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An Easy Repair With A little Thought
Posted to Technical Discussion Forum on 1/22/2017 21 Replies

A few days ago, an Express cargo van was towed in. It was a 2016 with about 500 miles on it. The complaint was "vehicle stalls on road, will restart." It seems strange to tow in for a stall only. It was almost closing time but I thought I'd drive the vehicle inside for the night. I went out to start it and when I turned the key, nothing happened. No click, nothing. The IPC illuminated and the headlights were bright. The Service Writer said there was no complaint of a no crank. Of course, with a commercial vehicle, you're not talking to the actual driver.

The next day, I went out in the lot to look at the truck and it cranked strongly and started right up. I drove it inside and scanned it and saw this....

[2006 Chevrolet Express 2500 LS, ECM/Inputs/Outputs Scan Data]

No codes set in any module...strange with a no crank situation. I let it idle in the shop for 2 hours with no stall and then shut it down to heat soak for an hour. After this, I turned the key and no crank. The security light was not illuminated. I didn't want to open the hood yet or disturb anything so I looked at scan data first. Let's see what the Theft Module is doing.

[2006 Chevrolet Express 2500 LS, ECM/Inputs/Outputs Scan Data]

All these scans are with the key held in the crank position. The power mode shows a crank request being sent . Note the ignition voltage shows 0 volts. How about the BCM?

[2006 Chevrolet Express 2500 LS, ECM/Inputs/Outputs Scan Data]

The ignition switch data shows a crank request also. Let's look at the ECM.

[2006 Chevrolet Express 2500 LS, ECM/Inputs/Outputs Scan Data]

AHA. The ignition signal also shows 0 volts. The crank request shows yes, but the starter relay command is off. The most important clue is the engine control ignition relay feedback is 1.4 volts. Note several scan glitches...the oil pressure shows 7.5 psi and the VSS shows 158 mph. Scan data glitches are fairly common so be careful of what data is correct and which is false.

OK What can cause a stall and a no crank without setting codes? If the modules see no ignition voltage, they will assume that the key is off to shut down the engine and will set no codes. We show no ignition voltage in the TDM and ECM. G.M. has relabeled some relays. The engine control ignition relay is actually the run/crank relay.

During a start sequence, the ignition switch signals the BCM of crank or run mode and the BCM supplies voltage to the run/crank relay to actuate it. This supplies ignition voltage to several modules including the ECM. When the ECM receives this signal, it turns on the powertrain relay which powers up most components needed to run the engine. If the ECM does not receive this signal, it will not allow a crank and if it loses it while running, it thinks that the ignition switch was just turned off and shuts down the engine. neither case will set codes.

Our relay feedback is only 1.4 volts. I guess I could proceed with v-drop tests but the UBEC is buried on the Van chassis. It's easier to just swap the relay first. I swapped the A/C compressor relay with the ign relay and the vehicle cranked and started. I swapped them back and had a no crank again. At first, when I unplugged the relay with the key on, I listened for the relay to click then I realized that solid state relays are being used now and I wouldn't hear any noise.

In any case, the problem was only a defective relay and it was mostly diagnosed from the driver's seat with a little thought and a scanner.

James from Florida

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