Quote:
Originally Posted by Aberdare
Vic:
This is could be more than a simple ground issue. With Key On Engine Off (KOEO), the VSS should be a flat line on your oscilloscope. With Key On Engine Running (KOER), the VSS should still be flat line until put into gear and speeding up should begin to show a frequency change.
Checking the grounds is certainly not going to hurt... The Flashing Check Engine Lamp while the engine is running is a good indicator of a bad/missing ground somewhere. Are you using a NEW TCM? GM TCM's require programing/software download to function correctly, as they are BLANK (no software) out of the box. You may want to consult a local professional who deals in this sort of thing who can do a software load for your TCM.
Just pointing you in a new direction... hope it helps.
Abe
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Thanks, Abe. As you said, it never hurts to insure you have good grounds, so I added several more today to connect the trans case to the chassis/cab and to tie the instrument panel ground post to another spot on the cab. While I had the instrument panel out I went back and re-tightened all the posts on the gauges and checked resistance values. I also verified continuity from the VSS TISS sender plug to the TCU plug and from the TCU plug to the SPEEDOUT wire that should be carrying the variable frequency square wave to the speedometer. I don't have a schematic for the ECU and its harness, so I can't see how it connects to the TCU. Which means I can't verify those wires. Long story short on the grounds, nothing I did made a difference.
The GM TCU (TCM) I'm using came pre-programmed to run the 4L65E transmission. When I connect the laptop cable to the port provided on the TCU harness, it downloads the trans calibration and I can see the maps and it will allow me to input the rear gear ratio and tire size to adjust for them. With the engine running I can see vital statistics like rpm, line pressure, voltage, coolant temp, trans temp, gear selection and whatnot. What I don't see is any movement on the virtual speedometer with the truck in gear and the tires rotating, even though everything else works. All of this makes me suspicious of the TCU. If it's getting the signal from the sender on the tailshaft, but not putting out a signal on the SPEEDOUT wire and the harness wiring is all good, it would seem the issue may be with the module.
I have a friend coming by tomorrow with an expensive, high end GM scanner. He's very knowledgeable in its use, so I hope we can come up with additional info from the OBD2 port. I've also heard that perhaps a dealership can "bench test" the computers to verify their operation. Whatever we learn tomorrow I'll document, then chat with the GM Powertrain helpdesk on Monday to see what they may suggest as well. I agree that the noise I observed with the scope is out of line with what I'd expect. If the noise is coming from some other source, shielding might help as you say. I'm a bit swamped at this point, so before I head down that path, I'm going to gather some more data with the scanner. Stay tuned...