Get to the route of AMM problems

Air Mass Meter considerations by Chris Manton, Technical Manager at SMPE

Intermotor SMPE

Published:  29 July, 2014

Within the complexities of modern engine management systems, Air Mass Meters (AMM) rate as one of the most difficult components to diagnose accurately. Their role is simple and their output "understandable" but the difficulties arise due to the number of other components that have an effect on their function. Couple this with the ambiguous/misleading information presented by fault codes and we start to understand the difficulties.

The meter's role is to provide a signal to the ECU in proportion to the mass (volume and quality) of air entering the engine. The ECU utilises this information to determine the engine's fuelling requirements.

The ECU has only one source of this information, the AMM, but it does have the capability to verify the information by checking other engine critical factors, i.e. at a certain engine speed, load and temperature, a certain amount of fuel should provide the correct fuelling. If the information back from the lambda sensor suggests all is not well, when compared to the fuel maps in its on-board memory (firmware), the ECU has two choices:

Build in a correction factor (short/long term fuel trim)

What are the other factors that can affect the AMM's ability to do its job?

The answer is anything that affects the flow of air entering the engine and the list is a long one. The obvious ones are:

The integrity of the intake system, the intake system is designed as a sealed unit from air filter to engine inlet, any component "intruding" into this sealed unit has the potential to fail and adversely affect the role of the meter. Initial AMM diagnostics should include a full and thorough check of the intake system. A smoke tester would be the preferred tool for this task. The "integrity of the intake system" also encompasses the EGR system any vacuum operated components, the integrity of the intake trunking and intake seals/gaskets.

The condition of the engine; due to the sensitivity of many of the EMS sensors a check should always be made as to the basic engine condition. Any "breathing" issues should be considered. Including compressions, valves, valve timing etc. A reasonable first test of engine condition is to monitor the output signal of the ECU with an oscilloscope at low rpm, compare the peaks of the signal for their relative "height", a low compression shows as a regular discrepancy on the trace.

Lambda sensor; a faulty or lazy lambda sensor will often show as an AMM fault. The ECU uses the lambda to check the fuelling settings post combustion, i.e. has the correct amount of fuel been burnt. If there is an issue with the Lambda signal the ECU often misinterprets it as a fault with the fuel mixture, not a sensor fail. Hence the potential of an AMM fault

Fault code analysis

The nature of these issues illustrates the potential for erroneous fault code information. AMM's rarely fail altogether, they generally lose accuracy. The AMM will still respond to air flow changes but the response will be inaccurate. The ECU will receive information on varying air flow, ergo it perceives that the AMM is working. The ECU is aware of a problem somewhere but its limited 'intelligence' results in an inaccurate fault code. The indicated faults may be air flow related (EGR) or, often, totally spurious (lambda).

A common fault code of P0102 which is commonly translated as "Mass Air Flow Circuit Low Input" reads like an electrical fault and might get you searching for your 'scope whereas the correct interpretation is the AMM circuit had a lower than expected air flow output which means either an electrical issue (from AMM fail to poor connection) or the volume of air entering the engine is low, which could be as obscure as a blocked exhaust.

As I overheard one of our technical sales staff explain to a customer, the fault code can be compared to a cough, you go to a doctor and he decides what the cough is a sign of, it may be a common cold or something more serious that needs further investigation, you decide.

Individually calibrated

All of SMPE's Intermotor and Fuel Parts range of UK manufactured Air Mass Meters are produced using the latest programmable electronic technology to ensure reliability which, unlike the more common (cheaper) variable resistor, will not fall out of synch when subjected to engine heat and vibration.

Each unit undergoes a unique electronic calibration and test process to ensure the highest quality standards, as opposed to batch testing which has proven to give unreliable results. A test certificate is included with the units as a mark of quality, and all have our SMPE logo permanently etched on.

For further information:

Technical helpline 01527 839307

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