I'm building a daily driver 1991 RS with a 305 TBI. The previous owner installed a new (used) ECM. There is a large yellow socket that has nothing plugged into it. Is this supposed to have a plug in it or is it supposed to be empty?

Could this be the reason I'm not getting ignition power to my fuel relay (I can jump it and it works) and also the reason my fuel injectors aren't spraying?

 

Did the '91s get an air bag? I know the '92s did. With an air bag car a yellow connector is usually for that system. GM went with yellow for air bag systems. Tracers on the wiring, the molded connectors and so on.

They even changed one of the '7730 ECM (not yours) connectors from yellow to light green.

It may be that yellow socket is for the DERM (diagnostic energy reserve module). And the module is missing.

Back to the no power, check the in line fuse under the hood. It resides along the fender well on the passenger side. Heavy gauge wire that runs from the front behind the headlight to the passenger side fire wall.

That fuse provides power to the fuel pump relay socket and the ECM.

RBob.

 

Thanks Rob! I suspected the plug might not be running related (It does have an airbag, but the steering wheel is rough and may get replaced).

The inline fuse you refer to, is that supplying the constant 12v feed to the relay? We do have 12v constant and are able to jump the 12v to the pump feed and get it to run, however, there's no switched power. Am I correct in thinking that the oil pressure switch has no effect on the pump relay until after the car actually starts and generates oil pressure?

 

Yes, it is constant battery +12 volts. So I'd say as you do that the in-line fuse is OK.

That leaves the ECM, which provides +12 volts to the relay coil to operate it. The TBI ECMs use a small transistor that tends to pop. Easy to replace with a PN2907 or 2N2907. If you are handy with a soldering iron there are pictures of it on this board.

The oil pressure switch is a fuel pump relay bypass. Once oil pressure is above about 4 psi the oil switch closes and operates the fuel pump even without the relay. This is a back up to the relay.

RBob.

 

Can you post the link for those pictures? I'm not terribly handy but I've got some "people" who are.

 

It sounds to me like there is a 1227730 (or similar) ECM in place of where a 1228746 (or similar) ECM should be.

 

How does one know which ECM is the correct one? (This was the one that the PO installed as a replacement)

 

You know six_shooter, I never thought of that. I figured that the yellow socket was hanging from the harness. And the DERM module is in the area under the passenger side of the dash.

1991_RS, is this yellow socket on the ECM? Along side of the two black sockets? If so it is the wrong ECM, that is a '7730 used on port injected f-body's.

The proper ECM for your TBI set up is the 1228746 ECM. This can be found as the Service Number on the paper label of the ECM. Should only have two black sockets to plug into the car harness.

Link to thread w/picture:

https://www.thirdgen.org/forums/dfi-...swap-ecms.html

RBob.

 

I'll be checking out the ECM shortly. I'll post back what I find. I was beginning to suspect the VATS wasn't working, but reading your posts and the link you posted, I'm thinking the PO didn't know what he was doing.

 

UPDATE: The only number that's still readable on the ECM label is 7750

How does that compare to what's supposed to be in there?

Also, I found one that is supposed to be from a 89-92 TBI car but it also mentions PROM ANTT What's that mean?

 

Talked to a service manager who confirmed that I need the 1227730 (not that I doubted anyone - he just happened to call me back) but could not identify the 7750 on my ECM. He also did not know what the ANTT meant.

Also mentioned that I would need to swap out the PROM chip. How do I know if my current PROM chip is the wrong one? Are there other model cars with the same one?

 

TBI does not use the '7730. The proper ECM for your car, as RBob said, is the 1228746.

The '7730 was used for the TPI cars.

I've also not come across a "7750", is it possible that the sticker is damaged, and it's actually a '7730?

 

Dang, you're good! Upon looking at it with a magnifying glass (no joking!) what's left of the printing looks like a 7730.

That being the case, I might have a line on the 8746, however, is it just a "plug-n-play" situation, or will I need to swap PROM chips inside it too? If so, how can I check to see if my chip is the correct one for my car?

 

The MEMCAL in the '7730 will not be correct for the '8746.

The '8746 uses an EMPROM that is a 24 pin and is in a small holder. The '7730 uses a MEMCAL that is larger with more inside it.

The 4 digit broadcast code will indicate what the EPROM is for. There is an excel file on the moates.net site that lists most of the BCCs available and what their application was. It's a great resource.

I may have one at home that I can check what EPROM it has, if you get one that is not correct.

 

Thanks SixShooter. I'll let you know what I end up with. The guy that may have one is about an hour and a half drive from me and I won't be able to get down there until the first weekend in May.

 

Apparently, I have a 7730 ECM with a 7419 chip (J-car). Thanks to 34blazer for letting me know what the chip was.
The whole ECM is for sale, $35. Pictures are here:
https://www.thirdgen.org/forums/album.php?albumid=4446
send me a PM if interested.


*** Due to no interest on thirdgen.org, the ECM has been listed on an auction site ***
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AWNX is the BBC on the memcal

 

See - that's how you know we're a newbie with the F-car.
What BBC should I be looking for when I pick up the replacement one? Our car is a 5.0 TBI with the 700r4 trans.

And for the sake of parts store people, it has AC and black carpet

 

ANTT will work. The latest calibration is AXKT (auto, FED).

RBob.

 

Old ECM has been sold.

Thank you everyone for the input. We'd still be cussing and scratching our heads if you all hadn't replied!