fuel
pressure and Air/Fuel gauge is indispensible.
[Last updated: 25.December.2014]