Installing Vehicle Speed Sensor in Ram Jet 350

 

I had the idea of interfacing a vehicle speed sensor (VSS)  to the MEFI4a controller so that I could get an instantaneous speed reading and miles per gallon reading in my ScannerPro software.  The MEFI4a controller in the Ram Jet 350 provides two different connections for a VSS as shown in the diagram below.

mefi_vss.jpg (62007 bytes)

The VSS on the left (labeled AN) uses a 5v DC signal from pin J2-2 and the VSS output line connects to pin J2-26.  This type of VSS outputs a 0-5v DC voltage depending on vehicle speed.

The VSS in the transmission of my car is a stock GM two wire unit that outputs a varying frequency AC voltage so I used the diagram on the right (labeled VF).  I also have a Cable-X unit in my car that drives the mechanical speedometer so I wanted to be able to continue to use it with the VSS connected. 

The Output line from the VSS is connected to the white input wire from the Cable-X and also to pin 25 of the MEFI4a J2 connector. The other line from the VSS is connected to chassis ground and also to the ground line from the Cable-X. In order to make the connection to the MEFI4a J2 connector you will need an additional connector pin which is available from Mouser Electronics (www.mouser.com), part number 12084913-L.

vss_wiring.jpg (41696 bytes)

 

Once the VSS is wired into the MEFI4a controller, you must also change some of the parameters in the controller so that it will know that a VSS is present and what type it is.  The following parameters were changed using TunerPro.

Vessel speed sensor present = set
Vessel speed based on analog input = not set
Number of vessel pulses per mile = 40613

By default the MEFI4a is set to use the VF type of VSS input, so you do not have to tell it what type of VSS to use.  If you were using the (AN) type of VSS, there is a setting that lets you set the VSS type to analog. 

Unfortunately turning on the VSS seems to cause other problems as now the idle speed wants to hang up at around 2000 RPM, almost like an advanced version of my "cruise control" problem; however, when I replaced my IAC "plug" this problem also went away.

So now when I log data I can record vehicle speed and also a calculated MPG value based on the vehicle speed and the GPH value.

vss_data.jpg (126755 bytes)