[Main C900 Bulletin Board | BBFAQ | Prev by Date | Next by Date | Post Followup ] Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos Packrats of a feather... Like This Post: - Subscribe to Daily Digest for this Bulletin Board Posted by RS [Email] (#15) [Profile/Gallery] (more from RS) on Tue, 2 Sep 2003 08:09:49 In Reply to: AMM 101, JeffS, Tue, 2 Sep 2003 07:46:06 Alert me when someone posts in this thread: Members do not see ads below this line. - Help Keep This Site Online - Signup

I have a similar collection of AMMs ... just in case, of course :-) I've picked a couple of horrendously bad ones, too.

The AMM measures the mass of the air going into the engine. It basically measures the current it requires to raise a hot wire to a setpoint temperature. Moving air carries heat away from the wire, so you have to keep pushing current into the wire to maintain that temperature. The cross-sectional area of the AMM is known, so you can calculate how much air is going into the engine.

The electronics in the AMM convert the current to a voltage signal that the fuel injection computer (ECU) uses to regulate the amount of gas to put into the engine. When the O2 sensor has warmed up, the ECU also uses that to trim the amount of gas to maintain a set amount of oxygen in the exhaust.

The eyes of an AMM are a thermistor. Thermistors aren't the most stable temperature sensing devices over the long haul, especially compared with thermocouples and RTDs. But they're inexpensive and can respond to changes in temperature fairly quickly. That's probably why Bosch used them. I suspect that what you're seeing in the bad AMMs is the result of thermistor drift coupled with degradation of the electronics from heat and moisture.



posted by 63.148.1...

• AMM 101, JeffS, Tue, 2 Sep 2003 07:46:06
  • Packrats of a feather..., RS , Tue, 2 Sep 2003 08:09:49 <-- Viewing This Message
    • Re: Packrats of a feather..., JeffS, Tue, 2 Sep 2003 09:21:47
      • I call the ones here ..., RS , Wed, 3 Sep 2003 06:20:58
    • can you test for a bad AMM then?, Trev, Tue, 2 Sep 2003 09:02:53
      • Re: can you test for a bad AMM then?, ChrisM, Wed, 3 Sep 2003 05:53:33
        • Re: can you test for a bad AMM then?, RS , Wed, 3 Sep 2003 06:07:17
    • RS, wow! 'Thermistor drift'! Wish I knew this stuff..., Micah in NC, Tue, 2 Sep 2003 08:14:55
      • When AMMs go bad - next, on Jerry Springer, RS , Wed, 3 Sep 2003 07:32:31

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