AFM question - DJ / MV / DH / GW engines

[Smiffo]

All.

Posting separately here outside of another thread, as these questions are specific to the Air Flow Meter that is between the air filter housing and the throttle body.

One of the tests to confirm all is OK - and the only one I am not sure of - is the resistance when moving the flap.
You measure this across pins 2+3 as per instructions I've found on here, but aall it says to look for is 'varying resistance'

Not helpful because mine seems to start low, increases as I open the flap, jumps around a bit between 33% and 50% open, then peaks in resistance around 50-60% open  before dropping off again as you approach what would be max airflow and the flap fully open.

Does anyone know what I should expect to see?
I'm struggling to find clarity online.


A picture of the insides of an AFM.
When the arm rotates as the flap in the housing opens, the resistance between pins 2+3 changes:

[Smiffo]

I've given it a clean with some carb cleaner on a cotton bud.
Plenty of dust came off.

Resistance is now more stable, but still increases toward 50/60% before dropping off again as the flap approaches fully open. I would expect it more like a scaelectric throttle, but maybe this is how it is supposed to be?

The idle switch on the throttle body goes from infinity to zero at both full and closed throttle positions too, and that is correct so it could be the way the AFM works too...?

Clarity, confirmation, or name calling to highlight my idiocy appreciated 

[colinthefox]

I've got one of these on my Volvo 850 tdi. I think it's probably OK as you describe. The actual resistance pattern is made up by the resistor elements in the chain at the top of the circuit board, not just by the resistance of the track itself. A few years ago mine went noisy (ie rough engine) so I filed the fixing holes for the circuit board so the wiper wiped a slightly different path. It's been OK for more than 100K miles ever since. If you don't have a rough engine I would leave it as is.

[Smiffo]

I've got one of these on my Volvo 850 tdi. I think it's probably OK as you describe. The actual resistance pattern is made up by the resistor elements in the chain at the top of the circuit board, not just by the resistance of the track itself. A few years ago mine went noisy (ie rough engine) so I filed the fixing holes for the circuit board so the wiper wiped a slightly different path. It's been OK for more than 100K miles ever since. If you don't have a rough engine I would leave it as is.

Agreed. I'm reluctant to play with it without a definitive idea of what 'good' looks like.

Problem is I am gathering the parts to eventually instal the digijet system in place of my Pierburg, so won't really know until I use it.
Because all the parts are from unknown history, I was keen to go through them all, so I have as little issues as possible when installing it all.

I will think of something. Your idea of moving the board so that the path the contacts follow shift slightly, is genius. That may be what I try to do if there as issues.  

[tencentlife]

A little late perhaps for your needs but may help future readers.

The book tests would point up a complete failure, as in open circuits, but don't actually confirm an AFM is functional. This does:

https://intrepidoverland.com/afm-testing-with-video/

[silverbullet]

Something else to watch for is variable clock spring tension, which results in different flap opening angles (and hence a change in fuelling) for a given throttle opening, when comparing units with the same part number.
I have been meaning to collect some data on this as I think it may contribute to so-called vanagon syndrome.

[Aidan]

from memory normal variable range is sort of 550ohm upto 1.2k then back down to 600 ohm  ..ish, I had a modified one that had a bigger peak; Dream and Drive in Holland can supply exchange reconditioned ones based on optical sensors, Mark is pretty happy they work after a couple of years experimentation, I have yet to drive a vehicle fitted with one; the AliBaba stuff is crap, the AFM doesn't fit without modifying the airbox and doesn't work for long (a few months on lpg), very flimsy mechanically and cheap and nasty compared with the Bosch units - hence the interest now in aftermarket engine management systems running modern components, this old world late 70s technology is well past it's best now that Bosch no longer support it

[Smiffo]

Just looking through my old DJ threads, and found these replies.

Many thanks all. Apologies in late response. I suspect I saw email notification, and intended to read properly at a later date, but got side tracked.

Really appreciate the input - very helpful.