AFM
The task of the Air Flow Meter is to measure the quantity of air as it goes into the engine. It sends this information to the ECU, so it can calculate how much fuel to inject into the motor.
There are 2 types of AFM used in the Nissan R31 models, depending on type of engine fitted ;
- 4cyl Pintara CA20E - this engine uses a vane type AFM, with spring loaded mechanical flap that moves when air passes through the meter. It should be noted that this mechanism only measures the volume of air passing through the meter, not the actual air mass. The actual air mass is calculated via a separate air temperature probe located in the airflow. Bosch call this device a volume air flow sensor (VAF). It was used with early Bosch L-Jetronic/LE-Jetronic ECUs -- the JECS ECCS ECU used on the Nissan CA20E, is based upon the Bosch LE2-Jetronic ECU.
- 6cyl Skyline RB30E - this engine uses a hotwire Mass Air Flow sensor (MAF) type AFM - this device measures the actual mass of air flowing through the meter, as a function of the hotwire control circuit. Bosch refers to this AFM as the HLM-1 type (1st generation) mass air flow sensor, and was used in the Bosch LH-Jetronic ECUs -- the JECS ECCS ECU used on the Nissan RB30E, is basically an LEx/LH-Jetronic based design, with hybrid circuitry to support the HLM-1 MAF sensor.
The following article focuses only on the RB30E type setup.
RB30E AFM[edit]
The AFM used by Nissan for the RB30E, is a Bosch HLM-1 type anemometer. An electrically-heated wire (hot wire) is placed in the intake air flow after the air filter, transverse to the flow direction. A temperature sensor is installed upstream of the hot wire. The hot wire is then heated to a temperature higher than the air temperature determined by the temperature sensor. As the hot wire is cooled by the incoming air, the heating current must be increased to maintain a constant wire temperature. In this method, the heating current represents a direct measurement for the incoming air mass, independent of the air density. Thus, the reduced air density at high altitudes is automatically compensated for.
Problems with the AFM not operating properly[edit]
Symptoms
- Car refuses to start or car starts but cannot maintain idle
- Car runs sluggish up to about 2000 +/- 200 RPM and then takes off after that
- Car almost cuts out on low throttle
Many ECCS fault symptoms like above, are common for other ECCS component failures. Symptom 1. above can also be caused by a faulty Crank Angle Sensor, and the Nissan workshop advises to check and clear that component first, when suspecting trouble with the AFM unit - if the connectors for CAS and AFM are in bad condition, it can also cause this symptom. Likewise, symptom 3. above, can also be caused by bad adjustment/failure of the Throttle Position Sensor idle switch. It is important to check, test, and verify other ECCS components are working correctly, before concluding the AFM is actually the fault.
The above symptoms can also be due to the fact that there is an air leak anywhere between the AFM and the engine, and the ECU is not aware of this 'extra' air. This could be a broken/split hose, or failed hose-clamp.
The reason why 2000 +/- 200 RPM is so, is because the O2 sensor is able to measure the exhaust gas ratio in this range, and the ECU will attempt to enter closed loop mode. Note that if your O2 sensor is damaged or has failed, the engine might not display this behaviour at all and simply stall if revved past 2000rpm.
When other possible causes of these symptoms are eliminated, they are often due to the failure of the AFM to be able to correctly measure the amount intake of air passing through it. It can also be due to the fact that the hot wire has become badly contaminated.
Self Cleaning[edit]
Self cleaning occurs for about 1 second during a 10 second period, after the vehicle ignition key is turned ON to OFF.
The ECU must have registered exceeding 2000 RPM and Speedo sensing greater than 20 KPH for AFM to self clean.
During the 10 second period, the ECU remains powered on even though the ignition key is turned OFF
This occurs, because the ECU uses the ignition key signal to switch on the ECCS relay.
The ECCS relay controls power to the remainder of the ECU circuitry and all EFI components (AFM, CAS, O2 etc).
The ECU uses a timer in the software to count the 10 seconds, and at the end will send a 12 volt signal to pin 4 on the AFM
Front view
Hot Wire activated during self clean
Sometimes self cleaning isn't enough when the AFM stops working.
Imagine a dirty soldering iron etc where although there is heat, there are particles which cannot be burnt off.
It is suggested that the platinum wire be cleaned with some form of electrical contact cleaner.
Spraying the cleaner might not be enough in some instances (as found from personal experience) and a very gentle brushing may be required.
Be very careful touching the wire, as it is quite thin, and can break very easily.
It may be possible to replace this wire also.
This is the main reason why AFMs refuse to work correctly, giving inaccurate results and affecting engine running.
Front view
Under the cover
AFM - ECU - Description
- 1 - 26 - Digital ground (ECU)
- 2 - 36 - Chassis ground
- 3 - 31 - Output signal
- 4 - 12 - Air wire self cleaning
- 5 - Battery power (from EFI relay)
- 6 - 30 - Mixture setting
To the left is a blue calibration potentiometer.
This has a range of 15 turns from 0 - 1K ohms.
This pot doesn't actually appear to use any of the circuitry within the AFM itself but is a calibration for the IDLE. Factory setting for this pot is about 382 ohms (which measures 3.58 volts when the ECU is powered on). This can be adjusted but is not recommended unless you have a wideband exhaust gas analyser. At 2000 RPM the Air/Fuel ratio should be at stoich 14.7:1.
The ECU requires two main sensors to operate the engine. Other components may fail and cause problems but it is still possible to keep the engine running the vehicle with a failed O2 sensor, AAC valve, Air regulator etc. even though there may be problems with fuel consumption and poor idle due to these components
Operation of an AFM[edit]
(Info adapted from a Toyota training guide but principles are the same)
The AFM converts the amount of air drawn into the engine into a voltage signal.
The ECU needs to know intake air volume to calculate engine load.
This is necessary to determine how much fuel to inject.
The air flow meter is located directly in the intake air stream, between the air cleaner and throttle body where is can measure incoming air.
The primary components of the AFM are a thermistor, platinum hot wire, and an electronic control unit.
The thermistor measures the temperature of the incoming air.
The hot wire is maintained at a constant temperature in relation to the thermistor by the electronic control unit.
An increase in air flow will cause the hot wire to lose heat faster and the electronic control circuitry will compensate by sending more current through the wire.
The electronic control circuit simultaneously measures the current flow and puts a voltage signal in proportion to the current flow
ECU mapping of the AFM[edit]
The measurement of voltage from the R31 RB30E AFM is within the range from 2-4 volts.
It also happens that the Z31 AFM shares the same characteristic. Later model Nissans (R32, Z32, S13 etc) have a voltage range from 0 - 5 volts.
This makes the replacement of AFM a difficult task as the voltage range needs to be restricted in hardware rather than software.
The voltage range is measured inside the ECU as a non-linear map, called the Voltage Quantifier (VQ) map. This consists of typically 64 values (not necessarily this many for all Nissan ECUs).
This map determines what voltage measurement in terms of % of voltage from AFM maps to what scaling value.
There are formulas for this but that wont be covered here.
My current program - Nisemu below demonstrates how the mapping is displayed.
When a demo version is available you will be able to change this.
Other vehicles where an AFM upgrade is beneficial (and more possible) can be done, for example a Z32 AFM on an R32 ECU.
But this requires remapping of the ECU for this map.
Fortunately this is fairly trivial, since the VQ map from another ECU (such as Z32) can be taken and written over the top of the original.
Some people/companies charge a lot of money for this procedure.
It is currently possible with a $12 EPROM chip, $2.50 socket and EPROM programmer using ROM Editor.
VQ Map adjustment in Nisemu (under development)
Pulling apart an AFM[edit]
The RB30E AFM has an internal diameter of 63mm.
For Naturally Aspirated applications it would be desirable to increase the side of the AFM without needing to substitute for another vehicle AFM.
The following are pictures I have taken during the disassembly
This is the top side, removal of the lid will expose the circuitry and screw holes
The top plastic part sits on top of this piece, the metal connectors feed through and solder to the metal tags.
It is suggested that a desoldering gun or solder sucker is used to remove these.
See how the screw holes line up between the AFM housing and the metallic base
There is heat transfer compound paste between the AFM metallic base and the plastic component housing.
Two screws hold this on, some force is required to separate these components
This is the inside of the AFM. There is a green coated copper connector inside which connects to the thermistor and hot wire. These go to the metal connectors.
Finally the inside from the front, you can see the thermistor.
The hotwire was broken after attempted cleaning on this unit.
It will require replacement magnesium wire.
This is probably what they use for reconditioned AFMs which cost about $260.
- No responsibility will be taken for damage to AFMs, ECUs, engines etc. from use/misuse of this information
- Incorrect setting of Air Fuel ratios can cause damage to your engine!
-darkhalf
Silvertop RB20 AFM[edit]
OK, so I was having a problem with my RB20 breaking down at around 2000 RPM. Replaced the plug on the loom as I suspected this was the problem BECAUSE when I lifted the plug (while connected) it seemed to contact better and I could hear a difference in the running of the engine and when driven was all OK until I hit another patch of bumpy road which I then thought would rattle the plug and so on.... So once plug replaced, it made a difference. However this only lasted a few kms until it broke down again.
I figured there had to be something i could do instead of getting another AFM which may not fix the problem.
Turns out it is as simple as the solder on the contacts inside the AFM not connecting as well as it used to, so here is the procedure for what I did and haven't had a problem since.
NO GUARANTEE THIS WILL FIX YOUR ISSUE, BUT IT WORKED FOR ME...
Tools needed:
- Screw driver (to undo clamps)
- Razor blade or Stanley knife
- Soldering iron
- Solder
- Sikaflex or something similar
Step 1: Remove AFM, undo the 2 clamps on either end of the AFM and unplug the connector plug.
Step 2: Using your choice of blade, CAREFULLY cut away the silicone sealant around the top of the AFM. Cut deep down the sides as it is really in there. And then gently pry the cover off to expose the copper plate. Use your soldering iron and melt the dab of solder which is holding the plate down and remove the plate
This is what you will see
Step 3: Simply add solder to the contacts already soldered... melt it down a little bit and re-apply solder, careful not to put too much heat into the rest of the circuitry or join them together. Let cool and re-attach copper plate.
Clean up remaining silicone, re-fit top of AFM, and apply sikaflex neatly and to whatever way you see most effective. Re-fit AFM to vehicle, reconnect plug and road test.
Thanks to some random out cruising who said he had the same problem with his 32 and told me about it.
-Cam_Dawg