AFM's and how they work

[geordieggg]

I'm not sure if this is the right place for this question, but here goes:

I know that the AFM's work by a flap opening further as the air intake increases, but what I'm wondering is what the output signal to the ECU actually is. Is it volts or resistance?

 

My understanding is that the resistance changes, not the voltage. Reason why I'm asking is that I have a chance to buy a digital fuel adjuster that can change 160 load points, but it works off voltage only (changes the voltage from the AFM to the ECU).

Can anyone give me a short, quick answer if this thing can be made to work, or what signal the ECU actually reads from the AFM?

Thanks again.

[alexideways]

Well, It works with a potentiometer, the more the air pushes on the flap, the more or less resistance you'll get depending on your car's setup.

 

The more resistance you have, the more the voltage will drop and this is how the computer interprets the signal.

 

let's say idle is 5V and full throttle is 1V., the more you push on the gas, the more the engine needs air and the more the flap will move inwards.

 

The more the flap moves the more the voltage drops and the computer will read ex. 2.5V as half throttle and so on.

[z-ya]

The AFM is a variable resistor, but it is biased (voltage applied) by the ECU. It acts as a voltage divider. The wiper provides a fraction of the input voltage back to the ECU.

 

How much is this device going to cost? What are you horsepower goals? You can do a fair amount of tuning using the AFM and by putting a resistor in series with the ECU coolant sensor. Or, if you horsepower goals are more lofty, get a Megasquirt or other programmable aftermarket ECU.

[geordieggg]

The more the flap moves the more the voltage drops and the computer will read ex. 2.5V as half throttle and so on.

Brilliant, thanks for that, means it'll work.

 

How much is this device going to cost? What are you horsepower goals? You can do a fair amount of tuning using the AFM and by putting a resistor in series with the ECU coolant sensor. Or, if you horsepower goals are more lofty, get a Megasquirt or other programmable aftermarket ECU.

The device (with hand-held controller, link cable, serial cable and PC software) is about $150 NZD$ + maybe $10 freight. It's more of a short- to mid-term solution without having to rewire the car for MS or aftermarket ECU. It can be tuned in real-time or after, and either by hand-held controller or laptop.

 

Basically I'm doing what I can at the moment to the motor while it's in one car (this car is getting taken back to as close to stock as I can), while prep the secod shell as a race car, where it'll end up in. As far as hp goes, the more the merrier!! Just really looking for something I can use to tune the motor a bit closer for now, so I can set it up for part-throttle cruise efficiency and WOT power, which this can do.

 

Thanks again for the quick responses guys. Much appreciated.

[mom'sZ]

The AFM provides the ECU with a voltage signal. Battery voltage (12 volts) is supplied to terminals 9 & 6. (the terminals are numbered on the connector on the AFM) 9 is positive, 6 is negative. Terminals 8 & 7 are the output to the ECU. The signal is 5 volt to 0 volts. 8 is the positive voltage signal and 7 is the reference ground. (negative) With the AFM fully closed (engine not running) the voltage to ECU is 5 volts. As the AFM opens, signal voltage drops to 0 volts fully open. What I've found on the one I've tested is that the signal varied (almost linear) between 5 volts and one volt from fully closed to half open. Then from half open to fully open it varied (almost linear) between 1 volt and 0 volts. I don't know if this is how it is supposed to work, but my car runs well, as I've only tested this one. (my car is 77 280) I have another out in the garage to test and see if it works the same, but haven't gotten to it. Has anyone else gotten similar results? I currently am logging it's output on the car with my innovate DL-32. This is probably more information then anybody cared to know,but I figured I'd share.

To the original poster, z-ya's idea about using a resistor between the coolant sensor and the ECU would do what you are trying to accomplish and would cost a lot less (a LOT!) If you use a potentiometer like alexideways suggests, it could be infinitly adjustable and if you use a long enough pair of wires, adjustable in real time as you drive. Blue from the Atlantic Z car club has an excellent write up with pictures on how to do it. I will try to find a link and post back.

 

OK here is a link to the article (hope he doesn't mind)

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html'>http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

Here's a link to the EFI page

second link from the bottom on this page is a link to the EFI bible (a must have for every 280z owner)

http://atlanticz.ca/zclub/techtips/EFI&fuel.htm'>http://atlanticz.ca/zclub/techtips/EFI&fuel.htm

And here is a link to all the tech tips

http://atlanticz.ca/zclub/techtips/

Thanks blue if you ever read this (he's a z car dot com guy) You are the GREATEST!!!

[cygnusx1]

The Z "enrichment circuit" has been for sale in catalogs for many years. Cockpit adjustable too.

[geordieggg]

To the original poster, z-ya's idea about using a resistor between the coolant sensor and the ECU would do what you are trying to accomplish and would cost a lot less (a LOT!) If you use a potentiometer like alexideways suggests, it could be infinitly adjustable and if you use a long enough pair of wires, adjustable in real time as you drive.

mom'sZ, gerat info again, thanks. However, putting a resistor in or a potentiometer like has been suggested isn't really the "best" solution. The resistor will make a certain adjustment across the board, and while the potentiometer is variable and can be done so remotely, fine tuning is a little harder (obviously depending on the range of the potentiometer).

 

The advantage of the adjuster I'm looking at is that once it's set, I don't need to change anything when driving, it has a lot more flexibility than the resistor and doesn't need to be changed manually when driving.

 

The other main reason is the fact that I can tune the car over 160 load points. At the moment, initial cold-start runs a little lean, when warm part throttle cruise runs way too rich, and WOT runs rich until about 5k, then dangerously lean after that. With the adjuster, I can set values to compensate for these conditions, especially once the new cam gets put into it.

[mom'sZ]

geordieggg: actually that sounds pretty cool! Do you have a link to it on the net where I can check it out or any other information?

[geordieggg]

I'm looking at getting a completed kit off someone over here for less than the retails shown, but they seem to have a few good little toys in here:

 

http://www.jaycar.co.nz/productResults.asp?FORM=CAT

 

Product codes are KC5385 for the adjuster and KC5386 for the hand controller. They've got another kit there for controlling an extra injector, a mixture display kit, and other boost-related things, all that can be run off this one hand controller.

 

Let me know what you think, as it is only a temporary thing until I either fully rebuild the EFI using an aftermarket system like MS, or ditch the EFI and use triples.

[mom'sZ]

Man that is slick.... very cool. Is it a kit? I'm a terrible solderer .. er I can weld up the crack of your you know what, but I can't solder to save my self. very cool indeed though, thanks for the link.

[geordieggg]

Yeah they come in a kit, and I'm the same, not too shabby at welding, can solder couple of pieces of wire together, but couldn't put that thing together to save myself! However, the one I'm looking at is second-hand and has already been put together and tested to ensure it's working, so it saves me the stress.

 

I'm still debating about if it's worth getting or not, if I do get it I'll post pics and my results.