AT/MT Ident pin question

Post by **Legacy777** » Tue Aug 31, 2004 5:56 pm

Ok.....so I'm a little confused.

From looking at the ECU I/O info I have, it says that on MT cars the MT/AT pin should read 5v, and 0v for AT.

Now on Dave's swap, he cut that wire, and it fixed his idle issues.

How can that be? How can cutting that pin put 5v to it?

mikec · Post by **mikec** » Tue Aug 31, 2004 6:20 pm

I haven't seen any wiring diagrams, but maybe its connected to ground, and floats at 5V normally?

vrg3 · Post by **vrg3** » Tue Aug 31, 2004 6:42 pm

Mike's got it.

Inside the ECU, that pin is connected through a pull-up resistor to the ECU's internal +5v rail.

If you leave it disconnected externally, the pin's got 5 volts on it. If you connect it to ground externally, you pull it to ground and the pin's got 0 volts on it.

Post by **Legacy777** » Tue Aug 31, 2004 6:53 pm

Sweeet!!

That makes sense. The brain was thinking something like that...but it didn't put all the pieces together

mikec · Post by **mikec** » Wed Sep 01, 2004 6:42 pm

Cool!

fishbone79 · Post by **fishbone79** » Fri Sep 03, 2004 3:28 am

Where is said pin located? I have this issue on my 92 legacy after putting in the 5MT. Also, if I floor it at, say 2500, it will bog until it hits a sweet spot at 3 grand and throws you back in the seat. If I'm light on the pedal below 3000 RPM, in any gear, the acceleration is much better than if its floored. it also cuts up above 5000 RPM. I have a hunch its entirely electrical; something to do with the ECU still thinking its dealing with an AT. Any insights? Thanks in advance.

vrg3 · Post by **vrg3** » Fri Sep 03, 2004 3:31 am

Well, first try cutting the wire and see what happens:

http://www.graphics.cornell.edu/~v/ecupins/

fishbone79 · Post by **fishbone79** » Fri Sep 03, 2004 3:51 am

I have no wire there at pin B48-20... which makes no sense, my car was an AT.. did i miss something?

vrg3 · Post by **vrg3** » Fri Sep 03, 2004 3:58 am

That ain't right...

Is it possible that whoever did your transmission swap removed the wire?

Are you sure you're looking at the right spot? Not upside-down or anything?

fishbone79 · Post by **fishbone79** » Fri Sep 03, 2004 4:09 am

I did the tranny swap, and didnt remove the wire.... I actually did the swap and found Dave's site afterwards (much to my shagrin). I had no idea these cars had the following until recenlty. Regardles, I am def. looking at the right spot, but there is simply no wire there... Are the ECU units for the MT and EAT the same? I had switched the AT ECU for a MT ECU, which i snuck out of the junkyard under my hat

, but it made no difference.

fishbone79 · Post by **fishbone79** » Fri Sep 03, 2004 4:15 am

Should i ground it, for shits and giggles, and see what it does to my idle?

vrg3 · Post by **vrg3** » Fri Sep 03, 2004 4:22 am

In 92 there wasn't a difference between AT and MT ECUs.

What year ECU did you swap in? Depending on what year it was, it could have caused some problems.

Yeah, go ahead and try grounding the pin. Let us know what happens.

fishbone79 · Post by **fishbone79** » Tue Sep 07, 2004 2:43 pm

I grounded that pin... and it made no difference. I also tried vrg's scan tool with the LPT hookup, which wouldent work. The program was unable to determine the ROM ID of my ECU... I'm sure there is another post concerning scan tool woes, so i will keep looking. Do i have an electrical lemon here? I have double and triple checked all of my work thus far and found no flaws.

Post by **Legacy777** » Fri Sep 10, 2004 4:04 am

Oh....I learned A LOT today.....

Got the 1992 ECU from vrg today. Unhooked the 1990 one....and plugged in the 1992 ECU. Started the car, and it started up.....seem to run ok....but idle was a little high. I hooked up the select monitor to look at the values, and things were a little off, and knock correction was all the way at -32. I looked at the on/off pins. The ECU was showing as an AT!!! Which also caused the operation of the Neutral switch to be backwards. It read N when it was really in gear.

I got to wondering if the MT/AT identifier pin was different. So I reconnected the wire, and restarted the car. The idle bounced around right away at first startup and then settled back down nicely. Checked the select monitor and BAM, shows that it was a MT. Neutral switch was working properly as well. Engine values, and knock correction were all normal.

Looks like I found another snafu in the FSM's. Unless there are only a few odd ball ECU's like this. I'm curious if all the 92-94 ECU's are like this, or if there's just a few.

I also checked the voltage on the identifier pin when it was in "AT" mode. It read 5 volts.

So the whole MT/AT identifier pin may be a little more complicated, depending on the ECU & MY.

Results however were quite good with the ECU swap. The car seems to run much better, much like it did before I did the injector swap. I don't think the 5MT swap affected my original ECU. I think it could have run things just fine. I do however think the injectors are different enough between the 1990-1991 MY's and the 1992-1994 MY's that you really need the corresponding ECU.

vrg3 · Post by **vrg3** » Fri Sep 10, 2004 4:10 am

Woah. Huh. Maybe you're right -- maybe in different years they used different AT/MT identification pins.

Did your stock 1990 ECU say it was an MT when you disconnected the wire the first time?

I suspect the difference between your stock injectors and the 92-94 injectors was probably in the opening time. One of the designs probably opened faster than the other; it's something the ECU isn't designed to compensate for, and so your mixtures would always be very slightly off.

Post by **Legacy777** » Fri Sep 10, 2004 1:15 pm

Yeah my 1990 ECU said it was MT, and the neutral switch displayed properly too.

Who knows......It's weird....especially since the voltage seems to be the same....it's just how the ECU interprets it.

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AT/MT Ident pin question

AT/MT Ident pin question