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Thread: Round MFD

  1. #1

    Question Round MFD

    My 94 SLT750 doesn't have the MFD and it's a pain not having one. I was thinking about trying to make up my own instramentation. I want over temp indication, fuel level and a tach. From what I have researched, the temp sensor is just a contact closing, so that is easy. Anyone know what the analog signal range on the fuel sender is? I guess I could measure it, but the ski is up at the lake. As far as the tach goes, anyone know how many pulses per revolution the signal is? It seems that outborad 3 cylinder 2 strokes are 20 pulse. Is this the same for the Polaris 750?

    I see some have used a tiny tach. The look really cost effective, but the base model updates every 2.5 seconds. This seems quite slow for trying to tune a two stroke. Do they work OK for this application or should a person spend the extra cash on the commercial model which updates every half second?


  2. #2
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    It would be cheaper to just buy a round MFD,Check with John Zigler,a member here.

  3. #3
    Quote Originally Posted by Keddano View Post
    It would be cheaper to just buy a round MFD,Check with John Zigler,a member here.

    I know, but I have cheap access to some parts that I might be able to make work. It's more of a challenge thing than least cost. I just need to know what types of signals I have to try and make work.

    However, a $40 tiny tach would be tough to beat if it works OK....

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Yammerhead View Post
    ...MFD...thinking about trying to make up my own instrumentation.

    I want over temp indication, fuel level and a tach. From what I have researched, the temp sensor is just a contact closing, so that is easy.

    Anyone know what the analog signal range on the fuel sender is? I guess I could measure it, but the ski is up at the lake.

    As far as the tach goes, anyone know how many pulses per revolution the signal is?
    ...Is this the same for the Polaris 750?...
    We might get along nicely

    Fuel and oil level senders are variable resistance. 33 ohms = full, 240 ohms = empty.

    The level sender has multiple internal reed micro-switches, and switches the resistance value as the fuel level rises or falls. I think there are 8 or nine discreet resistances. I can check if you need to know exactly how many steps there are.

    Voltage supply to the level senders must be limited to 3 volts or less. This prevents the possibility of a spark inside the fuel tank, even if the sender goes open circuit.

    Engine RPM is ten times Hertz value. 120Hz signal is 1200RPM.

    All Polaris PWC have the same fuel and oil level sender values, and the same RPM signal to frequency ratio.

    The RPM signal on the Yellow wire is not a clean sine wave. The same Yellow wire is also used by the LR voltage regulator module to control battery charging current, so the waveform on the Yellow wire is an odd sort of half-wave pseudo rectified AC.

    The LR module controls the battery charging current (and therefore the battery charging voltage) by controlling the connection to ground on the Yellow wire.

    When the engine is not running, the Yellow wire has battery voltage present. To power your display, tap into the Red/Purple wiring, and use a low Amp fuse for your display. Polaris used a 1/4 Amp fuse.

    Make sure your design can handle reverse battery polarity on both Red/Purple and Yellow wires. Sometimes batteries get installed backwards

    The killer with Marine electronics is long term survival, despite the ever present moisture, engine vibration, hull impacts, weather exposure, user abuse, corroded conenctions, and the passage of time.

  5. #5
    **Update**

    I ended up buying a MFD from a member. I finally had some time to plug it in and try it. It works not too bad, the display is faded a little, but it is probably good enough. However, the fuel guage does not show any bars. I pulled the sender and it tests OK, 30 to 240 ohms. When I measure the open circuit voltage between pink and black, I get about 8V, although it swings around a bit. As soon as I put the sender back in the circuit, the voltage goes to zero. Well, almost, I can actually read between 4 and 40mV across the sender as you move it across it's range of motion. Any ideas? The MFD appears to supply the voltage to the sender, but seems to collapse as soon as you connect the sender. Do I have to look into the MFD next?

    One other thing I noticed. One time in the many that I powered the MFD up and down, it powered up with the fuel / oil bars and the top numbers display on, and VERY BLACK. I wasn't sure what was happening, so I pushed the mode button for a while and eventually it went back to the faded "normal" display. Any idea what was going on? It would appear the MFD does have the ability to display with good contrast. Is there some way to adjust it????

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Did you buy a round or oval MFD?

    The oval MFD are known for faded digits, and can often be repaired.

    The round MFD might be repairable, but there is less information about how to do so. They are constructed differently inside, so the fix (if it can be fixed) would be different.

    Lots more info here;
    Multi-Function Display (MFD)

  7. #7
    It's a round MFD that I have. I messed around with it some and now have the fuel guage working. I gooped a bunch of dielectric grease in the connector on the back of the MFD when I first connected it. I think I had too much and was making a bad connection. Cleaned it up a bit and the guage now works.

    The faded digits are still an issue. I have to wonder if it has the same issue of a bad connection like the oval MFD. Sometimes, after cycling power to the MFD, it still comes up with some of the display on, very black. I snapped a couple of pics. I don't know what I did to make them display dark. It seems random, but makes me think the connections to the LDC screen must be OK. Must be another component then? Anyone have any experience with these?
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  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Arrow Dielectric grease is an electrical insulator - keep metal contacts grease free

    Quote Originally Posted by Yammerhead View Post
    It's a round MFD that I have. I messed around with it some and now have the fuel gauge working.

    I gooped a bunch of dielectric grease in the connector on the back of the MFD when I first connected it. I think I had too much and was making a bad connection.


    Cleaned it up a bit and the guage now works...
    Many people think of dielectric grease as some sort of electrical connection enhancer. Dielectric grease does not improve electrical connections.

    Dielectric grease is an insulator - it does NOT conduct electricity. That is why it is called dielectric.

    The primary use of dielectric grease is to lubricate and seal the perimeter of electrical connectors. Many water-proof connectors have a silicone or rubber outer seal, and the grease can be used to lubricate this seal.

    If the connector body has openings for the individual connector pins that extend from the outside to the inside, the grease can be used to plug the holes, so that water cannot creep into the connector along side the wires or pins. The grease should not be coating the contact portions of the pin mating surfaces.

    Applying dielectric grease directly to metal contacts or pins is not a good thing. If the grease gets between the metal to metal points of contact, it can reduce or prevent the metal to metal electrical contact, increasing contact resistance and sometimes even preventing proper operation.

    There are products designed to enhance electrical connections; dielectric grease is not one of them.

    The entire purpose of dielectric grease is to seal and lubricate the perimeter of an electrical connector, and protect the actual contacts from exposure to water and/or air. The metal contacts themselves should be free of dielectric grease.

    If you have contacts with excessive dielectric grease, they can be cleaned using an electrical contact cleaner, preferably one that does not leave any residue.

    After an electrical contact has been tightened or assembled, you can smear dielectric grease around the outside of the connection, and it will keep water and air from corroding the connection.

    If you apply dielectric grease to metal contacts before assembly, then you are relying on the forces within the connection to squeeze the grease out from between the metal surfaces, and the only actual electrical connection will be in those small spots where all of the grease has been displaced. Even a very thin layer of dielectric grease will not conduct electricity.

    As long as the connector mating pins have sufficient internal mechanical pressure to force the grease aside and make solid metal-to-metal contact, the dielectric grease does not cause a problem. When the contact's internal pressure is not sufficient to squeeze the grease away, then the grease does cause trouble.

    If the connector is old, and has some internal corrosion, then it is more difficult for the connector to create a solid metal-to-metal contact, as the corrosion prevents some portions of the contact from actually touching. If there is grease in other portions of the same connector socket preventing contact, then there may be no actual electrical connection.
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  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Arrow How to cut open a round MFD display

    Quote Originally Posted by Yammerhead View Post
    ... The faded digits are still an issue. I have to wonder if it has the same issue of a bad connection like the oval MFD.

    Sometimes, after cycling power to the MFD, it still comes up with some of the display on, very black. I snapped a couple of pics. I don't know what I did to make them display dark. It seems random, but makes me think the connections to the LC screen must be OK. Must be another component then?

    Anyone have any experience with these?
    If you cut open the round MFD casing, you may be able to repair the display.

    The cut line would be around the perimeter of the thick rim portion of the display casing, about 5/64" in front of the rear lip of the front bezel. The case wall is about 3/16" thick.

    I used a Dremel cut-off wheel - cut slowly and carefully, as there are wires and such right inside there. The part number on my round MFD is 3280147.

    Update: BryanP uses a thin blade handheld trim saw which cuts on the pull motion. Used with care and perhaps a jig to keep the blade aligned, it is possible to make a clean cut groove through the plastic.

    Care must still be taken when working in the ribbon cable vicinity directly below the two buttons.

    Refer to this thread for photos and more details.

    The LCD is electrically connected to the circuit board using a pair of conductive rubber strips. These strips are conductive in only one dimension, and act like many tiny parallel conductors.

    Sometimes the connection between the conductive rubber and the circuit board (or the LCD) weakens, and the display segments fade.

    If you disassemble the display, and clean the contacts and conductive rubber strips, you can often reinvigorate the display. The rubber strips are fragile, as they are made using carbon.

    I use chemical contact cleaner on the circuit board, and perhaps a white eraser (gently).

    Do not use any chemicals on the conductive rubber strips, and only the mildest of mechanical cleaning (gentle rubbing with a clean non-shedding cloth is all they usually need).

    Be sure to keep your fingers off the contact surfaces, as skin oils can cause trouble. Work in a clean area.

    The LCD is fragile, as are the rubber strips, so work carefully.

    Last edited by K447; 08-14-2014 at 12:17 AM.

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