Alright, here it goes. Car is a 94 Z28, LE1 Heads and Cam, long tube headers, ORY, Electric water pump, bigger throttle body and other supporting mods. Car has had these mods and been running great for at least 8,000 miles. I took it out for a drive one day, next day hop in it and it won't start. It cranks over fine but does not ground the injectors or send signal to ICM.

I've gone through the trouble shooting guides and have found that the PCM ground to the Opti reads less then 1 ohm with the key off, but when the key is turned to run the resistance to ground reads about 1600 ohms, clearly outside the 60 ohms or less it should read. My problem is trying to figure out what is causing this.

So far I have tried a PCM from a running 95 Firebird, also tried the ICM and coil from the same car. I've also tried checking the ground with the MAF, MAP, Temp sensor, and air inlet temp sensors disconnected. Car will not start and the resistance to ground is still high. I've been looking at schematics and it looks like the Theft Deterrent Module may also be on the same PCM ground wire. I have double check the PCM ground to starter are as well as all other grounds and they are good.

If anyone can give me some insight as to what to check next I would greatly appreciate it, I am pretty sure if I can get the resistance to ground down I will get my signals back from the opti to get the car running.

Thanks,
Bill

 

Have you scanned for trouble codes? Where your resistance readings the same with the other PCM?

 

No trouble codes when scanned. Resistance readings were the same, so ruled out the PCM. Also, when I turn the car to run, everything looks okay with a laptop, all sensors are reading, but RPM stays at 000 even when cranking.

 

Check the resistance of the PASSKEY chip in your ignition key, and go buy a the matching resistor (must be the exact same resistance), remove the kick plate, if yours is the same as mine, you should be looking for an orange wire (thin) disconnect that and put the resistor in series with the wire going back to the pcm.
If your passkey lost it's resistance, or somehow changed, your car will not start no matter what you try.
It is a long shot but I could be the issue..
Also, before doing the above, check the resistance of the wire(connector) to ignition switch, and connector to the PCM. If there is a short / open in one of the wires it would cause the same issue.

 

So, do you get signals to the ICM connnector pin B? (you said you did all the tests...)

 

Shoebox, I did test for a fluctuating AC signal on PIN B and did not get anything, well it says 3 volts for a split second then drops to nothing. I know I'm not getting spark, checked with a tester, also tried a new coil and PCM from a known good running car.

I will check from ignition switch to PCM, but the PCM is getting power and the car is cranking over, just no fuel or spark. I can also connect to the PCM and read some sensors plus get the VIN #.

This weekend I tried running two additional grounds from the PCM case directly to the frame and battery terminal. Key off I get 0 resistance, but as soon as I turn it on, I get around 500 ohms, this is what is confusing me...

 

I'm not convinced the ground is your problem. I would be more suspicious of the opti in this case. Got another one you could plug in and spin and see if you are getting your signals?

 

Unfortunately I don't, well I have the OEM that I had to replace after it died, so I know it isn't any good. I could hook it up and try it just for fun, can't hurt. I will dig it out and give it a shot just for giggles.

 

Scan it again. With no RPM signal to the tach and no AC voltage on the white wire to the ICM you should have codes for the optical cam position sensor in the Opti.

 

Fred, I will scan it tonight or tomorrow. I think I read somewhere that it takes X number of times trying to start the car or X amount of time of the car trying to start before it will throw a code. Is this true, or will it set a code right away?

Thanks,
Bill

 

Well I scanned the car this morning and no trouble codes popped up. Everything else looked okay, I will double check tonight and hopefully have an opti I can use to plug in and test the signals. At this point I really hope it's the opti so I have an answer and it's not too hard to replace.

 

Codes set based on operating conditions. Obviously, you aren't going to set a code for EGR flow if the engine isn't starting. But DTC 16 for the low resolution Opti cam position sensor signal sets when the engine is cranking, and the PCM does not find the pulse. I have never heard anything regarding a specific number of start attempts being required before codes will set.

 

Well, I am happy to report that I believe I have found the problem, and it is... Optispark!!! I retrieved my old optispark that I replaced a few years ago when it would intermittently misfire and die. I plugged it into the harness and turned it by hand, sure enough the injectors grounded and I got a good AC signal to my multimeter....

I pulled the water pump, harmonic damper and all other parts this weekend. It looks like the mode of failure is a leaky optispark / cam seal in the timing cover allowing oil to eventually leak into the distributor and kill the sensor. I could see the oil on the timing cover and when I disassembled the optispark I found some oil on the optical pick up and a little bit on the wheel. I'm glad I was able to diagnose the failure and will of course be replacing the seal before putting the new optispark on.

Thanks for all the help guys.

 

One more question for you guys. On this opti I want to mod it to use the vented design like the 95+. I already found the article on how to do it, my question is do you guys recommend sealing up the opti with RTV after I have taken it apart to lock tight the rotor screws? Not sure if it is worth it or not...