My car is street driven and was having the engine stall about 70% of the time when cruising in a mid to upper gear when pushing in the clutch. I also was having some slight bucking issues when creeping along at lower speeds – such as driving in traffic let’s say. I have the GM perf. LS376 (480hp) crate motor with the GM spec’d ECM and wiring harness for this engine.
After a bit of web research I found my answer, and hope this may provide others with a solution if encountered. There is a contingent of people that say it doesn’t matter and you can tune around it, but I believe they are mistaken. If you have a track car – this likely won’t matter. My Porsche 96.50 box does not have a VSS sender. There is/was a unused harness plug that is named “VEH SPEED”. The instructions that came with the GM ECM/Harness bluntly states that no VSS signal is necessary to run the engine. They are correct... the engine will run & drive without one – albeit with the above noted issues. Here’s a couple quotes from the WWW that nailed my issue:
The biggest problem I've seen on swaps with no VSS is idle instability and/or stalling after coasting to a stop.
Picture a car coasting down hill with no VSS. The PCM sees 0% TPS but high RPM. Since there is no VSS the PCM doesn't know the car is moving; it just thinks the idle speed is high and needs to be brought down, so it runs the IAC all the way in. Then when you finally stop, the IAC can't always recover quick enough causing a crazy speed fluctuation and sometimes a stall.
VSS - If using a non electronic trans, You should KEEP a vehicle speed sensor regardless of what transmission type you use. The ECM needs to know if vehicle is moving. Without the speed sensor, you can suffer from reduced fuel economy, reduced power, and idle problems."
"The VSS has two primary functions for our ECM's. It has a basic idle control function where if the VSS says the vehicle is sitting still, and there is no throttle input it will tell the idle to go into closed loop (where the IACV is used to control engine RPM directly) Otherwise, Idle will be in an open loop mode known as Commanded airflow, and it will also use the throttle follower. Without the VSS, the computer won't know which mode to control the idle with. This induces stalling in some situations.
Now, if the ECM is not receiving a VSS pulse you would/may have some stalling problems. It will always try to control the engine down to an idle when your throttle is closed. If your engine is being spun up by the transmission like when you let off without pressing the clutch, the idle valve will close. Then when you press the clutch the engine will stall almost instantly since there isn't enough time to open the idle valve and recover. So normally, a VSS is required."
DD also makes a GPS unit that can programmed to put out a sine wave but it’s pricy and does much more than I need: Dakota Digital Speedometer GPS Interface Modules GPS-50-1
So the outcome of this all: my car didn’t stall once today, runs much smoother at small throttle openings and the exhaust “note” doesn’t burble away when off the gas.
Attached are several photos of the above noted pieces and my install to the G96.50 trans-axle.
Regards – M. Shark
After a bit of web research I found my answer, and hope this may provide others with a solution if encountered. There is a contingent of people that say it doesn’t matter and you can tune around it, but I believe they are mistaken. If you have a track car – this likely won’t matter. My Porsche 96.50 box does not have a VSS sender. There is/was a unused harness plug that is named “VEH SPEED”. The instructions that came with the GM ECM/Harness bluntly states that no VSS signal is necessary to run the engine. They are correct... the engine will run & drive without one – albeit with the above noted issues. Here’s a couple quotes from the WWW that nailed my issue:
The biggest problem I've seen on swaps with no VSS is idle instability and/or stalling after coasting to a stop.
Picture a car coasting down hill with no VSS. The PCM sees 0% TPS but high RPM. Since there is no VSS the PCM doesn't know the car is moving; it just thinks the idle speed is high and needs to be brought down, so it runs the IAC all the way in. Then when you finally stop, the IAC can't always recover quick enough causing a crazy speed fluctuation and sometimes a stall.
VSS - If using a non electronic trans, You should KEEP a vehicle speed sensor regardless of what transmission type you use. The ECM needs to know if vehicle is moving. Without the speed sensor, you can suffer from reduced fuel economy, reduced power, and idle problems."
"The VSS has two primary functions for our ECM's. It has a basic idle control function where if the VSS says the vehicle is sitting still, and there is no throttle input it will tell the idle to go into closed loop (where the IACV is used to control engine RPM directly) Otherwise, Idle will be in an open loop mode known as Commanded airflow, and it will also use the throttle follower. Without the VSS, the computer won't know which mode to control the idle with. This induces stalling in some situations.
Now, if the ECM is not receiving a VSS pulse you would/may have some stalling problems. It will always try to control the engine down to an idle when your throttle is closed. If your engine is being spun up by the transmission like when you let off without pressing the clutch, the idle valve will close. Then when you press the clutch the engine will stall almost instantly since there isn't enough time to open the idle valve and recover. So normally, a VSS is required."
The VSS harness connection/ECM needs to see a sine wave pulse – I think any pulse will work. A good tuner can later check and see the VSS signal within their tuning program and adjust the PPM the ECM is seeing... if you need this signal to drive other systems like an automatic trans, cruise control or speedometer signals (which utilize square waves from what I understand).
A magnetic pulse generator is what I installed – it was Dakota Digital and appears the same unit is made by Rostra. SpeedHut also sells a VR sensor that has the magnetic pick-up within the bolt and one could use a metal tooth wheel or some metal pins/keys glued on the rotating axle shaft to interrupt the magnetic signal and the sender will generate the pulse to the ECM.
Here’s the DD unit: SEN-4165 - 2,000-8,000 PPM Pulse (Using 1-4 Magnets) Magnetic Pick-up coil for transmissions without a speed cable or VSS output.
Here’s the Speedhut: Magnetic Htz. Universal Speedometer Sender: G-SNDR-13 $38.00 Sender can be mounted anywhere that counts ferrous metal objects passing in front of sender. Sender works with all of our speedometer models. -Sine wave output. -connect directly to Speedometer to measure speed. -M18x1.5 sender body threads with jam nut. -Sender length 2.300" long threads -8 feet of sender wire.
A magnetic pulse generator is what I installed – it was Dakota Digital and appears the same unit is made by Rostra. SpeedHut also sells a VR sensor that has the magnetic pick-up within the bolt and one could use a metal tooth wheel or some metal pins/keys glued on the rotating axle shaft to interrupt the magnetic signal and the sender will generate the pulse to the ECM.
Here’s the DD unit: SEN-4165 - 2,000-8,000 PPM Pulse (Using 1-4 Magnets) Magnetic Pick-up coil for transmissions without a speed cable or VSS output.
Here’s the Speedhut: Magnetic Htz. Universal Speedometer Sender: G-SNDR-13 $38.00 Sender can be mounted anywhere that counts ferrous metal objects passing in front of sender. Sender works with all of our speedometer models. -Sine wave output. -connect directly to Speedometer to measure speed. -M18x1.5 sender body threads with jam nut. -Sender length 2.300" long threads -8 feet of sender wire.
DD also makes a GPS unit that can programmed to put out a sine wave but it’s pricy and does much more than I need: Dakota Digital Speedometer GPS Interface Modules GPS-50-1
So the outcome of this all: my car didn’t stall once today, runs much smoother at small throttle openings and the exhaust “note” doesn’t burble away when off the gas.
Attached are several photos of the above noted pieces and my install to the G96.50 trans-axle.
Regards – M. Shark