belaw

Let me start off my saying that I'm a a total TBI newb, so please bear with my basic questions.

I'm rebuilding a L31 Vortec that will go into a 4WD in front of a 4-speed transmission to replace a tired SBC. The build will be fairly stock except for an RV cam and headers. Once the engine is set up I won't be modifying it frequently. To be honest, I want to tune it and forget it.

I'm planning on ditching my carb and going with TBI and need some guidance on which ECM I should use. The most common suggestion for applications like mine is the C3 '7747 ECM, but after spending some time on this forum- which is lightyears ahead of any other forum I've read when it comes to TBI- I'm also considering using a P4 '7427 ECM from a '93 - '95 5.7 truck.

Since I'm running a manual transmission I don't need all of the bells and whistles on the 7427, but based on what I've read it will require less tuning work, which is definately a plus to someone like me who's new to TBI. Long story short, can someone give me some guidance on which ECM I should go with?

EDIT: On a related note, most of the articles I've read suggesting sourcing the parts for a TBI conversion from an '87- '92 vehicle. Why is that? Did anything change in '93 other than the ECM?

belaw

Well, after more reading I think I've found the answer to my question. One detail I didn't mention in my first post is that I don't have a speed sensor. If I understand things correctly (which is doubtful) I really need to start with a bin file with the dedicated idle functions for a manual transmission. Well, since I posted I found out that GM continued to use the C3 ECM's in 1993-1995 in trucks with 5.7's and manual transmissions, so there's no stock bin file for a P4 PCM with 5.7 and manual.

And that means a C3 ECM is my "best" choice by default. Dang...

RFmaster

That's not true - surprisingly P4 PCM was used with LO5 and manual trany in G20 and G30 vans and step vans. Partial list of calibrations provided below.

Word of advise - P4 PCM can be used with any trany (MT/THM350/700R4/4L60E, 80E). You may need to incorporate SA and AE changes into a baseline AT bin file to make PCM play nice with a manual gearbox.

BDTZ 1994 16168625 G2 L05 5.7 MAN GU6
BDUA 1994 16168625 G2 L05 5.7 MAN GT4
BJYH 1995 16197427 G2 L05 5.7 MAN GT4
BJYJ 1995 16197427 G3 L05 5.7 MAN GU5
BJYN 1995 16197427 G2 L05 5.7 MAN GU6
BJYP 1995 16197427 G2 L05 5.7 MAN GT4
BJYR 1995 16197427 G3 L05 5.7 MAN GU5
BJYZ 1995 16197427 G2 L05 5.7 MAN GT4
BJZH 1995 16197427 P3 L05 5.7 MAN
BJZP 1995 16197427 P3 L05 5.7 MAN

//RF

belaw

Thanks, this is very useful information!

One quick question: what does the column with G2/G3/P3 represent? And what about the last column with GU4, GU5, GU6, GT4? This is all new territory to me!

Also, this brings me back to the second half of the question in my original post. Most of the articles I've read suggesting sourcing the parts for a TBI conversion from an '87- '92 vehicle, which would have used a C3 processor. Why is that? If I want to use a P4 PCM is there any reason I wouldn't just pull the parts from a '93-'95 truck or van? Did anything other than the ECM change in the TBI system in 1993? I know that the addition of transmission control means that I will have to strip more wires out of the harness, but that sounds easy enough- and far easier than repinning an early harness for a P4 ECM.

RFmaster

G2 = G20 van (aka 1/2 ton rating) G3= G30 Van (aka 3/4 ton rating) P3 = P30 step van chassis = GM nomenclature for various vehicle lines.

GU4, GU5, etc are RPO codes for rear end ratio's. There are some minor differences in SA and AE table depending on rear end ratio.

I am just going to touch on the basics. The P4 ECM includes internal KS filter module - the old ESC module used with C3 is no longer required. The KS signal is routed directly into PCM. The E-trany is a big addition to the harness. There is also provision for a linear feedback EGR.

I have pulled both the C3 harness and P4 harness from JY donors (1990 burban and 95 C truck). The C3 is simpler and smaller - pre 1989 (C/K) are stand alone EFI harness. It takes twice as long to 'skin' P4 harness. Re-pinning ECM - PCM plugs is very easy with 12094429 tool (alternatives work just as well). With enough determination either harness can be modified to work with C3 or P4 PCM (usual disclaimer: some functions will be lost, or unsupported) to work with a typical SBC.

//RF