Just bought me a Stock '89 vette and it doesnt pass smog. Shop tech hooked up his snapon computer scanner and he says it is displaying multiple continuous errors. It shows stuff like coolant temp -40 degrees. Smog guy says he thinks it might be the wrong computer for the car but doesnt know for sure. btw, when i check the error codes through the dash it just reads 12, no hard codes. Anyone seen this before or have any ideas what i could check for first. thanks

Thanks for all the Info, bought the factory service manual, definitly a must have.

Resolution: It was the computer!!! I replaced it with a reman from Kragen. I bought it online at http://www.advanceautoparts.com/ for $89.98 + $140 core + tax and picked it up at a local Kragen store on the west coast. ps, the reman does not come with the chip, you have to use the old one from your old computer. thanks again to everyone that spent the time to help.

 

Something doesn't sound right-12 is proper. But if you had a problem you'd more than likely throw another code.That's not to say you can't have a problem without throwing a code. What was the reason they gave you for failing emmissions? Could be something as simple as an 02 sensor, or a faulty oil temp sensor,I've seen the latter go bad without throwing a code.

Is it stock or are there mods? Cats?

 

Based on the info from the tech, the first thing I would check is the ECT sensor. Can you get a more complete list from the tech? An incorrect ECM seems unlikely, but... You DO have the Factory Service Manual, right? If not, click HERE to order one.

RACE ON!!!

 

Just a shot in the dark but coolant temp sensor maybe. There are two I believe with different purposes. One controls when the fan comes on, and the other sends the info to the computer, I think. Its been several years since I fought this problem.

 

The coolant sensor acts like a choke and is what computer uses to enrich the mixture at startup. The -40 degrees is an open in the coolant sensor circuit and should set a Code 15. If the code is set, the fans will allways be on and even if it isn't on, the computer is going to think that the engine is cold, supplying addtional fuel and raising idle speed. Check for the reference voltage on the Yellow wire at the sensor connector. It should be 5 volts. If it isn't there, remove the passenger's hush panel to access the ECM. The yellow wire connects at Pin C10 and it's fairly easy to access as there's nothing in C9 or C11. Check again for 5 volts. If there, the wire is open between the ECM and the connector. If it's not there, the ECM is shot.

If the 5 volts is at the connector, check the ground circuit (black) by jumpering the harness terminals with a paper clip. A scanner should read 306 degrees and a Code 14 should be set. If not, the ground is open or it's the ECM. The ground is shared with the TPS and MAT sensor, so if those sensors aren't throwing codes (but maybe they are given what the mechanic found), you only have to trace the circuit to the splice in the harness. Otherwise, it terminates at ECM D2. A good ground will read less than .4 volts at the ECM (it really should be 0).

PS - just went through something similar on mine - only the CTS was toggling between 205 and 250, then sometimes working normally. I bought another sensor (they're only 5 bucks) and got the same results when I plugged it into the harness and both read really close on the ohm meter at 70 degrees. The wiring checked out and I was able to generate a 21, but neither a 14 or a 15. Replacing the ECM fixed it. There is only one ECM for the '89 which is shared with the '90. There are about 4 MEMCALS, depending on axle and tranny, but you old one should be fine. You can verify the correct MEMCAL by removing yours and comparing the # with those listed in the Parts Catalog at www.acdelco.com