Yanmar 2GM tach

[dennismenace111]

Im having a strange issue with my tach on my 1987 Yanmar 2gm20F.

In the morning when I start up the engine the tach operates perfectly. I have about a 2 mile cruise down the river, and must open 5 drawbridges to get out to open waters. I cruise down the river at about 75% power till I come to that first bridge, then idle back to about 950 RPM..... the tach dies. It goes to zero.

Its a Yanmar 2GM20F with a brand new tachometer running from a Mag sensor on the fly wheel. Ive checked all the connections, cleaned the connection plugs...

When the tach operates normally, I get about 1.8v alternating current from the two wires coming from the magnetic pickup. When it dies, I get zero volts. NADA, ZILCH... What would make the magnetic pickup stop when I throttle back?

If I let the boat sit overnight, it works fine again in the morning till I run the engine for 20 minutes or so at cruise RPM and throttle back quickly.

Could the magnetic sensor be overheating somehow?

[Sailmonkey]

It may be possible that the pickup isn’t close enough to the flywheel. When the bell housing heats up and expands the pickup has moved further away from the flywheel.

Pure conjecture on my part.

[dennismenace111]

It's either related to that or possibly the sensor itself or the wiring is getting hot and resistance is going up...it's only 1.8 volts. It won't take much to drop that to zero...the interesting thing is that it quits as soon as I back off the throttle to idle! Then it won't work again till it sits over night.

Maybe next time it happens, I'll put an ice pack on the sensor to see if that changes anything! It may identify the issue.

[dennismenace111]

Measured again today. I'm getting 1.8v AC on the tach sensor leads at the plug on the instrument panel. On the back of the rack, I'm getting only 1.4 volts AC.

I guess when I throttle back, it doesn't get enough voltage to the tach and it dies. But why when I throttle up again, the tach diesnt come alive again till it sits over night.

[Sailmonkey]

Is the tach sender screwed all the way down? Or is it spaced away from the flywheel with some washers?

[dennismenace111]

It appears to be. It's in a Watkins 29... One must be a contortionist and 15 years old to be able climb through the hatch in the lazarette to reach it....or from the front of the engine, one can actually go into the engine dungeon head first, laying on top of the engine. It's difficult to ascertain! I can only assume that it is! Short of pulling the engine, it's going to be difficult to put a feeler gauge in there to find out!

[Wotname]

I don't think it will be sensor to flywheel teeth distance problem unless the sensor has come loose.

It might be heat related but again I think unlikely.

The 1.8 volts seems to me to be reasonable but not the 0.4V drop. There is almost no current flowing in this signal wire so a drop of this magnitude suggests a very high resistance connection in the wire, most likely corrosion at the engine harness connector.

My take is that when it is running at cruise rpm, the engine is relatively steady but when you reduce RPM to idle, the engine vibration increases causing the poor connection to become virtually open circuit. I know my 2GM20 could get some shakes going around idle rpm.

Go looking for a corroded plug / socket or loose terminal.

Just a thought .

[dennismenace111]

I've opened and checked every plug in line from the sensor to the tach. When I throttle back to read idle, the tach quits and won't indicate again till it sits for an extended time....usually overnight. That's the wierd part. The tach is brand new.

[Wotname]

If the wires are good and the tach is new, it sounds like the sensor might be intermittent - perhaps heat or vibration related.

[dennismenace111]

My thoughts exactly.... But it's basically a magnet with two wires. Not alot to fail.

[Wotname]

Or more likely a Hall Effect sensor which has solid sate components.

[dennismenace111]

It doesnt look like there are any components other than a magnet and two wires...

As I understand the operation, these sensors use HALL effect to count the teeth and transmit alternating positive and negative current to the tach. I dont know that the Yanmar sensor would have any components other than a magnet.

[a64pilot]

Originally Posted by Sailmonkey

It may be possible that the pickup isn’t close enough to the flywheel. When the bell housing heats up and expands the pickup has moved further away from the flywheel.

Pure conjecture on my part.

I think you may have something there, back in the day when automotive electronic ignition first came out they used the standard distributors and put a Hall effect sensor in place of the points and a paddle wheel reluctor where the cam that operated the points used to be, anyway the gap between the two was pretty critical and if you have seen a feeler gauge with one brass leaf in it, that’s what it was for, you didn’t use a steel blade because of the magnet, you used the brass leaf. But a little too much gap and the ignition would begin to misfire.

It’s worth checking anyway.

I would expect that excessive gap would give erratic RPM readings though.

His failure does seem to be heat related and coils do break down when hot, and he has a voltage reading straight from the sensor, that goes away when the tach quits working.
I believe he has almost certainly trouble shot the problem down to the sensor, or that’s where my money is anyway.

[Wotname]

Originally Posted by dennismenace111

My thoughts exactly.... But it's basically a magnet with two wires. Not alot to fail.

Originally Posted by Wotname

Or more likely a Hall Effect sensor which has solid sate components.

Originally Posted by dennismenace111

It doesnt look like there are any components other than a magnet and two wires...

As I understand the operation, these sensors use HALL effect to count the teeth and transmit alternating positive and negative current to the tach. I dont know that the Yanmar sensor would have any components other than a magnet.

I guess it's times like these to Read The Manual
Yanmar simply describes it as a MPU (Magnetic Pick Up) sender. Although there is two wires on the terminals, the case is earthed potentially making it a 3 wire terminal. The internal resistance across the the two terminals should be 1.6Kohm and a minimum output voltage of 1V(AC). The gap should 51 thou.

The wiring diagram appears to have a typo as it shows the two wires joined together at the tacho end but I know this is incorrect!

[DougR]

Sometimes these inductive pickups become erratic because they build up a layer of ferrous particles on the end of the pickup and this prevents a "clean" flux break from the gear tooth to the air gap between teeth.

Try cleaning the magnetic tip of the sender.......

DougR