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Amedee, Cadillac Technician
Category: Cadillac
Satisfied Customers: 24141
Experience:  ASE certified Technician advanced level specialist. Wisconsin certified emissions state inspector
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I have a Cadillac CTS 3.6 2009 model. It has an intermittent

Customer Question

hi, I have a Cadillac CTS 3.6 2009 model.
JA: Thanks. Can you give me any more details about your issue?
Customer: It has an intermittent fault whereby it sometimes will not start. The PRNDL lights up but no letter is highlighted in red as it does when it does start. The instrument panel displays 'shift to park' and no moving of the shift lever makes any difference. At other times the car works normally.
JA: OK got it. Last thing — Cadillac Mechanics generally expect a deposit of about $18 to help with your type of question (you only pay if satisfied). Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 7 months ago.
Category: Cadillac
Expert:  Amedee replied 7 months ago.
Hello and welcome! Thank you for requesting me! I am currently at work now but get off at 6pm eastern time. Would that be ok if I helped you then? We can even talk on the phone if your willing to do so. Please let me know!
Customer: replied 7 months ago.
Hallo, I'm in South Africa. Yes, when you are ready, that will be fine. Online it seems that the system automatically attempted to subscribe me to just answer, I do not want this. How can I verify that I am not subscribed?
Expert:  Amedee replied 7 months ago.
I will check into it for you when I get home.
Customer: replied 7 months ago.
More background on the car - I bought the car about a year ago, the car had been in a RHS front smash, drivers airbag and tensioners had been deployed. These and the airbag module have been repaired / replaced. Two specialist auto electricians have looked at the vehicle previously and have been unable to get it started. No fault codes present at all. I noticed the power brake booster had previously been removed for repair and after disturbing cables under the dash on the driver's side (it's a right hand drive model) the P of PRNDL lit up in red and the car started immediately. Unfortunately the problem has reoccurred more than once and currently it does not work no matter how much I disturb the wiring under the dash.
Expert:  Amedee replied 7 months ago.

Hello and sorry for the delay! Let me start by explaing how the starting circuit works. When turning the key to the CRANK position, the ignition switch sends a signal to the BCM. The BCM then sends this signal to the PCM over the serial bus circuit. Then the PCM grounds or activates the starter relay. When this happens, the contacts inside the relay close and it then allows power down to the starter motor. Here is a wiring diagram for this circuit.

Expert:  Amedee replied 7 months ago.

So, not only we need to check for power down at the starter, we need to check the starter relay and inputs, we need to check the PCM as well as the BCM for codes. Are you sure that the body control module was checked for codes? If so, does it see the start command from the ignition switch?

Customer: replied 7 months ago.
Thanks Amedee, if the starter relay is manually closed it cranks the motor, so I don't think the issue lies there. Surely the BCM and/or PCM need a signal to ensure the car is in Park before a start command is issued? This is what's not happening. The PRND indicator is illuminated but P is not highlighted in red as it is when the car does start. Even when moving the shifter the selector indicator does not show which gear the car is in!
Where does the PRND indicator get its signal from?
Is it from the switch at the transmission or is there a switch in the shifter? Or both?
Can you send me a diagram of the PRND indicator circuit?
Again, when the PRND indicator shows the trans is in Park, the car works normally. When there is no indication of gear selected it won't crank or start, dash display says "shift to park" even though it's in Park!
Expert:  Amedee replied 7 months ago.

Hello and sorry for the delay! YES, that is correct. The ECM does need to see the position of the transmission in order to activate the starter relay. The part is called the internal mode switch. It is actually on the diagram I sent you above. However, here are two more diagrams of this circuit that may be better (show in more detail)

Expert:  Amedee replied 7 months ago.


Expert:  Amedee replied 7 months ago.

You should be able to check this using a scanner. With the transmission in park check the internal mode switch. It should read the following values.

Signal A Low

Signal B HI

Signal C Hi

Signal P Low

Should it read something different than this, check wiring side the transmission for short to ground or short to voltage. If all ok, then replace the internal mode switch.

Customer: replied 7 months ago.
Hi, the scanner is unable to connect with the ECU :-(
Expert:  Amedee replied 7 months ago.

This is the problem. Believe it or not, this is wny the engine is not starting. The computer is not "awake" with the key on. How can it activate the starter relay when it is not powered up?

Expert:  Amedee replied 7 months ago.

So we need to check all powers and grounds going into the computer (with the key on) as well as the BUS circuit to ensure the computers inputs are correct and that it is not being shorted out.

Expert:  Amedee replied 7 months ago.

If you want to check the ignition switch, check for battery voltage coming out of the switch on the yellow wire with the key in the CRANK position.

Expert:  Amedee replied 7 months ago.

If you want to check the park nuetral position switch, check for power going in on the yellow wire and power coming out of the switch on the purple wire with the transmission in park or nuetral.

Expert:  Amedee replied 7 months ago.

Here is a wiring diagram of your starting circuit.

Expert:  Amedee replied 7 months ago.

I am glad I can help!

It has been a pleasure working with you. Please dont forget to honor your good faith deposit by rating my answer as "Excellent Service" for assisting you today.

Please let me know if you have any further questions.

Expert:  Amedee replied 7 months ago.

Sorry about that. Please scratch those last few replies. For some reason my screen switched questions so those last few replies do not apply to your vehicle.

Expert:  Amedee replied 7 months ago.

The computer should have two powers and at least two grounds going into it with the key in the ON position. If you need help locating these at the computer, just let me know and I can send you some diagrams that will help. Also, the BUS circuit is the circuit that all of the modules talk to eachother on. IF one module or computer shorts out, then it can and will short out other modules on the BUS circuit. For example, if the ABS module shorts out, it can affect the entire BUS circuit , short it out and prevent the PCM from working so the vehicle will not start. The best way to diagnose this is to see which modules are and are not communicating by hooking up a scanner and checking each module for codes. Have you tried checking for communicating with any other modules besides the PCM? ABS, Air bad, instrument cluster, security module etc.. If you find one that is shorting out, you can disconnect from the circuit and sometimes it will allow the vehicle to start.

Expert:  Amedee replied 7 months ago.

This may also explain the goofy readings your getting on the instrument cluster. The cluster may be shorted out or the transmission control module (or mode switch) may be malfunctioning.

Expert:  Amedee replied 7 months ago.

I do work all day today and will not be back on line until late tonight. Please post your reply back on this page and I will respond after work tonight. Thanks and have a great day!

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