good morning everyone,
i am new to the forum
I have a 1984 corvette that as soon as I start it up and until it gets to 170 degrees it is fine then it starts to shut down as soon as I put it in gear. It has an automatic transmission. Can anyone help me, thanks in advance

 

I bet the coolant temp sensor is going out, reading cold when the engine is hot. It would be easier to check it with a scanner. It that's the case then make sure you upgrade the sensor and connecter.

 

Open/Closed loop issue.

If you start the car, the ECM is in open loop operation, its only reading Coolant temps and deciding the fuel pulse widths based on that and throttle body position. It ignores other readings essentially. Once a timer expires and it sees temp readings hit the right range, it starts listening to all the other sensors. If some of them don't make sense to it, it can run like a dog or not at all.

The CTS sensor can be a culprit, but you really need to look at several things if it becomes a closed loop problem. If a code is set you need to pull it.

 

Thanks for the reply, the sensor is the one located behind the smog pump ? do you know the code ? do you know where I can buy it ?

 

Yes, the one behind the smog pump, threaded into the front of the intake. The original (1st design) can be very temperamental, as far as grounding/functioning properly....the go to upgrade for this, is this updated version, ACDELCO 213-52 kit. It gives the sensor a dedicated ground wire (instead of relying on threads) and a better overall connection of the wires/pins themselves. This example is from RockAuto (1984 Corvette, Cooling System, Temperature Sender/Sensor). Although some aftermarket brands are cheaper, I personally would recommend buying the Genuine GM/ACDELCO kit (includes sensor, pigtail and instructions) Hopefully this helps, good luck👍

 

You should get a OBD1 cable and/or scantool and pull the code or see if there is one before you buy the CTS sensor. Don't throw money at problems.

You should also have an 84 Helm Factory manual, you will need one.

 

The CTS was and is a 2 wire sensor and one side is not grounded. 5v is sent from the ecm and what comes back to it is what determines temperature.

 

hello everyone, I replaced the temperature sensor that communicates with ECM as recommended. Unfortunately, the situation has not changed. When the temperature reaches 170 degrees the inconvenience occurs that as I engage R or D gear the car shuts down. Among other things, the idle speed is also unstable. I have also changed the fan switch that activates at 210 degrees. By jumpering A and B on the odb I have no error codes ... only 12. What can I do?

 

ps: when i replaced the temp sensor for ECM no coolant came out while when i replaced the cooling fan switch a lot of it came out. is this normal?

 

Try unplugging the CTS and see what happens once car gets warm. It will be harder to start cold, but if car runs better once at operating temperature, you'll know that it is an issue with the CTS.

 

the same defect also occurs when it is at a temperature of 210 degrees when the fan starts. When cold the minimum is at 1500 rpm while when hot is 900 rpm and as soon as I put the R or D gears it turns off. Do you know if there is a sensor on the automatic transmission?

 

Now you have me confused. In original post you said it dies when it reached closed loop. You now say it actually dies once temp reaches 210.

 

I have never seen a one-wire Coolant Temperature Sensor. And I haven't seen a CTS on a C4 that didn't have a blk/white and a yel/blk wire. Blk/wht is Circuit 450: Data level Ground.

Um, one side of the sender is indeed "grounded". The blk/wht wire is a Ground. The 5V is supplied by the ECM on the yel/blk wire and the resistance of the sender changes according to temperature. Low Temperature = High resistance, and High Temperature = Low resistance. High resistance keeps the voltage high as the 5V can not be shunted off to ground. Low resistance allows the voltage to fall, as more of the 5V is shunted to ground. Ohms Law stuff. The ECM monitors this voltage on the yel/blk wire. High voltage = Low coolant temp, and low voltage = High coolant temp.

As long as it doesn't flood, it is easier to start cold because the lower the temperature, the more the ECM increases fueling. The ECM will interpret the unplugged sensor as its maximum coldest temperature calibration for fueling.

 

There is a Park/Neutral Safety Switch mounted to the side of the shifter. It has (3) sets of contacts: One set for the starter motor (Park and Neutral), one set for the Back Up lights (Reverse), and another set that is closed in Park and Neutral that tells the ECM that the transmission is not in P or N. The ECM uses this input for Idle Control, EGR, and Canister purge. The ECM wires on the safety switch are orn/blk and blk/wht.

You have some other idle speed control anomalies. (I hope that word translates correctly.) The high idle speed until it warms up, the shut off at a certain temp when taken out of Park leads me to suspect that you have a bad ECM. My personal experience has been that the ECMs I've replaced (not that many) have been for idle control issues. Read on.

The ECM has several routines for Idle Speed Control. One of the inputs is the P/N switch. When in P/N and closed throttle the ECM controls the IAC to a pre-determined RPM. At cruising speed, when the throttle is not at idle, the ECM commands the IAC open a predetermined amount so that if the throttle is closed suddenly, a stall won't occur. The ECM uses the P/N switch input to control the IAC appropriately.

Obtaining a 1984 ECM in Italy may be challenging. I would try this, as it will cost nothing. Remove the top plate on the center console. (You don't have to remove the shift ****, just lift and twist the top plate out of the way.) Find the orn/blk and the blk/wht wires going to the P/N switch. Make a little jumper wire with stripped ends and stuff them into the connector cavities for these two wires. Be sure stray strands don't touch adjacent cavities. Put the top cover down and try it. If the engine no longer turns off at 170F, it is probably the ECM. If the car is driveable with the jumper in place, you can drive it, but stalling with a sudden close of throttle may occur. EGR and cannister purge will also not occur, but these probably aren't a concern in Italy.

You may have a bad ECM. Try jumping the orn/blk wire to ground and see if it stays running when taken out of Park.

 

Thank you so much for your professionalism and patience with me. I am happy to be part of this forum. I will try to jump the orange-black wires with white-black

 

Hello, I tried to short the orange-black and white-black wires as suggested but unfortunately I have the same problem described before. I was wondering if the oxygen sensor could cause the defect.
What do you think?

 

Darn. It seemed like it would react to not knowing that it was not in Park. So it may be the temperature.

Jumper the diagnostic connector A and B. Start it up. Watch the Check Engine Light (CEL) and the temperature. When in OPEN Loop the CEL will flash at about 3 flashes per second. In CLOSED Loop it flashes at about 1 per second. As it warms up, watch the CEL. See if it goes into closed loop prior to 170F. It should.

Have you checked for Diagnostic Codes? Jumper the same A and B. Turn key to RUN. Watch the CEL. It will flash each code beginning with "12" which is flash----flash-flash,,,,,,,,,,,flash----flash-flash..
Each code is displayed 3 times then the next one. When Code 12 is again displayed, or if only 12 is displayed, there are no further codes.

Code 13 is Oxygen Sensor. Code 14 and 15 are Coolant Temperature Sensor Codes. If you have other codes, report back, or there are many sources online that have the code descriptions.

F - E - D - C - B - A
G - H - J - K - L - M

 

Good evening, as you suggested I put a wire between A and B and checked the CEL for alarms. It only shows 12 and I think there are no alarms recorded.
When starting the engine the CEL flashes quickly until the temperature shows 115 degrees F. then it starts flashing slowly. Does this mean it goes into closed circuit mode at 115 degrees F? What can I do? I was thinking of replacing the oxygen sensor...

 

12-only means there are no codes/alarms. It is going in to Closed Loop at 115 F. The oxygen sensor is likely not the problem.

Does it stay remain in Closed Loop (flashing slowly) until it turns off at 170 F?

When it turns off at 170, it must restart if it can get to 210 to start the fan. Yes? Is it easy or difficult to restart?
When it turns off at 210, is it always at 210, or is it always when the fan starts?
Try this, please: Unplug the fan switch. Watch the temp rise. Allow the temp to get to 220F, and if it does without turning off, that's a hint that the defect has something to do with the load of the fan on the alternator. Plug the switch back in to run the fan to allow the engine to cool before turning it off with the key.

Last question for now: When it turns off is it sudden, like using the key, or gradual like it ran out of gas/petrol?

 

Thank you for your precious help. Today I will try what you advise me to do. Originally the fan did not start because the afferent temperature sensor was broken. I replaced it and at 215 F the relay engages and the fan starts.
The car up to about 115 F has the minimum at 1800 - 1900 rpm and then slowly drops to 900 800 rpm as the temperature rises and then turns off, but if I put the gear in, at this stage, the rpm drop drastically and it turns off. I manage to restart it and bring it to the point where the fan starts but as soon as I try to engage both R and D it suddenly turns off.