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Controlling stock oil temp gauge

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    Controlling stock oil temp gauge

    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.
    e30sport.net
    '15 Porsche GT3 - 7-speed PDK - Daily Driver
    '86 325es - s54b32tu - 6-speed - Mtech 1
    '89 325is - m20b25 - 5-speed - Individual

    #2
    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.

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      #3
      Originally posted by bmwman91 View Post
      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.
      e30sport.net
      '15 Porsche GT3 - 7-speed PDK - Daily Driver
      '86 325es - s54b32tu - 6-speed - Mtech 1
      '89 325is - m20b25 - 5-speed - Individual

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        #4
        Yes, it does...power/ground/sender. Order up a factory M3 temp sender and install it.
        "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

        85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
        88 M3 - LACHSSILBER/SCHWARZE
        89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
        91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

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          #5
          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.


          All other E30 wiring diagrams:

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            #6
            Originally posted by bmwman91 View Post
            a reed switch in the differential.
            I'd be surprised if it was a reed switch.
            Originally posted by Matt-B
            hey does anyone know anyone who gets upset and makes electronics?

            Comment


              #7
              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?
              e30sport.net
              '15 Porsche GT3 - 7-speed PDK - Daily Driver
              '86 325es - s54b32tu - 6-speed - Mtech 1
              '89 325is - m20b25 - 5-speed - Individual

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                #8
                I assume it originally worked on resistance of the oil pressure sender. How about one of these suckers or similar.

                Potentiometers are incredibly useful, whether you're controlling the volume on your stereo or the 'mood lighting' in your room. The problem with traditiona


                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?

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                  #9
                  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.
                  Originally posted by Matt-B
                  hey does anyone know anyone who gets upset and makes electronics?

                  Comment


                    #10
                    Originally posted by george graves View Post
                    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?
                    e30sport.net
                    '15 Porsche GT3 - 7-speed PDK - Daily Driver
                    '86 325es - s54b32tu - 6-speed - Mtech 1
                    '89 325is - m20b25 - 5-speed - Individual

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                      #11
                      What is the PWM frequency?

                      How clean are the oil temp values given by the CAN bus? As in, do you need to do some digital averaging on the received values, or are they pretty well low-pass filtered for noise before the MSS54 sends them? I'd assume that the ADC input stages in the ECU have at least a little filtering on them, but who knows.

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                        #12
                        I'm a little confused what you're asking at this point. Is your issue driving the gauge? Or? Can we start over? :)
                        Originally posted by Matt-B
                        hey does anyone know anyone who gets upset and makes electronics?

                        Comment


                          #13
                          Originally posted by george graves View Post
                          I'm a little confused what you're asking at this point. Is your issue driving the gauge? Or? Can we start over? :)
                          Sure. SO I built a gauge for the center vent like an Alpina gauge. Some of the info for it I am pulling from the s54 CAN bus (intake temp). I have access to the everything being sent on the CAN bus including oil temp. Oil temp is broadcast every 100ms in 8bits on 0x720. the 8 bits, bin->dec then subtract 48, corresponds to degrees C which the reading the ECU is using for oil temp. So if the ECU sends 0b11001000 = 0xC8 = 200 = 152degC.

                          I am running an e30 M3 gauge cluster that has the oil temp gauge built in. I know to get this gauge to function I just need to hook up the e30 M3 temp sender which is a thermister essentially. It seems wasteful to have 2 temp senders: one for the MSS54 ecu and data log, and another just for the gauge. Ideally I thought it would be more efficient to have the module that was receiving CAN and logging everything to control the cluster oil temp gauge so the temp displayed is the oil temp that the ECU is actually using. Sanity check on what the ECU is seeing as well as monitor oil temp

                          The microcontroller that I am using for the gauge has a PMW out for 10bit resolution. I have successfully used the PWM to drive a transistor on the signal line for the e30 M3 oil temp gauge and control it. So my next step was going to be to make a table to compare the oil temp I am receiving to what the PWM out should be.

                          The jump between pwm steps at the high end of the gauge, >210degF, gets a little choppy on the gauge. Its nonlinear from PWM steps to what is displayed on the gauge. But I figure that isnt that big of an issue since temps getting much above that mean I have bigger cooling issues.

                          For this thread I am asking for a sanity check if anyone has feedback or knows of a better way to do this. Or in the alternative tell me I am crazy and just to hook up the e30 M3 sender.
                          Last edited by BeirBrennerE30; 11-09-2018, 09:51 AM.
                          e30sport.net
                          '15 Porsche GT3 - 7-speed PDK - Daily Driver
                          '86 325es - s54b32tu - 6-speed - Mtech 1
                          '89 325is - m20b25 - 5-speed - Individual

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                            #14
                            So at the high temperature end, due to the non-linear transfer function, a one LSb change in the oil temperature value leads to a large jump in the PWM duty cycle / needle position? If so, there are a couple of fairly easy solutions since the needle is a relatively slow device (compared to the 100ms sampling period of the input).

                            What type of microcontroller are you using?

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                              #15
                              Originally posted by george graves View Post
                              I'd be surprised if it was a reed switch.

                              IIRC it was called out as such in my old Chilton's guide which I can't find. Searching around the web, it seems to be a common arrangement on older BMW's, including the E30. One of the two "prongs" in the sender contains a magnet, the other the reed switch, and when one of the vanes on the diff sender wheel passes between, it interrupts the magnetic flux and opens the switch.


                              /Hijack

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