Splitting the output of a VRS magnetic pickup

Thread Starter

MB107

Joined Jul 24, 2016
108
I have the challenge of adding cruise control to an older vehicle. The cruise control requires a VRS sensor with a 48 toothed wheel. I have a 48 toothed wheel already installed but its being used for the anti lock brake system. I would like to split this signal to go to both the ABS and Cruise control. I am concerned that the voltage may be attenuated in doing this or I may cause some kind of interference. These sensors have twisted pair wiring. The attached picture shows what I am trying to do.

Does anyone see any concerns with this or have any recommendations on how to do it without having RF or noise issues.

1593792047891.png
 

crutschow

Joined Mar 14, 2008
25,275
Check if one side of the VRS sensor is connected to vehicle ground.
If so, then make sure that ground wire goes to the ground connection on the Cruise Control.

If it's not grounded, then you likely will need to add a differential isolation amp between the sensor and the Cruise Control.

I don't see a particular problem with noise.
Just use a twisted-pair wire configuration similar to what goes to the ABS.

An alternate would be to add a separate sensor for the Cruise Control
 

Irving

Joined Jan 30, 2016
961
Do you know that the output of the ABS sensor is the same form of output you need for the cruise control?

On some older cars the ABS is a passive reluctance coil (2-wires) and outputs an AC signal at about 1vp-p whose frequency is related to road speed (approx 922Hz @ 100km/h). They cannot detect speeds below about 2km/h.

Later ABS are active hall sensors, with 3-wires, DC supply, ground and output. The output is a clean square wave at around 10v and can go to as low as 0.1km/h
 

Thread Starter

MB107

Joined Jul 24, 2016
108
Do you know that the output of the ABS sensor is the same form of output you need for the cruise control?

On some older cars the ABS is a passive reluctance coil (2-wires) and outputs an AC signal at about 1vp-p whose frequency is related to road speed (approx 922Hz @ 100km/h). They cannot detect speeds below about 2km/h.

Later ABS are active hall sensors, with 3-wires, DC supply, ground and output. The output is a clean square wave at around 10v and can go to as low as 0.1km/h
These are two wire passive sensors the both are the same and use a 48 tooth wheel. As far as I know voltage is also increases with speed. If its like my speedometer voltage will exceed 100V P-P at higher speeds. They are the stock sensors from a 1996 Mercedes Benz SL600.

They will be used exactly as they were in the SL600 except the car that they will be going into only has a 48 tooth wheel on each of the rear wheels. The 600SL had four, one at each wheel and used the front left wheel speed for cruise control. So I need to split one of the rear wheel signals to get an output for the cruise control. Or I could fabricate another mount and have two sensors picking up from one wheel. I would prefer to avoid that.
 

Thread Starter

MB107

Joined Jul 24, 2016
108
Check if one side of the VRS sensor is connected to vehicle ground.
If so, then make sure that ground wire goes to the ground connection on the Cruise Control.

If it's not grounded, then you likely will need to add a differential isolation amp between the sensor and the Cruise Control.

I don't see a particular problem with noise.
Just use a twisted-pair wire configuration similar to what goes to the ABS.

An alternate would be to add a separate sensor for the Cruise Control
No sensors go to ground, at least not directly so. But they may be grounded internally in the control box. The separate sensor is something I'm trying to avoid.
 

Irving

Joined Jan 30, 2016
961
Simple answer, try it and see. If the cruise control doesn't load the output of the sensor too much so that both it and the ABS get enough signal, then it'll be fine.

If it doesn't work, and I suspect it will, then your options are to fit another sensor, or create a simple buffer amplifier to feed the cruise control. Relatively easy to do.

Or fit another disc to one of the front wheels?

I knew the output increased with speed, but 100v p-p sounds massive!
 
Last edited:

Thread Starter

MB107

Joined Jul 24, 2016
108
Simple answer, try it and see. If the cruise control doesn't load the output of the sensor too much so that both it and the ABS get enough signal, then it'll be fine.

If it doesn't work, and I suspect it will, then your options are to fit another sensor, or create a simple buffer amplifier to feed the cruise control. Relatively easy to do.

Or fit another disc to one of the front wheels?

I knew the output increased with speed, but 100v p-p sounds massive!
100 p-p was what I was getting with my speedometer sensor at 130MPH, when you read the sensor by itself connected to an oscilloscope, but its only like 50V p-p with the speedo connected to it.
 

Thread Starter

MB107

Joined Jul 24, 2016
108
I'm talking about if the destination (ABS circuit) has one line connected to ground.
Also if the Cruise Control has one lead grounded (through the battery supply ground).
I can put an ohm meter on it and check fro continuity to ground, but the ABS sensor wires both go directly to the ABS control module. The cruise control sensor goes directly to the Engine Control module. So if one wire eventually gets to ground the done through the control units.

Right now I'm leaning to the conservative approach of installing a second independent sensor. Mostly because the engine control module is no longer available and it was probably $3000.00 when it was. Overhauled units are still available for about half that.
 
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