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Bentley AMM Test is limited
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Posted by Notnoel [Email] (#23) [Profile/Gallery] (more from Notnoel) on Thu, 31 Jul 2014 06:58:41 Share Post by Email
In Reply to: Bentley AMM Test?, Paul D, Wed, 30 Jul 2014 19:12:25
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IIRC, the Bentley test just has you examine the wire to see if it heats up. It doesn't cover anything related to the resistance changes or degradation of the circuit board over 25 or so years. I think I'm safe in saying that most of us here have a spare (they are plentiful and cheap on FleaBay). I suggest you drop $40-50 and buy one. If it turns out that the AMM is not the problem after installing it, you'll have a spare. At some point the one you have in the car is going to fail.

I had a very interesting conversation with the AMM rebuilder who rebuilt my LH2.2 unit last month ($175 BTW). The wire's resistance changes over the years and the electronics try and compensate, but at some point they simply can't anymore. Additionally the electronics themselves also age with time and use.


The LH2.2s are worse than the later LH2.4s, but both units were manufactured at the dawn of the modern electronic fuel injection era. Earlier CIS units used a metal plate and mechanical arms to provide feedback on airflow.

You can really gauge this effect on the LH2.2s because there is a manual setting for the resistance. Something like 380 ohms is the starting point for adjusting one in good condition. The used ones in my car (I had three)variied from 650 to 800 ohms. So you can see how the tolerances could start to become more and more problematic with time.

A good rebuild costs about 3 times what a used unit does. I went for that because I wanted to totally rejuvenate my FI system. But I still carry a spare.

->Posting last edited on Thu, 31 Jul 2014 08:36:28.
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