Adjusting CO With A Dwell Meter

[kings1527]

I purchased this multimeter with dwell function from Amazon the other day:

http://www.amazon.com/gp/product/B0002LZU7K/ref=olp_product_details?ie=UTF8&me=&seller=

Looks like a pretty decent device for the price. But my big question has to do with adjusting the dwell on the engine. I know this has been covered before but I do have some specific questions. Here's what I know:

1) I'm supposed to use the 4-cylinder setting on the meter, even though it's a 6-cylinder engine. Plus, you have to have all idle issues worked out and idling at around the factory spec of 775 or so.

2) I need to have the engine warmed up to operating temperature. As I understand it, this is to get the O2 sensor functioning correctly.

3) The reading I'm looking for is at about 40, with a low point at 30 and a high point at 50.

4) As I make an adjustment on the mixture screw located under the rubber bumper (clockwise = more CO and richer, counterclockwise = less CO and leaner), I need to either cover the hole with the bumper/wire again or cover it with my finger.

5) Once the hole is covered, take a look at the reading again and try to get it at 40.

So my questions are the following:

1) What exactly does the 30-50, centered on 40 actually represent? What does that number mean?

2) How do I get a reading that tells me whether or not my O2 sensor is functioning correctly? How do I test the O2 sensor with my meter?

3) D:04:01 in the manual says that I also need a "red exhaust gas analyzer" in order to perform a CO emissions adjustment. I thought with the dwell function on the multimeter, that that's what I'm doing? I think I might be looking at two different measurements but I'm confused when I read the manual.

4) Do I ever need to disconnect the O2 sensor during this test? The manual alludes to it.

I did a lot of work to my fuel system and engine and I'm currently heavy on fumes right now; in need of an adjustment. If someone can point me in the right direction, I'd appreciate it.

Alex
6575

[Bitsyncmaster]

All your doing is adjusting the center point of where the Lambda system is running. The idea is that with the system "centered", the cold start and wide open throttle fixed values are where things should be.

The center of the lambda system would be when the frequency valve is running at a 50% on and 50% off (duty cycle = 50%). The cold start holds a fixed 50% (60% if really cold) and the wide open throttle holds 60%. Those should be where the engine is a little rich. So you adjust it to run about 30 to 40% to get the mixture where it should be. That gives the lambda system a good range to adjust for altitude and other variables to keep the engine running at 14.7 AFR closed loop.

Dwell is used because it's a more common tool than a meter that reads duty cycle. On a 4 cylinder setting a 100% duty cycle would read 90 degrees. So those % values I stated would be about 10% higher than values read in degrees (what's in the service manual).

The adjustment is normally pretty sensitive. Usually a 1/4 turn on the mixture screw will drive the lambda system to it full maximum (about 95% duty cycle) and it full minimum (about 10% duty cycle). That would be a good indication that your O2 sensor is working.

[Ron]

+1 Dave


3) The reading I'm looking for is at about 40, with a low point at 30 and a high point at 50.

Note: 40 to 50 is with the O2 disconnected (steady)...for normal operation (everything hooked up and tuned up), the manual calls for pulsating from 35 to 45 (so, centered about 40). It's not supposed to be steady (and might vary wider)...An analog meter will swing back and forth and fwiw is easier to see/use to me.

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1) What exactly does the 30-50, centered on 40 actually represent? What does that number mean?

2) How do I get a reading that tells me whether or not my O2 sensor is functioning correctly? How do I test the O2 sensor with my meter?

3) D:04:01 in the manual says that I also need a "red exhaust gas analyzer" in order to perform a CO emissions adjustment. I thought with the dwell function on the multimeter, that that's what I'm doing? I think I might be looking at two different measurements but I'm confused when I read the manual.
First note the Lambda ECU has several other inputs, so the following is "over simplified"-
When exhaust gases flow over the O2 sensor there is a chemical reaction which generates a voltage. This voltage is fed to the ECU. The ECU uses this voltage to determine the Pulse Cycle for the Frequency Valve [This is what you are measuring] The Frequency Valve controls the fuel pressure in the lower chamber of the Fuel Distributor. The lower chamber pressure helps control the fuel output of the fuel distributor. So, say the mixture is too lean, the ECU will know from the voltage input from the O2 sensor and adjust the Pulse Ratio to the Frequency Valve to richen the mixture by reducing the pressure in the lower chambers of the Fuel Distributor (...this part may sound backwards because the lower pressure is used as a counter force to regulate with against upper chamber pressure, which feeds the injectors). Similar for too rich...
...Rather than viewing the ECU as "setting/holding" the A/F at a target value, it may help to think of it as the ECU says start going richer if too lean and V/V, making it bounce within two acceptable borders (35-45) to keep it near the ideal Air Fuel Ratio, which should result in a dwell of 40.0° Dwell (IF everything else is correct!). This is why an analog meter will sweep back and forth, and a digital will fluctuate +/- 5° or so.

Connect the orange wire in the loose two wire plug/pigtail running along the firewall, or, tap the orange wire in the Blue Bulkhead Connector. The other to the engine block.

Follow the 5 steps (A through E) in Section D:01:14 & 15. The chart was created by taking measurements taken on a good system -- If it passes all 5, the system (including the O2 sensor) should be good to go...


4) Do I ever need to disconnect the O2 sensor during this test? The manual alludes to it.
Yes, during tests B, C, and D.

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For test E, just press the WOT switch instead of holding the throttle wide open....

Hope this is more helpful than confusing...

[Bitsyncmaster]

A little more info:

A narrow band O2 sensor just measures when there is no oxygen in the exhaust. Oxygen in the exhaust means the mixture is to lean so all the fuel has burned so remaining oxygen does not get used. So the lambda ECU has to keep making the mixture richer and leaner to keep the sensor just reading where there is just no oxygen. This is mostly why you see the varying "dwell" reading. That is also why you can not use a narrow band sensor to hold anything but AFR 14.7 (Lambda 1.00). The unheated narrow band does not work if it does not stay hot. That is why the manual says to bump the idle up to 900 RPM. But I find 775 RPM seems to work fine setting the mixture.

The wideband sensor is different. It uses an oxygen generator to be able to measure the AFR. It takes some smart electronics to run the sensor and read accurately the AFR value. The wideband is a heated sensor and starts working in 15 seconds after it gets power.

[kings1527]

Guys, I can't thank you enough. That made total sense, even though I did have to re-read those posts a couple times! Ron, your explanation really did simplify what the manual was getting at. I'm big on understanding WHY something is doing what it's doing. It makes it easier for me to troubleshoot a problem down the road.

:question: Just one other question: my O2 sensor connection is going to be on the bulkhead, just below the engine compartment light switch? I believe that's the right one. I want to say it's an orange connection.

I'm heading out of town for this week but when I get back my multimeter should be here and I'll give it a run-through. I think my idle will be ok, even though I'm a little high at about 1000 rpm once warmed up. I'm thinking it's a deceleration spring problem (sound familiar, Dave M?). But regardless, I'll go ahead and make the adjustment. I'll let you guys know what I find out with the readings. Here's a few pics of my springs. I feel like I've ruled out pretty much anything else related to the *slightly* high idle (and not to hijack my own thread).

150481504915050

Alex
6575

[DMC5180]

It definitely looks like your deceleration springs are collapsing. Mine have nice equall spacing of the coils.

Btw I just noticed your Actron multimeter you bought. I have one of those also, which I bought for the dwell function. Mine never worked properly. Dave S. might be able to shed light on it. He made reference to issues with some digital dwell meters in an article he wrote a few years ago for Delorean World mag.

[kings1527]

It definitely looks like your deceleration springs are collapsing. Mine have nice equall spacing of the coils.

Btw I just noticed your Actron multimeter you bought. I have one of those also, which I bought for the dwell function. Mine never worked properly. Dave S. might be able to shed light on it. He made reference to issues with some digital dwell meters in an article he wrote a few years ago for Delorean World mag.

Thanks for the info on the decel springs. Dave S. replied and said they looked bad, too. Looks like I'll be in the market for a new throttle body at $84.

Not exactly the glowing review I was looking for on the Actron! Oh well...worst case, I'm out $45 for a dwell meter but hopefully I'll still have basic multimeter functions. Was only the dwell function not working on your unit? Or was it the whole thing?

Alex
6575

[DMCMW Dave]

Was only the dwell function not working on your unit? Or was it the whole thing?

Alex
6575

Try hooking the meter up backwards. The dwell meter is tryng to read the pulse width of a square wave. Apparently some meters are not sensitive enough to work at all, some will work if you connect them backwards but the reading will be reversed, i.e. 45 will still be 45 as that is the center, but it will read 75 when it should be 25. A bit confusing but you could still set it.

You can also try reading this at the engine diagnostic socket or the wire that goes it it (orange wire - to - ground) rather than across the freqency valve, sometimes that works better with certain meters.

[Ron]

:question: Just one other question: my O2 sensor connection is going to be on the bulkhead, just below the engine compartment light switch? I believe that's the right one. I want to say it's an orange connection.



If I understand correctly, that is the O2 sensor wire on the way to the UCU (blue) through the firewall, not the one you want:

My connections are here:
15076


A different harness/location on a project:
15077


(Sorry about the crappy cam ;-)

[Jeff K]

I have that unit, which I ordered from Amazon. I could not get it to work and Dave here said hook it up backwards and invert the numbers. (Subtract from 90 if I remember right)


I did that and it seems to work, but I want to get a better one when I have the chance. The one Dave mentioned has in his article is about 170 now I think.

[kings1527]

If I understand correctly, that is the O2 sensor wire on the way to the UCU through the firewall, not the one you want:

My connections are here:
15076


A different harness/location on a project:
15077


(Sorry about the crappy cam ;-)


Thanks for the pics, Ron. I don't think I was clear on that last one. I meant if I had to disconnect my O2 sensor, that would be the best location to do it.

Thanks again,
Alex
6575

[DMCMW Dave]

Thanks for the pics, Ron. I don't think I was clear on that last one. I meant if I had to disconnect my O2 sensor, that would be the best location to do it.

Thanks again,
Alex
6575

The only place you can disconnect the O2 sensor is at the connector where it goes into the body. Just forward of the left rear tire in the fenderwell.

[Ron]

+1

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And you do have to disconnect it to perform all of the tests....:deadhorse:

[kings1527]

+1

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And you do have to disconnect it to perform all of the tests....:deadhorse:

Understood. Thanks again everyone for all the help. I'll let you know what I find out.

Alex
6575