Scanmaster shows knock when another scantool is plugged into ALDL port?

[Steve]

I am working on a friend's GN trying to help get some small driveability issues ironed out. He has a Scanmaster in the car and I have access to a Snap On Verus Pro scantool with graphing so I was planning to use the Verus to data log while driving the car. The Verus seemed to be working fine but I noticed that the Scanmaster read around 22 degrees of knock whenever the Verus was plugged into the ALDL port and as soon as I unplugged it the knock dropped to zero. My theory is that having 2 different tools pulling from the data stream at the same time is somehow loading the system down. Has anyone else experienced this. If it is just having 2 tools at once I will just install a switch on the scanmaster and use them one at a time. Thanks

 

[garyk1970]

I've seen this too...I've seen Cars run rough too...I have no idea why.

 

[davelostmissing]

Could the snap on be forcing the ecm into one of the other modes?

 

[Steve]

Could the snap on be forcing the ecm into one of the other modes?

I don't think so. The Verus does have bi-directional control but that was not enabled I was only using it to datalog the data stream. I am curios if it would work correctly without the Scanmaster being connected to the system at the same time and may try that but I want to modify the wiring in my buddy's car as little as possible.

 

[davelostmissing]

I would think if both together were overloading the ECM drive capability that the frames would be corrupt and show errors in many fields on both devices

 

[davelostmissing]

only connecting ALDL and ground signals to the snap on would eliminate the possibility of other modes, that requires the use of other pins IIRC.

 

[TLIMITED]

I am working on a friend's GN trying to help get some small driveability issues ironed out. He has a Scanmaster in the car and I have access to a Snap On Verus Pro scantool with graphing so I was planning to use the Verus to data log while driving the car. The Verus seemed to be working fine but I noticed that the Scanmaster read around 22 degrees of knock whenever the Verus was plugged into the ALDL port and as soon as I unplugged it the knock dropped to zero. My theory is that having 2 different tools pulling from the data stream at the same time is somehow loading the system down. Has anyone else experienced this. If it is just having 2 tools at once I will just install a switch on the scanmaster and use them one at a time. Thanks

I hooked my OTC 4000E up to get multiple readings on one screen and got the same thing on the scanmaster.Unplugged the OTC and scanmaster worked fine.I guess the scanmater is jealous and acting like a BITCH toward other scan tools.

 

[garyk1970]

I don't know about the pre-obd computer been ng bi-directional....but I have seen different modes for the ecm during scan tool sessions with several of my scan tools ....someone with a GM scanner of the era can shed some light on diiffernt modes?