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Old 04-13-2009, 05:28 PM   #1
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Default MS not working with LC1/ possible ground problem


after starting my 94 miata the LC1 works great - the signal in MS is the same as on the LC1 gauge, it reacts the same in whole range. but after I drive for a minute (or the car warms up) the signal is distorted - it'a best visible with throttle off: the LC1 gauge shows AFR 22, but the gauge on my laptop shows about 16-18. After turning the engine off for 5 min. its ok again but after a minute drive the same error happens. The signal to MS is ok for sure, because after changing outputs from LC1 the lc1 gauge works great on both of output and MS gauge has the same error on both outputs.

So my guess is GROUND PROBLEM
I don't have a stock ground connector (under TB) on my 94 miata. Everything worked great wit the ECU ground connected here (it was there when I bought the car):
Name:  ground.jpg
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but it seems that LC1 needs a more stable ground. I tried to get the ground from the chassis, from the engine block, even tried to share ground from these both sources at once - the same ground for LC-1 system and MS of course, I even tried to get the LC1 heater ground from other source - but it doesn't work. I'm not 100% sure if MS gets ground from the source on the photo but I haven't seen any other ground connections going to my MS...

I want to get a separate ground cable directly from the battery and connect the MS ground and LC1 ground to it. hope it will help, Where can I find the MS pnp 94-95 ECU pinout diagram, so I could check which cables to connect to battery ground?

Any comments on my problem? Maybe someone has another idea what distorts my LC1 signal in MS and how could I fix it?

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Old 04-13-2009, 11:10 PM   #2
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A wire jammed into a bolt and fender washer is a pretty **** ground, take it all apart and crimp it all properly. I actually have a braided ground strap running from the engine (same place as the engine strap (not a Miata) going back to where all thge MS and LC1 are, and everything is connected to that. I can zero my multimeter on head versus that terminal, it works great.

You SHOULD connect both LC1 grounds together, and share the same ground point as the MS, thats probably why your gauge doesn't work, there is ground potential. Did you read your LC1 manual? Its mentioned in there at least once.
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Old 04-14-2009, 02:33 AM   #3
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I know it's a shitty ground - but that wasn't my idea. the picture is made before I started to do anything - the LC1 was very bouncy on it and the signal on laptop gauge was by one point different than on LC1 gauge.

The first thing I've done was crimping the ground cables together on a solid copper clamp, then I tried to get a stable ground from many places under engine bay using a solid thick ground cable... now the signal is very stable and accurate but only a short while after I start the car - the idle AFR is the same, with over run fuel cut set to 0.5 sec both gauges show the same max AFR 22 when I let off the throttle from 5.000 rpm but after I start to drive/the engine gets warm, the signal on laptop is distorted by about 20%... and it shows eg. 16 instead of 22 AFR...

Both LC1 outputs have the same settings in LM Programmer, Megatune has also correct settings for wideband sensor, I've checked all the connections with LC1 manual so many times that I can remember cable colours...

There's something in my car that causes MS to loose 1 volt from the LC1 signal when the car gets warm and drives for a minute... Any ideas how to fix it?
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