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Discussion Starter · #1 ·
Hello,

Just started my 85 LeBaron today after replacing a blown head gasket and getting the head straightened and surfaced. Car runs AWESOME - never been so smooth. I'm experiencing a strange thing with the check engine light:
  1. Cold start (starts immediately)
  2. No check engine light
  3. Drive for 1-2 minutes, check engine light is very dim, slowly fades up (compared to parking brake which when on is BRIGHT)
  4. After ~5 minutes of driving the Check Engine light is on full brightness
  5. Car still runs great but the light stays on
I checked the codes and the only code is 12, which is expected and shouldn't (from what I understand) cause the Check Engine light to light up.
I also think it's odd that it slowly fades up instead of just coming on.

My questions:
  1. Anyone ever seen the slowly fading up Check Engine light?
  2. Does a code 12 cause the Check Engine light to illuminate?
  3. If #2 is no, is it possible the Check Engine light will be on and no additional trouble codes supplied?
 

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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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Welcome to Allpar. When reading fault codes with the blinking 'ck eng/power loss' light, the code stream will always begin with '12' and end with '55'.
Fault code 12 may or may not be real.
Check all engine compartment connectors and grounds that may have been disturbed, damaged or left off while doing the head gasket? There should be a couple of grounds at the rear of the head and one between the intake manifold and firewall. Check for bad connections first.
Because it illuminates slowly, it sounds like something possibly may have happened inside the PCM?
 

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Discussion Starter · #3 ·
Thanks for the reply. I've check the resistance from negative battery terminal to the various ground straps near the intake and they are all fine (~1ohm). The 12 code is legit, the battery was disconnected recently.
I'd still be interested in answers to my questions 2 and 3 if you or anyone else knows?

Thanks again. Car is driving great, just bummed out I have a red Check Engine light staring at me the whole time, makes me nervous.
 

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If your battery is marginal and its voltage drops too low during cranking can set a 12. As mentioned, there are around 5-6 ground connections. Theres one under the battery that corrodes. I would even check the power connection, it has a splice point under the battery as well. I believe that's for the processor.
 

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Discussion Starter · #5 ·
I'm hesitant to purchase a new battery unless absolutely necessary. I have observed the voltage to the computer drop below 11v when cranking (clamped on to +12 to computer and monitored on DMM when starting).

Does the following test seem like it could work?
  1. Turn car off
  2. Disconnect negative battery terminal to dump ECM memory and clear codes
  3. Reconnect battery
  4. Verify codes are cleared by doing the on-off-on-off-on dance
  5. Connect to my truck that is running with jumper cables
  6. Start engine
  7. Check codes
My truck will hopefully provide enough extra current to keep the volts from dropping during cranking and the codes should be clear (i.e. 55). If that works and I drive it and the Check Engine light comes on still, well... then I'm stumped.
 

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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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According to this, code 12 does NOT light the 'ck eng' light:
Chrysler ECU Computer Code 12
It will be stored in memory as the fault occurs.
I do think that something is happening internally with the PCM since the lamp starts to glow dimly and then gets brighter. I have only thought that the light comes on bright and stays on.
Does the light exhibit the same dim-to-bright behavior if you introduce a 'man-made' failure? Say, disconnect the TPS while it is running?
Have a helper perform this as you sit in the car and watch the instrument cluster for the light to come on.
 
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Thanks for the reply. I've check the resistance from negative battery terminal to the various ground straps near the intake and they are all fine (~1ohm).
First, 1 ohm would be excessive and cause problems.
Second, measuring resistance is not a sufficient test. You need to flow enough current to see if there is a voltage drop from whatever resistance is there. Turn on headlights and measure between negative battery post and negative cable clamp; same for positive terminal; same between negative battery clamp and any and all of the ground straps. Any reading over 0.3 volts is bad.
 
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For what its worth, I have seen multiple warning lights come on dimly with low voltage from a dying battery, but they usually stay dim. My vehicles always threw a code 12 every time no matter how long ago the battery was disconnected. If your truck battery won't fit the LeBaron, how about disconnecting it from the truck, disconnecting the LeBaron's battery, and connecting the truck battery to the car's cables with jumpers? Of course you can't drive it this way, but if the CEL behaves the same in the driveway, you could see if the battery was involved.
I think I'd also reseat all electrical connections that were moved when the head was done.
 

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Did you ever check for codes using the key dance? I agree with Imperial - a code 12 will not set the check engine or power loss light.

How old is the battery? If older than 3 years you may want to have it checked. It can have good voltage, but not handle a load.
 

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Discussion Starter · #10 ·
Thank you all for the input. Over the weekend I installed a temperature gauge ( to avoid overheating and blowing a head gasket again - how can this car not have a stock temp. gauge?!). While I was under the hood working with the wiring harness, routing my new signal wire, etc. I checked the various things mentioned on this thread. No smoking gun.

When I was done I fired up the engine and drove around to check the temp gauge and notice no more Check Engine light. I didn't change the sending unit, I wanted to see if the existing one would work and it is accurate (+- 10F from my external readings) The only difference is that the stock purple temperature wire is now disconnected in favor or my wire to my gauge. It's my understanding that this purple wire is the sensor that, along with the oil pressure sensor can drive the Check Engine light. So it would appear that the sensor was allowing some current to flow to illuminate the Check Engine light. That's my theory, at least.
 

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Thank you all for the input. Over the weekend I installed a temperature gauge ( to avoid overheating and blowing a head gasket again - how can this car not have a stock temp. gauge?!). While I was under the hood working with the wiring harness, routing my new signal wire, etc. I checked the various things mentioned on this thread. No smoking gun.

When I was done I fired up the engine and drove around to check the temp gauge and notice no more Check Engine light. I didn't change the sending unit, I wanted to see if the existing one would work and it is accurate (+- 10F from my external readings) The only difference is that the stock purple temperature wire is now disconnected in favor or my wire to my gauge. It's my understanding that this purple wire is the sensor that, along with the oil pressure sensor can drive the Check Engine light. So it would appear that the sensor was allowing some current to flow to illuminate the Check Engine light. That's my theory, at least.
The wire from the coolant temp sensor provides no power, it's just a variable ground for the gauge. Anyway, I hope you've found the issue.
 
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