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For ground, you really want the same ground reference that is your DME's ground reference especially when measuring low voltage signals like an O2 sensor output. In the passenger foot well down by the DME wire harness connector, tap into one of the DME wire harness GND wires using a T-Tap or something similar. I have a few GND pig tails permanently hanging there because I'm always connecting a DVM, oscilloscope, or other test equipment to look at signals in the DME.
Have you ever thought of just unbolting the DME/KLR from the bracket and letting them lay on the carpet in the foot well. With the cover off the DME you can just clip onto whatever pin/signal you're interested in? I drove around for several weeks like that last summer trying to fix a flaky ISV that kept blowing the driver transistors in my DME.
When you get to measuring the O2 sensor voltage and see that it swings above and below 0.45 volts then your closed loop system is probably doing what it's supposed to be doing. If it's stuck constantly above or below 0.45 volts then either your sensor is bad or there's something else going on that is outside the control range of the close loop control system.
The DME isn't trying to control mixture by measuring the O2 sensor voltage directly. The sensor can only sense a rich or lean condition. It doesn't know by how much it's rich or lean. The sensor provides an error signal for the control loop running in the DME software. The control loop software knows the AFR is either rich or lean but not by how much. The software tells the fuel injectors to add a little fuel or remove a little fuel constantly trying to keep the error to a minimum. There's a limit to "add a little remove a little" so as long as the control loop can stay within those limits it's in "closed loop". Outside of that control window, the system gives up and default to other methods of controlling fuel.
Oh man it's never easy or simple to do anything, is it? lol
I don't fancy pulling the DME box yet again--the on my knees experience is getting stale. I'd rather start punching out that spare cat... and if I'm not too hung over in the next few days, might just do that. WBO2 is the Next Big Project. Right after the smaller projects
But thanks for the grounding tip..good to know. All this is making me realize how cool it would be to have something like that Vitesse system where you see all the DME sees on a screen in realtime. Imagine a bluetooth feed to your phone any time you want without detaching a single plug. But for one less zero on the tag lol
What you need is something that logs data while you drive. A real time display is nice but you can't watch all that and drive at the same time.
If you gut your cat, you can get started with a wideband O2 sensor / gauge, etc. by using a tail pipe adapter. No need to remove the exhaust system & weld on a bung. I did that for a while until I had time to make a more permanent installation. It's not the best solution but it's way better than no wideband capability at all.
What you need is something that logs data while you drive. A real time display is nice but you can't watch all that and drive at the same time.
If you gut your cat, you can get started with a wideband O2 sensor / gauge, etc. by using a tail pipe adapter. No need to remove the exhaust system & weld on a bung. I did that for a while until I had time to make a more permanent installation. It's not the best solution but it's way better than no wideband capability at all.
Oh yes forgot that with no cat material you can read off the tailpipe. I guess that's why they call them "test pipes" lol
Moot point for me though as I'm going to take advantage of the gutted cat currently not installed (it's a spare) to get that bung put in. I once got a real nice offer from a fellow Rennlister who can do bungs at his workplace shop
To accurately measure the same voltage the DME is seeing, I agree the same ground is best. For this purpose, however, I'd argue it's not worth the effort. All you are trying to do is see if the motor is in closed loop, so the actual voltage isn't the key thing. The key thing is to make sure the DME is constantly adjusting fuel above and below its target, which you should be able to see whether your set up has an accurate .45v center line or something slightly higher or lower due to the chassis ground.
Snuck in some time for a quick peek with the 'scope... just parked though. It seems proper except for an odd behaviour when releasing the throttle after holding about 2500rpm for a bit: it takes a very long time to get back to the pattern.
I start at idle, rev it to hold about 2500rpm, then release:
Here is the idle-only using a voltmeter; clearly, having all three wires hooked up with the jumper results in expected voltmeter behaviour--as opposed to the steady .9v I got by only measuring the signal pin with plug disconnected
I have to report that my last tank of gas went about 350 miles, better than previous couple, but not the 380+ miles before AFM cleaning/wiper reset. I did re-open the AFM and can see that the new grooves follow the old perfectly.
I have also noticed the off-throttle occasional popping has disappeared (Tom you might recall that). Perhaps I've been running lean for a while and the AFM service has put me back to regular. I knew the consistent 380+ tanks since the spring were too good to be true lol
12-23-2018, 02:15 PM
