i have an old dodge truck with a 225 slant motor in it. i am in the process of hooking up a cfi setup from an early ea to it. the only real hurdle so far is the speed sensor input. does anyone know if the waveform from it is a square wave, or is it just your average ac sinewave ? any help would be appreciated.

anyone ? do you know if it has to be a true representation of vehicle speed ? or is it just to show the computer that the show is moving ?

unfortunately i dont have an answer ...

dad's in the process of doing a similar thing himself so he will likely hit this issue too. im interested to see what answer you come up with

I think you get away pretty lightly with an EA setup... because even the automatic transmission has a seperate trans controller and as such, there are no really critical functions inside the ECU which rely on valid VSS input.

I offer you the following notes from some documentation i have on the EEC-IV (american context):

"VSS gets used for many things, even in the manual gearbag calibrations.
1. Used to drive coast-down strategies. EEC will use longer throttle dashpots if it sees a non-zero signal from the VSS at closing throttle. This prevents lift-throttle snatch, especially in lower gears and at lower road speeds.

2. Drives radiator fan strategies. Fan is usually turned off above 45 MPH unless coolant temp is above fan-on setpoint.

3. Cruise control on vehicles with the cruise control integrated into the EEC, instead of a separate electronic module.

The engine should run fine without the VSS. I have disconnected the VSS in my ’89 SHO and ’98 Contour V6 to get rid of the long dashpot. If the strategy doesn’t see a VSS signal, it drops the throttle immediately, which puts ME in control of the shift rev-matching, not the bloody computer.

The cruise control is separate from the EEC, but they share the same sensor. For a manual transmission MAF Mustang, the VSS is used to maintain an 1100 idle when coasting to a stop until you drop below ~8 mph. I’ve known a few people who converted their speed density cars to MAF that had stalling problems when pushing in and holding the clutch at speed until the VSS sensor was hooked up. The speed density cars did not use the sensor. I’m not aware of the sensor being used for anything else except on the ’93 Cobra, where it was used to change the timing curve when above 100 something miles per hour."




Sorry i dont have any information on the actual form of the VSS signal...

thanks for that. in my copy of electraspec, it said the vss was tied up with the closed loop at cruise setup as well. does this sound right ? or in your experience, doen't it affect the show that much ?

The only thing i can think of is that EEC *may* target a different a/f ratio in closed loop when speed reaches a given level (let's say, 90kmhr for highway purposes).

i.e. instead of targeting 14.7:1 it may chase something leaner for fuel economy purposes... but then, considering the limitations of a narrowband oxy sensor... is this even possible?

I used to disable the VSS in my EF to stop the box from changing gears on its own... but im not even sure the VSS went to the ECU on the EF V8??

I think you'll get away with it....

Is electraspec EEC specific or just general EFI?

electraspec has info on most efi cars sold here.

The VSS (speed sensor) give a 0-12v square wave, the ea ecu uses the vss for lean cruise in closed loop mode.
You should be able to get away with not having the VSS signal on the CFI ecu.

cool. thanks guys.