No Start Cadillac - Service Induced Failure

Our shop was contacted by a local auto service center owner asking if we would be able to address a no-start on 2008 Cadillac that was towed into their shop. The shop reported that they retrieved DTC U0073 and didn’t have any service information that would allow them to proceed. The customer reports that they parked the vehicle for approximately 30 min and when they returned, the engine failed to crank.

We accepted the job and had the vehicle towed to our shop. Upon arrival, our initial findings verified a no-crank condition and that the system battery voltage was at 4.2v. After charging and verifying that the battery was good, the no-crank condition continued. A full vehicle scan for DTC’s produced the following: (figure 2) 

With most of the DTC’s related to network communications; we felt that they were likely induced by a low voltage condition. We saved the DTC report and attempted to clear. Most cleared except for the power window learned position DTC’s which we can relearn later. The vehicle at this point was still in a no-crank state.

The next logical step for us was to consult service information. Looking at the factory diagram revealed that the starter solenoid power was controlled by the PCM via a relay located in the under-hood PDC (figure 3) (Source GM). We then proceeded to locate the relay (figure 5) and set up for circuit testing. My preference in this situation is to use the AES uActivate 07-80 relay circuit tester (figure 4) because it allows for quick circuit checks of both the primary and secondary circuits without the relay in the circuit.

On the high current circuit, (due to tool design) the light at the top of the tool should have been illuminated since the circuit is provided with B+ and the other side going to the starter solenoid windings to ground. But before we moved on to analyze the circuit further, we wanted to know if the PCM was controlling the relay. With the uActivate, all we needed to do was position the tool so it could be observed from sitting in the driver’s seat. A quick exercise of the dash-mounted start push-button produced a green control light. This confirmed that the ECM was requesting starter operation ruling out any inhibitors such as theft control or aftermarket interrupt devices.

Next, we went on to investigate the high current side of the circuit and discovered a blown 40-amp starter relay fuse (figure 6). Instead of replacing the fuse and possibly blowing a second, we used the uActivate to route substitute power (figure 11) through the tool’s built in switch which is protected by an on-board 15-amp circuit breaker. This resulted in the circuit check light illuminating as expected. We then we used the momentary switch (M-on) to attempt to exercise the starter. When we exercised the switch, the internal circuit breaker immediately tripped. This told us that we have something in the circuit is drawing too much current. Next we raised the vehicle to have a look at the starter connection.

Although the starter solenoid signal wire (Circuit 6) was properly seated into the solenoid socket we noticed damage to the wire which was in close proximity to the heat shielding wrapped around the harness (figure 7). We temporarily isolated the shield (assuming conductive material) from the circuit and rechecked starter operation. This resulted in tripping the uActivate 15-amp breaker again. Now we know we have a short either in the wiring or the starter so we then unplugged the starter solenoid wire from the starter. With an open circuit, the uActivate output light was shining bright. This told us that there indeed was a short to ground in the signal wire circuit between the relay and the connector we unplugged from the starter.

Next, we decided to step back and take a broader look at the vehicle. We uncovered evidence that the vehicle had been involved in a front-end collision at some point in its life and it was clear to me that some of the repairs performed were not as clean as they should be, IMO. Next, we began tracing the circuit from the starter and discovered that the signal wire joined a main harness that was routed up to the left front of the engine and then travelled along the left side cylinder head, around the back of the engine and then forward along the right side of the cylinder head, leading to the under-hood fuse block.

With the uActivate in sight, we began to manipulate the harness looking for the light to go out which would indicate where we were close to the short. As we moved to the back of of the engine, the uActivate light began to flicker. Moving on to the the right side of the engine, we began gently manipulating the harness until we could command the light on and off. What we found was that the wiring harness at the right rear of the engine passed inside of a support bracket. (figure 8) (Source GM) This should have been secured via special tie wraps which suspend the harness from the lower side of the harness support bracket. After lifting up the harness and inspecting with a mirror (figure 10), we could see where the wiring harness insulation had been penetrated by the support bracket. Here is a closer look at the bracket.

Next, we reconnected the solenoid wire at the starter and used the uActivate to exercise the starter, this action produced the sweet sound of a 12v starter motor in action. After replacing the blown 40-amp fuse and reinstalling the starter relay, we were able to bring the vehicle back to life. Once we secured approval for wiring harness repair, we repaired the wiring and secured the harness as designed.

Although the vehicles we see today have many sophisticated systems on-board, the basics should always be considered before seeking out the complex angle, which is not always an easy thing to do. I've beaten myself up many times in the past where I failed to break it down to what’s working and what’s not. For me, this is usually a result of trying to go too fast. Taking a step back and formulating a process one can use to help identify faults and their causes usually produces positive results. With the right service information, tools, and knowledge, this problem was solved with only a few parts and enough labor to ensure profitability. 

Thanks for reading.