nothing

My 1993 90 with an MMS200 ecu, which came with a skinny housing 4 wire MAF sensor, is now happily running a fat 3 wire MAF, sensor electronics and all. Pins 1-3 are a straight hookup. Pin 4, the "CO potentiometer" signal, is wired to a standard 2Kohm pot connected to 12v. An initial setting of 1500 ohms will get you in the ballpark, and <a href="https://www.audiworld.com/forums/showpost.php?p=22403251&postcount=13">fine tuning</a> with a VAG scan tool will get you a nice smooth idle - you WILL get a CEL if the setting is too far off.

The 3 wire sensors seem to come stock with a calibration that is a better match for the fatter MAF housings that we all want to use. And now Abnormally Aspirated Stage 3 MAFs are useful to MMS200 owners.

Update:
For my final install I used a <a href="http://www.radioshack.com/product/index.jsp?productId=2062307&cp=">1kohm potentiometer</a> and a <a href="http://www.radioshack.com/product/index.jsp?productId=2062323&cp=">1kohm resistor</a>. This is a much sturdier pot than the one I originally used, and fits inside the rubber boot on the MAF connector. I ground down the socket on the 3 wire MAF to accept a plug with 4 wires, eventually I will replace the plug with a 3 wire junkyard unit. Thanks to Craig Yandrow for his help on the final refinements.

-Simon

<img src="http://pictureposter.audiworld.com/70334/newcopot.jpg">
<img src="http://pictureposter.audiworld.com/70334/bare.jpg">
<img src="http://pictureposter.audiworld.com/70334/final.jpg">
<img src="http://pictureposter.audiworld.com/70334/installed.jpg">
<img src="http://pictureposter.audiworld.com/70334/messy.jpg">

quattro.pilot

so the question is... how does the car perform overall now?

check yah later

nothing

UPDATE: Everything is great now, my car runs awesome - even when hot! The little blue radioshack potentiometer is perfect.

Original post:
WOT runs with the engine cool had steady rich a/f ratios throughout, open loop. But as the engine warms up, some dithering at WOT happens, but still much richer. The cheap potentiometer I used might be heat sensitive. I used what I had on hand, the good electronics stores around here are closed on weekends. It's much better at partial throttle even hot, however - I didn't even realize what I was missing there. I still need a bit of enrichment with <a href="http://pictureposter.audiworld.com/70334/maf.jpg">my setup,</a> but an AA Stage 3 MAF (did you keep yours?) with o-rings should be perfect. I have an SXTB sitting here, ready to be installed, so we'll see how that affects things.

And to be honest, I haven't had to chance to fiddle around with it much, yet. I'll probably hook straight into the ECU for the permanent install. And replace the 4-pin plug with a 3-pin from the junkyard so no grinding is necessary.

quattro.pilot

I broke even on it's purchase, so I'll be in the market for another one at some point.

But I'd like to tinker with what you've just discovered before I go chaning MAF's again. CAI &amp; S3 hoses are the next things to come, &amp; hopefully I can get the S2 MAF installed in and around that time as well.

When you said that you'd just go with a straight 3pin plug &amp; wire it all into the ECU, what did you mean by that? Won't you still need to have the stand-alone potentiometer in order to fulfill the "4th pin requirement"?

Also I do not have a VAG. Isn't there a computer program floating around that I can d/l and then just makeup a patch cord for the OBD 1 output plugs to computer serial plug? Any suggestions on making this rig work without VAG?

these be exciting times.... i'm gonna join asap

check yah later

snowman '92 100

so that there is actually NO wire/pot in the engine bay. That's where the three-pin harness and wiring come into play. If he's got the three-pin from the MAF all the way back to the ECM, then he can just use a short tap into a +12v plus the resistor. Badda bing!

MikTip

Could you of ran the wire inback of the connector, through the rubber boot?

patience

Time for me to try one of those 3 wire sensors too.

What lead you to trying them?

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In the end I decided to mod my junkyard MAF, rather than my car's wiring harness. If I could have pulled out the #4 pin from the plug it would have worked with no permanent damage, but that pin didn't want to come out, so I would have had to cut or tap into the wire. Had I been 100% sure it was going to work, I would have probably just done so, it might have been easier to do, but harder to reverse. I still would have had to mod the MAF to accept the 4 pin plug, however.

nothing

...that the stock 4 wire sensor puts out. I looked into that, and I realized the voltage range was the exact same as the later models, the only difference was the CO pot signal. Further digging revealed the the CO pot actually has nothing to do with the MAF, it just shares a housing and a power source (a Jaguar website helped me realize that, some Jags use a similar MAF with CO pot.) It was all over at that point.

nothing

...by using "Single Readings" channels 00 and 01 instead of "Basic Settings" channel 08 (Basic Settings is disabled in the shareware version.) <a href="http://www.planetfall.com/~jeff/obdii/">Here</a> <a href="http://www.andywhittaker.com/ecu/obdii_hardware.htm">are</a> <a href="http://www.aggressive.batcave.net/">some</a> <a href="http://www.arlab.it/hardware.html">DIY</a> interfaces that you can make, and the last link also contains a freeware VAG scan tool application for Pocket PC. I have had some trouble with an optoisolator based interface on my MMS200, however - communication with the ECU is very flaky, and some things that should be accessible are not. It might be easier to buy a new interface from Ross Tech (the new interfaces are fundamentally different than the DIY and older units,) or to <a href="http://www.steve-hall.com/cgi-bin/VAG-Locator.pl">borrow</a> one to make the adjustment.

Snowman is right about the ECU tap-in thing.