88Form

First time posting, this has me stumped. Recently my engine started idling poorly and the check engine light came on. I tried put a paper clip between a & b(5&6) in the diagnostic port but the check engine light stays solid. No flashing codes.
I replaced the iac valve and it's running much better but I still have a check engine light and can't get any codes out. Has anyone encountered this?
I have a 1988 formula 305 tbi with 700r4.
Thanks

Tuned Performance

iTrader: (92)

If ground and command wire for ses has good continuity at aldl then sounds like a bad ecm.

88Form

Damn. I tried to bridge a9 to a12 on the ecm, blk/wht to wht/blk. Still nothing. Anything else it could be other than the ecm gone bad? It drives fine.
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88Form

I thought I replied already but I can't see it. I bridged a9 and a12 blk/wht to wht/blk at the ecm and still no flashing. Anything else to check before writing off the ecm? The car runs fine.

Tuned Performance

iTrader: (92)

Sounds like you’ve tested properly. Something internal in the ecm causing shorted lamp circuit.

Tuned Performance

iTrader: (92)

https://www.thirdgen.org/forums/engi...bi-t5-ecm.html

just use your chip.

88Form

I might have to buy one alright but first I decided to open the old one the guy before me had replaced. It looks like someone hooked the battery up wrong/tried to jump it wrong. Diodes cr 2 and cr3 are in a bad way.
Does anyone know what diode I would need to get for cr2? I can still read cr3 but cr2 is completely melted.
Hoping the ecm is still fine and whatever about driving with it, if I was able to get codes to flash it would get me over this hump.
Also if I were to open up the current one, where should I look for a fault with the diagnostics?
Thanks

Tuned Performance

iTrader: (92)

https://pcmhacking.net/ludis/c3xref.html#1228063

unfortunately no schematic for that ecm

RBob

iTrader: (1)

Ya, hooking up the battery backwards doesn't do those two diodes and a trace any good. Easily repaired (done enough of them). Every one I've encountered the ECM itself has survived.

Gimme' a few minutes to get my notes...

RBob.

RBob

iTrader: (1)

OK, here it is. CR2 is a common 1N4001 - 1N4007 diode. Pick any one in that line up.

CR3 is a Zener diode, 6.8V and either 3W or 5W. By the physical size of them I believe that GM used both power levels. Whatever was available at the best price.

And don't forget the short trace on the bottom of the board that acted like a fuse.

RBob.

P.S. Note that for the most part the 17 TBI C3 ECMs are all the same physically. So can use the '8746 schematics on all of them. One main area that varies among the PCB versions is the fuel pump drive circuit. Sore spot that GM worked on for a while in trying to make it more reliable.

RBob.

88Form

Thanks RBob,
It's odd how it happens identically every time, even down to that trace.
I'll see how I get on with repairing this one first but in regard to the current ECM in my car I might try opening that next and looking for any damage(particularly along the pin a9 trace). Have you any ideas of stuff to look out for that might cause the ses light not to flash codes?

RBob

iTrader: (1)

The ECM isn't powering up. Or the fuse for the SES/CEL light is popped. Or the q-driver that is used for the SES/CEL is bad.

Does the fuel pump run for 2-seconds at key-on, engine-off? If not check the 20A inline fuse under the hood.

There is also a fuse for the ignition that goes to ECM pin A6. Power on that pin tells the ECM when to run, which is does from the B1 & C16 battery inputs (fuse under the hood).

RBob.

88Form

The ECM currently in the car is fine, everything runs except ses will not flash codes, ses is solid. It only came on when it started idling poorly so I think it is genuine, and it should flash 12 even if there wasn't a problem.
What is the q driver for the ses? If that was bad would the ses not come on ever?

RBob

iTrader: (1)

A q-drvr is a quad-driver. IT is a chip in the ECM with 4 outputs(quad).

RBob.

88Form

I repaired the original ECM and after swapping it the check engine light went off and it flashes only 12, so we're all good. Except for the part where the ses is stuck on with the newer ecm. I inspected the board and saw nothing obvious will have to dive deeper. At least now I have a spare that will drive the car just no diagnostics

RBob

iTrader: (1)

There are two q-drvrs in the ECM: U14 & U15. Easy to tell them as the center 4 pins go to a heavy ground plane for heat sinking.

U15 is used for the EGR, TCC, and SES light. Most of the time they have physical damage to them, but not always.

Nat-Semi still makes a q-drvr chip that can be used, drop in replacement.

RBob.

88Form

I'm not seeing any physical damage to u15 but i'll try replace him. It has
delco 8820
274 z 60904
written on it. Is the DS3668 by nat semi/Texas Instruments the chip to replace it with?

RBob

iTrader: (1)

I believe that is the correct replacement chip.

RBob.