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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 04-07-2013
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MD7A Tach Sending Unit

Hi all. I've asked this before, but does anyone know (for sure) what the AC voltage is supposed to be off the original tach sending unit on the Volvo Penta MD7A engine? Mine produces
.062-.080 volts AC from low idle to a high idle, which would seem low, but I'm clueless as to what voltage is needed to power the tach. At least it increases with engine RPM, which is supposed to happen.

The reason I am asking is I installed a new tach from Faria and am not sure what AC voltage is needed to power the new one. I may have to buy another sending unit from them to make sure this thing starts working. On that note, does anyone know if the Faria sending unit will directly replace the existing unit on this engine?

By the way, that's the new engine instrument panel I just put together out of a piece of white acrylic. :-)
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Old 04-07-2013
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Re: MD7A Tach Sending Unit

Does the tach run off your alternator? or is there a separate pickup (on the flywheel, probably)
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Old 04-07-2013
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Re: MD7A Tach Sending Unit

The existing pick up is off the flywheel, or something inside the back of the engine case, because it is not on the flywheel you can see on the front of the engine anyway. The existing pick up has two wires coming out of it from the upper right (looking toward back of boat) side of the engine.
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Re: MD7A Tach Sending Unit

The flywheel is generally at the back of the engine.. between engine and gearbox, it's what the starter contacts when turning the engine. The tach pick-up in that housing is either counting teeth or ferrous slugs in the outer perimeter.

The frequency is probably more important than voltage, but if the signal is weak it's possible the pickup has backed out (vibration?) and is no longer close enough to the flywheel to get a solid signal.

You'll have to get data for the new tach and see what kind of input it's looking for - I'm surprised it didn't come with it's own pickup, which makes me think it's meant to use the stator output of the alternator. But some research is in order.

EDIT: Is this the one you ordered?

Quote:
TD9733
33818
7 59266 33818 0
USA
N
0.68
4
Tachometer (4000 RPM) (Diesel) (Mag Pick-Up)
Looks like it's meant for your style of pickup....
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Last edited by Faster; 04-07-2013 at 08:40 PM.
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Old 04-08-2013
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Re: MD7A Tach Sending Unit

Faster, it is actually this one: Faria, 4 In. Tachometer (4000 RPM) (Diesel) (Mechanical take-off & var ratio alt) Dress White, Model 33142.

By the looks of things, it will either work off the alternator or a mechanical (magnetic ?) pick up. The original tach was a VDO Tach and it ran off the mechanical pick up. I would prefer to keep using the same wiring from that to this new tach. I have voltage, it is just very low, or normal, if .062-.080 is normal anyway. I will have to see if the existing sender may have an adjustment for proximity to the gear or magnets.

Note my "flywheel" is actually on the front of the engine and the starter motor is up front too on the lower right front of the engine. The pick up, however, is mounted to the upper right rear of the motor as you look toward the back of the boat and is not near the flywheel, so there must be another gear with teeth back there in the engine case.

Thanks for replying and trying to help anyway. If you have any ideas, I would welcome anything.
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Re: MD7A Tach Sending Unit

Was there no manual with the instruments? if it's capable of reading either a pulse from a magnet or an AC alternator signal, maybe there's a switch somewhere to select the input type? Or at least information on what input it needs to function?

Found this on-line: http://faria-instruments.com/site_manuals/IS0030D.pdf

It talks of very low sensitivity, so the low level signal shouldn't be a problem... read through the manual and it refers to adjustability/calibration settings too (in the Addendum section). I'd expect/hope you received a copy of a similar manual with your new instruments.
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Old 04-09-2013
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Re: MD7A Tach Sending Unit

Thanks again Faster. I did get the installation manual. The sending unit I currently have installed has two wires from it. Based on the instructions, I attached those wires to the tach in the correct locations, but still did not get a functioning tach afterward. I am getting power to the needle, because it moves slightly when the ignition switch is turned on, just no rpm readings. Thus, the questions about voltage being produced by the sending unit in my first post.

In the meantime, I have been in touch with Faria. They advise the voltage necessary starts at 2 volts, far greater than I am getting now. Because of that, I have ordered a Faria sending unit, the Faria 90901. I'll see if that is the answer. If not, and if I still can't get the thing to work somehow, I'll just have to do with a pretty white non-functioning tachometer in the instrument panel. :-)
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Old 04-09-2013
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Re: MD7A Tach Sending Unit

How are you measuring the sender output? I'd think you'd need an oscilliscope or something similar, not a DVM (due to the nature of the signal).

Still, if the sender is bad why not use the alternator as the tach source? You mentioned that the tach supports types of input.

BTW he panel looks great. I like the white gauges.
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