Rover SD1 Efi System - (Flapper) Air Flow Meter

Description, Testing and Internal Adjustment

Part 1 of 2

Rover SD1 Efi Air Flow Meter

Rover SD1 AFM - Article Contents



Testing the Functions

Initial Set-up and Testing CO Content

Reasons for Opening the AFM

Crude Adjustment

Calibrated Adjustment



• With the exception of the ECU, the flapper Air Flow Meter (AFM) is the most complex and expensive original component of the Rover SD1 Efi system.

• Under utopian conditions and despite a delicate mechanism they rarely fail yet do have a reputation for unreliability most likely due to abuse because a prior owner or inexperienced technician has messed with the internal workings often evidenced by observing the plastic cover has been removed and untidily replaced.

• However utopia has ceased to exist for Rover Efi SD1’s so AFM problems become more likely given the age of the systems (21-25 yrs) and as cars change hands often. Thus there exists a need for today’s owners to better understand the functions of the AFM and appreciate its potential for adjustment or repair..

• There is also the possibility of age-related relaxation of the internal spring where such weakening of the factory-set tension results in a richer mixture, increased carbon deposits on plugs/heads and excess Carbon Monoxide (CO) emissions.

• Fortunately there are still plenty of inexpensive used items available through the second hand spares market or popular auction sites. Alternatively, given access to the correct diagnostic equipment and a degree of common sense it is possible to reset the internal components to restore normal functionality

• It is also possible to adjust it empirically if one is willing to test performance between adjustments. Later sections cover both these aspects.


• The AFM is located between the air filter and the throttle butterfly housing. Air flowing to the engine is monitored and the information sent to the ECU.

• Incorporated in the AFM is an adjustment screw to set the mixture and CO levels.Turn the screw clockwise to richen the mixture and anticlockwise to lean it.

Fig. 1

to its stop by a light spring (FR), and is opened by the air being drawn into the engine and as the measuring flap opens, the compensating flap (CF) moves into the damper chamber.

• A potentiometer (AP) is connected to a flap spindle so movement of the flap alters a voltage signal sent to the ECU which compares the value with its memory and, together with information from other sensors, computes the duration of the injector 'open' time.

• There is another electrical function controlled by the flap spindle to operate the switch contacts (FPC) in the circuit to the fuel pump.

Fig. 2

• At maximum throttle,fig.4,the flap rests against the full open stop and depression is maintained in chamber ‘D’ by the air rush passing the small gap shown at ‘G’.

• Both flaps twist in opposite directions to ensure progressive pressure changes in chamber ‘D' and smooth movement of the flap when opening or closing.

• This adjustment screw enables fine control of the actual airflow to alter the mixture strength and therefore CO content levels at idle speed.

• Turning the screw clockwise blocks the bypass port and so richens the mixture and turning it anticlockwise opens the passage to lean the mixture. The air in the bypass channel is not detected by the measuring flap so the ECU does not compensate for it by adjusting the fuel input.

• The throttle butterfly (TB), which controls the speed of the engine, is also equipped with a potentiometer (TP) to provide the ECU with information on throttle position.

• Also shown is the throttle butterfly by-pass port and idle speed adjustment screw (IS) that operates similar to the mixture screw.

• While very little air is passing the throttle butterfly, the idle screw alters the total volume of metered air entering the engine to control the idle speed.

Fig. 3

• The flap is stabilised throughout the speed range. When the throttle is opened as seen in fig.3, pressure at ‘B’ falls due to the depression in the manifold and atmospheric pressure ‘A’ moves the measuring flap against the spring tension as more air enters the engine.

• At the same time air in chamber ‘D’ is momentarily compressed, damping the rate of movement of both flaps.

• If the throttle is now held steady, the air pressure in chamber ‘D’ will also fall until it is equal to the pressure at ‘B’. This balance of pressure on each side of the damper flap ensures that the flap unit remains stable at any throttle opening.

• neat design feature takes advantage of flap momentum as it overshoots its position of equilibrium during acceleration to briefly signal an even higher airflow and boost the mixture. Upon deceleration a similar effect occurs to reduce mixture and aid the braking effect. Both these effects give good throttle response.

Fig. 4


• In principal then, the AFM is simple in operation.

• Air entering from the air filter pushes a flap open against a calibrated spring. As the volume of air increases, it pushes the flap open farther.

• The moving flap is linked to a potentiometer (AFP) whose wiper moves over a resistance to vary a voltage at pin 7 of the connector. The resulting signal is sent to the ECU to determine the volume of air entering the engine at any given time.

• The AFM incorporates an Air Temperature Sensor (ATS) to measure the temperature of the air which, of course affects air density, and sends the resulting signal to the ECU via pin 27..

• These measurements are combined by the ECU to determine the total number of air molecules entering the engine. The ECU uses this information to help decide how much fuel the engine needs and adjusts fuel injector duration accordingly.

• The ECU also gets fuel mixture adjustment information from three other sources, battery voltage, the coolant temperature sensor, and the air temperature sensor but the AFM's signal is the primary input to which minor adjustments of injector duration are carried out based on the above three inputs.

• Essentially a safety feature, the remaining function is a pair of Fuel Pump Contacts (FPC) on pins 36 and 39 which signals the fuel pump relay that air is entering the engine and it is safe for the fuel pump to operate. Conversely, when the engine stops the contacts open and disconnects power from the fuel pump.

• For maximum power and efficiency, a properly operating AFM is critical. Over time, the internal spring tension may weaken resulting in a richer mixture or it may have been manually disturbed, resulting in an incorrect mixture. However, this can be adjusted either way.

• A faulty AFM can prevent starting, an irritating start/stall pattern, low power, high emissions or unstable idle. Unfortunately most of these symptoms are not exclusive to the AFM as other components might also create similar problems.

• Therefore, when it comes to initially testing a unit it is reasonable to assume that the standard measurements for the potentiometer, air temperature sensor and fuel pump switching contacts will, at the very least, determine basic functionality.

• Given that the unit does actually work it is then possible to perform a whole range of tests on the Rover SD1 Efi System and only if it proves necessary, gain access to the inner components of the AFM for more complex adjustment.

• If the AFM fails any of the basic tests or is otherwise feared to be faulty, then substituting a known good unit may allow testing of the system to proceed.

Testing the Functions

Check voltage to the fuel pump via Air Flow Meter switch, steering module and fuel pump relay

• Conditions:

• ECU multiplug disconnected.

• Connect a voltmeter between ECU multiplug pin 20 & earth.

• Ignition ON.

• AFM flap closed.

• Reading should be zero volts.

• Manually open AFM flap and listen for fuel pump relay and fuel pump operation.

• Voltmeter should read 11 to 12.5 volts. If below 11 volts check all wiring and connections shown in Test 3 diagram.

• If still below 11 volts, substitute steering module.

• If voltmeter reads correctly but relay or pump are not heard to operate, substitute steering module and then pump relay.

• If pump still fails to operate, suspect a faulty fuel pump.

Check voltage across the additional resistor in Air Flow Meter potentiometer

• Conditions:

• To gain access to ECU terminals with multiplug fitted, disconnect multiplug, release its cover and refit the multiplug.

• Ignition ON.

• Connect voltmeter +ve to ECU multiplug pin 8 and -ve to pin 9.

• AFM flap closed.

• Reading should be 1.55 ± 0.1 volts.

• If the reading is incorrect, peel back rubber boot covering connections at AFM and connect voltmeter to terminals 8 and 9.

• If the reading is incorrect, peel back rubber boot covering connections at AFM and connect voltmeter to terminals 8 and 9.

• If reading is still incorrect, substitute AFM.

• If OK, leave the ECU connections exposed and continue with next test.

Check voltage through Air Flow Meter potentiometer and wiring to the ECU

• Conditions:

• ECU multiplug connected (cover removed).

• Ignition ON.

• Connect voltmeter +ve to ECU multiplug pin 6 and -ve to pin 9.

• Reading should be 4.3 ± 0.2 volts.

• If voltmeter reads 0, check all wiring and connections seen in diagram.

• Leave voltmeter -ve connected to pin 9 and move voltmeter +ve from pin 6 to pin 7.

• Reading should be 3.7 ± 0.1 volts.

• If voltmeter reads low, check wiring for high resistance.

• With voltmeter still connected to pins 7 & 9, slowly open the air flap.

• Reading should gradually decrease to 1.6 ± 0.1 volts.

• If results are not within those specified above replace air flow meter.

• If OK, refit multiplug cover and continue with next test.

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• MF Measuring flap.

• CF Compensating flap.

• FS Flap spindle.

• FR Flap return spring.

• AP AFM potentiometer.

• FPC Fuel pump switch contacts.

• Fig.1 (sectioned) shows that the AFM contains a double flap which pivots on a spindle (FS) mounted in the housing.

• The measuring flap (MF) is closed on

• CO Air by-pass port and CO adjustment screw.

• TB Throttle butterfly.

• TP Throttle potentiometer.

• IS Throttle by-pass port and idle speed screw.

• Fig.2 shows that whilst the bulk of air enters the engine via the measuring flap, a by-pass port and adjustment screw (CO) is also provided.

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