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7/20/15 9:29pm - Original Message: 'CEL won't turn off'
foca1
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Joined: 7/20/2015
Posts: 7
so, my 1981 seems to have some sporadic CEL behavior. when cold, light turns off after starting, i can short the check pins under the ash tray, and i get a code 12 and can here the clicking solenoid. all good...

now, when i drive around for a while, shut car off, then turn key to on, the cel light stays on. further, i can no longer pull code. when i short the test pins, it stays on solid and there is no solenoid clicking, also no blinking. i'll start the car and drive, CEL does not turn off...

found a related post, but there is no reolution. guy said he is replacing the coolant temp sensor, then the thread dies.


http://www.c3vr.com/vrforums/check-engine-light-will-not-go-off-or-flash_message456462_page2.html

|UPDATED|7/20/2015 7:29:59 PM (AZT)|/UPDATED|
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7/21/15 6:28am - Reply: 'Re: CEL won't turn off'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
My first thought was something was shorting the Diagnostic Connector Link (DCL) where you short the A B terminals to get the light to flash.  But then if it got shorted with the car running, the CEL would flash at different rates depending on if it is in closed loop operation or not.
It's not doing that.  
 
That means the computer is probably turning on the light.  But with the engine not running and key on, it would still give you codes and it is not.  So you have a computer problem.  It may or may not be the computer itself.
You need to check ALL the power and ground leads to the computer.  You have more than one of each.
 
Some early cars did in fact have a problem with the coolant temp sensor.  If it has a round plug it is the original style.  If the plug is rectangle, it is the updated replacement style.
The original leaked coolant which wicked all the way through the wiring harness and actually leakes coolant into the computer.  Some were so bad the computer would leak antifreeze.  Coolant and antifreeze do not get along.   Sometimes you can open the computer case and use electrical contact cleaner to gently clean the computer boards.  If you have the coolant problem, and the cleaning won't help, then you have to replace the computer.


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On the "Tail of the Dragon"
(some day, no strike that, October 2008 it turned red, still in progress!)

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7/21/15 8:22am - Reply: 'Re: CEL won't turn off'
foca1
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interesting, i cant beleive it actually gets all the way to the ecu. wow.

so,i opened up the ecu, cleaned all terminals, removed and reinserted the EPROM. All looks good... no change in behavior.

the fact that it changes behavior leads me to believe its ecu related also. like a capacitor on the board being marginal and not working properly after it warms up from being on. before iget a new ecu, lets go back to the coolant sensor. maybe its doing something funny when it gets warm or has current running through it for a little while...

i have the one with the round connector. maybe there is something amiss with that part of the circuit, whether its the sensor or the ecu itself.

what sort of resistance should i be reading on those wires, when cold? how about at operating temp?
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7/22/15 7:58pm - Reply: 'Re: CEL won't turn off'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

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1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
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I will get the resistance readings for you as soon as I can.  Presently I am teaching a double shift, takeing care of the wife and the house.   Sleeping about 5 hrs a night.
 
None of that matters, but give me a bit of time and I will have the specs for you.


____________________________________
On the "Tail of the Dragon"
(some day, no strike that, October 2008 it turned red, still in progress!)

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7/22/15 8:21pm - Reply: 'Re: CEL won't turn off'
foca1
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incorrect, friend. THAT is actually ALL that matters... of course. no rush at all. if you can get to it, great. no worries
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7/23/15 12:52pm - Reply: 'Re: CEL won't turn off'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
ECT Resistance     (Engine Coolant Temp)
 
These are not all numbers, but a good sample to get you there
 
Temp F     Ohms
212           177
176           332
140           667
104           1459
68             3520
32             9420
-40            100700
 
If the sensor checks out, check the wires at the computer and see if they say the same thing. 


____________________________________
On the "Tail of the Dragon"
(some day, no strike that, October 2008 it turned red, still in progress!)

(click either image to see a bigger version)
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8/10/15 9:36pm - Reply: 'Re: CEL won't turn off'
foca1
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checked resistance, all good. thanks again for those figures so, ordered a ecu. will update soon
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8/11/15 4:54pm - Reply: 'Re: CEL won't turn off'
kstyer Lifetime Member
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CUYAHOGA FALLS, OH - USA

Vette(s):
1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

Joined: 12/2/2003
Posts: 6410
Be sure to check ALL the power and ground leads into the computer before replacing it.  To find them look at a good wiring diagram.

____________________________________
On the "Tail of the Dragon"
(some day, no strike that, October 2008 it turned red, still in progress!)

(click either image to see a bigger version)
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8/15/15 8:00am - Reply: 'Re: CEL won't turn off'
foca1
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Joined: 7/20/2015
Posts: 7
yes, had done that and looked good

new ecu is in, so far so good. let the car heat up, did several engine start cycles. shorting the test pins gets me the clicking solenoid and 12 code every time, so all clear on codes.

going to put some miles on it to be certain. fingers crossed...

|UPDATED|8/15/2015 6:00:18 AM (AZT)|/UPDATED|
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8/16/15 8:29am - Reply: 'Re: CEL won't turn off'
Adams' Apple Lifetime Member
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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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