RACER 1993

Just a quick question. I have 1993 LT1 Vette. Has anyone had a PROM chip fail without any indicators or codes? I work on industrial computers and PROMS just never seem to fail but I'm am not sure about their reliability in cars.

Installed new ECM. Installed 200 Ohm resistor at plug for ECT sensor. This is direct input to ECM. Digital dash reads 205 degrees steady. Car runs for 20-25 minutes and gets DTC15 "Coolant temp too low" . Does the ECM need to see a transition when the thermostat opens or the fans come on?

The sensor is not in the circuit any more so that only leaves the wiring, (which checks good with the harness unplugged at both ends) no shorts or grounds and the ECM with the old PROM.

I don't understand why it takes so long to get a "Code" either.

Any good lead would be appreciated. Thanks

SunCr

Why would you do such a thing? ECT controls fuel delivery. How do even start it - remove the resistor so it's on backup (which will generate the code)? Anyway, either the circuit is opening or the ECM is funky. Take your pick. There's no transition, etc.

RACER 1993

Hello,
I was just trying to fool the computer with the resistor. If its already been running and is warmed up I wouldn't think it would make a difference.
I appreciate your input.

I have already replaced the ECM, (I did have to use my old Prom) and its still doing the same thing. I have a new Prom coming in the mail so I can see if the old one is flaky. The ECT sensor is also new Delco.

As far as wiring goes the 5 volts has been there at the sensor every time I checked it and wiggling wires showed no change. The one thing I'm not sure about is whether I have found all the ground straps and cleaned their connections.
I found 2 chasis grounds right under the battery, found another coming out of the harness by the water pump. I cleaned these but I
haven't found a couple that the book shows on the back of the engine that are supposed to be ECM grounds.
I drove the car for about 20 minutes today and it ran pretty good after the SES light came on but the gauge read less than 180 and the digital readout never went above 194.
I was considering a hotter thermostat, (it has a 180 now).
Thanks again.

SunCr

Seriously doubt the PROM has anything to do with it. Diagnostics is a RAM function and that's on the Main Board.

Quick test is to jumper the harness. That should generate a 14 and if it does, the wiring is good. Inspect the connector and if you're sticking leads into an ECM Pin, make sure that there isn't an intermittent at the ECM connector (and given your description, this sounds like an intermittent). If jumpering the harness still generates a 15, go directly from the signal (yellow) to the Block. If you still get get the 15, the wire is open and/or recheck the ECM connection. You really should capture data and also note whether the code is an "H" (history) or "C" (current).

GM usually shares the CTS ground with the Air Temp Sensor and TPS ground circuits. Recheck codes to make sure that there aren't additional codes for these sensors. If so, high probability that the ground is open, even if only intermittently. You should also trace the ground from the CTS to the splice.

RACER 1993

Hello,
A jumper on the plug does generat a DTC14, and taking the yellow wire to ground results in a DTC15 again.

I believe you're right about the internittent. Everything checks out fine in the garage and then the road test brings back the code. The roads here in Illinois are the worst and takes some of the enjoyment out of driving a car like this.

I did open the harness far enought to get to 2 splices, a group of Blacks and a group of greens. Untaped them and inspected them, they both looked good so I taped them up again. I guess I'm going to have to unplug the ECM and pull the harness up away from the firewall and open it end to end.

I am a retired industrial electrician, I worked mainly on CNC machines but while in research I have built many harnesses, I work part time building test equipment. If I could get a good source for the connectors and lugs I could build myself a brand new harness in a couple of hours. I haven't been able to find any of these connectors for less than about $16 dollars each and who knows how much the ECM plugs would be.

Thanks for your help. Time to tear into the harness.

I don't have the stuff, but I am looking at purchasing Datamaster for logging a footprint on this car. EDAQ would have probably solved this for me 2 months ago.

RACER 1993

Got thinking about what you said about the ECT, TPS and MAP having same ground. The ECM has only displayed 2 codes through all of this trouble DTC15 and DTC14. Since it shares a common ground to the other two sensors if I was loosing the ground I should be getting codes associated with the others.

Sometimes when you are Too Close to a problem you can't see it. The yellow wire almost has to be the culprit. I will pull the ECM connectors and go over this circuit. The should be no splices in this wire.

SunCr

No - you won't get codes for the other sensors if the ground opens between the splice and the CTS.

Generating a 15 and having an accurate temp display is a bit curious. When that 15 pops, you should be seeing -32 degrees (and with a 14, something around 300). I'd check again and if the temp readout remains accurate that's either further proof that the ECM is bad or there's something odd like an intermittent short to ground on the Check Engine Light circuitry. You should still scan it for clues.

RACER 1993

Hello,
Well I tore into the harness today and found the ground splice for the ECT,TPS,and EOT. The connection looked pretty good. I did buy a new pigtail for the ECT so I decided to run new ground from splice to ECT. Cut wires back and made new splice. While I had the wires cut loose I checked them with DVM again the black and yellow all the way to ECM connectors. ECM was removed at the time and has never been backprobed. The ground splice for the sensors was in harness between #4 and # 6 injectors. Put everything back together and drove car for about 30 minutes before it died and set DTC15 again. I did get a copy of Corvette Fuel Inj. and engine management. Very informative book but only mentions DTC 15 and "Open" circuit.
When this thing goes to "SES" its pretty violent shaking for a few seconds and usually dies. Have you ever seen these computers latch the wrong code? My new Ford truck set a code that called for replacing a certain sensor and the dealer said they found the trouble to be another sensor and replaced that one instead. Took care of problem.
Note: I have replaced the ECM, the PROM, The ECT sensor, wired in new pigtail and cut wires at ground splice and crimped and soldered new. Interested in data logging but may take car to dealer for tech 1 evaluation soon. Feel like I have a 1 gallon gas tank and run out of gas every twenty minutes.