Converting 5V input to VR output???

[Crapmaro]

My engine has been running a lot better after switching the auto trans PROM chip to a manual PROM chip, go figure... Unfortunately the ECU still wants to see a vehicle speed input to make idle adjustments and without it I have to keep my base idle a little high to keep the engine from stalling when I come to stop and engage the clutch.

The car in question is an 85' Cherokee (5 speed) which has no vehicle speed sensor and any speed sensor I could easily rig up will output a 5v square wave signal and my ECU (from a 91' Camaro) expects a variable reluctance signal. I could hook up either a hall effect sensor and point it at the u joint coming out of the rear end or use a sensor from a 91' Jeep Wrangler which allows me to keep my current cable driven speedometer while also giving me that 5V signal, but converting it is apparently a complicated and/or expensive issue. I've found a signal conditioner from Dakota Digital that would take that 5V signal and convert it to VR, but that's a 130+ dollar solution for a Jeep I got for free and put a 50 dollar engine in. Yeah, I'm cheap. Every google search I make ends up with some string of resistors and capacitors that *might* work or a cheaper signal conditioner that only works in the opposite direction I need it to.

Any ideas? Thanks!

[SappySE107]

Converting a square wave (like a Hall effect signal) to a Variable Reluctance (VR) signal typically involves creating a sine-wave-like analog signal from digital pulses, often achieved by passing the square wave through an RC network or transformer. Common methods include using passive components or active signal conditioners to mimic the magnetic pulse of a VR sensor. [1, 2, 3]
Methods for Square Wave to VR Conversion

• RC Network (Passive Converter): A simple 3-stage RC network using capacitors and resistors can convert a square wave input into a rough sine wave output.
• Transformer Coupling: Driving a small transformer through a capacitor with the square wave signal can generate a VR-like signal.
• Signal Conditioner Module: Use a dedicated converter module, such as a "VR to Hall" converter connected in reverse, or a specialized conditioner to convert a 5V square wave into a 2-wire analog VR signal.
• Alternative - Hall Effect Sensor: Instead of conversion, replacing the VR sensor with a 3-wire Hall-effect sensor often provides the desired 0-5V clean square wave, bypassing the need for signal conversion. [1, 3, 4, 5, 6]

This video explains how to build a circuit to convert a square wave to a sine wave:
Important Considerations

• Negative Input Grounding: If feeding a square wave into a VR input circuit (e.g., on an ECU), connect the negative VR input to ground and use only the positive input.
• Capacitor Coupling: To prevent DC offset issues, use a $1\mu F$ capacitor between the square wave source (e.g., Hall sensor) and the VR input.
• Signal Amplitude: Note that VR sensors generate a sine wave that increases in voltage with speed, while a converted square wave may have constant amplitude. [2, 7, 8, 9]

For automotive applications requiring high precision (e.g., cam/crank sensors), using a dedicated converter module (like the AXM-120) is recommended to ensure the signal is properly interpreted by the ECU. [2]

AI responses may include mistakes.
[1] https://www.electronics-lab.com/foru...signal.245203/
[2] https://diyautotune.com/products/axm-120
[3] https://circuitdigest.com/electronic-circuits/simple-square-wave-to-sine-wave-converter
[4] https://protunerz.com/products/axm-1...verter-to-hall
[5] https://www.facebook.com/FuelTechUSA/posts/-new-releasethe-vrhall-converter-was-designed-to-read-an-analog-vr-signal-and-tr/10163385005420165/
[6] https://www.syvecs.com/forum/viewtopic.php?t=828
[7] https://forum.allaboutcircuits.com/t...vertor.188642/
[8] https://patents.google.com/patent/US3982189A/en
[9] https://www.autometer.com/blog/understanding-speed-senders-sine-wave-square-wave-and-gps-explained/

[SappySE107]

130 might be it. The one the AI gave is to convert the other way for 70 bucks. Im not an electrical engineer so all of this is way over my head.

[bszopi]

Ben, is there a way to remove the speed sensor from the code? Or would that cause more issues...?

[bszopi]

Here is my ChatGPT response, lol...

You may not need to “convert” the 5V square wave into a perfect VR sine wave. The cheap path is usually to make the ECM see changing AC pulses.
Best cheap options

1. Try AC-coupling the 5V square wave

Wire the Hall/Jeep VSS signal through a capacitor into the ECM VSS high input:

Hall VSS signal ---- 0.1µF to 1.0µF capacitor ---- ECM VSS HI / B10
Hall ground -------------------------------------- ECM VSS LO / B9 / sensor ground

Optional cleanup:

10k resistor across ECM VSS HI and VSS LO

That capacitor blocks the DC part of the 0–5V square wave and leaves positive/negative “edge” pulses, which can often satisfy a magnetic/VR-style input.

Better junkyard/simple solution

Look for a 2-wire magnetic pass-through VSS that keeps the cable speedometer. Dakota’s SEN-01-4160 is basically this: an inline cable-drive pulse generator with an 8,000 PPM sine wave/AC output, while retaining cable-driven gauges.

That is closer to what the ECM wants than the Jeep Hall sensor. The 1227730-style GM ECM pinouts list VSS Magnetic Lo and VSS Magnetic Hi, matching the B9/B10 style input.

My practical recommendation

Try this order:

1. Use the 91 Jeep cable/VSS sensor if it fits your speedometer setup.


2. Run its 5V signal through a 0.1µF or 1µF non-polarized/film cap into ECM VSS high.


3. Ground the VSS low side to ECM sensor ground/VSS low.


4. Test for idle behavior and Code 24/VSS behavior.


5. If it does not work, grab or buy a 2-wire magnetic pass-through sender instead of buying the $130 conditioner.



The capacitor trick is cheap enough that it is worth trying first. It will not recalibrate pulses-per-mile, but for idle dashpot/decel logic the ECM mostly just needs to know “vehicle moving vs. stopped,” not perfect speed.

[SappySE107]

Removing the speedo code would still leave you with the same issue. It won't know you are moving or not moving to know what to do with the IAC.

How about GPS to VSS? I don't know what those output for signal.