Coast Down Drivability Issue-1995 LT1

[ltxdave]

I'm having a hard time figuring out how to address this strange driveability issue with my 95 LT1 Corvette. When I coast down to a stop in gear with my foot off the gas, the car will almost stall when I push the clutch in and come to a complete stop. Reviewing my datalogs, I noticed that the IAC is going to zero as I'm slowing down. The datalog shows speed at 0 even though I am going 20 MPH. It looks like the divisor for the scan tool vehicle speed is wrong and the computer thinks the car is stopped, idling over speed, and closing the IAC to bring the idle speed down to spec. I have noted that the datalog will show me at 0 mph until I'm 20 mph or so and then it just jumps to some speed (below my actual speed). I thought maybe the VSS was bad, so I replaced it and the new one does the same thing.

I have adjusted the divisor for the SCAN tool and it will change the speed that the computer reads at 60 mph, but it seems to still skip over the lower speeds. I converted this car from an automatic to a 6 speed and the dash readout for speed is correct and works great. I'm using data from a 6 speed car for my scan tool divisor. Anyone know what I'm overlooking???

I'm datalogging with EEHack.

[lionelhutz]

There is a pulse delay for stopped in Tunerpro or Time between pulses for stopped in CATS. Maybe you've got it set too small and the PCM thinks you're stopped under 20mph. The tune I looked at was set to 38mS.

[steveo]

that's a weird one. the stopped vehicle value would have to be insanely low to actually stop at 20mph.

are you using a y-body t56 bin? i know the y-body transmission is 2 pulses per driveshaft revolution and the f-body one is 17....

[ltxdave]

The time between pulses value is 499.98 ms. This is the same as my 96 Corvette that used the same sensor/transmission.

I started with my stock y-body automatic BIN file and modified it to the y-body six speed settings.

I downloaded a stock y-body six speed BIN and cannot find any related differences.

[lionelhutz]

Does the speed signal pass through other electronics before the PCM on that car?

[ltxdave]

I will have to look tonight. It's possible that it goes through the BCM, but I would think it would be the same as the fbody and b bodies.

[steveo]

The time between pulses value is 499.98 ms. This is the same as my 96 Corvette that used the same sensor/transmission.

I started with my stock y-body automatic BIN file and modified it to the y-body six speed settings.

I downloaded a stock y-body six speed BIN and cannot find any related differences.

i'd recommend you try a 6 speed bin. i bet there's some unseen variable biting you.


The time between pulses value is 499.98 ms

wont hurt to try turning that way down. try 50ms.

[lionelhutz]

Ya, I'd start with the 6-speed bin too.

[ltxdave]

Well, it does appear to have been some hidden parameter. I modified the 6speed bin to match my auto program and flashed it last night. No more problem at coast down! Thank you guys for the suggestion!

[johnny_b]

Glad you got it figured out, flashing a 6speed bin is def the way to go.

I would add, and steveo has documented this on his site,

looking into the stall saver feature thats within the 6speed bins.

When coasting down my m6 camaro would surge and act weird, I noticed timing was all over the place when this occurred. after "tuning out" the stall saver the car is now rock solid when coasting down and feels much better.