Help understanding signals from these switches

Thread Starter

john2k

Joined Nov 14, 2019
30
I'm new to this forum and looking for some help. I am a novice at electronics, so please excuse me if I don't understand all the terminology :)

I have a instrument/speedometer cluster from a car that I am looking to simulate bit by bit. I've tackled a few of the signals and managed to get them working. I want to tackle bit by bit so i don't get confused. I need help with the following pictured diagram.



It shows two switches. The left switch that has up down, left right and enter buttons. The signals from the switch somehow go to the instrument cluster on pin 2 and pin 3. The pin description from the manufacturer is as follows:

Pin 2: Ignition on, enter, top and back switches on steering pad switch assembly not pushed = 4.8 to 5.2 V
Pin 3: Ignition on, up, down, right, and left switches on steering pad switch assembly not pushed = 4.8 to 5.2 V

And then there is the ODO/TRIP switch on the right. This switch connects to pin 4, 25 and 26. The pin descriptions for these are:

Pin 4:
Ignition on, ODO/TRIP switch not pushed = 4 to 6 V
Ignition on, ODO/TRIP switch pushed = Below 1 V

Pin 25:
Ignition on, up switch and down switch of trip switch not pushed = 4 to 6 V
Ignition on, up switch or down switch of trip switch pushed = Below 1 V

Pin 26: Ground for trip switch

Basically, I am looking for help on how I can simulate or re-create these signals so that I can send it to the cluster. Can anyone kindly explain to me how these switches are working and possibly how I can simulate these signals to the cluster?

Thank you
 

Tonyr1084

Joined Sep 24, 2015
3,651
Can't answer your question directly. But I can comment on the fact that I've seen this sort of configuration before. My wife's car radio can be controlled from the steering wheel. Inside that stack of switches are resistors similar to what you show in your diagram. Each resistor has a different value and forms some sort of voltage divider network that the chip in the radio reads and determines to be "Volume UP" "Volume DOWN" "Seek" "Scan" "Next" "Previous" and "Mode". All that information can be conducted over just a few wires.

Whatever your switch is doing the computer is deciphering the meaning and responding accordingly.
 

Dodgydave

Joined Jun 22, 2012
8,466
It's a Resistor switch, you need to measure the resistance of each button then you can use a couple of CD4016 quad switch chip to substiute your switches.
 

MrSoftware

Joined Oct 29, 2013
1,573
I only glanced but I'll take a stab. It looks to me like an analog output, as opposed to digital. That is they are reading an analog voltage value instead of simple on/off. For example, on the left switch the center pin is the ground or supply. The top resistors maintain a minimal current flow (minimal voltage) so the ECU can tell that the switch is connected. Then depending on which switch is connected, the voltage will be different depending on how many resistors are in series for that particular switch. It is also possible that a minimal current flow is required to consider the connection good. Allowing some current to flow reduces the chances of bad readings due to EMF in the area. So to emulate it, you're going to need a DAC, and it's going to have to be able to output the required current. If you can somehow determine the resistor values then that will help you.
 

Thread Starter

john2k

Joined Nov 14, 2019
30
Thank you all for your replies. @Dodgydave I don't have access to the original switch to measure the resistance unfortunately.

I did some voltage measuring with the cluster turned on. And basically pins 2 and 3 both seem to read around the 5 point something volts. That's if I place the positive probe of my multimeter on to one of the pins and the negative probe onto the metal body of the cluster. Pin 4 for the ODO switch seems to also read 5 point something volts with a body ground. Does anyone have any suggestions as to what I can do to try and simulate a button press of any kind and try and figure things out?
 

Thread Starter

john2k

Joined Nov 14, 2019
30
Am I correct in understanding that pins 2 and 3 are being supplied a independent + voltage from the cluster and the switch is essentially linking both of them to ground but via resistors which allow the ECU to determine the voltage? So could I connect some sort of variable resistor across pin 2 and earth terminals and another variable resistor between pin 3 and earth and then try and adjust the resistance and see which does what? Is it safe to connect a resistor crossing over from + and ground? I thought usually resistors were inline on the positive line
 

Dodgydave

Joined Jun 22, 2012
8,466
Thank you all for your replies. @Dodgydave I don't have access to the original switch to measure the resistance unfortunately.

I did some voltage measuring with the cluster turned on. And basically pins 2 and 3 both seem to read around the 5 point something volts. That's if I place the positive probe of my multimeter on to one of the pins and the negative probe onto the metal body of the cluster. Pin 4 for the ODO switch seems to also read 5 point something volts with a body ground. Does anyone have any suggestions as to what I can do to try and simulate a button press of any kind and try and figure things out?
Then unplug the switch unit and measure the resistance at the steering wheel, otherwise you're going to have to use a Variable resistor to get the resistance values.
 

Thread Starter

john2k

Joined Nov 14, 2019
30
Then unplug the switch unit and measure the resistance at the steering wheel
That's what I meant, I dont have access to that particular switch. It's a second hand cluster. You mention I can try with a variable resistor and try and work out the resistance from there. Any particular variable resistor? Do I connect one end of the variable resistor to pin 2 and the other end to ground? and then another variable resistor on pin 3 and ground?

Is it safe to connect a resistor crossing over from + and ground? I thought usually resistors were inline on the positive line
 

Thread Starter

john2k

Joined Nov 14, 2019
30
OK some update: I've managed to located the technical datasheet for the switches itself and here is the list of resistance that the sheet specifies:

Left switch pushed = Below 2.5 Ω
Up switch pushed = 313 to 345 Ω
Down switch pushed = 950 to 1050 Ω
Right switch pushed = 2955 to 3265 Ω

Enter switch pushed = Below 2.5 Ω
Top switch pushed = 313 to 345 Ω
Back switch pushed = 950 to 1050 Ω

From looking at this it seems the enter, top and back switch are on their own positive power line which get a resistance value and the left, up, down and right are on their own line too. What's the easiest way to replicate these values?

For the first 4 resistance, can I just buy resistor lets say 2ohm, 320ohm, 980ohm, 3000ohm? Do I need a particular type of resistor and any particular wattage?
 

Thread Starter

john2k

Joined Nov 14, 2019
30
So i purchased a few packs of 1.5, 330, and 1k. I couldnt find 3k. it's either 2.7k or 3.3k which doesnt fall in the parameters of the right switch pushed value. Can i use 3x 1k resistors to make 3k ohms? how will i wire it? one after the other or all the legs on one side together and the other 3 legs on the other side to the negative?
 

Thread Starter

john2k

Joined Nov 14, 2019
30
Yes 3K use 3x 1K in series or use 2x 6K in parallel,, anything less than 10 ohms just link it out. Use 1/4 W.
Thank you. I will try this out as soon as the resistors arrive. So the 2.5ohm one I should be able to just link the + and ground pins together to create that? It won't short?
 
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