91rs4life2

1990 firebird formula (5.0 TBI to carb)

So I accidentally purchased a cap rotor and distributor base for a big cap (internal coil) HEI setup, when the car originally came with the small cap (external coil). I had planned to keep the ECM installed, to continue to control vital things like the electric fuel pump and the electric fan, etc.

My concern is if I isolate and extract the pink ignition switch 12v supply wire from the two-wire harness that has pink/white wires, put a female connector on it and plug it into the battery position on the cap, ignoring the other two wire harness with pink/black wires and ignoring the four-wire ignition module harness, will this do anything to the ECM other than turn on my MIL?

I apologize if this has been posted elsewhere, I went about 10 pages in and didn't see anyone talking about this situation.

Tuned Performance

iTrader: (92)

The ecm doesn’t control the fan. The cylinder head switch on the passengers side cylinder head between cylinders 6 & 8 controls the fan. The four wires coming from the icm are for distributor reference pulse to the ecm and ecm control of timing.
why not keep the tbi ?

91rs4life2

Because the guy I bought the car from had already started the process. I would have to buy an intake, EFI unit and computer controlled distributor, (car had no distributor so I bought a vac adv mechanical distributor, after reading the conversion post here). I'm on a very limited budget with this build. So i'd rather just buy an FPR instead.

Tibo

iTrader: (15)

Are you wanting the distributor to control timing or be a vacuum advance? The TBI ECM controls timing so as long as the large cap distributor is capable of controlling timing you should be able to still control the timing.
On the ignition control module of the large cap distributors they are usually a total of 7 terminals and the small cap electronically controlled timing distributors have 8 pins total. The small cap control modules have 1 extra terminal that is a sensor ground to the ECM which is pin G. You could just cut that wire and leave it disconnected but do not ground that wire.
If you want to totally remove ecm control of timing then yes, the pink wire is just a 12V switched wire. You could also trace that wire back towards the C100 connector solder on another wire to it that would activate the fuel pump relay. Down side to that is anytime the key was on the pump would be running.

91rs4life2

I ran into a guy that had the same situation. He told basically what you guys said:

A. The cooling fan switch on the block runs to a relay that then activates the cooling fan when necessary but, that there's also the temp sensor that actually does run to the ecm, that is used to interact with the oxygen sensor(s) (which I will not have)

B. I can use that pink wire to energize the coil, as it is a 12+ switched hot that comes from the ignition switch.

He gave me copies of schematics, and ran his finger all over the page explaining stuff, explaining along the way:
No O2 sensor to control, no job for the temp sensor accept the gauge. I explained I'm buying one of those fancy $300 LED gauge clusters and he said, "OK well, that's most likely gonna have it's own special input sensor" and that I've left the ECM void of at least 2 jobs. Controlling timing and reading temperature. Then he asked if I was going to use an EGR, IAC and all the other emissions sensors and I said, "No. Trying to keep it simple. He said, "there's more unemployment for that ECM" He finally asked why I was even keeping the "d*mn thing in there" and I replied, fuel pump... He said, "you can buy a toggle and some 12 ga. wire for that" I said A/C and he said, "ok yeah, for the compressor, that's 2 reasons" then I said, transmission VSS and temp switch and he said, "yup! Alright, leave it in then."

Then we got into the lockup mod I'm gonna have to make.

I'm trying to ... "hybridize" this thing to keep the things I like about OBD I and just get rid of the crap (emissions) I don't like, but, I'm also continuing what someone else started and trying to stay within a budget. . . which never happens anyway. I've never done this kind of work to an OBD I car before. All my other 3rd gens were either stock replacement work or I was working with electronic carb. (my 83 sport coupe and 84 RS) my 86 and 87 were both 6 cylinders and my 91 RS was TBI . I never had a TBI to carb conversion to deal with before. The block casting comes back to 1980-86 2 bolt so when I ordered the distributor parts and put it all together, I was thinking of the engine, not the car. The money is spent, I'm still barely under budget in the engine bay so I have to make this work and I want to thank you guys for confirming that pink wire and the fan. Truly appreciated.

91rs4life2

Guys, I wanted to bring a conclusion to this thread and tell y'all what I did. I ended up using that pink wire to BATT on the cap, white wire to TACH and it worked. The fuel pump was priming but not staying on because there's a circuit that is dependent on a signal from the oil pressure switch, (which was not an easy install. The headers crowd the oil filter area, hence the unit was literally 3 sheets of paper from the header. I was able to find a 135° brass angle, which required an adapter from coarse thread to fine, which allowed me to mount it up top next to the distributor). After all that was done, my ride started and stayed running.

I accidentally hooked up the blue wire to my cooling fan switch .. Because it fit... After realizing my mistake, I got the green one down there and tied the blue one up. Fan still runs under KOEO and I noticed my temp sensor (it's new) not moving the gauge. She's idled for a half hour and left to cool down to kind off loosen up any stiffness, burn out moisture, blow out any internal oxidation. I've timed her, set her fuel pressure at 5.5 per Edelbrock instruction. She misses when she's cold but, roars to life after she's warmed up (which I time since I don't seem to have a working gauge)

There's other stuff going on that'll get me off topic if I discuss it here. Minor quirks and kinks. Thanks again for your help guys. Just felt like I owed you a conclusion.

What have I gotten myself into? 🤣