Testing Duty Cycle

[Funracer]

I have two DVMs with %HZ and both do the same thing 95% of the time.

If I attach black COM probe to green test port pin and red probe to ground, the engine bogs down a bit, about 75-100 rpm.

If I disconnect the red probe from ground and leave the black on the test port, RPM goes back up.

Is this supposed to happen?

Thanks

[Funracer]

Asking because I have a really hard time getting the FV to about 45%. In the 30 or so times I have tried to set it over the last couple of months I have only got it to 45% twice.
I set it up the same way all 30 times and it always shows 65-85% but will not go lower.
Then once in a blue moon I hook it up and it works like a champ and I can put the DC right on 45% plus or minus a bit. During those very few times when this happens hooking up the DVM does not bog down the engine RPMs. When I am able to get it to 45% the car runs well and hot starts fine but next morning will cold start perfectly but then idles down and dies after 2 or 3 seconds.

Tony is helping me with fuel pressures and the WUR on another thread. Anyone got any ideas on what is going on with my FV?

Thanks

[Bob Kontak]

I have always used an analog dwell meter set on 4 cylinders.

[Funracer]

Originally Posted by Bob Kontak

I have always used an analog dwell meter set on 4 cylinders.

I have tried a 2 wire Dwell meter (no battery) that has 4,6,and 8 cylinder settings and could not get it to work. Does it have to be 3 wire? Can I use the 6 cylinder setting or only 4? What numbers am I looking for from 4 or 6 cylinder setting?

Thanks

[tirwin]

Why are you putting red to ground? Isn’t that effectively grounding the circuit? If you’re grounding it and RPM changes then it would seem the FV is working.

One thing about using a digital meter is you have to figure out if the meter is telling you the % of time ON or OFF. They don’t all report the same way. 50% is obviously half ON & OFF. But if the meter says 40% there is a big difference between 40% ON and 40% OFF (ie, it’s really 60% ON). This is where an oscilloscope would be really handy.

There is a way to determine which way your meter is reading. I remember this was discussed a long time ago but I don’t remember the specifics. The basic idea is you want to get the FV to go in a known direction (probably by grounding a pin) and then see how your meter reports. You’ll either be able to use the displayed value or you have to subtract it from 100.

[dhanl82]

There are a few ways to determine which direction your meter is reading duty cycle - on or off. The first is to observe your meter as you open your oil fill cap – this introduces additional air into the mixture simulating a lean mixture, the duty cycle will increase for a few seconds until the 02 control system brings the mixture back to “Stoich” (close to 14.7 A/F ratio), therefore if you were reading a duty cycle of , let’s say 55% and it temporarily goes to 70% as it increases the duty cycle in reaction to the all of a sudden lean mixture that it sees. This will tell you what direction your meter is reading.

You can also preform the “rich stop” or the “lean stop” tests as described on page 22, chapter 6 of the Charles O. Probst book “Fuel injection and engine management” Rich stop you disconnect your O2 sensor connector and ground the connector end going to the to the O2 sensor control box to simulate a lean condition – you will see the system significantly increase the duty cycle as it reacts to seeing a “lean” condition.

The “rich stop” test is where you connect the positive terminal of a “D” cell flashlight battery to the Lambda sensor wire and ground the negative terminal to the engine case - this simulates an overly rich condition and you will see a sharp decrease in duty cycle.

These tests will also confirm whether your Lamda control system is working as it should.

I have a broad band A/F ratio gauge in my 82 SC and have been able to validate all of this by watching the gauge.

Hope this helps.

Dave

[Bob Kontak]

Originally Posted by Funracer

I have tried a 2 wire Dwell meter (no battery) that has 4,6,and 8 cylinder settings and could not get it to work. Does it have to be 3 wire? Can I use the 6 cylinder setting or only 4? What numbers am I looking for from 4 or 6 cylinder setting?

Thanks

Two wire Actron. Tim sent me his as well I believer to validate. He could not get his to work iirc.

Four cylinder mode. Not six or eight.

Dwell is a 90 degree scale. For best emissions at open loop you should dither around 45 degrees post closed loop. I set my car a little rich so I think it's reading high 30's.

Here's the vid of both mine and Tim's.

https://www.youtube.com/watch?v=Et9tE-10mYI

Of course, an exhaust analyzer is the correct answer but a dwell meter is a decent poor man's proxy.

[AndrewCologne]

Originally Posted by tirwin

Why are you putting red to ground? Isn’t that effectively grounding the circuit? If you’re grounding it and RPM changes then it would seem the FV is working.

On a DMM you put red on ground and lead the black to the test wire cause the pwm of the dutycycle affects the ground signal to the FV.