Losing MPH input @ ECM
07-19-2012, 12:04 PM
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Losing MPH input @ ECM This is so strange and I could use some advice. When the ECM doesn't get a MPH input signal from the VSS it will not shift gears. Well, I finished a 6.0 / 4l60e swap into a 72 chevelle for a friend of mine and everything seems okay. I have driven the car around my streets for a few days and everything seemed to be okay. It was shifting good, sounds good, no serious issues. ( I don't have the gauges set up yet so in order to read my MPH I would use my OBD II auto scanner with the live feed mode and it would work fine ) ... Well, Last night I took it over to the dyno for a good solid tune and once again everything was working great.. we made 7-8 pulls with no issues and half way through the last pull the car down shifted and would no longer shift. Come to find out we lost the MPH input at the ECM. On my scanner it shows 0 mph when the car is in gear and running. I have no idea what happened, nothing is against the headers, no wires melted, we have continunity from the VSS sensor to pin 20/21 at the ecm and the vss pins have the proper signals coming out of it... The only logical thing to me is a malfunctioning VSS but I have my doubts a new one will fix it. Does anyone have any advice or input for this issue? Other then the ECM getting a solid MPH signal the car will be done.. Thanks in advance.. Nothing worse then being at the dyno and have something like this happen.
07-19-2012, 10:41 PM
#2
Check the wires for continuity from the PCM plug to the VSS plug, both of them. Then check for continuity to ground to make sure they're not shorted to ground. There's only 2 wires so it should be pretty cut and dry checking them.Moving the wires around can cause shorts to appear to come and go, so take that into consideration when checking them. I had a bad wire on my transmission and it took me a long time to track it down because it would temporarily fix it's self from time to time, but it was in fact shorted to ground. I ran a new wire and the problem hasn't returned. Good luck with it.
