No PCM readout to OTC scanner??

[1968 Pontiac]

I had trouble yesterday and today starting my truck. Today I thought I would plug in my OTC 4000E scanner to see if there was any trouble codes, etc. Well, lo and behold, I wasn't getting any readout from the PCM. First I thought that maybe the lighter socket fuse was blown, so I attached the scanner directly to the battery, but still nothing. Then I thought, damn, the scanner died, but I tried it on another car and it worked fine.

I don't have an electrical manual for the car, but could there be a fused circuit between the PCM and the diagnostic plug causing a problem?

One thing I did notice though when I had the scanner plugged into the trucks diagnostic port, it would idle really rough while attached to the scanner, and idle fine when I unplugged the scanner. This scanner worked fine on the truck about a month ago, so this is something new.

BTW, the check engine light on the dash comes on when you turn the ignition. And, once the truck does start it has an occasional slight miss at idle, but runs excellent at anything more than idle. The truck has new cap, rotor, wires, recent fuel pump, and complete new exhaust with edelbrock headers. Any help would be appreciated. Thanks.

[jdl]

Are you talking the 92 chevy truck? Did you try to access codes by using the jumper on the proper terminals at the data link connector? Will the lite flash? Did you try wiggling the wires on the back of the data link connector, with the scanner hooked-up?

OTC has a web-site, you might talk to them. Even if there is nothing wrong with the scanner, they might have some ideas.

[1968 Pontiac]

Yes, this is regarding the 92 K2500. Yes, I get access codes if I jump the terminals, the error code is 51. Not good, according to my manual that would be eprom/ecm problem. Scanner tested and is working; still no readout to scanner from truck aldl terminal. But, now that the sun has warmed everything to a balmy 28 Deg F, the truck starts like a summer day. Wouldn't start this morning or yesterday at 10 Deg F. At that time I was getting error code 15 and 33 when jumping the aldl terminals. Very strange.

[GMMerlin]

Code 51 is a terminal code (death of a module)
Make sure you have good connections at the module and that the prom is properly seated.
Check all powers and ground to the module using a volt drop tests.
If all that passes, time for a module

[1968 Pontiac]

I don't doubt what you're saying, as the module was my first thought. So, I'll run the tests you mention, and then have the module checked if necessary.

The GM manual for my truck does say that code 51 diagnosis is to first check eprom, if that's ok replace the ecm. I realize that manuals can have mistakes, and field experience is far superior to a manual printed before a vehicle has a chance to show it's faults, but could a bad module/module circuit cause the ecm/eprom not to send data through the serial wire and cause various different error codes?

The eprom is seated properly. I took it out yesterday and replaced it with one from a 89 truck to see if that would work. Well, my truck started, but the SES light was flashing like a strobe light. It looked like it was connected to a timing light!! So I switched the original one back. Of course I'll check again that the eprom is seated properly. I've also checked CKT 461 for continuity and it was ok.

[1968 Pontiac]

Well, went out this morning to start the truck, but it wouldn't start . Then I thought about GM Merlin's suggestions for a minute. I removed the air filter housing, then I grabbed the wiring harness at the coil, shook it a couple of times, turned the key and voila, the truck started.

Still no readout to the scanner, but no error codes when jumping the aldl terminals.

I checked the coil, pickup coil, and wire harness from the coil to the distributor, and all were fine. The only wire left to check at the coil is the thick pink wire. Where does it come from? The selenoid? Could it be grounding itself to the air cleaner housing which sits right on top of the pink wire? I don't know, but I've now taped up the wire at the connector area just in case. Funny enough, each time I had trouble with the truck not starting, it would start once I had the air cleaner housing removed. I thought it had more to do with an intermittant sensor/module problem causing the engine flooding then needing a less restrictive source of air to lean out the mixture ... but maybe not.