GM Auto Start/Stop Fault
Hope get some other thoughts on this one. Vehicle originally came in with P0305 after an engine was installed due to customer driving into standing water, Tech had replaced the Aux battery (CB1). A month later vehicle was back with same fault code, tested primary battery (C1) with Medtronic's tester, test good. The customer was waiting and strapped for time and my supervisor got involved in trying to help diagnosis vehicle he ended up telling me to put a Dual Battery Control Module when customer returns to drop it off. Of course gut instinct told me this wasn't the issue according to what I understood from the GM code set description. Car is dropped of and I install the DBCM which is also the fuse box on top of the Positive battery post. Service info shows no programming needed for this module. I cleared the code and went on a long test drive. about 4 miles in and at 3rd auto start stop event the CEL is on after start up. Driving back to the shop I come to another stop start event that lasted a couple minutes, but this time the car will not start and had to be started with the key. All data provides a TSB PI#1495 for the issue of the no start from a auto stop start event in which the key needs to be used. TSB directs you to reprogram the ECM K20 which I went a head and did using SPS.
The fault code description talks about how the DBCM switches between primary and aux batteries at appropriate times to support vehicle load and charging. During engine crank the aux battery is switched to provide power to all loads ( except the starter) and the ECM monitors the Aux battery voltage through the IGN input.
Code setting conditions say the ECM sees the voltage during crank at the IGN input of ECM is below 8.9v for 2 out of 3 start events.
I decide to check the F7UA 5A fuse at the under hood fuse block that is fed power from KR73 Ign main relay and is IGN in put for ECM. I also check F18UA 5A which is fed power from KR73 relay and is power input to DBCM. Both check out to be intact. I take my U Scope and probe into pin 51 VT/GN at X1 ECM connector. Scope is set to 10v scale, 200 ms time base, horizontal cursor is set at 9v because code setting info says IGN input below 8.9v. Code is cleared from ECM, auto start stop data pids are brought up on scanner monitoring the following AT IDLE
System Malfunction- No
Low batt state of charge- 80%
ECM ignition Relay feed back signal-System voltage
IGN 1 Signal- system voltage
Auto start reason- system low voltage- No
Low Battery SOH- 99%
System Status- Running
During Test drive and start up after a stop start event and CEL illuminating my scope reads lowest cranking voltage to be 9.8v however this is what I get from scan data,
System Malfunction- No
Low Voltage SOC- 80%
ECM igniton Relay feed back signal- 7.6v
IGN 1 Signal- 9.6v
Auto start stop reason- System Low voltage- No
Low Battery SOH 99%
Power Mode- Run
Stop Start system- Engine Running
I trust the scope reading more than I do the scan data for the voltage PIDs, I have also checked to make sure the grounds underneath the primary battery are tight, I would think there would be multiple issues if I had a loose ground there. the primary battery has been tested again and shows to be good. Perhaps I'm over looking to check Voltage at the ECM from F10UA and F9UA fed by KR 75 Engine controls ignition relay. Your help is much appreciated.
In addition to the TSB you found, there was also TSB #16-NA-383: Information on Rough Idle, Crank No Start, Extended Crank or Misfire Due to Excessive Carbon on Top of Valves or Sticking Valves, Malfunction Indicator Light (MIL) Illuminated - DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308 Set - (May 8, 2020)
You said that it drove to standing water, so i'm thinking in water corrosion on the cables or the DBCm pins. That would make a voltage drop for sure
Testing directly at each battery, what are the main and aux battery voltages when sitting at idle? And refer to #18-NA-367.
13.6 at primary and 13.5 at aux at idle, blower on, headlights on, and radio on
What is the max voltage drop of the main battery when cranking?
With DVOM hooked up to main battery, rest voltage is 12.5volts, lowest voltage during cranking is 11.7v. I did a voltage drop from G 103 to BK wire pin 4 ground of DBM at idle and read 20mv
What is the voltage drop between the positive man battery terminal and the aux battery terminal when starting?
From the main batt pos post to aux batt pos post during cranking voltage drop is 70mv which looks like alot
It can be up to .5V so it is good. Now, Battery negative unhooked, with X1 of the ECM unplugged, ground pin 72, and take the ohms reading from battery positive to pin 73 and 82 and light a test light through the circuits. It should be 2ohms or less with a bright test light.
Further checking of other grounds I found the ground that hooks to chassis under main battery to transmission loose at the trans I have tightened it and will let you know after test driving.
After tightening the ground at the transmission bell housing that is the ground that goes to chassis under the main battery and test driving under multiple start stops events the code has not returned and my IGN voltage input pids on scan tool no longer drop from 12.1 volts to 9.6volts starting, instead they are 12.1 to 11.9 during crank. Lesson from this one is like it usually is.. don't over…
Hi Thomas. At least you found the fault and learned a bit too. In GM SI, following the Diagnostic System Check-Vehicle (DSC-V) from the Diagnostic Starting Point in the Quick Access tool bar, the preliminary fuse and ground checks are called out and would be the first steps to perform, since the history included engine replacement. While it's easy to see more clearly in hindsight, in reality…
Always a learning experience for me. I despise the trouble trees, how many times I’ve unplugged This and check resistance between x and y and it checks out. But it could have been helpful this time hard to say. This ground cable was just ever so loose that it makes me wonder if I would have found it with the ohm meter. Guess I’ll be doing some tinkering when I get into work to see what the…
Hi Thomas. FWIW, GM hasn't used “trouble trees” (flow charts) since the ‘80s. They moved to 5 column charts through the ’90s that provided more information than was available in the flow chart diagrams. Then came the current scripted diagnostic process. Unfortunately, many will skip steps because they don't like to read and perform measurements, a step is misunderstood or believed to be…