working on a 94 P code motor

owner towed it to my house because it wouldn't start
"security" lamp operates when car is off, turns off while cranking, making me think VATS system is OK. Can't remember if VATS cuts ignition or fuel or both, it's been a long time since I worked on a GM.
has fuel pressure
has oil and full tank
checked every fuse (2 boxes under the hood, 1 inside the car)
verified cam is turning/valves opening closing
coil and module have voltage, replaced them anyway because I know what a PITA it is to change an opti on these motors, praying it was one of those two, no luck
C12 displays on DTC when I use the paperclip trick, ecu is probably OK
it chugs like all cylinders have compression
!!But i get no spark!!
I've poured over information on this for the last 2 days

Sit rep:
The same night it was dropped off after various attempts to start it and after checking all of the above I decided to check spark, my friend cranked the motor and it started right up on the 7 cylinders. plugged the spark plug wire back in, turned it off and started it up 4 or 5 times, drove normally for about four blocks and it died, no more spark since.

I am down to the opti, ecu, or bad wiring somewhere

*is there a way I can check if the ECU is getting a cam/crank pulse (RPM) with my multimeter or a generic scan tool I can borrow from Autozone/Advance? Maybe a way to display digital RPM with the paperclip trick? I want to know if the Optispark is delivering an RPM pulse to the ECM

*Is there any oil pressure/oil level cutoff type of system that would kill the spark? many people say this is a myth, I would love to see the wiring diagram. All the one's I've seen tie it into the fuel relay, not spark.

*What systems can cut the spark besides VATS?

*There is also a strange rectangular box, small (about 1/2 by 1/2 by 1 inch), taped to the wiring by the coil and module with 3 wires going into it, looks factory, no plug, the wires go directly into it, what is it? I suspect it might be some sort of noise filter????? I would like to test it if it is relevant.

*Autozone electrical system diagrams (which come from Alldata) also list a small box in the passenger side fenderwell by the blower motor/washer fluid tank, they call it a "TP sensor interface module", could it be related?


Please help, I want this thing out of my driveway :b

 

Having the exact same problem with my 94 Lt1 P (101k miles).

Just out of the blue, went to take it for a ride and it wouldn't start, thought maybe to low on gas, but added 4 gallons, and confirmed that wasn't the problem.

Have confirmed no spark, and was thinking the coil or coil module, so interesting you replaced those and it didn't solve the problem.

I have FSM and decision trees show ways of checking what you interested in, but busy today but might be able to scan/email some pages tomorrow -- so PM me with your email address.

Hopefully if we get your friends running, it will help me find my problem too.

BTW, I had presumed as did you that C12 (no DTCs) imply ECU is good, and thought a bad opti would give codes.

 

if the distributor cam sensor is bad, then the ecm assumes the engine has stopped and it's not going to look for codes. The best way to check would be to have some kind of digital rpm signal from the distributor. If it is 0 rpm you will knot the distributor is not turning or has failed.

The owner was in a hurry, so I decided the Opti is the most likely problem. I replaced it and everything is now perfect, I did not replace any other parts.

 

Hi keepinitoldskool,

Glad you got it fixed, will consider what you said about the ECM not getting any pulse and thereby not setting any codes as I finish troubleshooting mine - so thanks.

To follow-up on your earier questions and for future reference:

Question: *is there a way I can check if the ECU is getting a cam/crank pulse (RPM) with my multimeter or a generic scan tool I can borrow from Autozone/Advance?

Answer: from FSM book 2 - 6E3-C4-4 -- yes, but not directly.

warning: disconnect the coil 'gray' connector and jumper the 'b' power feed from 'gray' connector to the coil, so as to disconnect/protect the tach feed ('a' connector) and its 'tach filter' before doing any of the below.

The Distributor pulses (grounds) low and high resolution 5v feeds from its 'A' and 'B' connections and the PCM detects the 'pulse' (possibly also via the 'D' connector which grounds via the PCM -- I am also wondering if a very high ohm VOM could test the 'a' and 'b' feeds and detect the pulse, but not mentioned in manual so beware my thought as could damage the PCM).

The PCM then interpretes the distributor pulse and sends a AC feed to the Ignition (coil) control module via its 'd' connector.

Thereby, per manual "with DVM on AC scale, connected from ignition coil module connector terminal 'b' to ground, crank engine and observe voltage. Is voltage between 1 and 4 volts?". If no, then check distributor.

So above test is an indirect way to tell if Distributor's 'crank' position sensor is working.

Question: Maybe a way to display digital RPM with the paperclip trick? I want to know if the Optispark is delivering an RPM pulse to the ECM


Answer: not really. The 'tech 1' can display RPM, but its likely receiving feed from same place as Tach -- the coil. So if the PCM doesn't tell the Ignition (coil) control module to do something, then it doesn't tell the coil and thus no RPM signal.

Hope this helps for next time...

 

You CAN view both high resolution signal and low resolution signals from the Opti-Spark on a GOOD scan tool like a Tech 1 or my OTC Nemisys.

Parts store code reader probably would not even pull codes on a 94. Those code readers are OBDII 96-up only.

 

another forum member posted this a while back, it gives a good description on how our LT ignition systems work, and some very good detail on a decent troubleshooting Approach.

http://www.youtube.com/watch?v=lFCja...ature=youtu.be