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AFM - EFI Bible Help


KyleG

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Ive been going through the EFI Bible trying to trouble shoot my '75 280z's engine troubles. Ive run in to an issue checking the AFM.

 

The bible says there should be continuity (small resistance) between pins 7&8. I interpret this as <10 ohms, but read ~450 ohms when doing this check.


The AFM performs fine in all the other tests, including the ones in the FSM.

 

Has anyone done this check? What sort of resistance do you read between 7&8? Id be grateful for any responses!

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I haven't done that specific test but. I have done similar tests and I found the resistance didn't change in a linear way. I guess it is something to do with the circuit between the flap and the plug. I suggest trying the measurement while moving the flap and see what it does. In fact that might be your issue in the first place....is the flap moving freely?

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I just tested a spare that worked well but was just a little bit rich (new MSA, swapped it for a used factory unit).  175 ohms at rest, bounced around from 175 to 250 over the range of the vane.  I think that the 7-8 circuit might be the feed for the potentiometer.  The ECU compares the outputs from 6 and 8 or maybe 6 and 9 to determine where the vane is, I believe.

 

What kind of troubles are you having?

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I just tested a spare that worked well but was just a little bit rich (new MSA, swapped it for a used factory unit).  175 ohms at rest, bounced around from 175 to 250 over the range of the vane.  I think that the 7-8 circuit might be the feed for the potentiometer.  The ECU compares the outputs from 6 and 8 or maybe 6 and 9 to determine where the vane is, I believe.

 

What kind of troubles are you having?

 

 

Thanks for the feedback Galderdi and NewZed, its very helpful.

 

My engine misfires and lacks power between 1000-3000 rpm. I tried the test in the FSM, and hooked up a power supply to the AFM. The results seemed smooth and linear as I moved the flap, and the Flap did not bind. Ive also checked for vacuum leaks with a can of carb cleaner, and nothing has showed up.

 

All the other electrical checks have come back fine, so i suppose its on to the fuel circuit next. If either of you had any suggestions, id be willing to give them a shot.

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Fuel pressure is critical.  The ECU program is based on constant pressure at the injector tip.  Some have run the gauge out from under the hood so they could see it while driving.  You can't go wrong knowing that your fuel pressure is right.

Well, my fuel pressure seems fine at just over 35 psi, and everything else i have checked from the efi book has performed properly too. If you had any advice on what else to look at, id be very appreciative. 

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Mine wouldn't fall out of a tree between idle and 3,000 RPM, after 3k it would pull like a tractor.  I thought it was the Throttle Position Switch and carried it to a mechanic for adjusting, that didn't work.  He slightly clamped off the return line to increase fuel pressure on the rail and said I needed a Adjustable Fuel Pressure Regulator turned up to 45psi.  I ended up doing this for $5 from Radio Shack and now I'm able to lay down some rubber from a dead stop, it's like a whole different car.  http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

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fuel pressure seems fine at just over 35 psi, and everything else i have checked from the efi book has performed properly too.

Have you measured resistance on the 9-8 and 8-6 pins?  My first AFM measured high on those pins, 126 and 227 instead of 100 and 180 and the engine would only idle and go full throttle.  It just popped back through the intake and bucked in between.  Which, in retrospect, was probably a lean condition and 45 psi might have "fixed" it.  But then it would have had a rich idle and burned a lot more fuel at full enrichment. 

 

The resistor in the coolant temp. circuit might give you a clue.  It's pretty easy to do and cheap.  Get an audio taper potentiometer they're easier to adjust. If it works you can decide if you want to keep it that way or get a different AFM.

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Yea that's what mine was, audio potentiometer, it's a volume knob for an amplifier or something.  I put mine inside the cabin, ran the wire through a firewall grommet, then I could "tune" it while driving.  Paired speaker wire worked for me.

Edited by siteunseen
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  • 2 months later...

I managed to resolve this issue, so I figured I'd post my solution incase someone else stumbles across this thread looking for answers.

The engine was rebuilt by the P.O., and the shaft driving the distributor was off ~220 degrees. The P.O. tried to compensate by moving the plug wires 3 over (180 degrees) and adjusting the distributor for another 20, leaving the timing 20 degrees off still.

 

The solution was to cut a slot into the distributor adjustment channel and rotate it further to get the correct timing.

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