Hate to crosspost, but haven't gotten any response in 2 days in tech/performance section:
OK, been slowly going through all the systems. Now I am back to the distributor. Car runs rich still. Idle doesn't increase with AC, but it does dip for a second when AC is cut off and then it seems to hunt a little. Measured at the ECM terminals for the distributor outputs. EST signal on white wire should be 1.3 V running, I'm getting 2.25 - 2.35. Reference (purple/white wire) shows up at 2.56 V, manual states that it should be 1.3. Bypass wire (tan/black) has reading of big fat 0, should be 4.5V. I have replaced everything in the dist. but the pick up coil. Trying to put off replacing ECM until after everthing else has been gone through. Any suggestions or insight would just be outstanding at this point. Thanks.

 

Well, using this later model wiring diagram as a reference - http://temp.corvetteforum.net/c4/doc...j_wiring_s.jpg

1. The EST/DRP signals you are looking for are square waves. If you want to check them accurately you need an O-scope. A DVM will tell you if a signal is present, but that's about all you can infer from it.

2. The DRP is a square wave originating in the module & going to the ECM on the purple/white line (D13 in this diag). If the engine starts and runs, you are getting a DRP - you can stop worrying about that one and the pickup coil.

3. The EST signal is another square wave, orinating in the ECM (C9 here). Put a timing light on the engine - if the timing moves around when you rev the engine (for example if the timing mark jumps from 15-30* BTDC) then you are getting an EST signal, and you can stop worrying about it.

4. Whenever the engine is above 400 rpm, there should be constant 5 v (ref to ground D19) on the bypass line (C3). If that v is not present, there is either a wiring, ground, or ECM problem. BTW, your DVM needs to be on a DC scale to see that v.

What are you trying to diagnose?

FWIW - while a DVM is a useful aid (and you are applauded for using it to try to find a problem), ALDL scanning software like Diacom or Datamaster is a far more powerful tool to see what is going on - or going wrong - in the engine systems & ECM.

HTH




[Modified by DOCTOR J, 2:23 AM 9/19/2003]

 

Well, to start with Doc, you have no idea how glad I am to get an answer!! I used the the DVM based on schematics in the manual for the ECM terminal wire identification and gives expected voltage readings with ignition switch on and with engine running. Thought there must be a problem since my voltage on one is twice the given specs. The bypass line shows no voltage at ECM terminal or anywhere else down the line. Checked continuity from module connection in base of dist to ECM terminal and it was good (after rewiring four plug connector - someone saw fit to butcher the wiring before I bought it).I think that I can spell oscilloscope, but don't know anybody with one and am currently clueless as to how to operate it.
I am just trying to get this car to run smoother and not as rich. Plan to go on vacuum safari this weekend (have gone on several before, always new creatures to discover!). I am actually trying to rule out the ECM as the culprit for code hell. For example, Pull a code 14 more often than not. Checked wires for continuity (OK), voltage (correct for temp), tested CTS (OK) and then replaced it anyway, replaced connector at sensor. Yet it still wants to taunt me. The cooling fan comes on with the ignition key on run. Check shows cooling fan relay grounds from ECM. Still does it if you disconnect AC switch. Does not run if you disconnect relay. Could be a short to ground that I overlooked, but they're not booking that trip again until later next week. May be that the false Code 14 is triggering the ECM to create the rich condition. If EPROM was cracked, would it do this? I would like to have a good scanner, but no time to learn it/no money to buy it right now. Thank you for the response, and any advice or thoughts are welcome and appreciated.

 

Simply put: does this sound like the ECM is bad or is there another possibility? Ready to do what I have to, but not sure what that is.

 

You didn't say what ecm you are looking at, but generally for GM:
"Code 14 - Coolant temperature sensor signal indicates a temperature of over 266° F for 3 seconds."

If the CTS unit is working, the code means that signal must be shorting out somewhere. Without knowing what wiring diagram you are looking at, there is no way to know if the code is related to other problems you percieve.


WRT the cost/time to use a scanner: If you read the FAQ above, sources are listed for various kinds of FREE software that I think work on anything up to '92. A cable can be made to connect them to a laptop or desktop - I believe the 2-transistor cable (~$5) works with most of the free-ware. If not, the max232 cable can be made for <$25 in one or two evenings. http://temp.corvetteforum.net/c4/doc.../akm_box_s.jpg
Scanning software receives and RECORDS several dozen engine variables at the same time. That's faster than trying to do the same thing with a DVM & a pencil, particularly while driving.

Or perhaps there is someone else in SC who owns a GM scanner already, such as a local shop.

Without some engine data to go on, your guess is as good as mine. In my experience, one look at the data is worth 1,000 guesses. Good luck in your endeavors.

DrJ

 

Sorry, vette is an early 86, automatic. Code 42, 14 and occasionally 32 and 54. These recur even if sensors and wiring have been changed, not just checked. And the cooling fan stays on with the ignition on, unless the relay is disconnected. The ecm is stock for the year. As far as the CTS, measurement at the ECM pin and the sensor(yellow wire) match, so there shouldn't be a short I hope. Car runs rich at idle, fairly strong, but stumbles hard if you stomp the pedal and then resumes when the sudden load is taken away. The voltage measurements are straight from the diagnostic checks listed in the service manual for ECM terminals. I am quite the novice with the car computer situation, this is the first GM I've owned that had one. Broke my foot last night, so a little limited as to what I can do right now. I do want to get a scanner soon, but would like to drive the car some (only 400 miles since purchased over a year ago!) and not be embarrassed by it's antics. So, you see, I'm ignorant and impatient. I will try to search and become more educated about the software and scanners. Thanks for trying to help, I know that my rambling nature doesn't make that job any easier.