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Discussion Starter · #1 ·
Most of the time I try my best to maintain my old ship and its motor, a Volvo MD17c (1977) but I don't need lessons like this.
The tacho dropped out and was not reading.
I looked at the wiring diagram below, and looking to the leftmost terminal behind the tacho.... it says it is +12V .... I used my multimeter and it read only about 0.3 V (or something)... way too low.
"So", said I, "I have lost continuity with the power supply. I will rig up a shotgun wire from a 12V source and re-establish power to the leftmost terminal".
So I did, and immediately I got a wisp of smoke telling me not to.
"How can that be?", said I, "all I did was apply 12V to a terminal that was supposed to accept it.

So I powered down all, and pulled the wires off the back of the tacho.

In the fine print embossed there, and barely visible was not + G W -, but, wait for it.... G W + -.

Yes, I have just put 12 V on the crankshaft position sensor blowing it wide open and probably blowing the tacho too.

Now what? From where the heck will I source that lot, and what will that cost?

So, in future, what do you believe? The wiring diagram (below), or the tiny details on the back of the tacho?

You know folks, sometimes I want to give up.

Rockter.
 

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Don't fret. I've been through this all. Tach was under rereading... checked with a laser tach... In the process I determined that it wasn't the sender... (replaced that first)... it was actually the tach. Could only find a used replacement. I think G & W are gray and white wires going to the sender. They might both be gray. Each gauge needs 12v for the light.
In the process... one project leads to the next... I decided to redo the panel... and made a wiring diagram. I replaced the Oil Pressure sender not the gauge (returned). And found a used tach.

The engine is a Volvo MD17D so this diagram may be helpful:

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Discussion Starter · #3 ·
Sander.....

Thanks.

A used tacho is a King's ransom. They are very expensive and no guarantee they will work.

What about an after-market tacho' like the one below?

I am puzzled how it manages to read the correct revs for all flywheel tooth speeds but it does, apparently. There must be some form of calibration possible.

Rockter.
 

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Sander.....

Thanks.

A used tacho is a King's ransom. They are very expensive and no guarantee they will work.

What about an after-market tacho' like the one below?

I am puzzled how it manages to read the correct revs for all flywheel tooth speeds but it does, apparently. There must be some form of calibration possible.

Rockter.
I got a money back guaranty from two sellers... and I did return a few until one worked.
I wanted one that fit into my instrument panel and did not consider an after market.

Let us know how it works out for you.
 

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Discussion Starter · #5 · (Edited)
I got a money back guaranty from two sellers... and I did return a few until one worked.
I wanted one that fit into my instrument panel and did not consider an after market.

Let us know how it works out for you.
Sander....

I will go with a Chinese unit, an 85 mm model complete with 18 mm sender probe.
I will have to enlarge the instrument aperture a little to accommodate it, but it should not be that difficult.
I also will have a portable hand-held RPM sensor to calibrate the tachometer.
It has to be adjustable, somehow, as it is effectively a gear tooth-counter and must be calibrated to the particular motor.

The Volvo wiring diagram in the manual is simply and subtly wrong. Best not to learn it the way I did.

Rockter.
 
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