LSx Vehicle Speed Sensor feed to PCM to Ultima speedometer
Discussion
ultimichael said:
If PCM means ECU I have done it this way but using a VDO speedometer.
Why do you ask?
Michael
Well right now I don't have a VSS for the ECU. The current hall sensor drives the Ultima Speedometer. So it would be nice if I could replace the existing hall sensor and then switch over to the ECU wires and have the ECU VSS out send the signal to the Ultima speedometer. Why do you ask?
Michael
No geek speak. Fingers out of the ears.
Question was if the vehicle speed sensor (VSS), usually connected directly to the speedometer, can be connected to the ECU´s VSS input (I assume a GM ECU) and the ECU´s speed signal routed to the speedometer. Thats the way it is done in modern cars as the Corvettes with LS engines.
Why does the ECU need to know the car´s speed in an Ultima?
Haven´t found the answer yet but some say the GM ECUs (E38 in my case) will not work properly without the VSS signal. Need to do some more research.
So for now my speed sensor is connected to the ECU. I use a BMW crankshaft sensor as speed sensor that I had available, works well.
Due to the Corvette service manual the GM speed sensor is a simple inductive sensor
(just a copper coil around a magnetic core) that creates an A.C voltage.
What I found is that the GM ECU alters the signal´s frequency. The speedometer needs to be recalibrated if connected to the ECU.
Michael
Question was if the vehicle speed sensor (VSS), usually connected directly to the speedometer, can be connected to the ECU´s VSS input (I assume a GM ECU) and the ECU´s speed signal routed to the speedometer. Thats the way it is done in modern cars as the Corvettes with LS engines.
Why does the ECU need to know the car´s speed in an Ultima?
Haven´t found the answer yet but some say the GM ECUs (E38 in my case) will not work properly without the VSS signal. Need to do some more research.
So for now my speed sensor is connected to the ECU. I use a BMW crankshaft sensor as speed sensor that I had available, works well.
Due to the Corvette service manual the GM speed sensor is a simple inductive sensor
(just a copper coil around a magnetic core) that creates an A.C voltage.
What I found is that the GM ECU alters the signal´s frequency. The speedometer needs to be recalibrated if connected to the ECU.
Michael
ultimichael said:
...Why does the ECU need to know the car´s speed in an Ultima?
Haven´t found the answer yet but some say the GM ECUs (E38 in my case) will not work properly without the VSS signal. Need to do some more research.
....
I don't know how dependant your ECU will be on speed.Haven´t found the answer yet but some say the GM ECUs (E38 in my case) will not work properly without the VSS signal. Need to do some more research.
....
Many just use speed to detect you are still moving but with no throttle such as running down to a junction or traffic lights. In this case it will hold the idle up at say 1100 or 1200 rpm for a few moments after you stop the idea being that it prevents a stall or stumble. When it knows you are stopped it will drop the idle to approx 900.
Steve
Steve_D said:
Many just use speed to detect you are still moving but with no throttle such as running down to a junction or traffic lights. In this case it will hold the idle up at say 1100 or 1200 rpm for a few moments after you stop the idea being that it prevents a stall or stumble. When it knows you are stopped it will drop the idle to approx 900.
Steve
Regarding the advantages for a VSS feed to the ECUSteve
1. Same understanding as Steve regarding raising the RPMs when slowing to a stop/junction.
2. When cruising on the highway the engine can adjust to run more efficient.
3. LS Tuners that provide a street tune, capture ECU stats as the car is driven. If the tuner has the speed recorded as well there is more information for the tune.
If the VSS is calibrated with the ECU the exact speed of the car can be read from the ODB port as you drive.
Steve_D said:
I don't know how dependant your ECU will be on speed.
Many just use speed to detect you are still moving but with no throttle such as running down to a junction or traffic lights. In this case it will hold the idle up at say 1100 or 1200 rpm for a few moments after you stop the idea being that it prevents a stall or stumble. When it knows you are stopped it will drop the idle to approx 900.
Steve
There will be a whole host of "driveability" functions, like oscilation damping, dashpot trimming, idle-drive entry smoothing, ignition and throttle management during gearshifting etc that rely on having a vehicle speed input present. The car will still drive and "work" without one, but drivability will be reduced. (of course, driveability is already reduced by sticking the engine in an ultima, which has very different characteristics (like drivetrain stiffness for example) than the original donor vehicle!Many just use speed to detect you are still moving but with no throttle such as running down to a junction or traffic lights. In this case it will hold the idle up at say 1100 or 1200 rpm for a few moments after you stop the idea being that it prevents a stall or stumble. When it knows you are stopped it will drop the idle to approx 900.
Steve
You can usually "reshape" a hall effect signal into one that looks enough like a VR signal by using a series capacitor (AC coupling the signal) and a high value resistor to PCM ground (to zero volt bias the signal). That will allow you to put the hall effect signal from the speedo sensor into the PCM that expects a VR signal. (of course, the scaling may not be correct in the PCM, depending on how many teeth your target wheel has per wheel revolution etc, but generally, the most important calibration effects for driveability are simple one of a vehicle moving/not moving nature (although correct gear determination will be degraded)
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