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rough idle then stall; multimeter smooths it out? 200

posted by golfer on Wed Aug 4 14:09 CST 2004
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Good evening,

My kids 92 244 w/ 96k has been having idling problem. I suspected the AMM (when I unplugged it she at least wouldn't stall) and got one from the PNP, installed it and the car ran fine for a day. Ordered another from FCPGroton, figuring the junkyard piece was bad, installed it and it still idled like crap.
When the car is cold it starts right up and idles fine. As she comes up to temp, the idle starts to lope and the car will eventually stall. If I keep the rev's up she sounds normal and seems to run fine. Changed out the plugs, checked the vacuum lines and all looked OK.
Bought a multimeter to check the AMM. I have no idea how it works so I followed the Haynes directions. With the car running I put the leads to the #2 and #3 contacts and the idle smoothed out. What's up with that? The AMM was not connected (obviously) but the car idled like it should. My codes on the OBD says I've got the car compensating for lean/rich at idle and a faulty AMM sensor signal. Like I said, I really don't know how to use the multimeter but was hoping one of you guys could tell me why the thing would smooth out my idle and if doing so either identifies a specific problem or eliminates a piece of the puzzle.
Thanks for any help.
Eric
Blue 84 front/Gold 92 rear

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rough idle then stall; multimeter smooths it out? 200

posted by david hunter on Thu Aug 5 04:44 CST 2004
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last visit: Mon Jun 30 22:10 CST 2014

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Assume that your car has the LH2.4 FI system and not the 3.1. There are differences in the AMM connections between the two.

Your test measurements and problem description is confusing. The resistance measurement between 2 and 3 is merely testing for continuity of the heated wire inside the AMM. Obviously it is done on the bench with the AMM disconnected. If you were trying to measure at the plug on the car that would be incorrect and the meter may be upsetting the operation of the ECU and that result can be ignored.

With an AMM disconnected the engine should idle perfectly but will not rev up very well. Therefore if your idle is bad with it disconnected the AMM is not the source of your problem. From your description of the problem whereby it idles poorly after warm up I would suspect the ECU coolant temp sensor. Remember there are 2, one for the guage and one feeding the ECU. If the ECU temp sensor is bad the ECU thinks the engine is always cold and the mixture will stay rich and idle will be rough but higher RPM's will seem OK.

I
--
David Hunter

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rough idle then stall; multimeter smooths it out? 200

posted by jkordzi on Thu Aug 5 02:03 CST 2004
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last visit: Wed Sep 12 02:43 CST 2001

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Like Art, I don't see how measuring the resistance across the terminals of the AMM with the car running could have cleared up the idle. I'm thinking you were leaning on some wire/connector while you were measuring the resistance.

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rough idle then stall; multimeter smooths it out? 200

posted by golfer on Thu Aug 5 02:23 CST 2004
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jkordzi,
I'm not kidding. I would touch the contact and the idle would smooth right out; pull the lead off and back to rough. Did it half a dozen times and it did it every time. The meter was set on ohms and at the 200 spot on the meter.
Eric

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rough idle then stall; multimeter smooths it out? 200

posted by jkordzi on Thu Aug 5 03:00 CST 2004
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last visit: Wed Sep 12 02:43 CST 2001

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and the AMM was completely disconnedted while you were measuring it, right - you weren't back probing the terminals while the AMM's connected was attached?

If so, then (and this is a wild guess) maybe you were altering the ground circuit. I'd check the various grounding points of the car and make sure you're not allowing any part of your body to come in contact with the car while you checking the resistance.

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rough idle then stall; multimeter smooths it out? 200

posted by golfer on Thu Aug 5 03:43 CST 2004
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Right. The AMM was unplugged completly. I will get some contact cleaning today and go through all the electrical connections and hope that a liitle crud on a contact is the culprit. Thanks for your help.

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rough idle then stall; multimeter smooths it out? 200

posted by Morsing on Wed Aug 4 20:58 CST 2004
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What's an AMM?

There should be a resistor for the ignition coil mount just left to the radiator. These things somtimes fail and could cause rough idle.

Cheers

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rough idle then stall; multimeter smooths it out? 200 1992

posted by Art Benstein on Wed Aug 4 15:06 CST 2004
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last visit: Fri Oct 17 20:10 CST 2014

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You measured the resistance per the manual with the car running, but the AMM was not plugged in? And that had an effect on the idle? I'm imagining you carrying the amm/multimeter contraption away from the car to see how far you can get before its influence is lost. Sorry, I'm sure that is not what you meant.

Or were you probing the connector on the wiring harness with the meter set to measure ohms? If so, you may have invented a new troubleshooting technique to substitute for the AMM output using the voltage the meter applies to measure resistance. Not a bad idea, except multimeters use varied designs with different probe voltages.

Only suggestion I can think of now is to make sure the connector pins on the harness plug are tight and corrosion free. A bad connection here would explain all the symtoms you had with any number of good AMMs. Also, nobody has ever tested an AMM with those Haynes instructions to prove one good. The readings are in spec on the majority of bad AMMs. There are some better ideas about how to use the multimeter to assess your AMM in the 700/900 Frequently Asked Questions pull-down from the Features tab above. But the best way of all is to find someone else with an LH2.4 car (89-93 240 series and some 700/900 cars) and swap parts. Of course if you could have you would have. But only after you make sure the plug is clean and good-- that's where a lot of AMM troubles get fixed.

--
Art Benstein near Baltimore- whose golf swing would make you laugh

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rough idle then stall; multimeter smooths it out? 200 1992

posted by golfer on Thu Aug 5 02:20 CST 2004
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Art,
I did have the meter set on ohms and did consider duct taping the meter to the fender; the meter is a lot cheaper than another AMM!
I hate like hell to take the car to the local volvo independent, but I'm really getting pissed off at the car for not cooperating. Could the fuel pressure regulator be a part of this? Isn't there a cold start device of some sort that stays on until she warms up and then shuts off? Does the code for "rich/lean compensation" narrow the problem down at all? I'll give the problem another shot tonight and see if I get lucky and stumble across something I may have overlooked.I'll also make certain that the connection at the AMM is clean and shiny. Thanks for your help.
Golf swings come in all shapes and sizes. There is an infinite number of ways to miss hit the ball. I learned long age not to laugh at anyones swing; it just makes it more painful when you're paying the guy at the end of the round. There was a guy a couple of years ago with a really funny swing named Jim Furyk. He was laughing his ass off on a Sunday afternoon when he was holding the US Open trophy over his head!
Thanks again.
Eric
blue 84 front/gold 92 rear

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rough idle then stall; multimeter smooths it out? 200 1992

posted by Art Benstein on Thu Aug 5 13:22 CST 2004
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last visit: Fri Oct 17 20:10 CST 2014

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Hi Eric,

I just read David Hunter's analysis. It seems well reasoned especially mentioning the change during warm up. Your new multimeter would do well to check the ECT (engine coolant temp) sensor operation, using it at the ECU plug (motor off of course!) as the book suggests. There's a chart showing resistance vs. temperature so you can know it is in the ballpark (on the green) and that it functions.

Those codes are primitive. You'd think the ECU would know whether compensating for rich or for lean-- which one, and tell you! But David is correct; if everything else is in order, the disconnected AMM should allow a good idle, just no guts.
--
Art Benstein near Baltimore

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rough idle then stall; multimeter smooths it out? 200 1992

posted by golfer on Fri Aug 6 00:33 CST 2004
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Thank you to all that replied to my question. I think that David may be correct in thinking that the coolant sensor may be the problem. The symptoms certainly point in that direction. Didn't have time last night to look things over. Played golf and was overserved (again!) so figured I'd give it a rest until I was thinking a little more clearly. Tonight it get's my full attention.
Thanks again everyone. Have a great weekend.
Eric

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