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Thread: HM Tach

  1. #1
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    HM Tach

    I’m finally going to install my new-old HM gauge panel, and I need some help.

    When I purchased my boat way back when, my then-original HM tach was inop. I installed Teleflex Heavy Duty gauges to replace the originals. I wired the tach like I’ve always done. Boat would not start, it would kick then immediately die. Someone on the Donzi board posted that the old tachs were wired strange, (the ignition ran through the tach maybe?) I don’t remember how I did it, but I rewired the tach to make it work.

    Now I am wanting to install an old school panel. The main plug is exactly the same, (the only difference in the new panel is the lack of fuel gauge, I think it was for a 16. Im still looking for one to match…). I would like to be able to just plug it in, but I am a little worried about frying the pristine 45 year old tachometer…

    I haven’t traced the wires out yet, but could someone explain how the originals were wired? I want to undo the “fix” I made to the wiring before I hook it up…

    Thanks!!

    Jim
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

  2. #2
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    Jim,

    there were a few wiring variations with the ammeter and the external volt reg not sure if that included the HM tach. I might have a diagram in all my stuff I'll check.

    What are the lugs labeled on the tach? they usually are I,S and G
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  3. #3
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    this given to me by Gary S when i was working on my wiring

    it shows a lead to the ign switch "I", a lead to ground "G" , a lead to the coil "S" and it also shows a lead to the light switch depending on the gauge some have a short pig tail directly to the bulb and the bulb picks up ground thru the gauge casing like stewart warner, or the gauge has an internal wire and a lug for the bulb like teleflex. hope that helps
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    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



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  4. #4
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    You should find the info you need in this thread or this one. Too bad some of the photos are missing...


    Frank
    Triple Hatch.
    It's a classic Donzi, not a Donzi Classic.

  5. #5
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    No lugs on the old tachs. Just 4 loomed wires coming out of the same hole, yellow blue green and white... My panel is a factory pre- wired unit, with a white eight lug plug that plugs into the wiring harness.
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

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    Penbroke, thanks for the links! Thats what I was looking for!!
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

  7. #7
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    Is there a special sender I have to use for the old gauges to work?

    I plugged the panel in and started the engine… the 45 year old panel lives!! Lights work, tach works, switch works! Amazed!

    The oil pressure gauge is not working. I have an 80psi Teleflex sender on there now, and I cannot get a reading. (I know the pressure and sender are good, plugged in the other gauge…)

    Any ideas?
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

  8. #8
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    the sender impedance needs to match the gauge, each mfg'r has their own range usually pretty close. most are 0-240 ohms

    240 ohms = 0psi
    0 ohms = gauge pegged on the high side usually around 30 ohms will be full gauge rating ie 80 psi on a 80 psi gauge (on stewart warner stuff)


    have you verified the gauge? have you measured the sender?
    not sure what the specs are on the old hm stuff . if your sender is a teleflex i would see if you can find a stewart warner sender and see if that makes a difference.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



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  9. #9
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    I haven't measured anything but the voltage going into the gauge.

    Is there a specific 80psi sender that HM used for this gauge panel? When I bought the boat, it had aftermarket (Teleflex I think) gauges. I'm assuming the sender is a Teleflex. Works great on the Teleflex Heavy Duty oil pressure gauge currently installed on the boat.

    Thanks!!
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

  10. #10
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    not sure who made the gauges or senders for holman moody.

    to verifiy the sender with a meter in ohm reading, ground side of meter to engine block positive lead to sender terminal with the wire to the gauge off.
    with no oil pressure reading should be 240 ohms, then start the motor and let pressure build then take the same reading , approx 100 ohm should read half way on gauge 40 psi on a 80 psi gauge

    to test the gauge short the ground lead and the sender lead on the back of the gague with the ignition on, that should peg the meter on the high side . the other catch would be if the sender is for single or a dual gauge setup that can make a difference in senders output.

    I would assume you're going to have an issue with the water temp as well.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

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  11. #11
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    The gauges were made by Teleflex.
    1970 18 2+3 Hull #18-355 H/M 351W Volvo 250
    07 Cayman IT IS BACK! - '13 Abarth Cabrio

    PRESERVE, DON'T RESTORE

  12. #12
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    I would test the gauge to see if it moves at all, verify voltage , polarity, ground and sender leads. then i would contact teleflex and see what senders were spec'd for their late 60 early 70's gauges.

    you can id the sender you have there should be a code on the hex head of the sender( if it is teleflex)
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    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  13. #13
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    Could it be that since this is a never installed NOS (New Old Stock) HM/Donzi gauge panel that the oil pressure gauge never worked when new; and that is why Donzi never used it ?
    Or~
    Since it was never used could the oil pressure gauge's needle movement lube have dried-up & the needle is now stuck ?

    I would try putting full ground for a moment to the gauge's sender input as Matty suggested to see if the gauge is in fact good & to possibly jog a stuck needle movement loose.
    The needle should then peg at full scale.
    Just touch the full engine ground for an instant while watching that gauge needle to avoid a gauge burn-out.

    If you indeed have the incorrect sender you still should get at least some readings on that oil pressure gauge other than near zero.

    Check for a complete circuit on that molded-in harness plug also.
    Some of these molded-in plugs are not that great and can give you a a bad connection.

    I believe that these old Teleflex gauges used the same exact senders as the similar old vintage Stewart~Warner marine gauges.
    In your case you need an 80 PSI sender which is basically a variable resistor rheostat in a sealed can connected to a pressure idicating bordon tube.
    The sender needs a good screw in pipe engine ground to work correctly as it works as a one wire only connection to the engine's ground.
    The oil pressure gauge itself is a simple voltmeter calibrated in PSI to read the sender's variable ground voltage signal from it's internal rheostat.

    The temp gauge is basically a millivolt voltmeter that uses a self voltage generating thermocouple sender that varies out put voltage according to the engine's coolant temps..

    Matty~
    Great oil pressure sender ID chart you posted !
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  14. #14
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    Jim,

    let me be clear I didn't say apply voltage to the sender lead i said apply a ground or short the sender lead to ground. if your grounds are truely bonded it would be like the sender sending 0 ohms which will peg the meter on the high side.

    since your HD gauge works i would bet the older gauge works on a different range which would be outside of the newer more common range of 240-0 or the gauge is bad

    your gauges are hard wired so an easy test would be to remove the sender lead at the sender and hold it to the block in a spot that is a good ground. with the ignition on the gauge should move and peg. what voltage do you have between the ign lead and ground at the gauge?
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  15. #15
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    ~~sg nice edit~~

    the gauge is a simple meter but not a volt meter it is an ohm meter that measures resistance in a defined scale. just has your old analog sweep dial meter or new digital multi meter which has an internal battery and two test leads .

    the gauge(meter) has internal battery usually 12 volts on it's I ignition and G ground leads from the systems 12 volt battery then uses the ground and the S sender connections as test leads to measure the resistance between block ground and the changing resistance provided by the sender. so take your meter set it in the 10's ranges for ohms and try to measure a circuit outside that range either the gauge pin pegs usually a dead short ( 0 ohms) or it doesn't move at all, on a digital meter OL might be displayed. the newer gauges all look for a range of 0 to 240 ohms from the sender this setup is the same for the temp sender. if the range on the sender and the range on the gauge don't match but do overlap the needle may move and give a false reading , depending on where the ranges overlap the needle might not move at all.

    Jim
    your meter isn't working so these are the possible causes

    the meter's internal battery is bad :
    there is not the correct voltage on the I and G leads

    the meter has a broken test lead: the S lead is open i doubt this cause the old meter worked so the lead and the sender work

    the meter is in the wrong scale:
    the sender is in one scale and the meter is looking for another making the needle not move

    the meter is bad:


    Jim I went thru the same on my 16 wanted to keep the holman moody stuff but couldn't find the senders my holman moody oil gauge with a teleflex sender barely read 10 psi at wot and zero at idle ,mechanically the psi were at 37 psi wot and 19 at idle. I gave up on on trying to go old school and put in gauges and senders that were matched.

    good luck hope it works out for you.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

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