I've posted this message before to the forum, and all responses seem to point at the opti-spark, which now has about 57k on it. I haven't gone to pull it yet since its so damn cold here in Michigan, and my son has taken over the garage with his cars in various states of distress.

Anyway I've still been driving it about 100 miles per day to work and back. When it warms up it goes into its misfire, backfire & stumble routine above 2000 rpm. If I'm cruzing or rev it up a few times, it will set the SES lite and run fine till I stop it. Some times, the SES lite will go out by itself, and then the problem comes back.

Well I wanted to confirm the opti-spark, and see what code it sets, so I got a cheapee OBD1 code reader, its a Actron 9001..... makes the SES lite flash the code. When I follow the directions and plug it in, turn on the key...it does not turn on the SES lite, or flash it even. When I set it for the ABS codes (No ABS Brakes for awhile, just the inop lite) its the same thing, with no flashes for the codes.

So this evening (it got up to 30 deg. during the day) I did the simple diagnostic from the Helms manual for checking the "serial data line". This involves connecting an Ohm meter to the A and M ports of the data link connector. I disconnected the 3 modules connected to the serial data line, the PCM, Diagnostic Energy Reserve Module, and the Electronic Brake Control Module. This then showed an infinite ohm reading, or open circuit.

Next step is to reconnect each module, turn on key, and check for any voltage. I connected each module individually and got a 4.5 vdc reading for each, including the "black" connector on the PCM.

The way I read the manual says that if I get a voltage reading, then that module needs to be replaced.

Are there any other tests that should be done to confirm these modules are bad?
Can all three modules really be bad at the same time?
Should I even be trying to read the codes with this cheapee code reader?
Could the PCM be warming up, and messing up the opti-spark?

I hate to buy all three modules (probably $100 each), and find out that they are not it.

As usual any help is greatly appreciated!

'94 z-28, m-6, and now 217k

 

Nothing is wrong with your computer. That cheapie code reader will NOT work on your car. A 1993 is the only f-body that can use it and even then it is no more than a glorified paper clip. Sorry, but it is a simple waste of money. You must have a real scanner to look at trouble codes on any f-body after 1993.

Another item that can begin to fail when your engine gets hot is the Ignition Control Module.

 

I already replaced the coil and the ignition module last month (still does it). So i'll have to get a fancier scan tool? But the diagnostic routine I did on the serial data line was for naught also?

 

What procedure in the book were you using?

 

Shoebox is correct, there was rumor that it worked on early 94 cars, but I've seen no proof of it, I dont believe it works with the 94+ PCM at all. I went through the same ordeal myself before getting a cable, and using FreeScan.

 

I heard that (pleas don't let lightning strike me) that Autozone (there I said it in public) has code scanners and can scan your vehicle for free. They only tell you what the code says, not how to fix it. This could be a cheap way of getting the code without buying or waiting for a scanner. I use and love the Snap-on scanner but normally they ae expensive. I got mine from my dealer that repo'd it. they repo stuff all the time. Free from autozone, try it

 

I used the Helms service manual (Carline F for 1994 camaro/firebird). Serial Data Line Line Diagnostics 8A-50-2. So even if the scan tool wasn't going to work, the diagnostics would work out for checking the modules.

 

I actually bought the cheapee scan tool from Autozone. They only have the OBD2 scan tool for checking codes. I had contacted Actron to verify I had the right tool....the 9001 model for OBD1. No big deal, I'll just take it back.

Still back to original question, if I do the diagnostics on the serial data link with a volt/ohm meter, it tells me all 3 modules are bad?

 

hey sounds like im having the exact same problem as you..ill keep an eye on here..post what fixed the problem if you figure it out.
goodluck

 

I have not located tests like that in my 1995 manual, but I really think that if there was something wrong with the pcm, you would be getting a SES with a code (or at least, just a code) to tell you that there was something wrong. A real scanner is what you need to verify presence of any codes and would let you monitor the sensors to see if there is something else awry.

Info about your Actron scanner pulled off the internet:

GM Code Scanner
The CP9001 GM Code Scanner will retrieve engine and transmission trouble codes from most domestic GM & Saturn vehicles built from 1982 through 1992.
The GM Code Scanner can also retrieve Antilock Brake System (ABS) codes from certain GM Vehicles.
Plugs into the on-board computer.
Trouble check codes flash on dashboard "Check Engine" light.
Compatible with 1982 - 1993 GM vehicles
1994 and 1995 may vehicles require a Scantool in order to access the engine computer.

 

You still need the proper Scan Tool...
[8a-50-2 is to diagnose if the scantool,or serial data connector,
is working or not... ]
For 94, you need the proper scanner[OBDI GM Auto Xray, etc]