Wildblue19
I'm Awesome
BLUF: Scan tool shows 4WD LOW signal "ON" when in 2WD and 4WD High, and goes to "OFF" when in 4WD Low, essentially the opposite of what I would expect. Followed the service manual troubleshoot guide and everything appears to function correctly on the truck, mechanically and electrically.
Additional information on recent work done and issues that might be related:
VEHICLE
1993 GMC Suburban, K1500, 5.7 TBI 4L60E w/AC
BACKGROUND
2nd owner, very original truck, 1st owner replaced engine and trans after a collision around 120K with Goodwrench off the shelf crate parts, truck now at 190K with no rust and overall great shape. Got the community scan tool on my truck, historic DTC showed 43 (knock sensor), 67 (torque converter lockup solenoid), 81 & 82 (shift solenoid A and B circuit issue).
KNOWN ISSUES
1) 4WD isn't engaging on my truck
2) in the rain it will stall occasionally
3) shifts (esp 1-2) have been harsh under load
4) Main issue: 4WD LOW signal from PCM to scanner seems inverted of what it should be
DIAGNOSIS
1) 4WD engaging issue has been diagnosed due to the front actuator (electric) being dead; all wires and connections are good on voltage, resistance, continuity.
2) Stalling was likely due to a burned wire on the knock sensor that would contact/ground to the block in the rain. Replaced wire and shielded; sensor ohm'd correct at 3.9K and good voltage from the PCM.
3) Harsh shifts might have to do with DTC 67, 81, 82, or the 4WD low signal being inverted. I cleared the codes a week ago and none of them have come back since clearing. May be intermittent?
4) The reason for this thread: The scan tool shows 4WD Low "on" at all times except when in 4 low, at which point it reads "off".
Wiring Diagrams
Things I've checked based on these diagrams:
-Good grounds from Transfer Case to trans, block, and body. Good ground on transfer case switch pin B to transfer case in 4WD low; open in 4wd high and 2wd as expected.
-12v (B+) from PCM (both at connector at PCM and at transfer case switch) on DK BLUE in 2WD and 4 High. Properly switches to ground (0v) in 4 low.
-12v in run on BRN.
-LT BLUE gets 12v in 4WD high and Low; 0v at 2wd, as expected.
According to this, in my opinion the PCM is sending and receiving the correct signals. The only indication of an issue as discussed is the old historic DTCs which haven't returned since clearing, harsh shifts (which might just be 4L60E getting old), and the scan tool showing 4WD LOW signal "on" when it shouldn't. So....
Do we think this is a PCM or scan tool issue on the sensing of 4WD LOW? Or perhaps I left something out of my tests that you might have seen. Thanks!
WB
Additional information on recent work done and issues that might be related:
VEHICLE
1993 GMC Suburban, K1500, 5.7 TBI 4L60E w/AC
BACKGROUND
2nd owner, very original truck, 1st owner replaced engine and trans after a collision around 120K with Goodwrench off the shelf crate parts, truck now at 190K with no rust and overall great shape. Got the community scan tool on my truck, historic DTC showed 43 (knock sensor), 67 (torque converter lockup solenoid), 81 & 82 (shift solenoid A and B circuit issue).
KNOWN ISSUES
1) 4WD isn't engaging on my truck
2) in the rain it will stall occasionally
3) shifts (esp 1-2) have been harsh under load
4) Main issue: 4WD LOW signal from PCM to scanner seems inverted of what it should be
DIAGNOSIS
1) 4WD engaging issue has been diagnosed due to the front actuator (electric) being dead; all wires and connections are good on voltage, resistance, continuity.
2) Stalling was likely due to a burned wire on the knock sensor that would contact/ground to the block in the rain. Replaced wire and shielded; sensor ohm'd correct at 3.9K and good voltage from the PCM.
3) Harsh shifts might have to do with DTC 67, 81, 82, or the 4WD low signal being inverted. I cleared the codes a week ago and none of them have come back since clearing. May be intermittent?
4) The reason for this thread: The scan tool shows 4WD Low "on" at all times except when in 4 low, at which point it reads "off".
Wiring Diagrams
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Things I've checked based on these diagrams:
-Good grounds from Transfer Case to trans, block, and body. Good ground on transfer case switch pin B to transfer case in 4WD low; open in 4wd high and 2wd as expected.
-12v (B+) from PCM (both at connector at PCM and at transfer case switch) on DK BLUE in 2WD and 4 High. Properly switches to ground (0v) in 4 low.
-12v in run on BRN.
-LT BLUE gets 12v in 4WD high and Low; 0v at 2wd, as expected.
According to this, in my opinion the PCM is sending and receiving the correct signals. The only indication of an issue as discussed is the old historic DTCs which haven't returned since clearing, harsh shifts (which might just be 4L60E getting old), and the scan tool showing 4WD LOW signal "on" when it shouldn't. So....
Do we think this is a PCM or scan tool issue on the sensing of 4WD LOW? Or perhaps I left something out of my tests that you might have seen. Thanks!
WB
