Trouble code 15 CTS signal high voltage

Tiny
THESANDMAN
  • 1984 OLDSMOBILE CIERA
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 100,000 MILES

Rebuilt southern car throws trouble code 15. Replaced thermostat sensor as part of rebuild. Only other issue is high idle for couple seconds at startup and upon parking after driving. No idle issues during driving. High idle issue does not happen if I disconnect high speed control motor. Don't know is this is a separate issue or an indicator of an electrical/wiring issue.

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Saturday, September 17th, 2011 AT 3:13 PM

26 Replies

Tiny
SATURNTECH9
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So you replaced the coolant temp sensor already?If so what color were the wires going to it?Also do you have a multimeter?

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Saturday, September 17th, 2011 AT 4:55 PM
Tiny
KHLOW2008
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High idling at start up is normal as the engine computer is designed to increase the idling speed and then lowers according to engine operating temperature.

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Saturday, September 17th, 2011 AT 5:28 PM
Tiny
THESANDMAN
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To KHLow2008's response. That there is a higher idle at cold start up. I understand and expect. But it does not explain why it does it after driving and placing in park before shutting engine down. Engine is at normal operating temperature and a high idle would not seem necessarily need to engage (even for a few seconds). To restate, neither situation happens if I unplug idle speed motor (but that seems to defeat the purpose of the control).

To saturntech9, yes the coolant temp sensor has been replace and the male plug (to the sensor) that came with it had two black wires coming out of it(at least the plug had been replaced sometime before I got the car because it was wired into place), but I will get the color of the wires coming out of the wiring harness tomorrow when I get the time to go to the car. And yes I have a multimeter.

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Sunday, September 18th, 2011 AT 2:09 AM
Tiny
SATURNTECH9
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Alright get me those colors we could have a issue with poor connector replacement causing that code. Keep me posted with the colors and we will start tracing wires. Your scanner doesnt read live data on your car does it?

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Sunday, September 18th, 2011 AT 2:24 AM
Tiny
KHLOW2008
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When the engine is shut off, the ISC would be be stopped at the cold start position. The computer does not have any idea of the condition of the engine operating temperature and when restarting, the computer would normally increase the air fuel ratio for a few seconds until it gets the the required data from the various sensors.

Tuen ignition switch on and wait 2 seconds and strat to see if problem persists.

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Sunday, September 18th, 2011 AT 6:05 AM
Tiny
THESANDMAN
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To saturntech9: The wires coming out of the wiring harness are solid yellow and solid black (or an extremely dark color that appears to me to be black). I have continuity from both wires from the wiring harness to the plug. I sent my meter on 12v and had a reading of 2.5 from the yellow wire out of the harness and it the plug. No reading from the black wire.

To KHLow2008: Turn ignition key on for several seconds but no change in cold start up (increased rpm's for a couple of seconds before returning to normal idle), the car is parked in a garage with a 60 to 70 degree temperature range. Did you want me to run car to normal operating temperature and test to see if idle increases upon parking the car using your 2 second wait time on start up?

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Monday, September 19th, 2011 AT 11:42 PM
Tiny
SATURNTECH9
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Does your scan tool read live engine data?I think the high idle could very well be related to the code 15 that is being set. So if we track the code 15 down it just might fix the idle concern.

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Tuesday, September 20th, 2011 AT 12:02 AM
Tiny
SATURNTECH9
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Here is the trouble tree for the code 15 let me know what you find.

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Tuesday, September 20th, 2011 AT 12:07 AM
Tiny
THESANDMAN
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When I click on thumbnails they are obscured by the message reply box. Can you send them to my email address?
Sanfordgray@comcast. Net

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Tuesday, September 20th, 2011 AT 12:18 AM
Tiny
SATURNTECH9
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I emailed you the trouble tree let me know what you find.

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Tuesday, September 20th, 2011 AT 12:38 AM
Tiny
THESANDMAN
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Okay, I'm confused, but here is what I found. To access the sensor wires on this 3.0L engine you have to move the alternator out of the way, so I took the alternator belt completely off removed the alternator from its brackets and I hung the alternator up out of the way, leaving all the wires connected to the alternator.

I hooked up my meter on the 12v setting, connected the red meter probe to the yellow wire and grounded the black meter probe to the chassis. I set the meter so I could observe from the driver's seat. Turned key on without starting engine and meter read.765v. Started the car and meter stayed in a range of.765v to.767v. Observed meter while car ran to normal operating temperature and the range still remained in the range of.765v-.767v.

My observations were that car "did not increase" rpm's (as in the past) at startup, nor did the check engine light come on at any time. I have car off the ground and I allowed the transmission to operate at a slow speed and placed it back in park to see if rpm's would increase (as it did in the past). Rpm's did not increase then either.

The only conclusion I can initially draw is that the alternator or its wiring is causing the problem somehow.

When I replace this engine I rehooked everything as I originally found it (I take pictures to assure this). Some of the wiring to the alternator had been spliced in and is not the original wiring. I just rehooked the wires in their same positions that I saw in the pictures I took before disassembly.

Could there be a wiring screw up from an "engaged" alternator that is causing the extra voltage? Or is my present reading of.765v an indicator of something else? I see that I wrote my initial voltage finding from the yellow wire was 2.5v. I don't have an answer to why the difference now in my two voltage readings from the yellow wire. As of this moment, the voltage remains steady at around.765v.

Just to let you know. The wires to the alternator are as follows, the "male-plug" coming from the wire-harness that is from the starter side of the motor is serviced by a red and black (or dark brown) wires, the alternator "post" has two red wires to it, one coming from the wire-harness from the starter side of the motor and the other coming from the passenger's firewall wire-harness area. For a total of four wires. The wire from the alternator to the firewall side wire-harness has obviously been replaced at sometime, the rest seem original.

Also tested sensor ohms and they fell within the range found in the trouble tree chart you emailed me. Thanks for sending the chart by email, I couldn't read it on line.

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Friday, September 23rd, 2011 AT 11:48 PM
Tiny
SATURNTECH9
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If you follow down to the left side of the trouble tree of the code 15 I emailed you it says warm the engine up to normal engine temp. Then key on engine off unplug the coolant temp sensor then take the meter and measure between the black and the yellow wires on the coolant temp sensor terminals and see what the voltage is. Lets follow the trouble tree before we make any assumations. That will lead us to the problem with are coolant temp sensor code issues and I think that it will most likely fix the car once we follow that trouble tree down. Let me know what you find.

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Saturday, September 24th, 2011 AT 5:01 PM
Tiny
THESANDMAN
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Engine at operating temperature, ingition on and engine not running. Unhooked plug from sensor and check voltage between terminals and have a zero voltage reading.

Sorry, had that information but neglected to include in last message.

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Saturday, September 24th, 2011 AT 5:28 PM
Tiny
SATURNTECH9
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Now you need to go to the right side of the trouble tree and and check the two circuits from the connector to the pcm.

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Saturday, September 24th, 2011 AT 7:45 PM
Tiny
THESANDMAN
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Never done this before. Could use a little help on what to do and look for.

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Sunday, September 25th, 2011 AT 12:50 AM
Tiny
SATURNTECH9
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Are you sure you got the meter leads in the terminals to check the voltage across the two terminals?

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Sunday, September 25th, 2011 AT 4:48 AM
Tiny
THESANDMAN
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From the nature of your reply/question it has me wondering if I checked the (what I thought to be) correct terminals. So let's start there. Where are the terminals that I should be checking the voltage located? And then if the voltage is under 4 volts then where do I find and how do I check to see if the circuits are opened or closed? (I believe I know the answers, but humor me anyways. I am more a pre-80's car guy.)

Sorry for any inconvenience that I am putting you through.

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Sunday, September 25th, 2011 AT 10:54 AM
Tiny
SATURNTECH9
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You would check the terminals inside the coolant temp sensor connectors you would check the voltage across those two terminals engine normal operating temp key in the run position. The reason I ask isg because some of the coolant temp sensor connect terminals have small resessed terminals that the meter tips wont fit in.I just dont want to go the wrong diafgnoses direction. Let me know and we will go from there.

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Sunday, September 25th, 2011 AT 3:43 PM
Tiny
THESANDMAN
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Okay, we are on the same page on the voltage test, which again was zero volts. But the next test I know I need assistance with.

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Sunday, September 25th, 2011 AT 6:11 PM
Tiny
SATURNTECH9
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There is going to be terminals C10 and D12 on the ecm connector the colors of the wires to the those terminals will be the same colors as the ones going to the coolant temp sensor. You match the colors up at each terminal end and see what the ohms reading is from the coolant temp sensor terminal to the ecm terminal end. The ecm is behind the right side of the of the instrument panel. Let me know what you find.

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Monday, September 26th, 2011 AT 2:12 AM

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