how do i read a dwell meter for lambda AF mix
 

83sc

I've read many threads about this, and I plan to get a reading at cold start then another after getting the car good and warm with a drive. I don't want to start making adjustments without getting an idea of what I'm seeing, as it's not what I expected.

Sears Engine Analyzer with which I can select these:

dwell
volts
4
6
8

Red alligator to fuse terminal
Black to ground
Green clip to green/white wire on test port

Set to dwell, cold start shows 70 on the 4 cyl scale.

First, should the meter be set to "dwell" or to something else? If it should be set to dwell and my reading is actually 70, does that mean I'm 12 units on the lean side?

Bump

So basically the needle pegs to the max when the analyzer is hooked to power and the knob is set to "dwell." When the engine warms up, the needle dithers. Car runs great, so it can't be too far off. Wondering why the needle isn't reading in the acceptable range. Any insight is appreciated.

OK - I can help you. Let me get my head around it though. It's been a year or two. Plus I have a youtube video of this out there.

Will be back in a few.

Dithering is normal with the frequency valve is doing it's job with the engine warm. It should dither around 45 on a 4-cyl scale which equals a 50% duty cycle.

As tirwin says - the needle when dithering, should swing over 45 when set correctly.

I can't remember some of the intricacies and I think what is confusing me is that psalt a while back indicated you can set your AF mixture a wee bit rich and the O2/frequency valve will still automatically dial in the desired AF ratio at idle. Then you get a wee bit of a rich bump under WOT but at idle and cruising you are at the stoichiometric ratio.

That said, you need to get your 3mm allen wrench down in the hole and adjust the af ratio. Always finish the adjustment with a clockwise movement. Meaning if you are turning counter clockwise, finish that turn with a clockwise movement. Something to do with screw lash.

See post #196 - video link at end of post.

http://forums.pelicanparts.com/porsche-911-technical-forum/587238-911-sc-starting-issue-10.html

I feel like I know the both of you from reading your many CIS-related posts---psalt, too. And Bob, I've seen your video. I feel like I have a good idea about what to do. Unfortunately, I don't understand why my needle is pegged to the right when I hook up the analyzer, and it doesn't move (except to dither). I wonder if the analyzer is not working or if I have it set incorrectly.

Also, I'm connecting the positive cable to the uppermost rear terminal, where the fuse is, on the panel in the engine bay. In Bob's pic from the old post, his fuse is red. In my car, it's light blue. I'm grounding it on one of the bolts on the body that attaches the trunk latch.

BTW, After leaning it out a bit to pass smog, I have the timing/idle/af mix dialed in as best I can by ear, seat-of-the-pants feel while driving, and smell. But I'd like to see what the instrument says about the whole thing.

Mine showed 65 on cold start. It's not in closed loop till it warms up. Disregard till dithering starts. That means O2 is kicking in and sending voltage to the lambda box.

When you say pegged is that 70?

Now I remember what was bugging me and the resistance to respond immediately.

Remember that 45 is the 50% duty cycle, right?

I am kind of remembering that psalt noted that if the gauge is reading high - over 45 - it indicates that the frequency valve is pumping in a richer mixture to offset a lean reading from the O2 sensor. I had been assuming (a couple of years ago) that if the meter said over 45 that it was sniffing too rich. I believe that is incorrect. It is sniffing too lean and the frequency valve is trying to offset the lean mixture.

I could have it totally reversed.

Have you stuck a 3mm wrench in the hole?

Bob is right.

When the O2 sensor detects a lean condition (high O2 content), the ECU increases the duty cycle. Increased duty cycle means increased volume of fuel to the injectors.

When a rich condition is detected (low O2 content), the duty cycle decreases.

Remember that the ECU doesn't always listen to the O2 sensor. At cold start, above 35% throttle and at WOT the O2 sensor is ignored.

Edit: I mispoke in the original post. I meant to say that ABOVE 35% throttle, the O2 sensor input is ignored. If fixed it.

The O2 sensor has to get warmed up before it starts to work (generate voltage). On a cold engine, the control unit sends a 60% duty cycle at first. The lambda thermotime switch starts out closed on a cold engine. When it warms up enough to open, the ECU sends a fixed 50% duty cycle. Once the O2 sensor has warmed up to operating temp, the ECU switches to closed-loop operation where it adjusts the mixture based on the O2 sensor information.

Tell us more about "pegged" post dithering.

Here's another check. When your dwell meter is pegged, disconnect the O2 sensor. Hook the + of a multimeter to the O2 sensor and - to ground. Check your voltage reading. Close to 1V is rich and close to .1V is lean. There is a wide margin of error around stoich so you can't reliably set your mixture based on the O2 sensor voltage. This is just a quick test to confirm if you're running to either extreme.

With the dwell pegged, the ECU should be trying to compensate for a lean condition, so you should see a low voltage on the O2 sensor.

Great info, thanks all. When I say "pegged,"
I mean when I hook up the analyzer it pegs to 70, even before I start the motor. On cold start it stays pegged as the car warms up. I disconnect the analyzer, drive for 10 minutes, connect it again, and the needle dithers at 70.

I understand that because the analyzer may read very high that the af may be very lean. When the motor is warm, I can pull the sensor plate down less than a mm and the idle dips. Any more and it stalls. I can lift up on the plate a good mm plus before the motor stalls. That tells me I'm at the lean end of the range.

I'll put a multimeter on the o2 sensor when I get home. Still bugging me that the needle maxes out even before the motor starts.

OK - cool.

4 cylinder is the correct setting and "dwell" is where it should be set.

$40 - $50 will get you a new Actron at the local parts store. Hate to ask this but you are sure the connector on the test lead is not touching any of the other two leads? I am sure it is not but just checking.

The MM test on the O2 sensor is actually pretty slick. Try to back pin it so you can have the system working while you read it.

You will get it.

So with the motor fairly warm but not as hot as it gets after 20 minutes driving the O2 fluctuates between .1 and 0.

Idle isn't noticeably different with the 02 disconnected.

Oh, and the analyzer clip is not touching any other probes on the test port. I stuck a piece of bicycle inner tube in there to prevent contact.

Think I should start richening via the af screw until the dwell falls into the correct range?

Yep! You have confirmed what you already suspected. You are running too lean. Turn the mixture screw until it dithers around 45 on the 4-cyl scale. Just for schitz'n'giggles check your O2 sensor voltage after you've adjusted the AFR and then plug it back in. It should be around .5-.7V. Then go for a drive and let us know how it's running on the butt dyno after that.

OK, thanks for the encouragement! I have turned the screw in 1/5-turn increments 1 full turn. Now the idle kind of picks up and adjusts down in a slower rhythm, and if i give it some throttle the rpms drop quickly almost to a stall before picking back up to idle. That's a sign of richness, right? Still the needle is pegged. Should I keep going, or am I already too rich?

It sounds like you are still lean assuming the oxygen sensor is unplugged - what happens when you plug in the oxygen sensor?

I believe that if your rpms rise when you plug the sensor in, the fuel mixture is lean and the freq valve then adjusts by adding more fuel to get to lambda...
Could someone knowledgeable confirm or deny this please? :)

Bryant

Oxygen sensor has no effect on rpm. If it's still too lean, how can I keep it running as I enrich the mixture? The idle swings up to 2k, then dives to 0, then swings back up. Seems like it's going to stall out if I make it any more rich.

Turned the screw 1/5 turn to lean so it idles better. Dwell needle still pegged. O2 sensor reading started at .4.something and kept climbing up to .7.something before I stopped reading.

If you went a full turn you're too rich now. Don't worry about idle. There is a separate adjustment for that.

Thanks, Tirwin. Yes, too rich. Chugging puffs of dark smoke. I was just trying to get the needle to move. Failing that, I leaned it out and got it to run well again. Using what is referred to elsewhere as the field test, I have it I think not quite as lean as it was when I started. O2 reads .5v. Timing is a few degrees off at 900 and spot on at 4k (@25 degrees advance). It idles smoothly again with no hunting and drives great.

Tuning by ear is fun, but I think, as Bob suggested, I need to try another meter. At my local stores I have seen only the two-prong digital meters. Will those work, or do I need the positive, negative, and clip?

Given you want to see the dithering, I would stick with analog.

My old Sears analog dwell meter had three wires. The Actron analog only has two wires and instead of selecting 4, 6 or 8 cylinders, the dwells scales are on the face of the gauge.

http://www.summitracing.com/search/brand/actron/product-line/actron-dwell-meters?autoview=SKU

I bought mine at Autozone five years ago. I looked for them online today at Pep Boys, AutoZone, O'Rielly's and Advance Auto. No one has them anymore. Summit Racing and Sears have them, though.

I am not sure if the thing takes a battery or not but it does not have a clip that hooks to a power source.

See pic below.

http://forums.pelicanparts.com/uploa...1375200566.jpg

I would imagine the analog would be a little easier to comprehend than rapidly flickering numbers. I may order one of those, but in the meantime I'm going to borrow another meter (digital) and see if it at least registers a change when the af mix is changed. I'll report later.

Here's a pic of my Sears unit connected. Wonder if I can get it checked out and repaired.



http://forums.pelicanparts.com/uploa...1375210590.jpg

When I bought the 911 I needed a timing light so I thought I would be smart and get one with the dwell meter built in. It's digital so you basically can't use it for the intended purpose. I are smart.

I ordered the Actron for ~$30 from Amazon. Haven't had a chance to use it yet but I need to.

I thought I had read somewhere that people were using digitals for this, but I'm probably mistaken.

Well, I got everything set up last week to get smogged and the guy is looking at his timing light and looking over at me in the waiting area like I'm an idiot, back at the light, back at me, etc.... Turns out he was right. My idle was way high, so my timing was way off. I was using the tach to gauge the idle speed. For some reason I had convinced myself that on the gauge the white line under the 1 was 900 rpm. I ran directly out to get a decent timing light that showed rpm. Fortunately, Sears serves fools.

I still have a lot to learn about K-Jet and making adjustments to it and such (no expert here), but I was wondering why you guys are tuning with a dwell meter as opposed to going by wideband AFRs?

I recently bought a wideband AFR with narrowband output. Haven't had a chance to install it. Have to figure out how to wire it in.

If you need to troubleshoot the FV working properly you need a dwell meter to see the duty cycle.

Thanks for the explaination - I have a random, extremely rich running condition that I am struggling with and checking the FV is one of my trouble shooting steps (maybe I can figure out how to do that thanks to this thread).

It's effective if it's working correctly. Would I bet my life on it, ummmmmm, probably not if I had to provide a perfect AF ratio number. The dwell meter is not helpful at WOT as the O2/Lambda system shuts down after 35-ish percent throttle engagement


Here's some talking. Probably not perfectly correct but will give you a flavor.

Dwell meter reads the output from the Lambda box (telling the freq valve what to do). When the needle starts dithering - around a 5-8 dwell range, the O2 sensor has warmed up and the lambda box is in closed loop AND the box is able to adjust idle/part throttle AFR via the frequency valve to the happy place even if CO is set a little rich or lean. As is (FV) adjusts lean or rich the O2 sensor picks up the change in the exhaust and the freq valve then adjusts pressure to reverse what is being sniffed. This cycling is what creates the dithering.

If CO is way too rich or too lean, the needle will not dither, again, that's if I remember correctly. Suspect there is some range from the O2 sensor that if outside may kick it out of closed loop and into open loop and the frequency valve uses it's default value.

When you adjust the dithering to bounce over 45 on the 90 degree dwell range you are real close to a 50% duty cycle which means that when the Lambda system is not operating (cold O2 sensor on cold start) your 3mm CO adjustment screw is set for the proper emissions friendly AFR. Still rich when cold but close to the best you can have.

Thanks, B.K.!

I would just add that the dwell meter method is very useful if you or some PO have screwed up the idle AFR and you just need to get it back in the right ballpark. It seems a lot of people mistakingly try to adjust the idle AFR thinking that it will cure CIS problems. It won't. It's all about checking that all of the components are working correctly, checking fuel pressures and making sure there aren't any vac leaks.

Closed-loop operation switches to open-loop in the following situations:
- O2 sensor not warmed up to operating temp
- engine is cold
- WOT
- coasting cut-off
- O2 sensor disconnected

Note that a DEFECTIVE O2 sensor is a different animal from NO O2 sensor (just running in open loop). A defective O2 sensor could give the WRONG reading.

Also I think it was psalt that reminded us recently that the "limp home" mode on these cars is to run LEAN, not rich. So if you think your car is running overly lean, that could be an indication that something is not working correctly.

I'll bet what happens is that the O2 sniffs so rich or so lean that the freq valve cannot counteract the condition so it stays in one (all lean or all rich) mode to try to fight the condition.

if you're comfortable experimenting, you could check dwell as you go fully rich then fully lean in closed loop. I've been meaning to do this but have some other issues that are keeping things inconsistent.

Btw how are you setting your idle bypass screw?

I pretty much did that, only I went lean to middle to rich. No action on the dwell. When heading for rich, I adjusted the idle bypass screw to keep it running.

Bob, when you connect your meter with the engine off, does the needle move? If so, where does it settle? Or does it stay at 0 until the engine starts?

Don't know for sure. My Car is in eastern PA in a pal's garage while I wrench on his car here in North Canton.

I don't remember it ever doing anything until the engine started.

Thanks. Man, even when I connect the meter to the battery it pegs when I turn the switch to Dwell. Guess I have to expect to get burned on ebay every now and then.

I'm off on vacation for about a week. I should have a new analyzer by the time I return, and I'll report back when I get a good reading. Thank you to all for your help.

OK, new Actron meter. Green lead clipped to green/white post in test port. Black lead clipped to ground. Now I get no reading. (Engine is already warm.) Needle stays at 0. What am I doing wrong??? :)

The needle does not move on mine until the engine is started.