how do i read a dwell meter for lambda AF mix

[tirwin]

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?

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

[Baby]

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?

[Bob Kontak]

Originally Posted by Baby

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.

[Baby]

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.

[tirwin]

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.

[Baby]

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.

[Ronnie's.930]

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?

[tirwin]

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.

[Ronnie's.930]

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).

[Bob Kontak]

Originally Posted by Ronnie's.930

.....why you guys are tuning with a dwell meter as opposed to going by wideband AFRs?

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.

[Ronnie's.930]

Thanks, B.K.!

[tirwin]

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.

[Bob Kontak]

Originally Posted by Bob Kontak

If CO is way too rich or too lean, the needle will not dither

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.

[blyguy]

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?

[Baby]

Originally Posted by blyguy

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.

[Baby]

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?

[Bob Kontak]

Originally Posted by Baby

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.

[Baby]

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.

[Baby]

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???

[tirwin]

Quote de Baby

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.

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