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I have an 88 240 that is running really rough at idle, but runs fairly well at high rpms (above 2,000). I thought that I had a bad AMM, so I purchased a used one on ebay, but swapping the two did not product a difference. The only other thing that I can think of that would cause these symptoms is a vacuum leak and since I am unable to find one I am still trying to diagnose the AMM.
If I unplug the AMM the car runs the exact same as it does with it plugged in, if the AMM was good I would think that unplugging it would cause the car to run worse, what do you think?
I followed the testing procedures in the Haynes manual that I have. It says that the resistance between terminals two and three should be 3.5 to 4 ohms. Mine shows no resistance, my voltmeter reads 0.00. I do not have much experience testing electrical parts, am I right to be using the 20K setting on the voltmeter for this type of part? I am reluctant to purchase a new AMM just based on this one test especially when I may have already wasted some money on a used one. Does anyone know of any other testing procedures for the 007 AMM?
Greg
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I don't have any confidence in the Bentley/Haynes meter tests, having read here that they "have never solved an AMM problem". I think a more positive test is to apply power to the AMM and check the voltage produced in a static condition.
To do this, you must peel back the rubber boot at the AMM plug, so you can "back-probe" the terminals, which are numbered from back to front, 1 thru 6. The output to the ECU is #3, the white-red wire on 240s (blue-red on 740s).
With Key On II, you should see at least 1.2 volts on this terminal. This voltage will be higher with air flowing thru the AMM. You can see this effect by taking the air ducting off so you can hand-fan air into the front of the AMM.
But I think the static test is sufficient to validate the AMM as good or not. I had a recent '87 no-start with an AMM that read only 1.14 volts. A spare AMM producing 1.3 volts is now running the car.
I use an extra AMM plug with 6" of wire cut from a JY car to test my spare AMMs, by connecting +12v to terminal 5 (orange), grounding (-12v) terminal 1, then reading the voltage output on terminal 3.
P.S. I'm no expert. This test is from the "Volvo Problem Solver" book, and is probably old hat to Art Benstein and the other Brickboard pros.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.
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Regarding using an ohm meter: you want to use the smallest setting that is above the resistance that you're reading. So for example, to read 3 ohms, if your meter has a 2 ohm, 200 ohm, 2k ohm, and 20k ohm settings, select the 200 ohm setting. If you reading 1 ohm, select the 2 ohm setting, 1.5k ohm, select the 2k ohm setting, etc.
Also, when taking the measurement, if there is no reading displayed (such as in the case of an out of range indication), select the next highest meter setting -- until you have a reading.
Another approach, if you're measuring resistance but don't know how much resistance to expect is to start at the high end of the meter settings and work back down until you reach the best resolution possible with the meter.
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dnvolvo '89 765T 200K - '91 245 100K
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Typically, if an AMM is bad, unplugging it and running without it causes a slightly richer fuel mixture, and should make the car idle a little better.
If I was you, I'd focus on the elements specific to idling: throttle position sensor (TPS) and idle air control valve (IAC), and probably the throttle body too. Check out this post.
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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Greg,
I thought I had AMM problems for a while (which turned out to be O2 Sensor) and here's what I learned.
1. When you unplug the AMM, the idle should become noticable rough... This is called "limp home" mode, where you CAN drive it, but it will run poorly.
2. Make sure you are checking voltage, not testing the circuit... In other words, make sure you arent sending a voltage TO the AMM instead of reading the voltage FROM the AMM (I've heard that this can damage the ECU)
3. What's the condition of the eBay purchased AMM? Here it is: If you unplug the "original" AMM without a change in idle, then you install (and unplug) the "ebay" AMM and the same result? Probably the eBay one is no good, and I feel bad tellin you that because it sucks you know?
I hope this helps a little, If I were you, I'd look on some of the 700/900 FAQ websites as the AMM works exactly as it does with B230F engines in the 7/9 models as well...
Happy trails and best of luck
Spencer
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