My 1995 Cutlass Supreme 3.4 has the 12 pin OBD connector with only three wires/pins in it. Is this right? Am I missing something?

Can you post a pic?

You are caught in the transition vehicle. Certain 94 & 95 models, because of changes in the ECM, are different from all those before or after them. That vehicle is half OBD1 and half OBD2. The ECM is OBD1 but the communication system is OBD2. I do believe you cannot get codes manually due to this fact. I had a 1995 *-10 Blazer that was the same way.

I believe he still has the OBD1 system.

During the transition to OBD 2, the 1.5 systems used the OBD2 connector, but used 1.5 software and had most 2.0 codes.

If he only has 3 physical pins, then this is OBD 1.0.

It'* dark here, but these show what I have. I will try better in the morning. The last photo is a loose 16 pin connector that is just floating under the dash?
Thanks.

I am thinking that may be correct. One wire to PCM, one wire to ABS.

As others mentioned, OBD 1.5 was a very dark time in GM'* history. I don't know personally about what the 3400 was doing that year, but I suspect it may be the OBD 2 side of things. My '94 Regal has the 3800, spits out OBD 2 style codes and parameters over an ALDL port (OBD 1).

I could get in to my first hand experience with this madness deeper, but it would be veering from the OP.

And I have no idea what that connector is.

If I recall correctly, the OBD 1 connector was rectangular in shape while the OBD 2 connector is more trapezoidal, that is, wider along one edge and narrower on the other. My '95 Aurora has the OBD2 style connector. When I connect my OBD2 scanner to it, I get a "communication error" message. My '95 Cutlass Supreme convertible has the same connector. I haven't tried to connect my scanner to it yet, so I don't know for sure what the result will be, but it'll probably be the same. Of course, the problem could be with the scanner itself. It'* just a cheap one from Harbor Freight but it has worked fine with the OBD 2 vehicles I've connected it to, including my 2011 Impala and the '97 LSS I used to have. I had to identify the LSS as a Buick on the scanner since Oldsmobile isn't among the brands listed in its database.

Mine is the square, 12 pin (holes) type. I have ABS and 'brake' light on, but I only have an OBD II reader. The dealer wants $150 to scan it. I haven't tried the paperclip trick yet, I'm afraid of frying the module. I might buy a scanner, but have heard so many opinions on which one to get, which ones are ABS capable and the adapter cable and all, I would probably get the wrong one.

Yeah, that'* a weird one. The only GM diagnostic ports I've worked with are the 12-pin ALDL and the 16-pin OBD2. That one looks suspiciously like a small version of a radio plug.

Now I'm thinking that odd plug may be leftover from the heads up display for the pre-'95 models. I'll just zip-tie it down and try to forget about it....