Hello all.
Working on sons 1991 Formula. All stock except transmission tail shaft housing. PO installed later model 700R4 with 40 tooth reluctor ring. I tried modifying a DRAC module from an S10 no luck. I have heard about Dakota Digital's signal buffer, but didn't want to admit defeat yet.

I need to know with 99% certainty what the signal and waveform that the magnetic VSS is putting out and what the stock buffer expects.

For $20 in electronics I believe I can modify the sine wave from the magnetic VSS to a square wave that the stock buffer can use.

Thanks.

 

i feel ur pain. i went dakota and was VERY happy!

but the 90 shop manual states... vin t.f.8 use 4k pulses per mile for speedo and 2k for cruise contrl

vin e uses 4k from the sensor to the buffer and 4k to speedo but only 2kppm to ecm and cruise...

sorry been a late night so if u have any more ? pm me

good luck!

 

Thanks for the reply BanditSBC.

This car has been a great opportunity to hit wrecking yards again. Something you don't do when your into 70's era Mopars.

Near as I can tell the old VSS signal was a DC signal that varied with output shaft speed. Does anyone know how high a voltage this VSS can send to the Buffer? My other option is to by the Older VSS and measure the output. Time to buy an Oscilliscope I guess.

The reluctor based VSS is an AC signal, but I'm not sure how high the AC voltage goes.

??????

 

Nothing like talking to yourself.
More details...

To quote the GM electrical diagrams book Regarding 40 tooth reluctor:

The Digital Ratio Adapter Controller (DRAC) Processes inputs from the Vehicle Speed Sensor (VSS), and outputs signals to the Speedomoeter, Electronic Control Module (ECM), Cruise Control Module and the Rear Wheel Antilock Brake (RWAL) Module.

The VSS is made up of a 40-tooth rotor and a coil mounted on the transmission. As each rotor tooth nears the coil, the coil produces an AC voltage pulse. Each revolution of the rotor produces 40 AC voltage pulses. As the vehicle speed increases, the number of AC voltage pulses per second increases.

The DRAC takes the voltage pulses from the VSS and uses them to open and close four solid state output switches to ground at a rate proportional to vehicle speed.

The Speedometer switch opens and closes at the same rate as the VSS pulses. The ECM and Cruise control switches open and close at half the rate of the VSS pulses. The RWAL switch opens and closes at a rate seven times the VSS pulses. ...

 

I grabbed an old gear driven VSS, To my surprise the output is 0 - 10 VAC. The voltage is proportional to the drill motors RPM.

 

from National Instruments:

Variable Reluctance = VR. This is key terminology to finding comparator circuits to modify the VR signal to digital.

VR and Passive Proximity Sensors
VR and passive proximity sensors are electromagnetic devices that consist of a permanent magnet wrapped with a wire winding. The sensor is sensitive to ferrous material, so when the ferrous material passes through the magnetic field of the sensor, a change in the magnetic flux induces a current in the winding that is proportional to the change in flux. The ferrous material could be in the form of gear teeth or a notch on a rotating shaft. The sensor generates a time varying voltage that increases in magnitude and frequency as the speed of the gear or shaft increases. Essentially, these sensors can be modeled as an AC source in series with an impedance equivalent to the output impedance of the sensor. The AC output of these sensors generally has little to no DC offset.


You can connect VR and passive proximity sensors to the conditioning circuit in the same manner. Figure 1 shows a schematic diagram of the circuit. It should be noted that the CS1124 IC provides two signal conditioning channels. The components R1 and C1 apply to the first channel (IN1) and R2 and C2 apply to the second channel (IN2). R3 is the comparator threshold bias resistor for both inputs. The remainder of this document focuses on the first input, but the same information also applies to the second input.

 

So the voltage changed too? Not just the hz?

-- Joe