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No 'correct' value Posted by Ari [Email] ![]() ![]() In Reply to: Correct AMM Resistance?, Jon W, Thu, 24 Oct 2002 12:54:07 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The AMM is adjustable to compensate for differences between cars. The 380 ohm value was chosen because at that value, most all the cars will at least start and run. You can't properly adjust the AMM unless the engine is running.
Once the engine is running, you adjust the AMM for proper operation. This involves putting a meter on the test connector, which is basically a buffered version of the O2 sensor. The O2 sensor output swings between 0.2 volts (lean) and 0.9 volts (rich). The test signal swings between 0 (lean) and 5 (rich). You adjust the AMM until the O2 sensor signal swings between the two voltages, spending about half the time above the middle, half the time below. It swings back and forth about 1/second.
This adjustment takes into differences in the O2 sensor, AMM itself, and condition of the engine. Each car is different. The ideal setting may be 200 ohms for one car, and 760 for another.
Actually, the burn-off feature does a lot. The AMM hot-wire normally operates at between 100 and 155 Degrees C above ambient (depends on model). That means it can be on the order of 300-400F on a warm day. During burn-off, the wire heats up much higher, closer to 1000F (I'm not sure of the exact value-I can look it up). That burns off most contaminants.
The absolute MOST I would do to clean the AMM would be to spray the wire (GENTLY) with car cleaner. Do this with the AMM OUT OF THE CAR. Don't spray it in while the engine is running, or within a few minutes of shut down. The last thing you want to do is hit a hot wire with a vaporized, flammible liquid. Let it dry out throughly before reinstalling.
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