Changing the calibration of an automotive electronic speedometer.

Thread Starter

MB107

Joined Jul 24, 2016
103
Hello Ladies and Gentlemen

I just completed a project of getting an electronic speedometer to work with an alternate transmission in my 1986 Mercedes 560SL. You can read about that project at this link.
https://forum.allaboutcircuits.com/threads/need-to-find-a-compatible-magnetic-speed-sensor-for-speedometer.156292/

Now with that complete its time to move on to what lies ahead and that will be changing the rear end gear ratio which will require re-calibration of the electronic speedometer.

To begin with I am well aware of the off the shelf frequency converters that are available to do this sort of thing externally. And it was my intent to use one of those until I read an article on the European version of my speedometer which was adjustable. So it got me thinking that I could make mine adjustable, probably just by changing one resistor. I have attached two pictures, the first is the European version of the Mercedes speedometer and the second one is my speedometer. The part circled in the first picture is the pot that is adjusted to calibrate the speedometer on the European version. Notice my speedometer is missing that part and there appear to be two resistors in its place along with slight changes of other parts in the vicinity.

So my question is how might I be able to figure out how to make the USA spec speedometer work like the European version shown in the first picture. I have a spare US spec speedometer to play with. I do not have access to a European speedometer.

Bouton trimmer.jpg Stock 560SL Speedometer.jpg
 

Hymie

Joined Mar 30, 2018
708
The two resistor values are not clear from your photo, I suggest you determine their value by careful inspection of their colour bands and measurement, then modify the circuit to include a potentiometer of suitable value that will adjust the voltage at the common point of the two resistors, which may allow adjustment of the speed output.

Be aware that modifying the circuit as above could result in damage to the circuit.
 

Alec_t

Joined Sep 17, 2013
10,357
It would be helpful if you could post the markings/values/colour band info of all the visible resistors and capacitors in both speedos. If you can't see the trim-pot markings, measure the resistances between all three legs.
 

jpanhalt

Joined Jan 18, 2008
7,881
Can you tell how he pot and resistors are connected on the bottom of the board by checking the traces?
 

Thread Starter

MB107

Joined Jul 24, 2016
103
This is about as best as I can do. All resistors were as measured in place.
If you looked at an earlier version of this I just found resistors 7 and 8 and added them in. Also I added the large coil which appears to be the output of the whole circuit. Looks like if I unsoldered those three pins the circuit board would lift off and anything beyond that would drive the motor.

Speedometer Circuit Layout.jpg P1000660.JPG P1000652.JPG P1000655.JPG
 

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Hymie

Joined Mar 30, 2018
708
Based on the in circuit measured resistances, I’d recommend you try the following:-

Lift the common end of both resistors R2 and R3 from the circuit board. Connect a 10kΩ pot between these two lifted resistor ends, with the pot wiper soldered to the PCB where the resistor ends were commoned.

You will then be able to adjust the relative values of R1 & R2, and if these are the resistors which determine the calibration point, you will be able to adjust it.

If the adjustment range is not sufficient for your purposes, you may need to increase the 10kΩ pot value, conversely if the adjustment is too course to allow setting to the required calibration point, you may need to decrease the value.
 

Thread Starter

MB107

Joined Jul 24, 2016
103
Based on the in circuit measured resistances, I’d recommend you try the following:-

Lift the common end of both resistors R2 and R3 from the circuit board. Connect a 10kΩ pot between these two lifted resistor ends, with the pot wiper soldered to the PCB where the resistor ends were commoned.

You will then be able to adjust the relative values of R1 & R2, and if these are the resistors which determine the calibration point, you will be able to adjust it.

If the adjustment range is not sufficient for your purposes, you may need to increase the 10kΩ pot value, conversely if the adjustment is too course to allow setting to the required calibration point, you may need to decrease the value.
Thanks Hymie. I figured it was something like that. Un fortunately I wont be able to try this for a while as I cant risk messing this speedometer unit up at least until I get the first other speedometer issue fully solved. The thread I referenced in my first post is still in work as I found out my speedometer would only work to 80 MPH. I would expect adjustment to be no more that 30% either way. I will only be going 7% for my current install.
 

shortbus

Joined Sep 30, 2009
6,977
How rare are the European speedometers? Maybe buy one on Ebay from Europe to find if there are other differences? Before just randomly changing things on the US one, just going by a picture of a European one seems kind of 'iffy'.
 

Thread Starter

MB107

Joined Jul 24, 2016
103
How rare are the European speedometers? Maybe buy one on Ebay from Europe to find if there are other differences? Before just randomly changing things on the US one, just going by a picture of a European one seems kind of 'iffy'.
They are very rare on this side of the pong and usually sold with the rest of the instrument cluster for about $500. Also Eros will be in kilometers.
 

Dodgydave

Joined Jun 22, 2012
8,384
What is the Chip number.
Also are you trying to lower or increase the output of the speedometer?
 

Thread Starter

MB107

Joined Jul 24, 2016
103
What is the Chip number.
Also are you trying to lower or increase the output of the speedometer?
Can read it without disassembly. Will be trying to both increase or decrease preferably up to 50%. But current project is -20%
 

Dodgydave

Joined Jun 22, 2012
8,384
It's probably a Frequency to Voltage converter chip, and it's just a matter of altering the time constant..
 

Thread Starter

MB107

Joined Jul 24, 2016
103
It's probably a Frequency to Voltage converter chip, and it's just a matter of altering the time constant..
That's what I understand. Its a frequency to voltage converter. So all I should need to do is modify the circuit after the Frequency to Voltage converter. I do know that the chip was made by ITT.
 

LesJones

Joined Jan 8, 2017
2,317
I think the chip does more than just frequency to voltage conversion.The wide tabs on either side of the chip suggest that it drives something that requires a reasonable amount of power. The tabs will be to conduct heat away. (The only chip I have seen constructed like this is a TBA800 which is a 5 watt audio amplifier but it is unlikely to be that device in a tachometer.) What type of technology is the actual display part of the tachometer ? (Moving coil, stepper motor, some type of servo like a model control servo or something I have not thought of ?) Can you post a picture showing the complete unit. I have not worked out what the large coil and the item to the left of it does.

Les.
 

LesJones

Joined Jan 8, 2017
2,317
I think that is VERY likely to be the IC being used. If so then increasing the value of the pink 47nF capacitor should make the displayed reading increase for the same frequency input as it will increase the length of the pulse from the monostable. R2 and R3 are in series and form the timing resistor for the monostable. Reducing the value of the timing resistor by shunting it with another resistor should make the displayed value decrease by reducing the length of the pulse from the monostable for the same input frequency. If your unit also displays distance then some other changes may be needed to correct this. I have not yet studied the data sheet to see how that part works.

Les.
 
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Alec_t

Joined Sep 17, 2013
10,357
Here's the Fig 1 application circuit from the UAF2115 datasheet :
UAF2115_speedodriver.PNG
If the speedo is indeed using that chip, which seems likely, then the R and C connected to pin 8 might well need adjusting if it is required to have this speedo work correctly up to 180mph.
 

Thread Starter

MB107

Joined Jul 24, 2016
103
Interesting discussion s on both changing capacitance and resistance.
I noticed the Euro speedo has 2 extra components. In the first picture of the first post, there is a small green cap just to the right of the chip and the pot which seems to be modifying the resistance of 1 or 2 of the resisters up front.

The best solution would be able to add a cap and change 1 resister or add a pot to be able to vary output +/- 50%.
 

Thread Starter

MB107

Joined Jul 24, 2016
103
I've located a similar but lower cost speedometer that I will be able to do some practice surgery on. Locating it and then getting someone to actually ship it is still a challenge. Stay tuned.
 
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