Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Hello guys
I'm working on a 1999 suburban 1500 with crank no start.
Few dtcs
P0122
P1122
P1361
Also code for fuel tank pressure sensor low voltage and bad MAF sensor
The truck was driving with all of those DTCs.
Fuel rail is full of fuel and #1 cylinder has spark.
I checked for rpm signal with scan tool and did not see any. So I focused on ckp. I backprobed the signal wire at the VCM and had my power probe reading Ovolt with sensor disconnected and 5v connected. I cranked the vehicle and could see the crank signal voltage fluctuate indicating that the crank is sending voltage but the scan tool is not showing any rpm change.
Also the fuel pump relay would go keep going on and off while in on position. The VCM would lose communication every 30 seconds or so.
I'm leaning towards a bad VCM. Your feedback is much appreciated
PS the ECT is at 300 F with key on. Unplugging it doesn't change data
Had one a few months back very similar results but no issues with the ECT. I pulled the VCM and found a diode corroded replace the VCM all was well.
That being said I assume no scope since you said seen the V fluctuate when cranking if this is the case then I would back probe the CKP and rotate the motor by hand to see what the volts are getting if I remember right the VCM wont read RPM signal unless it fully grounds and rises to 4.7-5v. If you do have a scope check your high and lows on it. i am pretty sure chevy is real picky on full ground and a bad sensor might pull down only to .02v instead of .0.
As for the ECT dont remember if its pull up or pull down but use a test light and ground the Power side and power the ground side you should see a change if circuit integrity is good.your safe useing a test light there.
Thanks for the input. Unfortunately I do not yet own a scope but it's definitely on my wish list. I've told customer to order a VCM ($85). Will install it and see what happens. The power probe showing variance in voltage while cranking shows that wire and sensor are ok. But VCM isn't processing that return
signal
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