Re: Mystery Hose/Cold Start Idle - Saab 900 Bulletin Board - Saabnet.com
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TBH, I don't entirely understand the relationship between what the AMM does, what the adjustment does, and what that means to the ECM - and truly I think that's generally the case. It would not surprise me to find people maxing out the adjustment trying to get a specific result. My experience is that the functional range of adjustment is tiny - if you have to go beyond a few dozen ohms you will never get good results. More than that, my suspicion is that the adjustment has VERY limited impact on the actual function of the AMM, and that the way the AMM typically fails cannot be corrected by adjustment of the AMM. What I think is that the edges of the AMM's sensitivity shrinks over time, sort of narrowing the bell curve, and the adjustment just moves the median around but doesn't grow the curve. Put another way, the adjustment is just trim, +/- some percent of reading, but what's failing is the ability of the AMM to read. That's why you find poor idle and poor WOT, but "normal driving" falls into the thick part of the curve and continues to be fine. So, TL;DR, I think the adjustment is generally (but not always) a boondoggle when replacement is what's called for. Whether this is science or not IDK, but it's definitely borne out in my experience - both the butt dyno and the smog machine. ;)
Newer versions of LH self-calibrate and deal with aging AMMs much, much more elegantly. Much more stable performance under all circumstances for much longer. But, even on newer versions of LH eventually the AMM needs to be replaced. There's no such thing as a lifetime AMM, and IME across a wide variety of cars (even much newer Saabs), you'll find that 100k-150k is max lifetime for any of them.
posted by 69.62.25...
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