I'm doing a modification to a 1986 Mercedes Benz 560SL. It involves swapping an old transmission with a magnetic pickup used to drive the speedometer to a newer transmission which has no provision for a speed sensor on the transmission. While I can use the old magnetic pickup, I would prefer to adapt an off the shelf part as the old ones are no longer available. So therefore I'm trying to find a magnetic pickup that will be compatible with the Mercedes computers. And of course Mercedes Benz does not publish spec for these parts. But I have made a few measurements and run an output test of the current sensor. The stock sensor is shown in the first picture. Where it will be mounted is shown in the second picture.

What I have done so far:

Measured resistance across the two terminals at 1.825 KOhm using digital meter.
Measured AC voltage output with with the sensor running at 2.85V @ 535RPM and 7V @ 1500RPM. This was done on a lathe with the sensor ~ 0.060" away from the chuck jaws as shown in the third picture.
I made a few calculations of the required physical installation

Wheel Diameter 560SL in 2.5
Wheel Teeth 4
Max RPM 6000
Max pulse speed Hz. 400
Max surface speed in/sec 314.1593

Not sure which parameters are important but I cannot find one at last in the Newark catalog with a coil resistance above 1 kOhm and it has a peak to peak voltage of 190VAC, which may not be too much as I notice they tested it with a 0.005" gap. I will be at about 0.040"

Any help on this would be appreciated.

Thank You
John

 

A big unknown is what the ECU does at power-up to check for the pickup's presence and condition. If the new coil is 1k but the ECU is looking for a coil resistance > 1k you would have to find a way of fooling it. The obvious answer is a series resistor, but that would reduce the input voltage to the ECU so some amplification of the coil's output might be needed.
I know nothing of Mercs, but my understanding is that ECUs generally aren't too fussy over pickup input voltage provided it exceeds some minimum specified amplitude at tickover rpm.
Another thing to consider is pulse frequency. It will need multiplying/dividing depending on the numbers of teeth on the old and new 'tone wheels'.

 

Thanks for the replay. I don't believe, in this car any ECU comes into play as it certainly would in the newer vehicles. This is an early old school electronic speedometer only sensor. No warning lights are displayed if it is not functioning. The sensor is wired directly to the speedometer, but the speedometer does have some electronics in it so who knows what is available to get fried if I do this incorrectly. Attached is a circuit diagram with the speedometer and sensor circled in red.

Also not sure if the voltage I measured with a DVM is accurate since this was the pulse of three jaws of a lathe chuck that have very large spaces in between each jaw. An oscilloscope would certainly be more appropriate.

As far as pulse frequency I plan to handle that with a pulse frequency modifier which are readily available from the aftermarket for people changing gear ratios.

 

As far as pulse frequency I plan to handle that with a pulse frequency modifier which are readily available from the aftermarket for people changing gear ratios.

I wouldn't expect that device to be too fussy about pickup coil resistance or output voltage, and it should provide its own output compatible with a range of speedos (check its spec).

 

Sounds promising. I think I will take the risk with my speedometer. I'm now leaning toward this one.

Thank You

 

Bendix makes a 1.5-2.5 Kohm sensor for its ABS systems. They can be found on most Class 8 trucks. It is a 2 wire magnetic pulse generator. You will need to figure out how to get your pulses per mile and that should do the trick.
Maybe I missed it but why aren't you using the original sensor?

 

Bendix makes a 1.5-2.5 Kohm sensor for its ABS systems. They can be found on most Class 8 trucks. It is a 2 wire magnetic pulse generator. You will need to figure out how to get your pulses per mile and that should do the trick.
Maybe I missed it but why aren't you using the original sensor?

Thanks for the reply.

Two reasons I/m not using the OEM. 1) it is no longer available. 2) it is too big. The yellow thing in the four view picture is a 12mm wide sensor. It is positioned ~3mm from hitting the already shortened drive shaft bolts. The stock 18mm diameter sensor will hit. Re positioning the setup is not favorable to installation of the tone wheel or protection from the elements.

Interested in the Bendix sensor do you know what diameter it is. Getting the pulses per mile is easy. The stock MB had a 4 tooth wheel and I have a 4 tooth wheel. I can also adjust PPM using a electronic device readily available to the automotive industry just for that propose.

After talking to the supplier of the sensor proposed I have concluded MB uses the 1.825KOhm sensor because there stock sensor when embedded into the transmission needs to have ~ 0.1" gap as the sensor is insulated from the hot transmission fluid by placing a plastic thimble type piece sealing off the sensor from the transmission internals. Mine will be external so I believe I can get away with a smaller gap and consequently a lower resistance sensor.

 

The outer diameter of this sensor is 0.627" and spacing to the tone ring is 0.015".
The sensor comes in a straight or 90 degree install. I think it is close to what you are looking for.

 

I'm doing a modification to a 1986 Mercedes Benz 560SL. It involves swapping an old transmission with a magnetic pickup used to drive the speedometer to a newer transmission which has no provision for a speed sensor on the transmission. While I can use the old magnetic pickup, I would prefer to adapt an off the shelf part as the old ones are no longer available. So therefore I'm trying to find a magnetic pickup that will be compatible with the Mercedes computers. And of course Mercedes Benz does not publish spec for these parts. But I have made a few measurements and run an output test of the current sensor. The stock sensor is shown in the first picture. Where it will be mounted is shown in the second picture.

What I have done so far:

Measured resistance across the two terminals at 1.825 KOhm using digital meter.
Measured AC voltage output with with the sensor running at 2.85V @ 535RPM and 7V @ 1500RPM. This was done on a lathe with the sensor ~ 0.060" away from the chuck jaws as shown in the third picture.
I made a few calculations of the required physical installation

Wheel Diameter 560SL in 2.5
Wheel Teeth 4
Max RPM 6000
Max pulse speed Hz. 400
Max surface speed in/sec 314.1593

Not sure which parameters are important but I cannot find one at last in the Newark catalog with a coil resistance above 1 kOhm and it has a peak to peak voltage of 190VAC, which may not be too much as I notice they tested it with a 0.005" gap. I will be at about 0.040"

Any help on this would be appreciated.

Thank You
John

View attachment 168832 View attachment 168833 View attachment 168834

Warner Group used to do reluctance sensors with integral magnet that sense plain teeth on the starter ring for a tacho. For speed; you could mount one through a hole for the planet ring in the diff, resolution wouldn't be great though.

 

The outer diameter of this sensor is 0.627" and spacing to the tone ring is 0.015".
The sensor comes in a straight or 90 degree install. I think it is close to what you are looking for.

After fine tuning the design I have found that 12mm (0.472in) diameter is the absolute limit. Its so close I'm considering using a 3/8" version which is also available. I hate putting SAE hardware on an all metric car.

 

Warner Group used to do reluctance sensors with integral magnet that sense plain teeth on the starter ring for a tacho. For speed; you could mount one through a hole for the planet ring in the diff, resolution wouldn't be great though.

There is actually already on in there for the ABS system using a 36 tooth wheel. I have considered splitting the signal and than trying to reduce the frequency by a factor of 9 of the signal going to the speedo. Not sure if that can be done.

 

reduce the frequency by a factor of 9 of the signal going to the speedo. Not sure if that can be done.

Don't know if there's an off-the-shelf module to do that, but a bit of DIY electronics could do it.

 

There is actually already on in there for the ABS system using a 36 tooth wheel. I have considered splitting the signal and than trying to reduce the frequency by a factor of 9 of the signal going to the speedo. Not sure if that can be done.

Messing with the ABS sensors is best avoided till you've eliminated any other option.

 

Measured resistance across the two terminals at 1.825 KOhm using digital meter.
Measured AC voltage output with with the sensor running at 2.85V @ 535RPM and 7V @ 1500RPM. This was done on a lathe with the sensor ~ 0.060" away from the chuck jaws as shown in the third picture.
I made a few calculations of the required physical installation

Wheel Diameter 560SL in 2.5
Wheel Teeth 4
Max RPM 6000
Max pulse speed Hz. 400
Max surface speed in/sec 314.1593

Not sure which parameters are important but I cannot find one at last in the Newark catalog with a coil resistance above 1 kOhm and it has a peak to peak voltage of 190VAC, which may not be too much as I notice they tested it with a 0.005" gap. I will be at about 0.040"

Any help on this would be appreciated.

Thank You
John

It appears that what you have is a fairly standard magnetic impulse sensor, generating an AC signal. As already mentioned I wouldn't be terribly worried about using a 1KΩ sensor, you can simply add an 820Ω resistor in series if absolutely necessary. The important consideration is the number of pulses per mile, and the signal level. You will need to determine what is the lowest signal voltage that the speedometer will work at and the peak voltage at max road speed.
A magnetic impulse crankshaft sensor or abs sensor will do the job, or if you are fussy about the voltage level, a 5V Hall type sensor (often used for camshaft sensors) should work with a voltage level spot in the middle of your desired range. It will have the added complication of requiring a 5V supply , which can be easily obtained, and it will have a square wave output. I would be surprised if the speedometer is fussy whether it is a square wave or AC pulse.
Before you start, you will need to accurately determine the number of pulses per mile. By far the best option is to use a signal generator to drive the speedometer and graph frequency V's speedometer reading. If you don't have access to one you can easily make one with an arduino.
. Your Tyres are 205/65-15 which turn 792 times per mile. Your top speed is 140MPH = so your tyre turns at max speed are 30.8 times per second.
Going by the max figure you quoted in your original post of 400HZ, you will need to generate 13 pulses per wheel rotation.(30.8x 13 = 400.4) which is accurate to 0.1%
This is very easily achieved by making a new split collar in two halves,the same diameter and approx weight as the existing one on the OE driveshaft, with 13 evenly spaced teeth attached to it. It is then a simple matter of mounting the sensor on a bracket so that it is pulsed as each tooth passes.

 

There is actually already on in there for the ABS system using a 36 tooth wheel. I have considered splitting the signal and than trying to reduce the frequency by a factor of 9 of the signal going to the speedo. Not sure if that can be done.

A factor of 3 would be my calculation!

 

Wow just got a new toy. GW Instek GDS-1054B Digital Storage Oscilloscope, 4-Channel. I didn't realize how much voltage these things put out. The stock 560SL puts out about 250v pk-pk at about 70MPH. That was as fast as I cared to run it on jacks. I hope I can find a sensor that powerful in a 12mm max diameter range.

 

You are measuring pulses, not voltage. Any sensor that outputs above the threshold voltage will be fine. The threshold voltage will be way less than 250V and whatever voltage you get at about 10 MPH will suffice. also remember that it is very likely that your instruments will have a way lower input impedance than your oscilloscope. This will reduce the measured voltage somewhat

 

Check your probe and scope settings. Is the scope set for a 10:1 probe or 1:1 probe?

 

Check your probe and scope settings. Is the scope set for a 10:1 probe or 1:1 probe?

I figured this out. Or should I say someone else here figured it out. Probes are as 1:1. but I didn't know how to scale the Fast CSV data files. See my other thread in test and measurement.

A factor of 3 would be my calculation!

This vehicle it is a three channel ABS system, with the rear wheel speed sensor on the input of the differential, so a factor of 9 would be appropriate.

 

You are measuring pulses, not voltage. Any sensor that outputs above the threshold voltage will be fine. The threshold voltage will be way less than 250V and whatever voltage you get at about 10 MPH will suffice. also remember that it is very likely that your instruments will have a way lower input impedance than your oscilloscope. This will reduce the measured voltage somewhat

I understand the measuring of pulses but would I need to be concerned about frying things if voltage went too high.