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