[Kevin 280/300TT]

I've done all the FSM tests and found an oddity with my first AFM. My 75 FI troubleshooting manual says to check continuity between pins 7-8, and there should be a small continuity. My 77 FSM mentions the same test, but says to put 12v across pins 6-9.

Now on my first AFM, here is the deal

1. 0 volts across 6-9, no continuity on 7-8 flap closed, varying resistance flap open
2. 12v across 6-9, continuity on 7-8 throughout flap range

On my second AFM from the junkyard, there was continuity (175 ohms) with the flap closed and varying resist. with the flap moving with no volts on pins 6-9.

So, which way is correct? Do you need voltage to check across pins 7-8. BTW, the first one wouldn't allow me to drive the car unless the full throttle switch was set to contact just off idle. It did however pass smog in all areas except 2500 rpm HC. The second AFM drives fine with the TPS set correctly when warm, but is way too rich when cold. It did, however, bomb the smog check in all areas (HC/CO at idle/2500). Thanks for the help.

Kevin 77 280Z

 

[Dave280ZX]

To make a continuity (resistance) measurement with a meter, you would NOT apply voltage to the circuit. A meter measures resistance by applying its own known voltage to the circuit and then measuring current flow. It can then compute resistance from the voltage and current values. Applying an external voltage will throw off the calculation.

Dave

 

[jeffp]

I just use the book to get an idea if the unit is operating at all or not. The fact that the resistance changes when the flapper is opened and closed is a good test. The voltage is not critical to test the unit. Also you should have the air temp sensor(there is one in the 280zxt AFM) and I am not totally sure of the different variations on the meters. So check the resistance of the Air Temp sensor by taking a resistance reading of the unit and then get a blow dryer and heat the part up and make sure the resistance changes. Dont worry to much about exact readings that are in the book but that they follow generally the curve the book shows.

The best test for the AFM operation is to check the scailing voltage coming out of the unit to the ECU, this is a critical maeasurement as this varying voltage will tell the ECU the engine is running faster, more air is being taken into the engine and adjusts the duty cycle of the injector accordingly to keep the right fuel/air mixture. The temp sensor has to do with the air density, and makes slight adjustments to the ECU duty cycle also, but that is a programed table in the ECU from Nissan. This also holds along the same operation for the cylinder head temp sensor.

Now, back to the AFM. The only way(unless you have a calibrated nose to smell the exhaust) to get accurate readings of the mixture is to take the O2 readings of the exhaust. If the car is rich while you are driving, tighten the return spring on the flapper, Readjust the bypass air at idle to get the idle readings correct. (the thing to remember is that the bypass air operates significantly at IDLE, so you can be driving the car and it is running good but fails at idle)
The scailing of the AFM can be probed while the car is running and you should be able to tell if it is working correctly. The faster the engine speed the higher the voltage will be, so if you test it and you get to say for example 3 volts @ 3000 rpm and you increase the rpm and the car starts running bad and the scailing voltage does not go higher, chances are that the AFM is bad. Remember when you open the flapper more you should get more scailing voltage out of the unit. I dont know the max voltage it puts out they all will be different, my car for example puts out 5.7 volts @ 20psi of boost, max air intake into the engine. The readings will not be linear either, you will see them jump up quickly from idle to say 3K and then the curve will become more gradual until max rpm.
Anyway hope that helps and gives you more understanding of what this thing is doing in the car, you can dial both units in to work on the car, and I am finding that allot of the AFM units you get have already been adjusted for the car it came off and may or may not be adjusted very well for your car, and there are allot of reasons for that also.