BAC/ECU Fix
 

If after performing all of your tests, you find that the BAC is not receiving it's signal from the ECU you may want to try this fix (As far as I know, this is for the Series 4 NAs). You will need to have a good small-point soldering gun, desoldering tape and TIP120 Darlington transistor (Radio Shack # 276-2068.)

1. Remove ECU from passenger footwell

2. Remove both covers from the ECU (lots of screws)

3. Carefully fold the top board out of the case

4. With the plugs facing you, (see attached image) you should see 2 rows of transistors—they are on the left side. The transistor you are going to be replacing is labeled on the circuit board as "T801"



5. Carefully flip the ECU over and find the three points that the transistor posts come through the board—desolder these three points using desoldering tape available at Radio Shack.

6. You will now need to bend the posts of the new transistor to fit in the now vacant holes on the board and solder the new transistor in.

7. You're done with ECU. Put it back together.

NOW BEFORE YOU RECONNECT CHECK THE CONNECTORS AT THE BAC!!!! If they are touching or are capable of touching you will need to make a new connector that will not short out. When those two wires touch it blows the transistor! I replaced my connectors with shielded Radio Shack brand spades. The OEM connector just plain sux. The plastic was brittle and the connectors slipped out and shorted. Good luck!

B

cool thanks

As a precaution you may want to disconnect the battery before pulling the ECU or checking the BAC connector.

B

I'm surprised that this thread didn't get more views!

The total time to do this is about an hour, including pulling the ECU from the car. Is it that people are fearful of touching their ECUs? I wouldn't even call that thing a computer—my electric toothbrush is more sophisticated!

I guess I should have stated that this fixes the problem with the engine idle when accessories (AC, PS) are turned on. So, if your car stalls when you turn the AC on or your idle drops when turning the wheel this may fix it.

I should also note that I gleaned this fix from several posts that seem to have gotten lost last year when the server went down.

B

Good job on the fix. Sorry you didn't get more views:) But hey, you fixed your ride:) I will rate it so it gets archived!

James

Yo thanx man i just asked this same question like two days ago but got nothing that was even revelent i have a 87 turbo 2 and the test for checking the ecu is using a 1/4 wat 10 ohm resistorand sticking it in the bac valves plug and meshure the ac voltage between between the resistor if it coes up between 6 and 7 volts you are in the green but any thing below 5 and your probley got a shot transistor and need to do this mod do you. the only thing is i dont know how to fox the 87 turbo 2 bord i know how to sodier but i dont know what transistor it is and ime no genioous with wiring charts so if you can help me that would be great

Great! Thanks for the writeup ... definately one for the archives.

Crap ... finally got around to yanking the ECU in my '90 Vert to do this. Then I realized this writeup was for the 86-88's. Does anybody know which transisitor is responsible for the BAC on the 89-91 ECU's?

http://www.teamfc3s.org/info/articles/idle.html

https://www.rx7club.com/forum/showth...threadid=90036

I've gone through the tests and came to the conclusion that I had a bad T801 transistor. I've got the replacement transistor from RS and I removed the ECU hoping to find this transistor. I see the T801 written on the board, but the transistor looks fine. Should it be burned in some way, or is all of the damage internal and will not show any signs of being bad?

I soldered the new transistor in, but I'm not getting the correct reading at the green wire on the BAC at idle. Is there a right way and a wrong way to solder the transistor in? I didn't see any polarity marks on the board or the piece itself........

:confused:

I believe the metal side of the transistor should face the wiring harness side of the ECU.

B

I don't remember which way I soldered it in. :(

Good write-up.

Reviving an old thread. Does anybody have a pic of the transistor that needs to be replaceon a '89 - '91 ECU? That pic is for a '86 - '88, and it's completely different.

That's a good question. I now have a series 5 and would like to know also. Where's Hailers and his scanner!?!

Dammit, HELP! I need to do this fix asap ... somebody is buying my car this weekend and I need to fix it before then.

I'm downloading the FSM now to take a look. Just have to get my son to stop bugging me to "change maps!" on Unreal Tournament...he's 5.

Crap! No wiring diagram for the S5!

While we are on the ECU--here's a question....I have an 86 NA, and I am now working on the "no starting" problem(gonna do the ATF trick tomorrow), but when it was running, I had the usual lag between 3700 and 4000. I have seen all the posts about how this is when the 5 and 6 ports open, and I have no desire to wire them shut. I heard someone once tell me that there was something you could do to the ECU to fix this....anyone know what it is??

Maybe somebody should PM HAILERS? Hmm? ;)

That hesitation is more a function of the crossover to the secondary injectors. A major hesitation is probably from bad grounds--search for the 3800rpm hesitation fix. It doesn't work for everyone, it dd work on my 87 n/a. It's basically regrounding the ecu, block and boost sensor. Good luck!

Amur-- LOL! Since I got the 91 TII I've really not had any problems so I haven't been contributing as much and picking HAILERS' brain. Where is he and that damned scanner!?! I love when he puts car parts on the scanner!!!

Brian :)

He puts car parts on a scanner??? :eek: I hope it's not his! :rofl:

Here, I finally found it: http://home.earthlink.net/~burntoast/3700.html

So what about the BAC output transistor?