Automatic ECM with Manual Trans question

[stemo76]

I have a 91 chevy TBI engine that I have put into my Z. Went through the wiring yesterdy and noticed that I am missing a computer module for the auto transmission that came with the engine. I am putting in a T5 so the question is can I run the VSS sensor straight into the ECM or will I need to get a ECM made for a manual trans? Current wiring has the VSS going into the trans module and then out to the ECM VSS wire.

[strotter]

I'm wondering what "module" your missing? IIRC, in '91 the Corvette / Camaro motors used the '7730 ECU. The good news is you can go down to your local Chevy dealer (or GMPartsDirect) and order a "Memcal", (which is the chip, some "limp-home" and other circuitry on a carrier you just jam into your ECU).

 

Looks like this:

 

 

It'll cost around $75. You can go to http://www.usol.com/~okfoz/91/promcodes.htm to figure out which one you want. The bad new, as you may have noticed from the table at the above site, is that there was no T-5 equipped 350 that or any nearby year, so if you have a 350 and you're not happy with the "automatic tune" (which is a bit different that that for the manual, especially w/r/t timing), you'll need to burn your own chip. Not difficult, but not trivial either. If it comes to that I can give you some pointers. Also, JTR has a good book, "Chevrolet TPI & TBI V8 Engine Swapping", which talks about some of this stuff (though not in great detail - none of the custom tuning stuff).

[piston]

dont you have to reprogram the pcm anyways?

[stemo76]

JTR has a good book, "Chevrolet TPI & TBI V8 Engine Swapping", which talks about some of this stuff (though not in great detail - none of the custom tuning stuff).

 

I have both the JTR books and didn't notice anything in them in regards to using the auto ecm with a manual gear box. The modelule that I am missing connects via a square white 4 post plug. it takes input from things like the VSS, AC and other load items. I believe it controls your downshifts since teh mechanical kickdown is no longer used.

[strotter]

I've been searching around for a TBI circuit diagram for your setup, but I haven't had any luck - regardless, to answer your original question (which I should have done yesterday, sorry, I'm easily distracted - Oh, look! A pretty penny!...)

 

The VSS lines run in two different ways:

1. Directly from the VSS (at least for the 1227730) at pins BB9 (purple wire, VSS low) and BB10 (yellow wire, VSS high). This is an *unbuffered* input, directly from the VSS, no boxes in between.

 

2. From VSS to a buffer (a yellow box most years) and from there to the ECU at pin BC6. This is the system used in many of the TBI motors (it was in an '89 I swapped).

 

You may be interested to note that there are two VSS output signals *from* the ECU, pins BB11 (4000 ppm output), BC1 (2000 ppm output), which you can use to drive an electronic speedo (such as the Autometer units) if you so desire.

 

Automatic transmission operations come from pins BD14 (the 4th gear switch signal from the tranny) and GF6, the TCC control signal from the ECU to the tranny, via a 4-pin connector - this may be the thing you're looking at. Not to worry, just leave it undone, your new Memcal will ignore it.

 

Hope this helps. I have .jpg's of the TPI setup, if you'd like them. Email me and I'll "attach" them back to you.

[stemo76]

thanks for the info. I will be out of town for about a week and a half. When I get back home I will look at it again. When I was looking a the wiring the two twisted wires from the VSS plug seemed to go into another plug inches from the ECM. But when I was looking at the wiring diagram for the that year vehicle it doesn't look that way.

 

As for the Memcal, as shown in the chart there is now 5.7 manual chip. Will I need to get a custom job done? Been doing a little searching on this tons of information to wade through.