Trouble code 12 and 15

I have a '84 Bonneville with the 305 V8( Chevy engine). It has about 140,000miles on it. The car idles rough(just started about 2weeks ago) and have dieseling( run on) sometimes, after I turn the ignition off. I get trouble code 12( diagnostic mode and also no RPM pulses to Electronic Control Module) and 15 ( Open Coolant Sensor Circuit). The check engine light have been coming on sometimes (off and on), for the last 4years. I am going to check the timing, dwell, and see if the plugs are getting a spark. Concerning 15, the car's temperature is fine. Any suggestions would be appreciated.
:confused:

The ecm is seeing an open circuit in the temp sensor which equates to 40 below, get this fixed first, bad sensor or wiring, it is the 2 wire temp sensor not the 1 wire for the gage.

Okay, I replaced the coolant temperature sensor, and the connector that attaches the wires to it. The computer no longer displays the 15 code. Now I have to figure out if the code 12 the computer is throwing out. Is Just the computer telling me, its in diagnostic mode, or there is a problem.

12 is the diagnostic recognition code, no problem.

Thanks MaxWedge.

'84 Pontiac Bonneville

12 No Reference Pulse
This incicates that the ECM doesn't see the engine's RPM, therefor it can't control the timing. When this happens, I belive it goes to pre-programmed timing table. This engine does have the HEI system, right?

EDIT: I assume your jumpering the ALDL to get codes.. How many times does 12 flash..3 or 4?

Good catch corning d3, your are correct.

Yes the engine has the Delco-Remy High Energy Ignition system. I use a special code key, made by Wells to connect the A & B contacts to obtain trouble codes. It will flash 12 untill I turn off the ignition, and remove the code key.

In this particular instance, since you are doing a key on/engine off test, the code 12 is nothing to be concerned with. Technically, Corning is correct, but for this example, Maxwedge is correct in that it is nothing to be concerned with. On an OBD1 vehicle, the only time a code 12 is a concern, is during a key on/engine running test with a scan tool. That would indicate no distributor reference signal, and the computer would default back to a pre-set value for the timing, essentially a "limp home" mode.

Thanks guys.

Geez I stand "uncorrected", kinda thought that's what I remembered!

"uncorrected", now that's funny!

You guys are great!