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One more time, with feeling Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 25 Sep 2008 13:36:08
In Reply to: thanks for the info, but i still need help with amm, t, Thu, 25 Sep 2008 12:55:55 Members do not see ads below this line. - Help Keep This Site Online - Signup |
First, the mystery of 380 ohms or 480 ohms.
Saab says to set the AMM to 380 ohms, because at that setting, most every engine will at least run. You can't adjust the AMM properly if the engine won't run. If your engine is running, then there is no need to change the adjustment.
The purpose of the adjustment is to use the AMM to compensate for a lot of other variables in the fuel injection system - small leaks, AMM to AMM variation, fuel injectors, the works. So saying a given resistance is right is like saying I set my toaster to 4 to get great toast. You may have a different toaster, and like toast differently.
In the end, the resistance will be what it has to be. I've seen cars adjusted perfectly, and the resistance is up over 700 ohms. 380 is just a starting point.
The AMM adjusts fuel flow based on air. The O2 sensor measure mixture - lean or rich. You want to adjust the AMM so that at idle, the mixture is right between lean and rich.
You need to measure the output of the O2 sensor. The output of the O2 sensor should vary between about 0.2 and 0.9 volts. This signal goes to the ECU.
What you need to do is get the signal wire from the O2 sensor. Typically there are three wires, two of one color, one of another. The two of one color are the heater - ignore that. Look for the wire of another color- that's the signal wire. (Yes, later cars have four wire O2 sensors, but they also don't have adjustable AMMs).
You need to measure the voltage on that wire without disconnecting it from the system. If you can get to the connector, stick the positive lead of the meter into the connector pin. If you can't get to the connector, what I do is stick a pin in the wire, through the insulation, and connect the positive lead of the meter to the pin. Connect the negative lead of the meter to a good ground, like the engine.
Set the meter to volts. Ideally, you want a good old analog voltmeter with a needle. You can also use a digital meter. Start the car, let it warm up, and make sure the idle is right. It should be around 850 rpm. You don't want a 1000 rpm idle. Watch the meter. If it is an analog voltmeter, you want to see the needle swing back and forth between 0.2 and 0.9 volts, averaging around 0.5 If it's a digital voltmeter, it's harder - you want to see numbers above and below 0.5. If the voltage is stuck up around 0.9, the engine is running rich. If it's stuck down at 0.2, it's running too lean.
You want to adjust the screw first so that the meter is changing - not stuck on 0.2 or 0.9. That's usually good enough. If you want to get really fine, try to adjust the screw so that the average is about 0.5 volts, spending 50% of the time above 0.5, and 50% of the time below 0.5 volts. That's harder without the right meter.
If your engine is already running OK, do yourself a favor and measure the resistance. That way, if you really mess it up, you can always re-adjust it back with a meter. If you don't want to do that, count how many turns you go one way or another as you adjust, so that you can at least go back to where you started.
Yes, there is a test point to check the O2 sensor voltage, but it varies from year to year and I've never been good at finding it. So I just stick a pin in the wire. I cover it with some electrical tape afterwards.
posted by 192.249....
, Thu, 25 Sep 2008 13:36:08 <-- Viewing This Message, Thu, 25 Sep 2008 10:30:15
, Thu, 25 Sep 2008 15:08:23
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