Controlling stock oil temp gauge

[BeirBrennerE30]

Digital pots are almost never the answer - in almost any circuit. They can only handle a tiny amount of current. And with anything automotive, you need to expect huch voltage spikes and even negative voltages. 100v volt spike or a -50v spike isn't uncommon on when working with automotive electronics.

Agreed with that. I may change out the transistor for a fet (any recommendations?). I have a 10ohm resistor inline with it so it never shorts out. Appears to work and likely more accurate than the stock sender would be.

Anyone see an issue I am missing?

[george graves]

Digital pots are almost never the answer - in almost any circuit. They can only handle a tiny amount of current. And with anything automotive, you need to expect huch voltage spikes and even negative voltages. 100v volt spike or a -50v spike isn't uncommon on when working with automotive electronics.

[e30davie]

I assume it originally worked on resistance of the oil pressure sender. How about one of these suckers or similar.

https://www.sparkfun.com/products/10613

Ive never used one, but if it does indeed work on resistance then i reckon it might be the go. That one is a 10k one. I assume you can get them in different sizes. if it were me i would get it working with a manual potentiometer first. then you know what size you need.

Alternatively if you already have it going with your pwm, can you put a capacitor on the output to help smooth it out?

[BeirBrennerE30]

Alright I used a transistor and had a microcontroller PWM it and I can control the oil temp gauge now but it is a little jumpy. Granted there is only 10bit resolution and its not linear on the gauge. Probably not a big deal because over CAN it is only 8 bit resolution for oil temp from the MSS54.

Anyone see an issue with this setup or a better way to do it?

[george graves]

a reed switch in the differential.

I'd be surprised if it was a reed switch.

[bmwman91]

Ah, sorry, I thought you were trying to repurpose the econometer. reelizimpro has it right for using the other analog gauges.

Page 6210-2 shows how the oil temp gauge is connected.

http://www.armchair.mb.ca/%7Edave/BMW/e30/e30m3_90.pdf

All other E30 wiring diagrams:

http://www.armchair.mb.ca/~dave/BMW/e30/

[reelizmpro]

Yes, it does...power/ground/sender. Order up a factory M3 temp sender and install it.

[BeirBrennerE30]

The econometer is controlled by two signals: vehicle speed and fuel injection duty cycle. The speed signal is routed internally in the cluster's main PCB, so that trace would need to be cut. It is a signal which is pulled-up to 12V and switched to ground by a reed switch in the differential. For the injector duty cycle, there is a pull-up to 12V on the input which is switched to ground in the ECU. The low-time matches the time that the injector is open.

So for your application, you would want to feed one of these a constant duty cycle signal and then vary the other. It'll take some experimenting to figure out the right values.

Thanks for the reply. I think the oil temp gauge is different and has a separate power, ground and then the sensor signal. The sensor being a normal thermister temp sensor. So I guess its about how best to replicate that temp sender.

[bmwman91]

The econometer is controlled by two signals: vehicle speed and fuel injection duty cycle. The speed signal is routed internally in the cluster's main PCB, so that trace would need to be cut. It is a signal which is pulled-up to 12V and switched to ground by a reed switch in the differential. For the injector duty cycle, there is a pull-up to 12V on the input which is switched to ground in the ECU. The low-time matches the time that the injector is open.

So for your application, you would want to feed one of these a constant duty cycle signal and then vary the other. It'll take some experimenting to figure out the right values.

[BeirBrennerE30]

Hello r3v. I have an M3 cluster with the stock oil temp gauge. I am collecting date from the canbus (s54 swap) for logging and display on a custom Alpina style digital gauge and I figured since I had access to the oil temp canbus data it didnt make sense to install another oil temp sender on the engine.

I know people have converted the econometer into boost gauges and stuff like that. What method of control was used for the gauge. Is 10bit PWMing a mosfet on the sensor line enough to drive this? I didnt want to reinvent the wheel since I know people are already doing this.