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TCU Compatibility

Posted: Tue Oct 23, 2018 9:02 pm
by Coyote
This is an update to my previous thread, "A long sad tale, etc."

I solved the problem by replacing the TCU (from a '94 with one from a '90.

While this solved all performance problems and got rid of the original TCU error code (14 - Shift Solenoid #2), it added a new wrinkle.

I have been getting intermittent TCU error codes 22 - Atmospheric Sensor (or Barometric pressure).

Turns out that the '90-'92 ECUs had an atmospheric sensor in them and told the TCU what the pressure was. '93 and later did not have this in the ECU. So now, I have a '94 ECU which doesn't have a sensor and can't tell the TCU what the pressure is and a '90 TCU which expects the ECU to tell it what the Barometric pressure is. When that input is not there (and it never will be), the TCU throws code 22. I have no information about TCU internals so I have no idea why the code is intermittent.

I've sent the bad TCUs off for repair but until I match the ECU to the TCU, this situation will exist.

Hopefully, this information will save someone some time and/or trouble at some point.

Re: TCU Compatibility

Posted: Tue Oct 23, 2018 11:08 pm
by mike-tracy
So you're saying you need a 94 tcu?

Re: TCU Compatibility

Posted: Tue Oct 23, 2018 11:33 pm
by Coyote
I suspect that a '93 OR '94 would work as neither will expect a barometric pressure input from the ECU, but I have two '94 TCUs that I've sent for repair ($149 + shipping with a lifetime R/R warranty) so I'll just live with the codes until I get them back.

Re: TCU Compatibility

Posted: Tue Nov 06, 2018 7:44 pm
by Legacy777
I'm a little surprised to hear the 93-94 ECU's don't have a barometric pressure sensor. They have to have one to be able to adjust the fuel injection system for altitude and pressure changes.

How did you come by the information they don't have one?

Re: TCU Compatibility

Posted: Tue Nov 06, 2018 8:23 pm
by Coyote
Josh -

The information is from a note on the '90-'94 ECU pinout page for pin B48.16, "Barometric pressure sensor 1N" (N: Naturally aspirated models only , 1: 1990-1992 models only )

I made a bad assumption (I know what that makes out of me. . .) and the facts do not support it.

The note pertains to the ECU output (and by inference the TCU input) but all it really says is that the ECU tells (''90-'92) or does not tell ('93 and later) the TCU on that pin what the pressure is. As you say, the ECU must have a way to adjust the fuel injection system for changes in pressure (altitude).

And, sure enough, when I got the '94 TCU back from repair, the problem went away. I guess Subaru found another way to deal with the TCU shift maps. . .

Re: TCU Compatibility

Posted: Wed Nov 21, 2018 7:57 pm
by Legacy777
Ok, thanks for the feedback :)