mikenycz

I have a 75 280z and am having problems with a rough idle when the engine is cold. After the engine gets to operating temperature it idles fine at about 800rpm. I've checked the ignition timing and it is right on at 10 Deg BTDC. I then started to check the electronics of the fuel injection system using the book "280z Fuel Injection System" that I downloaded. On page 49 there is a test called "Air Flow Meter - Resistance #1". It says that the resistance between pins 6 and 8 should be about 180 ohms. I measure about 201 ohms which is within 10%. On page 51 there is a test "Air Flow Meter - Resistance #3" that is measured between pins 8 and 9. It says the resistance should be about 100 ohms. I measure 109 ohms which is about 10% off in the same direction... So far so good. On page 50 there is a test "Air Flow Meter - Resistance #2". Here it says to measure between pins 7 and 8 and that I should see "Continuity (Small Resistance)". I measure about 150 ohms. I would have expected less than 10 ohms based on the book. After looking at the AFM's circuit board, I don't think that the book is correct for what I should read between pins 7 and 8. It looks like these pins are between one end of the resistor and the swipe of the variable resistor which is at the opposite end of the carbon trace. Can anyone verify this? If I believe the book the AFM has a bad reading and is defective. If I look at what "makes sense" it may not be a real problem. The other thing I did was look in the FSM for this test. Here they say to apply 12v between pins 6 and 9 (across the resistor) and measure the voltage between pins 7 and 8. The FSM says that I should see the voltage vary between 2 and 7 volts. I am seeing a voltage between .2 and 6.5 volts which seems in the ballpark. If anyone has any experience with this please pass on any experiences. Right now I would say that the AFM is behaving correctly however the 280z Electronic Fuel Injection book says otherwise. Any help would be greatly appreciated.

Thank you,
Mike Nycz

naviathan

From 180 to 201 is not within 10% tolerance. 10% on 180 bring you up to 198.

For Resistance #2 it's testing the potentiometer/wipe arm. Could be wearing out the resistive strip along the wipe pattern. Read HERE under the sections on inspection, cleaning and measurement.

mikenycz

Yes, for pins 7 and 8 is says that the resistance should vary. And it does. It just does not say what resistance you should read.

naviathan

I don't remember what the resistance is suppose to be exactly. I remember there being a 50Ohm resistance from one side of the carbon strip to the other. Somewhere I read about moving the circuit board a bit in the AFM to get wipe arm out of the groove it's been digging for years. You may want to try that. Just loosen the screws holding the board down and shift it slightly to start a new wipe pattern. Clean everything really good and it should be ok.