Jay

iTrader: (2)

91 Camaro Z28 TPI 305 auto. Cranks but does not start.

The car was a mess when I bought it. Previous owners yanked the stock LB9 in favor of a 70s truck 350 with a carb.

Engine was junk, piston had a big hole blah blah blah. I sourced a healthy stock 91 LB9 and went about putting the car back to stock TPI set up.

Seems like there is no reference signal going to the ECM from the distributor.

I went through a bunch of threads here and step by step with the factory service manual.

Scan shows code 12 and that's it. When connected to Tuner Pro it shows fuel pump voltage for prime and while cranking. Spark checker shows spark from coil and to the plugs. The ignition control module is brand new.
No RPM indicated on Tuner Pro while cranking.

For the heck of it I did the no blinking injector light and everything came out good. Same result with 5 ECMs and 2 stock PROMs.

Security light comes on for bulb test but then goes out.
Tried with a "custom" PROM that was all stock other than unchecking the VATS flag.

If I touch the test light with 12V+ to the reference signal line with the 4 way connector unplugged from the distributor as outlined in the manual there's no RPM indicated which according to the manual means short or open or bad ECM. Incidentally the test light does NOT light up while doing this, however it does cause the fuel pump to prime and the injectors pulse once.

I don't know what I am missing.

Vader

Your diagnostic effort seems to be suitable. The real meaning of the "Code 12" is "No distributor reference pulses" which is exactly what seems to be the problem. Since that code exists every time the key is on and engine is not running, it is frequently misconstrued as meaning "System O.K."

Since the distributor is apparently generating spark, the reluctor and pickup coil are operating. Also, since applying signal to the 4-way connector (PUR/WHT wire) does start a fuel pump cycle and pulse the injectors once, the rest of the wiring and ECM are operating as expected. It would seem apparent that the distributor pulse at that point is missing, and since it is derived from the HEI module it may be a good idea to check the connections at the module and distributor, and verify the operation of the module.

Vader

This has happened before:

https://www.thirdgen.org/forums/tech...ml#post5420102

Jay

iTrader: (2)

In that link something that stood out was crank speed. I've got the typical 90-92 factory tach which is kinda all over the place so its not accurate at all. If the engine isn't getting to 400 rpm, then the ICM doesn't send a signal?
It turns over...but I'm not certain its spinning at 400 or higher.

Jay

iTrader: (2)

I connected my digital voltmeter to the reference wire (pur/wht) with it disconnected from the ECM and tried starting. There was definite "pulse" signal with voltage readings going up and down. I don't know what the readings should look like for the ECM to "see" the correct reference, but there is definitely a signal of some sort going to the ECM.

I did notice that my noid light does not illuminate with the injector pulse from the 12V+ test. It doesn't light with straight 12V+ going to it either... I'm wondering now if it just doesn't work and I'm basing diagnosis off faulty test equipment.

But the other thing is there is no RPM indicated in Tuner Pro.

Vader

Does the engine have an MSD or other capacitive or multiple spark distributor or ignition amplifier?

Jay

iTrader: (2)

No it's all stock ignition.

Turns out my noid light no longer works. Doesn't seem like something that would burn out, but it's dead. Worked last time I used it.

Not sure why I'm getting no RPM indication when scanning, but it now appears everything electrically speaking is working. The injectors are in fact firing. I tested them and they range from 16.6-16.2

It appears to be a mechanical timing issue. When it didn't start and I had spark, I put the noid light on and found I had no blinking light and assumed the injectors weren't firing. Frustrating when diagnostic tool failure sends you searching in the wrong direction.

rusty vango

its easy to check the timing chain and gears WITHOUT disassembly. do this; remove the distributor cap, with a socket and breaker bar on the crank bolt ,turn the engine first one way then the other. if the rotor dosent follow the crank movement exactly. its a sure bet that the timing set needs replaced.

Jay

iTrader: (2)

Timing set is brand new. Crank and rotor move in unison.
Fuel pressure is 42 lbs.
Triple checked for compression stroke and rotor lining up with #1 post. It looks to be dead on, close enough to be able to start any way.

I'm going to re do the wires... maybe I messed those up when running them under the headers. Going to try pulling all the plugs maybe they got fouled with all this no start.
New battery and check all the starter wire connections as well.

1987 T/A GTA

Did you ever figure out the issue?