So to make a long story short, I am having an issue where the dash will go blank when i try and start the car. We've been chasing this for 6 months, new battery, new bcm, new engine harness, new ecm,new fuse box, still fighting the same issue.
The only difference now is the car will try and start every now and again will run for about 6-8 seconds then shutoff. Car started doing this completely out of the blue. Drove it one night and parked it and the next morning it started this issue ever since.
In relation to this issue the fuel gauge will show zero, we have noticed that when the fuel gauge is reading properly it will try and start but it wont stay running. Has anyone ever had anything like this happen before? I'm so discouraged with this pos now im ready to have it fixed so i can get rid of it. Thanks in advance, to whomever can help me fix this I'll be forever in your debt!!!!

 

You sure fired the parts canon !!…a no crank no start can be a communication issue…have you hooked up a scan tool that can read all modules and not a cheap code reader ??…if you can’t communicate with at least the ECM, BCM, and RCDLR the car won’t start or if the serial data or high speed CAN bus wiring is compromised like shorted to ground or to power or to each other the car won’t start…if an open or high resistance you may be able to communicate to some modules…unfortunately 99.99% of general repair shops or dealerships have no clue how to diagnose communication issues…if its over your head seek out an auto electric shop…I would jump the load side (terminals 30 and 87) of the crank relay #43 just to take that portion of the starting system out of the equation…if you have a 12 volt test light and DVOM you can try to diagnose this yourself...as a new member we encourage you to fill out your profile with model year, trans,and all other information.

 

We did fire the parts cannon at it. We plan on putting an o scope on it to see what the can data is doing. What puzzled me is the fuel gauge showing 0 when it won’t start but you can play with the fuse box a little bit and the fuel gauge will come back “sometimes” and then the car woll
try to crank and start. We did pull the cover off the starter relay and jumped it and it will turn the motor over and the motor will even try to fire off doing it that way but it won’t run.

 

You should check the coil side of the crank relay to see if you are getting a signal…depending on your model year it is either ground or power side switched…I still have no clue what model year you have…you have a 12 volt test light ??

 

Can high and low and serial data in green 0-7 volts

Resistance terminals 6 and 14 with battery disconnected.

Can low terminal 14

Can high terminal 6

 

Sorry I put my info in my profile but I guess it’s not working. 2008 base auto car

 

OK, great but don't see it in your profile...only see male and you live in the US...well you have a 2008 Auto like myself so do you have a 12 volt test light ??...if so we can see if the run/crank relay is being energized as well as crank relay 43.

Circuit Description

When the ignition mode switch is placed in the START position, a discrete input is sent to the body control module (BCM) notifying it that engine start has been requested. The BCM then verifies that the brake pedal has been depressed with a discrete input from the brake pedal position sensor and that the key fob, with the correct code, is in the vehicle. After this information has been verified the BCM grounds the control side of the RUN/CRANK relay closing the switch side of it and allowing battery positive voltage to flow from the BATT 3 fuse through the RUN/CRANK relay switch, engine control module (ECM) fuse to the battery positive voltage side of the CRANK relay coil. The BCM also sends a high speed GMLAN message to the ECM requesting engine start. The ECM will then verify that the internal mode switch (IMS) is in Park / Neutral or the clutch pedal is fully depressed. After this has been verified the ECM then supplies 12 volts to the control circuit of the CRANK relay closing the CRANK relay switch circuit and allowing battery positive voltage to flow through the STARTER fuse, the CRANK relay switch to the S-terminal of the starter solenoid cranking the engine. Ground is supplied through the engine block.

 

i do not have a test light. But i can pick one up tomorrow!

 

You can get a cheap incandescent test light (not LED) from Harbor Freight...I rarely use an LED one...you have a scan tool (factory level or equivalent) and not a code reader ??

 

A good friend of mine has a super nice scan tool, weve used it to force the fans on etc before. When this issue started we could get communication with the bcm and tcm with the scan tool but not with the ecm.

 

Well it’s best to use that one and see what modules you can or can’t communicate with…there are data PIDS like “crank request”, brake pedal sensor and a few more you can look at while starting… read the circuit description above…if you don’t know how the circuit operates you can’t diagnose it..I’ll put a few tests below you can do with the test light below.

 

Thank you sir! Could the symptoms im having be a power related issue as well or would you bet your money on it being a can data issue?

 

With 12 volt test light connect it to ground and while probing terminal 86 the test light should illuminate when you press the start button..diagram is if you are standing over the fender…if no light we will move to the fuses at the BCM but before hook the test light to B+ and probe terminal 85…that is the ground for the relay coil…test light should illuminate if you have a good ground…to see if the run/crank relay is being energized we will hook test light to a good ground and probe either test point on ECM fuse at the BCM…when starter button is pushed the test light should illuminate…you can start there !!

 

You can crank the engine over jumping 30 and 87 at the crank relay so it’s not the “power” side of the starting circuit…you’ll have to check the “control” side which is 86 and 85 !!

 

If you have to play around with the fusebox to get the car to turn over it may be in the fusebox…the purple/white wire which is the starter enable relay circuit…X2 connector terminal E12 if you look at C13 in the schematic.

 

50-50 until you put a good scan tool on the car.

 

I really do appreciate all the help. Ive searched all over for people having the same issue and i can't find it. feel like I'm the only person to ever have this exact issue lmao

 

I highly doubt you'll find someone with the EXACT same issue...you will just need to diagnose a no crank condition and follow that "diagnostic process" we call it.

 

This is the Information Bus schematic for your 2008…if you can’t communicate to say the ECM or BCM you check that modules powers and grounds FIRST before changing it…the BCM and ECM are on the high speed bus and the RCLDR is on the low speed bus…those are the 3 critical modules to start the car…now I’ve played around with shorting the high and low speed busses to power or ground to see what happens…no damage to the modules doing this as the modules are fault tolerant.

 

trying to do this with one person is tough but i never did get a start signal from 86