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Old 11-13-2003, 04:07 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 790
'91 fault code reader

Guys, I searched the archives, and I'm still confused.
How exactly do I get the engine codes from the diagnostic connector on my '91 300SE?

The connector is behind the battery, and has a big push-button next to it.
However, I see no LED.

I assume I need to rig a LED and connect a couple of the terminals, then push the button for 2-4 seconds, or ground something, or what?

Sorry to be so dumb, but I learned not to poke wires in expensive computers without 100% knowledge.

Any help is appreciated. I wandered thru the various posts, and found the blink harness, the code list, grounding termnal 3, etc;
but nothing exactly pertaining to a terminal that has a button and no LED.


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Old 11-13-2003, 09:04 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,802
The flat spot beside the button is the LED. It looks like pin 2 filled in. Key on, engine off, hold down the button for two seconds then count blinks. Hold down the button for two seconds then count the next set of blinks, etc. Hold down the button for 10(?) seconds to clear each code after reading it.

95 S420
91 300SE
87 300SDL
83 300SD
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Old 11-13-2003, 11:01 PM
Registered User
Join Date: Jan 2003
Location: Dallas
Posts: 790
Sixto, thanks. I see you have the exact same car, except maybe yours is a California edition? These Texas cars just have a big ball of bailing wire in there - well they're different apparantly.

Mine has nothing in the pin 2 hole, you can look right thru it.
I have metal sockets in holes 1,3,67. The others are empty.
The push button is right of the connector, near pin 10, but is mounted separately on the bracket.
I tried pushing the button 2 seconds to see if maybe that little LED Devil was hiding - but no luck.

There must be a lot of different varieties of the connector, depending on model, year, state, etc.

So, after a search, I assume pin 3 is the proper pin, and I read that the system grounds pin 3 for the "blinks". So that says I should hook up an LED between pin 3 and 12V.

But I am reluctant to hit pin 3 with voltage until I know for sure.

Anyone else got a suggestion?

Maybe I should leave well enough alone - 13 years with never a check-engine light or a stall. But I have this little chuggle when it starts hot, and an almost imperceptible occasional glitch in the idle. So this is a good excuse to fiddle with it.


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